DOCUMENT RESUME

ED 120 448 CE 006 703

TITLE Aviation Weather for Pilots and Flight OperationsPersonnel.

INSTITUTION Federal Aviation Administration (DOT), Washington,D.C. Flight Standards Service.; National Oceanic andAtmospheric Administration (DOC), Rockville, Md.National Weather Service.

PUB DATE 75NOTE 224p.; Photographic material will reproduce poorlyAVAILABLE FROM Superintendent of Documents, U.S. Government Printing

Office, Washington, D.C. 20402 (Stock No.050-007-00283; $4.55)

EDRS PRICEDESCRIPTORS

MP -$0.93 RC-$11.37 Plus PostageAircraft Pilots; Aviation Technology; *ClimaticFactors; Environmental Influences; *Flight Training;Instructional Materials; Manuals; *Meteorology;*Safety Education; Textbooks; *Thermal Environment

ABSTRACTThe revised Aviation Weather book discusses each

aspect of weather as it relates to aircraft operations and flightsafety. The book is not an aircraft operating manual and omits allreference to specific weather services. Much of the book has beendevoted to marginal, hazardous, and violent weather. It teachespilots to learn to appreciate good weather, to recognize and respectmarginal or hazardous weather, and to avoid violent weather. Thepurpose of the book is to promote safety and to help the pilot torecognize potential trouble and make sound flight decisions before itis too late. The manual is presented in two parts. Part 1 explainsweather facts every pilot should know covering the following topics:the earth's atmosphere; temperature; atmospheric pressure andaltimetry; wind, moisture, cloud information, and precipitation;stable and unstable air; clouds; air masses and fronts; turbulence;icing; thunderstorms; and common IFR (Instrument Flight Rules)procedures. Part 2 contains topics of special interest discussinghigh altitude and arctic,_ tropical, and soaring weathers. tumerousillustrations, photographs, and diagrams are inter versed throughout.The document concludes with a glossary of weather terms and a subjectindex. (Author/Br)

***********************************************************************Documents acquired by ERIC include many informal unpublished

* materials not available from other sources. ERIC makes every effort ** to obtain the best copy available. Nevertheless, items of marginal ** reproducibility are often encountered and this affects the quality ** of the microfiche and hardcopy reproductions EPIC makes available ** via the ERIC Document Reproduction Service (ED '!4. EDRS is not* responsible for the quality of the original document. Reproductions ** supplied by EDRS are the best that can be made from the original. ************************************************************************

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For Pilots andFlight Operations Personnel

Revised 1975

of Mose SAer

ow I DEPARTMENT OF TRANSPORTATION*

,...

FEDERAL AVIATION ADMINISTRATIONc1: # Flight Standards Service

stitas of F31'

DEPARTMENT OF COMMERCE

NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATIONNational Weather Service

Washington, D.C.1

AC 00-6A

11

V.5%

Preface

AVIATION WEATHER is published jointly by the FAA Flight Standards Serviceand the National Weather Service (NWS) . The publication began in 1943 as CAABulletin No. 25, "Meteorology for Pilots," which at the time contained weatherknowledge considered essential for most pilots. But as aircraft flew farther, faster,and higher and as meteorological knowledge grew, the bulletin became obsolete. Itwas revised in 1954 as "Pilots' Weather Handbook" and again in 1965 under itspresent title.

All these former editions suffered from one common problem. They dealt input with weather services which change continually in keeping with current tech-niques and service demands. Therefore, each edition became somewhat outdatedalmost as soon as published; and its obsolescence grew throughout the period itremained in print.

To alleviate this problem, the new authors have completely rewritten this editionstreamlining it into a clear, concise, and readable book and omitting all referenceto specific weather services. Thus, the text will remain valid and adequate for manyyears. A companion manual, AVIATION WEATHER SERVICES, Advisory Circular 00-45,supplements AvigrroN WEATHER. This supplement (AC 00.45) periodically is updatedto reflect changes brought about by latest techniques, capabilities, and service demands.It explains current weather services and the formats and uses of weather charts andprinted weather messages. The two manuals are sold separately; so 'at a nominal cost,a pilot can purchase a copy of the supplement (AC 00-45) periodically and keepcurrent in aviation weather services.

C. Hugh Snyder, National Weather Service Coordinator and Training Con-sultant at the FAA Academy, directed the preparation of AVIATION WEATHER andAVIATION WEATHER SERVICES. He and his assistant, John W. Zimmerman, Jr., didmuch of the writing and edited the final manuscripts. Recognition is given to thesemeteorologists on the NWS Coordinator's staff who helped write the original manu-script, organize the contents, and plan illustrations: Milton Lee Harrison, EdwardA. Jessup, Joe L. Kendall, and Richard A. Mitchem. Beatri -te Emery deserves specialrecognition for her relentless effort in typing, retyping, proofing, correcting, andassembling page after page of manuscript. Many other offices and individuals havecontributed to the preparation, editing, and publication of the two volumes.

.4111

*,

Contents

Page

Preface IIIIntroduction XIII

PART I. WHAT YOU SHOULD KNOW ABOUT WEATHER

CHAPTER 1. THE EARTH'S ATMOSPHERE 1

Composition.. 2Vertical Structure.. 2The Standard Atmosphere 2Density and Hypoxia 3

CHAPTER 2. TEMPERATURE 5Temperature Scales... 6Heat and Temperature 6Temperature Variations 7In Closing 10

CHAPTER 3. ATMOSPHERIC PRESSURE AND ALTIMETRY 11Atmospheric Pressure 11

Alimetry 17IL closing 21

CHAPTER 4. WIND 23Convection 23Pressure Gradient Force.. 24Coriolis Force 25The General Circulation 26Friction. 30The Jet Stream 31

Local and Small Scale Winds 31Wind Shear 34Wind, Pressure Systems, and Weather. 35

CHAPTER 5. MOISTURE, CLOUD FORMATION, AND PRECIPITATION 37Water Vapor 37Change of State 39Cloud Formation 42Precipitation 42Land and Water Effects 43In Closing... 45

CHAPTER 6. STABLE AND UNSTABLE Am 47Changes Within Upward and Downward Moving Air 47Stability and Instability 49What Does It All Mean? 52

Page

CHAPTER 7. CLOUDS 53Identification 53Signposts in the Sky 62

CHAPTER 8. An MASSES AND FRONTS 63Mr Masses 63Fronts 64Fronts and Flight Planning 78

CHAPTER 9. TURBULENCE 79Convective Currents 80Obstructions to Wind Flow 82Wind Shear 86Wake Turbulence 88In Closing. 90

CHAPTER 10. ICING 91Structural Icing.. 92Induction System Icing 97Instrument Icing 98Icing and Cloud Types 99Other Factors in Icing 100Ground Icing 102Frost 102In Closing 102

CHAPTER 11. THUNDERSTORMS 105Where and When ?.. 105They Don't Just Happen 111

The Inside Story 111*

Rough and Rougher 112Hazards 113

Thunderstorms and Radar 120Do's and Don'ts of Thunderstorm Flying 121

CHAPTER 12. COMMON .IFR. PRODUCERS 125Fog 126Low Stratus Clouds 128Haze and Smoke 129Blowing Restrictions to Visibility 129

Precipitation 130Obscured or Partially Obscured Sky 130In Closing.. 130

PART II. OVER AND BEYOND

CHAPTER 13. HIGH ALTITUDE WEATHER.. 135The Tropopause 136The Jet Stream 136

Cirrus Clouds... 139Clear Mr Turbulence 142Condensation Trails 143

VI

so

4-

Page

Haze Layers 144Canopy Static 145Icing 145Thunderstorms . 145

CHAPTER 14. ARCTIC WEATHER 147Climate, Air Masses, and Fronts 148Arctic Peculiarities 152Weather Hazards 153Arctic Flying Weather 154In Closing . 155

CHAPTER 15. TROPICAL WEATHER 157Circulation 158Transitory Systems 162

CHAPTER 16. SOARING WEATHER 171Thermal Soaring 172Frontal Soaring 191Sea Breeze Soaring 191Ridge or Hill Soaring.. 195Mountain Wave Soaring 198In Closing 200

Glossary of Weather Terms 201

Index 215

VII

illustrationsFigure Page

1. Composition of a dry atmosphere2. The atmosphere divided into layers based on temperature3. The two temperature scales in common use... 64. World-wide average surface temperatures in July 85. World-wide average surface temperatures in January.. 86. Temperature differences create air movement and, at times, cloudiness.. 97. Inverted lapse rates or "inversions" 108. The mercurial barometer 129. The aneroid barometer 13

10. The standard atmosphere 1411. Three columns of air showing how decrease of pressure with height varies

with temperature 1512. Reduction of station pressure to sea level 1513. pressure systems 1614. Indicated altitude depends on air temperature below the aircraft 1715. When flying from high pressure to lower pressure without adjusting your

altimeter, you are losing true altitude 1816. Effect of temperature on altitude 19

17. Effect of density altitude on takeoff and climb 2018. Convective current resulting from uneven heating of air by contrasting

surface temperatures 2419. Circulation as it would be on a nonrotating globe 2520. Apparent deflective force due to rotation of a horizontal platform 2621. Effect of Coriolis force on wind relative to isobars 2722. In the Northern Hemisphere, Coriolis force turns equatorial windi to

westerlies and polar winds to easterlies 2823. Mean world-wide surface pressure distribution in July 2824. Mean world-wide surface pressure distribution in January... 2925. General average circulation in the Northern Hemisphere 3026. Mr flow around pressure systems above the friction layer 31

27. Surface friction slows the wind and reduces Coriolis force; winds aredeflected across the isobars toward lower pressure... 32

28. Circulation around pressure systems at the surface 3329. The "Chinook" is a katabatic (downslope) wind 3330. Land and sea breezes... 3431. Wind shear 3532. Blue dots illustrate the increased water vapor capacity of warm air 3833. Relative humidity depends on both temperature and water vapor 3934. Virga 4035. Heat transactions when water changes state 41

36. Growth of raindrops by collision of cloud droplets 4237. Lake effects 4338. Strong cold winds across the Great Lakes absorb water vapor and may

carry showers as far eastward as the Appalachians.. 4439. A view of clouds from 27,000 feet over Lake Okeechobee in southern

Florida. 45

ix

8

Figure Page40. Decreasing atmospheric pressure causes the balloon to expand as it rises 4841. Adiabatic warming of downward moving air produces the warm Chinook

wind 4942. Stability related to temperatures aloft and adiabatic cooling . 5043. When stable air is forced upward, cloudiness is flat and stratified. When

unstable air is forced upward, cloudiness shows extensive verticaldevelopment 51

44. Cloud base determination. 5245. Cirrus 5446. Cirrocumulus 5547. Cirrostratus 5548. Altocumulus 5649. Altostratus.. 5650. Altocumulus castellanus 5751. Standing lenticular altocumulus clouds... 5852. Nimbostratus.... 5953. Stratus 5954. Stiatocumulus 6055. Cumulus 6056. Towering cumulus 6157. Cumulonimbus 6158. Horizontal uniformity of an air mass. 6459. Cross section of a cold front with the weather map symbol.. 6660. Cross section of a warm front with the weather map symbol 6761. Cross section of a stationary front and its weather map symbol 6862. The life cycle of a frontal wave 6963. Cross section of a warm-front occlusion and its weather map symbol.. 7064. Cross section of a cold-front occlusion. 7165. Frontolysis of a stationary front 7166. Frontogenesis of a stationary frpnt 7267. A cold front underrunning warm, moist, stable air.. 7368. A cold front underrunning warm, moist, unstable air 7369. A warm front with overrunning moist, stable air 7470. A slow-moving cold front underrunning warm, moist, unstable air 7471. A warm front with overrunning warm, moist, unstable air 7572. A fast moving cold front underrunning warm, moist, unstable air 7573. A warm front occlusion lifting warm, moist, unstable air.. ,,,..76

74. A cold front 1-..^.clusion lifting warm, moist, stable air 7675. An aerial view of a portion of a squall line 7776. Effect of convective currents on final approach.. 8077. Avoiding turbulence by flying above convective clouds 8178. Eddy currents formed by winds blowing over uneven ground or over

obstructions 8279. Turbulent air in the landing area 8380. Wind flow in mountain areas.. 8481. Schematic cross section of a mountain wave 8482. Standing lenticular clouds associated with a mountain wave. 8583. Standing wave rotor clouds marking the rotary circulation beneath

mountain waves 8684. Mountain wave clouds over the. Tibetan Plateau photographed from a

manned spacecraft 87

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F

Fisrurs Page85. Satellite photograph of a mountain wave and the surface analysis for

approximately the samc time 8786. Wind shear in a zone between relatively calm wind below an inversion and

strong wind above the inversion 8887. Wake turbulence wing tip vortices developing as aircraft breaks ground 8988. Planning landing or takeoff to avoid heavy aircraft wake turbulence. . 9089. Effects of structural icing 9290. Clear, rime, and mixed icing on airfoils 9391. Clear wing icing (leading edge and underside) 9492. Propeller icing 9593. Rime icing on the nose of a Mooney "Mark 21" aircraft 9694. External icing on a pitot tube 9795. Carburetor icing 9896. Internal pitot tube icing 9997. Clear ice on an aircraft antenna mast.... 10098. Freezing rain with a warm front and a cold front 10199. Frost on an aircraft 103

100. The average number of thunderstorms each year 106101. The average number of days with thunderstorms during spring... 107102. The average umber of days with thunderstorms during summer. 108103. The average number of days with thunderstorms during fall 109104. The average number of days with thunderstorms during winter .. 110105. The stages of a thunderstorm. 112106. Schematic of the mature stage of a steady state thunderstorm cell. 113107. A tornado 114108. A waterspout... 114109. Funnel clouds 115110. Cumulonimbus Mamma clouds. 116111. Tornado incidence by State and area 117112. Squall line thunderstorms ., ..7. 118113. Schcmatic cross section of a thunderstorm 119114. Hail damage to an aircraft 120115. Radar photograph of a line of thunderstorms 121116. Use of airborne radar to avoid heavy precipitation and turbulence.. 122117. Ground fog as seen from the air 126118. Advection fog in California 127119. Advection fog over the southeastern United States and Gulf Coast 128120. Smoke trapped in stagnant air under an inversion 129121. Aerial photograph of blowing dust approaching with a cold front 130122. Difference between the ceiling caused by a surface-based obscuration anti

the ceiling caused by a layer aloft 131123. A cross section of the upper troposphere and lower stratosphere.. 136124. Artist's concept of the jet stream 137125. A jet stream segment 137126. Multiple jet streams 138127. Mean jet positions relative to surface systems 139128a. Satellite photograph of an occluded system 140128b. Infrared photograph of the system shown in figure 128a 141129. A frequent CAT location is along the jet stream north and northeast of a

rapidly deepening surface low 142130. Contrails 144

xi

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Figure Page131. The Arctic 148132. Sunshine in the Northern Hemisphere 149133. The permanent Arctic ice pack 150134. Average number of cloudy days per month (Arctic) 151135. Visibility reduced by blowing snow 154136. A typical frozen landscape of the Arctic 154137. Prevailing winds throughout the Tropics in July. 160138. Prevailing winds in the Tropics in January 161139. Vertical cross section illustrating convection in the Intertropical Con-

vergence Zone 161140. A shear line and an induced trough caused by a polar high pushing into

the subtropics 163141. A trough aloft across the Hawaiian Islands 164142. A Northern Hemisphere easterly wave 165143. Vertical cross section along line A-B in figure 142 165144. Principal regions where tropical cyclones form and their favored direc-

tions of movement 166145. Radar photograph of hurricane "Donna" 168146. A hurricane observed by satellite. 169147. Thermals generally occur over a small portion of an area while down-

drafts predominate.. 172148. Using surface dust and smoke movement as indications of a thermal. .. . 174149. Horizontal cross section of a dust devil rotating clockwise 174150. Cumulus clouds grow only with active thermals 176151. Photograph of a dying cumulus 177152. Altocumulus castellanus clouds are middle level convective clouds 178153. Experience indicates that the "chimney" thermal is the most prevalent

type 179154. Thermals may be intermittent "bubbles" 179155. It is believed that a bubble thermal sometimes develops a vortex ring.. 180156. Wind causes thermals to lean.. 181157. Photograph of cumulus clouds severed by wind shear 181158. Conditions favorable for thermal streeting 182159. Cumulus clouds in thermal streets photographed from a satellite high

resolution camera 183160. The Pseudo-Adiabatic Chart 184161. An early morning upper air observation plotted on the pseudo-adiabatic

chart 185162. Computing the thermal index (TI) 187163. Another exampleof computing TI's and maximum height of thermals 188164. An upper air observation made from an aircraft called an airplane

observation or APOB 189165. Schematic cross section through a sea breeze front.. 192166. Sea breeze flow into the San Fernando Valley 193167. Sea breeze convergence zone, Cape Cod, Massachusetts 194168. Schematic cross section of airflow over a ridge 195169. Strong winds flowing around an isolated peak 196170. Wind flow over various types of terrain 197171. Schematic cross section of a mountain wave 198172. Wave length and amplitude.. 199

XII

i1

Introduction

Weather is perpetual in the state of the atmosphere. All flying takes place inthe atmosphere, so flying and weather are inseparable. Therefcre, we cannot treataviation weather purely as an academic subject. Throughout the book, we discusseach aspect of weather as it relates to aircraft operation and flight safety. However,this book is in no way an aircraft operating manual. Each pilot must apply theknowledge gained here to his own aircraft and flight capabilities.

The authors have devoted much of the book to marginal, hazardous, and violentweather which becomes a vital concern. Do not let this disproportionate time devotedto hazardous weather discourage you from flying. By and large, weather is generallygood and places little restriction on flying. Less frequently, it becomes a threat tothe VFR pilot but is good for IFR flight. On some occasions it becomes too violenteven for the IFR pilot.

It behooves every pilot to learn to appreciate good weather, to recognize andrespect marginal or hazardous weather, and to avoid violent weather when the atmo-sphere is on its most cantankerous behavior. For your safety and the safety of thosewith you, learn to recognize potential trouble and make sound flight decisions beforeit is too late. This is the real purpose of this manual.

AvanoN WEATHER is in two parts. Part I explains weather facts every pilotshould know. Part II contains topics of special interest discussing high altitude,Arctic, tropical, and soaring weather. A glossary defines terms for your referencewhile reading this or other weather writings. To get a complete operational study,you will need in addition to this manual a copy of AVIATION WEATHER SERVICES,AC 00-45, which is explained in the Preface.

We sincerely believe you will -enjoy this book and at the same time increase,your flying safety and economy and, above all, enhance the pleasure and satisfactionof using today's most modern transportation.

All

12

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VPart ONE

WHAT YOU SHOULD KNOWABOUT WEATHER

13

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Chapter 1THE EARTH'S ATMOSPHERE

Planet Earth is unit!! te in that its atmospheresustains life as we know :t. Weatherthe state ofthe atmosphereat any given time and placestrongly influences our daily routine as well as ourgeneral life patterns. Virtually all of our activitiesare affected by weather, but of all man's endeav-ors, none is influenced more intimately by weatherthan aviation.

Weather is complex and at times difficult to un-derstand. Our restless atmosphere is almost con-

standy in motion as it strives to reach equilibrium.These never-ending air movements set up chainreactions which culminate in a continuing varietyof weather. Later chapters in this book delve intothe atmosphere in motion. This chapter looks brief-ly at our atmosphere in terms of its composition;vertical structure; the standard atmosphere; and ofspecial concern to you, the pilot, density andhypoxia.

1 41

COMPOSITION

Air is a mixture of several gases. When com-pletely dry, it is about 78% nitrogen and 21% oxy-gen. The remaining 1% is other gases such asArgon, Carbon Dioxide, Neon, Helium, and others.Figure I graphs these proportions. However, in na-ture, air is never completely dry. It always containssome water vapor in amounts varying from almostzero to about 5% by volume. As water vapor con-tent increases, the other gases decrease propor-tionately.

OTHER GASES

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NITROGEN

FIGURE 1. Composition of a dry atmosphere. Nitrogencomprises about 78%; oxygen, about 21%; and othergases, about 1%. When water vapor is added, the per-centages decrease proportionately. Water vapor variesfrom almost none to about 5% by volume.

VERTICAL STRUCTURE

We classify the atmosphere into layers, orspheres, by characteristics exhibited in these lay-ers. Figure 2 shows one division which we use inthis book. Since most weather occurs in the tropo-sphere and since most flying is in the troposphereand stratosphere., ye restrict our discussions mostlyto these two layers.

The TROPOSPHERE is the layer from the sur-face to an average altitude of about 7 miles. It ischaracterized by an overall decrease of temperaturewith increasing altitude. The height of the tropo-sphere .varies with latitude and seasons. It slopesfrom about 20,000, feet over the poles to about

65,000 feet over the Equator; and it is higher insummer than in winter.

At the top of the troposphere is the TROPO-PAUSE, a very thin layer marking the boundarybetween the troposphere and the layer above. Theheight of the tropopause and certain weather phe-nomena are related. Chapter 13 discusses in detailthe significance of the tropopause to flight.

Above the tropopause is the STRATOSPHERE.This layer is typified by relatively small changes intemperature with height except for a warmingtrend near the top.

THE STANDARD ATMOSPHERE

Continual fluctuations of temperature and pres-sure in our restless atmosphere create some prob-lems for engineers and meteorologists who requirea fixed standard of reference. To arrive at a stan-dard, they averaged conditions throughout the at-mosphere for all latitudes, seasons, and altitudes.The result is a STANDARD ATMOSPHERE with

2

specified sea-level temperature and pressure andspecific rates of change of temperature and pres-sure with height. It is the standard for calibratingthe pressure altimeter and developing aircraft per-formance data. We refer to it often throughout thisbook.

s.

FI0URE 2. The atmosphere divided into layers based on temperature. This book concentrates on the lower two layers, thetroposphere and the stratosphere.

DENSITY AND HYPDXIA

Air is matter and has weight. Since it is gaseous,it is compressible. Pressure the atmosphere exertson the surface is the result of the weight of the airabove. Thus, air near the surface is much moredense than air at high altitudes. This decrease ofdensity and pressure with height enters frequentlyinto our discussions in later chapters.

The decrease in air density with increasing heighthas a physiological effect which we cannot ignore.The rate at which the lungs absorb oxygen dependson the partial pressure exerted by oxygen in the

air. The atmosphere is about one-fifth oxygen, sothe oxygen pressure is about one-fifth the totalpressure at any given altitude. Normally, our lungsare accustomed to an oxygen pressure of about 3pounds per square inch. But, since air pressure de-creases as altitude increases, the oxygen pressurealso decreases. A pilot continuously gaining alti-tude or making a prolonged flight at high altitudewithout supplemental oxygen will likely suffer fromHYPDXIAa deficiency of oxygen. The effectsare a feeling of exhaustion; an impairment of

163

vision and judgment; and finally, unconsciousness.Cases are known where a perion lapsed into un-consciousness without realizing he was sufferingthe effects.

When flying at or above 10,000 feet, forceyourself to remain alert. Any feeling of drowsinessor undue fatigue may be from hypoxia. If you do

4

not have oxygen, descend to a lower altitude. Iffatigue or. drowsiness continues after descent, it iscaused by something other than hypoxia.

A safe procedure is to use auxiliary oxygen dur-ing prolonged flights above 10,000 feet and foreven short flights above 12,000 feet. Above about40,000 feet, pressurization becomes easential.

17

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Chapter 2TEMPERATURE

Since early childhood, you have expressed thecomfort of weather in degrees of temperature.Why, then, do we stress temperature in aviationweather? Look at your flight computer; tempera-ture enters into the computation of most param-eters on the computer. In fact, temperature can

be critical to some flight operations. As a founda-tion for the study of temperature effects on avia-tion and weather, this chapter describes commonlyused temperature scales, relates heat and tempera-ture, and surveys temperature variations both atthe surface and aloft.

18

5

TEMPERATURE SCALES

Two commonly used temperature scales areCelsius (Centigrade) and Fahrenheit. The Celsiusscale is used exclusively for upper air temperaturesand is rapidly becoming the world standard forsurface temperatures also.

Traditionally, two common temperature refer-ences are the melting point of. pure ice and theboiling point of pure water at sea level.. The melt-ing point of ice is 0° C or 32° F; the boiling pointof water is 100° C or 212° F. Thus, the differencebetween- melting and boiling is 100 degrees Celsiusor 180 degrees Fahrenheit; the ratio between de-grees Celsius and Fahrenheit is 100/180 or 5/9.Since 0° F is 32 Fahrenheit degrees colder than 0°C, you must apply this difference when comparingtemperatures on the two scales. You can convertfrom one scale to the other using one of thefollowing formulae:

5C =9(F 32)

F = 9C + 32

where C is degrees Celsius and F is 'degrees Fahren-heit. Figure 3 compares the two scales. Many flightcomputers provide for direct conversion of temper-ature from one scale to the other. Section 16, AVIA-TION WEATHER SERVICES has a graph for temper-ature conversion.

Temperature we measure with a thermometer.But what makes a thermometer work? Simply theaddition or removal of heat. Heat and temperatureare not the same; how are they related?

FIouRE 3. The two temperature scales in common useare the Fahrenheit and the Celsius. 9 degrees on theFahrenheit scale equal 5 degrees on the Celsius.

HEAT AND TEMPPRATURE

Heat is a form of energy. When a substance con-tains heat, it exhibits- the property we measure astemperaturethe degree of "hotness" or "cold-ness." A specific amount of heat absorbed by orremoved from a substance raises or lowers its tem-perature a definite amount. However, the amountof temperature change depends on characteristicsof the substance. Each substance has its uniquetemperature change for the specific change in heat.For example, if a land surface and a water surfacehave the same temperature and an equal amount

6

of heat is added, the land surface becomes hotterthan the water surface. Conversely, with equal heatloss, the land becomes colder than the water.

The Earth receives energy from the sun in theform of solar radiation. The Earth and its atmo-sphere reflect about 55 percent of the radiationand absorb the remaining 45 percent converting itto heat. The Earth;' n turn, radiates energy, andthis outgoing radiation is "terrestrial radiation." Itis evident that the average heat gained from :r.-coming solar radiation must equal heat lost through

19

1.

V

Y

terrestrial radiation in order to keep the earthfrom getting progressively hotter or colder. How-ever, this balance is world-wide; we must consider

regional and local imbalances which create temper-ature variations.

TEMPERATURE VARIATIONS

The amount of solar energy received by any re-gion varies with time of day, with seasons, and withlatitude. These differences in solar energy createtemperature variations. Temperatures also varywith differences in topographical surface and withaltitude. These temperature variations create forcesthat drive the atmosphere in its endless motions.

DIURNAL VARIATIONDiurnal variation is Me change in temperature

from day to night brought about by the daily rota-tion of the Earth. The Earth receives heat duringthe day by solar radiation but continually losesheat by terrestrial radiation.Warming and coolingdepend on an imbalance of solar and terrestrialradiation. During the day, solar radiation exceedsterrestrial radiation and the surface becomeswarmer. At night, solar radiation ceases, but ter-restrial radiation continues and cools the surface.Cooling continues after sunrise until solar radiationagain exceeds terrestrial radiation. Minimum tem-perature usually occurs after sunrise, sometimes asmuch as one hour after. The continued coolingafter sunrise, is one reason that fog sometimes formsshortly after the sun is above the horizon. We willhave more to say about diurnal variation and topo-graphic surfaces.

SEASONAL VARIATIONIn addition to its daily rotation, the Earth re-

volves in a complete orbit around the sun once eachyear. Since the axis of the Earth tilts to the planeof orbit, the angle of incident solar radiation variesseasonally between hemispheres. The NorthernHemisphere is warmer in June, July, and Augustbecause it receives more solar energy than does theSouthern Hemisphere. During Decembor, January,and February, the opposite is true; the SouthernHemisphere receives more solar energy and iswarmer. Figures 4 and 5 show these seasonal sur-face temperature variations.

VARIATION WITH LATITUDEThe shape of the Earth causes a geographical

variation in the angle of incident solar radiation.

Since the Earth is essentially spherical, the sun ismore nearly overhead in equatorial regions than athigher latitudes. Equatorial regions, therefore, re-ceive the most radiant energy and are warmest.Slanting rays of the sun at higher latitudes deliverless energy over a given area with the least beingreceived at the poles. Thus, temperature varies withlatitude from the warm Equator to the cold poles.You can see this average temperature gradient infigures 4 and 5.

VARIATIONS WITH TOPOGRAPHYNot related to movement or shape of the earth

are temperature variations induced by water andterrain. As stated earlier, water absorbs and radi-ates energy with less temperature change than doesland. Large, deep water bodies tend to minimizetemperature cLanges, while continents favor largechanges. Wet soil such as in swamps and marshesis almost as effective as water in suppressing tem-perature changes. Thick vegetation tends to controltemperature changes since it contains some waterand also insulates against heat transfer between theground and the atmosphere. Arid, barren surfacespermit the greatest temperature changes.

These topographical influences are both diurnaland seasonal. For example, the difference betweena daily maximum and minimum may be 10° orless over water, near a shore line, or over a swampor marsh, while a difference of 50° or more iscommon over rocky or sandy deserts. Figures 4and 5 show the seasonal topographical variation.Note that in the Northern Hemisphere in July,temperatures are warmer over continents than overoceans; in January they are colder over continentsthan over oceans. The opposite is true in the South-ern Hemisphere, but not as pronounced because ofmore water surface in the Southern Hemisphere.

To compare land and water effect on seasonaltemperature variation, look at northern Asia and atsouthern California near San Diego. In the deepcontinental interior of northern Asia, July averagetemperature is about 50° F; and January average,about 30° F. Seasonal range is about 80° F. NearSan Diego, due to the proximity of the Pacific

7

lzmuaz 4. World-wide average surface temperatures in July. in the Northern Hemisphere, continents generally arewarmer than oceanic areas at corresponding latitudes. The reverse is true in the Southern Hemisphere, but the contrastis not so evident because of the sparsity of land surfaces.

FIGURE 5. World-wide average surface temperatures in January when the Northern Hemisphere is in theoceanic arat

corresponding latitudes, and in the Southern Hemisphere continents are warmer than oceans.

S21

Ocean, July average is about 70° F and January av-erage, 50° F. Seasonal variation is only about 20° F.

Abrupt temperature differences develop alonglake and ocean shores. These variations generatepressure differences and local winds which we willstudy in later chapters. Figure 6 illustrates a possi-ble effect.

Prevailing wind is also a factor in temperaturecontrols. In an area where prevailing winds arefrom large water bodies, temperature changes arerather small. Most islands enjoy fairly constanttemperatures. On the other hand, temperaturechanges are more pronounced where prevailingwind is from dry, barren regions.

Air transfers heat slowly from the surface up-ward. Thus, temperature changes aloft are moregradual than at the surface. Let's look at tempera-ture changes with altitude.

VARIATION WITH ALTITUDE

In chapter 1, we learned that temperature nor-mally decreases with increasing altitude throughoutthe troposphere. This decrease of temperature withaltitude is defined as lapse rate. The average de-crease of temperatureaverage lapse ratein thetroposphere is 2° C per 1,000 feet. But since thisis an average, the exact value 'seldom exists. Infact, temperature sometimes increases with heightthrough a layer. An increase in temperature withaltitude is defined as an inversion, i.e., lapse rateis, inverted.

An inversion often develops near the ground onclear, cool nights when wind is light. The groundradiates and cools much faster than the overlyingair. Air in contact with the ground becomes coldwhile the temperature a few hundred feet abovechanges very little. Thus, temperature increases

Flovae 6. Temperature differences create air movement and, at times, cloudiness.

9

22

with height. Inversions may, also occur at any alti-tude when conditions arc favorable. For example,a current of warm air aloft overrunning cold air

near the surface produces an inversion aloft. Figure7 diagrams temperature inversions both surface andaloft. Inversions are common in the stratosphere.

FIGURE 7. Inverted lapse rates or "inversions." A lapse rate is a decrease of temperature with height. An inversion is anincrease of temperature with height, i.e., the lapse rate is inverted. Shown here are a surface inversion and an inversionaloft.

IN CLOSING

Temperature affects aircraft performance and iscritical to some operations. Following are some op-erational pointers to remember, and most of themare developed in later chapters:

1. The aircraft thermometer is subject to in-accuracies no matter how good the instru-ment and its installation. Position of theaircraft relative to the sun can cause errorsdue to radiation, particularly on a parkedaircraft. At high speeds, aerodynamical ef-fects and friction are basically the causes ofinaccuracies.

2. High temperature reduces air density andreduces aircraft performance (chapter 3).

3. Diurnal and topographical temperaturevariations create local winds (chapter 4).

10

23

4. Diurnal cooling is conducive to fog (chap-ter 5).

5. Lapse rate contributes to stability (chapter6), cloud formation (chapter 7), turbulence(chapter 9), and thunderstorms (chapter11) .

6. An inversion aloft permits warm rain to fallthrough cold air below. Temperature in thecold air can be critical to icing (chapter10).

7. A ground based inversion favors poor visi-bility by trapping fog, smoke, and otherrestrictions into low levels of the atmo-sphere (chapter 12) .

t.

c

N

4

Chapter 3ATMOSPHERIC PRESSUREAND ALTIMETRY

When you understand pressure, its measure- pressure, you can more readily grasp the signif-men; and effects of temperature and altitude on icance of pressure and its application to altimetry.

ATMOSPHERIC PRESSURE

Atmospheric pressure is the force per unit areaexerted by the weight of the atmosphere. Since airis not solid, we cannot weigh it with conventionalscales. Yet, Toricelli proved three centuries agothat he could weigh the atmosphere by balancingit against a column of mercury. He actually mea-sured pressure converting it directly to weight.

MEASURING PRESSURE

The instrument Toricelli designed for measuringpressure is the barometer. Weather services and theaviation community use two types of barometers inmeasuring pressurethe mercurial and aneroid.

2411

The Mercurial BarometerThe mercurial barometer, diagrammed in figure

8, consists of an open dish of mercury into whichwe place the open end of an evacuated glass tube.Atmospheric pressure forces mercury to rise in thetube. At stations near sea level, the column of mer-cury rises on the average to a height of 29.92 inchesor 760 millimeters. In other words, a column ofmercury of that height weighs the same as a col-umn of air having the same cross section as thecolumn of mercury and extending from sea levelto the top of the atmosphere.

Why do we use mercury in the barometer? Mer-cury is the heaviest substance available which re-mains liquid at ordinary temperatures. It permitsthe instrument to be of manageable size. We could

Firma 8. The mercurial barometer. Atmospheric pres-sure forces mereury from the open dish upward into theevacuated glass tube. The height of the mercury columnis a measure of atmospheric pressure.

12

use water, but at sea level the water column wouldbe about 34 feet high.

The Aneroid BarometerEssential features of an aneroid barometer illus-

trated in figure 9 are a flexible metal cell and theregistering mechanism. The cell is partially evac-uated and contracts or expands as pressure changes.One end of the cell is fixed, while the other endmoves the registering mechanism. The couplingmechanism magnifies movement of the cell drivingan indicator hand along a scale graduated in pres-sure units.

Pressure UnitsPressure is expressed in many ways throughout

the world. The term used depends somewhat on itsapplication and the system of measurement. Twopopular units are "inches of mercury" or "milli-meters of mercury." Since pressure is force per unitarea, a more explicit expression of pressure is"pounds per square inch" or "grams per squarecentimeter." The term "millibar" precisely ex-presses pressure as a force per unit area, one milli-bar being a force of 1,000 dynes per squarecentimeter. The millibar is rapidly becoming auniversal pressure unit.

Station PressureObviously, we can measure pressure only at the

point of measurement. The pressure measured at astation or airport is "station pressure" or the actualpressure at field elevation. We know that pressureat high altitude is less than at sea level or low alti-tude. For instance, station pressure at Denver isless than at New Orleans. Let's look more closelyat some factors influencing pressure.

25

PRESSURE VARIATIONPressure varies with altitude and temperature of

the air as well as with other minor influences whichwe neglect here.

AltitudeAs we move upward through the atmosphere,

weight of the air above becomes less and less. Ifwe carry a barometer with us, we can measure adecrease in pressure as weight of the air above de-creases. Within the lower few thousand feet of thetroposphere, pressure decreases roughly one inchfor each 1,000 feet increase in altitude. The higherwe go, the slower is the rate of decrease with height.

ir

1

FIGuRE 9. The aneroid barometer. The aneroid consists of a partially evacuated metal cell, a coupling mechanism, and anindicator scale. The cell contracts and expands with changing pressure. The coupling mechanism drives the indicatoralong a scale graduated in pressure units.

Figure 10 shows the pressure decrease with heightin the standard atmosphere. These standard alti-tudes are based on standard temperatures. In thereal atmosphere, temperatures are seldom standard,so lees explore temperature effects.

Temperature

Like most substances, air expands as it becomeswarmer and shrinks as it cools. Figure 11 showsthree columns of airone colder than standard,one at standard temperature, and one warmer thanstandard. Pressure is equal at the bottom of eachcolumn and equal at the top of each column.

rt Therefore, pressure decrease upward through eachcolumn is the same. Vertical expansion of the warmcolumn has made it higher than the column atstandard temperature. Shrinkage of the cold col-umn has made it shorter. Since pressure decreaseis the same in each column, the rate of decrease ofpressure with height in warm air it less than stan-dard; the rate of decrease of pressure with heightin cold air is greater than standard. You will soonsee the importance of temperature in altimetry andweather analysis and on aircraft performance.

Sea Level Pressure

Since pressure varies with altitude, we cannotreadily compare station pressures between stationsat different altitudes. To make them comparable,we must adjust them to some common level. Meansea level seems the most feasible common reference.In figure 12, pressure measured at a 5,000-foot sta-tion is 25 inches; pressure increases about 1 inchfor each 1,000 feet or a total of 5 inches. Sea levelpressure is approximately 25 + 5 or 30 inches. Theweather observer takes temperature and other ef-fects into account, but this simplified example ex-plains the basic principle of sea level pressurereduction.

We usually express sea level pressure in millibars.Standard sea level pre'sure is 1013.2 millibars,29.92 inches of mercury, 760 millimeters of mer-cury, or about 14.7 pounds per square inch. Figures23 and 24 in chapter 4 show world-wide averagesof sea level pressure for the months of July andJanuary. Pressure changes continually, however,and departs widely from these averages. We use asequence of weather maps to follow these changingpressures.

'26

13

BEGIN USINGOXYGEN

it,er - 16.

1111116.

tiMer,, :1'4144

Fromm 10. The standard atmosphere. Note how pressure decreases with increasing height; the rate of decrease with heightis greatest in lower levels.

14

27

Sr

FIGURE II. Three columns of air showing how decreaseof pressure with height varies with temperature. Leftcolumn is colder than average and right column, warmerthan average. Pressure is equal at the bottom of eachcolumn and equal at the top of each column- Pressuredecreases most rapidly with height in the cold air andleast rapidly in the warm air.

Pressure Analyses

We plot sea level pressures on a map and drawlines connecting points of equal pressure. Theselines of equal pressure are isobars. Hence, the sur-face map is an isobaric analysis showing identifi-

able, organized pressure patterns. Five pressuresystems are shown in figure 13 and are defined asfollow:

I. LOWa center of pressure surrounded onall sides by higher pressure; also called acyclone. Cyclonic curvature is the curvatureof isobars to the left when you stand withlower pressure to your left.

2. HIGHa center of pressure surrounded onall sides by lower pressure, also called ananticyclone. Anticyclonic curvature is thecurvature of isobars to the right when youstand with lower pressure to your left.

3. TROUGHan elongated area of low pres-sure with the lowest pressur.- along a linemarking maximum cyclonic etuvature.

4. RIDGEan elongated area of high pres-sure with the highest pressure along a linemarking maximum anticyclonic curvature.

5. COLthe neutral area between two highsarid two lows. It also is the intersection ofa trough and a ridge. The col on a pressuresurface is analogous to a mountain pass ona topographic surface.

,,Upper air weather maps reveal these same typesof pressure patterns aloft for several levels. Theyalso show temperature, moisture, and wind at eachlevel. In fact, a chart is available for a level withina few thousand feet of your planned cruising alti-tude. AVIATION WEATHER SERVICES lists the approx-imate heights of upper air maps and shows detailsof the surface map and each upper air chart.

Plaintz 12. Reduction of station pressure to sea level. Pressure increases about 1 inch per 1,000 feet from the stationelevation to sea level.

2815

,

HIGH

)

COL

LOW

100

.FIGURE 13. Pressure systems.

Chapter 4 of this book ties together the surfacechart and upper air charts into a three-dimensionalpicture.

An upper air map is a constant pressure analysis.But, what do we mean by "constant pressure"?Constant pressure simply refers to a specific pres-sure. Let's arbitrarily choose 700 millibars. Every-where above the earth's surface, pressure decreaseswith height; and at some height, it decreases to thisconstant pressure of 700 millibars. Therefore, thereis a "surface" throughout the atmosphere at whichpressure is 700 millibars. We call this the 700 -millibar constant pressure surface. However, theheight of this surface is not constant. Rising pres-sure pushes the surface upward into highs andridges. Falling pressure lowers the height of thesurface into lows and troughs. These systems mi-grate continuously as "waves" on the pressure sur-face. Remember that we chose this constantpressure surface arbitrarily as a reference. It in noway defines any discrete boundary.

16

29

HIGH

The National Weather Service and militaryweather services take routine scheduled upper airobservationssometimes called soundings. A bal-loon carries aloft a radiosonde instrument whichconsists of miniature radio gear and sensing ele-ments. While in flight, the radiosonde transmitsdata from which a specialist determines wind, tem-perature, moisture, and height at selected pressuresurfaces.

We routinely collect these observations, plot theheights of a constant pressure surface on a map,and draw lines connecting points of equal height.These lines are height contours. But, what is aheight contour?

First, consider a topographic map with contoursshowing variations in elevation. These are heightcontours of the terrain surface. The Earth surfaceis a fixed reference and we contour variations inits height.

The same concept applies to height contours ona constant pressure chart, except our reference is a

v

constant pressure surface. We simply contour theheights of the pressure surface. For example, a 700 -millibar constant pressure analysis is a contourmap of the heights of the 700-millibar pressure sur-face. While the contour map is based on variationsin height, these variations are small when com-pared to flight levels, and for all practical pur-poses, you may regard the 700-millibar chart as aweather map at approximately 10,000 feet or 3,U48meters.

A contour analysis shows highs, ridges, lows, andtroughs aloft just as the isobaric analysis shows suchsystems at the surface. What we say concerning

pressure patterns and systems applies equally to anisobaric or a contour analysis.

Low pressure systems quite often are regions ofpoor flying weather, and high pressure areas pre-dominantly are regions of favorable flying weather.A word of caution, howeveruse care in applyingthe low pressure-bad weather, high pressure-goodweather rule of thumb; it all too frequently fails.When planning a flight, gather all information pos-sible on expected weather. Pressure patterns alsobear a direct relationship to wind which is the sub-ject of the next chapter. But first, lees look at pres-sure and altimeters.

ALTIMETRY

The altimeter is essentially an aneroid barometer.The difference is the scale. The altimeter is grad-uated to read increments of height rather thanunits of pressure. The standard for graduating thealtimeter is the standard atmosphere.

ALTITUDE

Altitude seems like a simple term; it meansheight. But in aviation, it can have many meanings.

True AltitudeSince existing conditions in a real atmosphere

are seldom standard, altitude indications on thealtimeter are seldom actual or true altitudes. Truealtitude is the actual or exact altitude above meansea level. If your altimeter does not indicate truealtitude, what does it indicate?

Indicated AltitudeLook again at figure 11 showing the effect of

mean temperature on the thickness of the threecolumns of air. Pressures are equal at the bottomsand equal at the tops of the three layers. Since thealtimeter is essentially a barometer, altitude indi-cated by the altimeter at the top of each columnwould be the same. To see this effect more clearly,study figure 14. Note that in the warm air, you flyat an altitude higher than indicated. In the coldair, you are at an altitude lower than indicated.

Height indicated on the altimeter also changeswith changes in surface pressure. A movable scaleon the altimeter permits you to adjust for surfacepressure, but you have no means of adjusting theinstrument for mean temperature of the column of

air below you. Indicated altitude is the altitudeabove mean sea level indicated on the altimeterwhen set at the local altimeter setting. But what isaltimeter setting?

OO

CC

INDICATED ALTITUDE 10,000 feet

r.

-10,000

ccto

29.92 in.

FIGURE 14. Indicated altitude depends on air tempera-ture below the aircraft. Since pressure b equal at thebases and equal at the tops of each column, indicatedaltitude is the same at the top of each column. When airis colder than average (right), the altimeter reads higherthan true altitude. Whey air is warmer than standard(left), the altimeter reads lower than true altitude,

3017

M .0

Altimeter Setting

Since the altitude scale is adjustable, you can setthe altimeter to read true altitude at some specifiedheight. Takeoff and landing are the most criticalphases of flight; therefore, airport elevation is themost desirable altitude for a true reading of thealtimeter. Altimeter setting is the value to whichthe scale of the pressure altimeter is set so the at-timeter indicates true altitude at field elevation.

In order to ensure that your altimeter reading iscompatible with altimeter readings of other aircraftin your vicinity, keep your altimeter setting current.Adjust it frequently in flight to the altimeter set-ting reported by the nearest tower or weather re-porting station. Figure 15 shows the trouble youcan encounter if you are lax in adjusting your al-tieter in flight. Note that as you fly from highpressure to low pressure, you are lower than youraltimeter indicates.

Figure 16 shows that as you fly from warm to coldair, your altimeter reads too highyou are lowerthan your altimeter indicates. Over flat terrain thislower than true reading is no great problem; otheraircraft in the vicinity also are flying indicated.

rather than true altitude, and your altimeter read-ings are compatible. If flying in cold weather overmountainous areas, however, you must take this dif-ference between indicated and true altitude intoaccount. You must know that your true altitudeassures clearance of terrain, so you compute a cor-rection to indicated altitude.

Corrected (Approximately True) AltitudeIf it were possible for a pilot always to determine

mean temperature of the column of air betweenthe aircraft and the surface, flight computers wouldbe designed to use this mean temperature in com-puting true altitude. However, the only guide apilot has to temperature below him is free airtemperature at his altitude. Therefore, the flightcomputer uses outside air temperature to correctindicated altitude to approximate true altitude.Corrected altitude is indicated altitude correctedfor the temperature of the air column below theaircraft, the correction being based on the esti-mated departure of the existing temperature fromstandard atmospheric temperature. It is a closeapproximation to true altitude and is labeled truealtitude on flight computers. It is close enough to

Rennes 15. When flying from high pressure to lower pressure without adjusting your altimeter, you are losing true altitude.

1$

31

1 I /\9 0 1/

4M1(014..0

ALTIMETER READS

TOO HIGHI

1I 1 I%, L 0 if /

7 A f 2°...... ,...

TRUE AND INDV

ARE

ALTIMETER READS

TOO LOW"'I I \%

FIGURE 16. Effect of temperature on altitude. When air is warmer than average, you are higher than your altimeter indi-cates. When temperature is colder than average, you are lower than indicated. When Eying from warm to cold air at aconstant indicated attitude, you are losing true altitude.

true altitude to be used for terrain clearance pro-vided you have your altimeter set to the value re-ported from a nearby reporting station.

Pilots have met with disaster because they failedto allow for the difference between indicated andtrue altitude. In cold weather when you must clearhigh terrain, take time to compute true altitude.

FAA regulations require you to fly indicated ahi-tude at low levels and pressure altitude at highlevels (at or above 18,000 feet at the time this bookwas printed). What is pressure altitude?

Pressure Altitudein the standard atmosphere, sea level pressure is

29,92 inches of mercury or 1013.2 millibars. Pres-suyt falls at a fixed rate upward through this hispo-thlt ,ral atmosphere. Therefore, in the standardat 31.Asphere, a given pressure exists at any specifiedaltitwie. Pressure altitude is the altitude in thesten:44rd atmosphere where pressure is the same aswhere you are. Since at a specific pressure altitude,pressure is everywhere the same, a constant pres-

sure surface defines a constant pressure altitude.When you fly a constant pressure altirude, you areflying a constant pressure surface.

You can always determine pressure altitude fromyour altimeter whether in flight or on the ground.Simply set your altimeter at the standard altimetersetting of 29.92 inches, and your altimeter indicatespressure altitude.

A conflict sometimes occurs near the altitudeseparating flights using indicated altitude fromthose using pressure altitude. Pressure altitude Qnone aircraft and indicated altitude on another mayindicate altitude separation when, actually, the twoare at the same true altitude. All flights using pres-sure altitude at high altitudes are IFR controlledflights. When this conflict occurs, air traffic control-lers prohibit IFR flight at the conflicting altitudes.

DENSITY ALTITUDEWhat is density altitude? Density altitude simply

is the altitude in the standard atmosphere whereair density is the same as where you are. Pressure,

3219

temperature, and humidity determine air density.On a hot day, the air becomes "thinner" or lighter,and its density where you are is equivalent to ahigher altitude in the standard atmospherethusthe term "high density altitude." On a cold day,the air becomes heavy; its density is the same asthat at an altitude in the standard atmospherelower than your altitude"low density altitude."

Density altitude is not a height reference; rather,it is an index to aircraft performance. Low densityaltitude increases performance. High density alti-tude is a real hazard since it reduces aircraft per-formance. It affects performance in three ways.(1) It reduces power because the engine takes inless air to support conibustion. (2) It reduces thrustbecause the propeller gets less grip on the light air

or a jet has less mass of gases to spit out the ex-haust. (3) It reduces lift because the light airexerts less force on the airfoils.

You cannot detect the effect of high density alti-tude on your airspeed indicator. Your aircraft liftsoff, climbs, cruises, glides, and lands at the prescribedindicated airspeeds. But at a specified indicatedairspeed, your true airspeed and your groundspeedincrease proportionally as density altitude becomeshigher.

The net results are that high density altitudelengthens your takeoff and landing rolls and re-duces your rate of climb. Before lift-off, you mustattain a faster groundspeed, and therefore, youneed more runway; your reduced power and thrustadd a need for still more runway. You land at a

..,

1500

1000

500

00'

1500

1000

500

20

FIGURE 17. Effect of density aldtude on takeoff and climb. High density altitude lengthens takeoff rolland reduces rate of climb.

3 3

,

w

faster groundspeed and, therefore, need more roomto stop. At a prescribed indicated airspeed, you areflying at a faster true airspeed, and therefore, youcover more distance in a given time which meansclimbing at a more shallow angle. Add to this theproblems of reduced power and rate of climb, andyou are in double jeopardy in your climb. Figure17 shows the effect of density altitude on takeoffdistance and rate of climb.

High density altitude also can be a problem atcruising altitudes. When air is abnormally warm,the high density altitude lowers your service ceiling.For example, if temperature at 10,000 feet pres-sure altitude is 20° C, density altitude is 12,700

feet. (Check this on your flight computer.) Youraircraft will perform as thOugh it were at 12,700indicated with a normal temperature of 8° C.

To compute density altitude, set your altimeterat 29.92 inches or 1013.2 millibars and read pres-sure altitude from your altimeter. Read outside airtemperature and then use your flight computer toget density altitude. On an airport served by aweather observing station, you usually can get den-sity altitude for the airport from the observer. Sec-tion 16 of AVIATION WEATHER SERVICES has agraph for computing density altitude if you haveno flight computer handy.

IN CLOSING

Pressure patterns can be a clue to weather causesand movement of weather systems, but they giveonly a part of the total weather picture. Pressuredecreases with increasing altitude. The altimeter isan aneroid_ barometer graduated in increments ofaltitude in the standard atmosphere instead of unitsof pressure. Temperature greatly affects the rate ofpressure decrease with height; therefore, it influ-ences altimeter readings. Temperature also deter-mines the density of air at a given pressure (densityaltitude). Density altitude is an index to aircraftperformance. Always be alert for departures ofpressure and temperature from normals and com-pensate for these abnormalities.

Following are a few operational reminders:1. Beware of the low pressure-bad weather,

high pressure-good weather rule of thumb.It frequently fails. Always get the completeweather picture..

2. When flying from high pressure to low pres-sure at constant indicated altitude and with-out adjusting the altimeter, you are losingtrue altitude.

3. When temperature is colder than standard,you are at an altitude lower than your al-timeter indicates. When temperature iswarmer than standard, you are higher thanyour altimeter indicates.

4. When flying cross country, keep your altim-eter setting current. This procedure assuresmore positive altitude separation from otheraircraft.

31

5. When flying over high terrain in coldweather, compute your true altitude to en-sure terrain 'clearance.

6. When your aircraft is heavily loaded, thetemperature is abnormally warm, and/orthe pressure is abnormally low, computedensity altitude. Then check your aircraftmanual to ensure that you can become air-borne from the available runway. Checkfurther to determine that your rate of climbpermits clearance of obstacles beyond theend of the runway. This procedure is ad-visable for any airport regardless of altitude.

7. When planning takeoff or landing at ahigh altitude airport regardless of load,determine density altitude. The procedureis especially critical when temperature isabnormally warm or pressure abnormallylow. Make certain you have sufficient run-way for takeoff or landing roll. Make sureyou can clear obstacles beyond the end ofthe runway after takeoff or in event of ago-around.

8. Sometimes the altimeter setting is takenfrom an instrument of questionable reli-ability. However, if the instrument cancause an error in altitude reading of morethan 20 feet, it is removed from service.When altimeter setting is estimated, be pre-pared for a possible 10- to 20-foot differ-ence between field elevation and your al-timeter reading at touchdown.

21

a

I

Chapter 4WIND

Differences in temperature create differences inpressure. These pressure differences drive a com-plex system of winds in a never ending attempt toreach equilibrium. Wind also transports watervapor and spreads fog, clouds, and precipitation.To help you relate wind to pressure patterns andthe movement of weather systems, this chapter ex-

plains convection and the pressure gradient force,describes the effects of the Coriolis and frictionalforces, relates convection and these forces to thegeneral circulation, discusses local and small-scalewind systems, introduces you to wind shear, andassociates wind with weather.

CONVECTION

When two surfaces are heated unequally, theyheat the overlying air unevenly. The warmer* airexpands and becomes lighter or less dense than thecool* air. The more dense, cool air is drawn to theground by its greater gravitational force lifting or

Frequently throughout this book, we refer to air aswarm, cool, or cold. These terms refer to relative tem-peratures and not to any fixed temperature reference or

forcing the warm air upward much as oil is forcedto the top of water when the two are mixed. Figure18 shows the convective process. The rising airspreads and cools, eventually descending to com-

to temperatures as they may affect our comfort. For ex-ample, compare air at 10' F to air at 0' F; relative toeach other, the 106 F air is cool and the 0' F, warm.90' F would be cool or cold relative to WO' F.

:3 5

23

plete the convective circulation. As long as theuneven heating persists, convection maintains acontinuous "convective current."

The horizontal air flow in a convective currentis "wind." Convection of both large and smallscales accounts for systems ranging from hemi-

spheric circulations down to local eddies. This hor-izontal flow, wind, is sometimes called "advection."However, the term "advection" more commonlyapplies to the transport of atmospheric propertiesby the wind, i.e., warm advection; cold advection;advection of water vapor, etc.

////////i/M,/, ,,,,, ,,,,WARM SURFACE /COOL SURFACE

/1

FIGURE 18. Convective current resulting from uneven heating of air by contrasting surface temperatures. The cool, heavierair forces the warmer air aloft establishing a convective cell. Convection continues as long as the uneven heating persists.

PRESSURE GRADIENT FORCE

Pressure differences must create a force in order,to drive the wind. This force is the pressure gradientforce. The force is from higher pressure to lowerpressure and is perpendicular to isobars or con-tours. Whenever a pressure difference develops overan area, the pressure gradient force begins movingthe air directly across the isobars. The closer thespacing of isobars, the stronger is the pressure gra-dient force. The stronger the pressure gradientforce, the stronger is the wind. Thus, closely spacedisobars mean strong winds; widely spaced isobarsmean lighter wind. From a pressure analysis, youcan get a general idea of wind speed from contouror isobar spacing.

24

Because.of uneven heating of the Earth, surfacepressure is low in warm equatorial regions and highin cold polar regions. A pressure gradient developsfrom the poles to the Equator. If the Earth did notrotate, this pressure gradient force would be theonly force acting on the wind. Circulation wouldbe two giant hemispheric convective currents asshown in figure 19. Cold air would sink at thepoles; wind would blow straight from the poles tothe Equator; warm air at the Equator would beforced upward; and high level winds would blowdirectly toward the poles. However, the Earth doesrotate; and because of its rotation, this simple cir-culation is greatly distorted.

I01

4.

a

AST

ei,

Fiouaz 19. Circulation as it would be on a nonrotating globe. Intense heating at the Equator lowers the density. Moredense air Rows from the poles toward the Equator forcing the less dense air aloft where it flows toward the poles. Thecirculation would be two giant hemispherical convective currents.

CORIOLIS FORCE

A moving mass travels in a straight line untilacted on by some outside force. However, if oneviews the moving mass from a rotating platform,the path of the moving mass relative to his plat-form appears to be deflected or curved. To illus-trate, start rotating the turntable of a record player.Then using a piece of chalk and a ruler, draw a"straight" line from the center to the outer edge ofthe turntable. To you,: the. chalk traveled in astraight line. Now stop the turntable; on it, theline spirals outward from the center as shown infigure 20. To a viewer on the turntable, some "ap-parent" force deflected the chalk to the right.

A similar apparent force deflects moving parti-cles on the earth. Because the Earth is spherical,the deflective force is much more complex than thesimple turntable example. Although the force istermed "apparent," to us on Earth, it is very real.The principle was first explained by a Frenchman,Coriolis, and carries his namethe Coriolis force.

The Coriolis force affects the paths of aircraft;missiles; flying birds; ocean currents; and, mostimportant to the study of weather, air currents.The force deflects air to the right in the NorthernHemisphere and to the left in the Southern Hemi-sphere. This book concentrates mostly on deflectionto the right in the Northern Hemisphere.

Coriolis force is at a right angle to wind directionand directly proportional to wind speed. That is, aswind speed increases, Coriolis force increases. At agiven latitude, double the wind speed and you dou-ble the Coriolis force. Why at a given latitude?

Coriolis force varies with latitude from zero atthe Equator to a maximum at the poles. It influ-ences wind direction everywhere except immediatelyat the Equator; but the effects are more pronouncedin middle and high latitudes.

Remember that the pressure gradient force drivesthe wind and is perpendicular to isobars. When apressure gradient force is first established, wind be-

3725

FIGURE 20. Apparent deflective force due to rotation of a horizontal platform. The "space path" is the path taken by apiece of chalk. The "path on the record" is the line traced on the rotating record. Relative to the record, the chalk appearedto curve; in space, it traveled in a straight line.

gins to blow from higher to lower pressure directlyacross the isobars. However, the instant air beginsmoving, Coriolis force deflects it to the right. Soonthe wind is deflected a full 90° and is parallel tothe isobars or contours. At this time, Coriolis forceexactly balances pressure gradient force as shown

in figure 21. With the forces in balance, wind willremain parallel to isobars or contours. Surface fric-tion disrupts this balance as we discuss later; butfirst lees see how Coriolis force distorts the ficti-tious global circulation shown in figure D.

THE GENERAL CIRCULATION

As air is forced aloft at the Equator and beginsits high-level trek northward, the Coriolis forceturns it to the right or to the east as shown infigure 22. Wind becomes westerly at about 30°latitude temporarily blocking further northwardmovement. Similarly, as air over the poles begins

26

its low -level journey southward toward the Equa-tor, it likewise is deflected to the right and becomesan east wind, halting for a while its southerly prog-ressalso shown in figure 22. As a result, air liter-ally "piles up" at about 30° and 60° latitude inboth hemispheres. The added weight of the air in-

3 8'

*

FIOURZ 21. Effect of Coriolis force on wind relative to isobars. When Coriolis force deflects the wind until it is parallel tothe isobars, pressure gradient balances Coriolis force.

creases the pressure into semipermanent high pres-sure belts. Figures 23 and 24 are maps of meansurface pressure for the months of July and Jan-uary; The maps show clearly the subtropical highpressure belts near 30° latitude in both the North-ern and Southern Hemispheres.

The building of these high pressure belts createsa temporary impasse disrupting the simple convec-tive transfer between- the Equator and the poles.The restless atmosphere cannot live with this im-passe in its effort to reach equilibrium. Somethinghas to give. Huge masses of air begin overturningin middle latitudes to complete the exchange.

Large masses of cold air break through the north-ern barrier plunging southward toward the Trop-ics. Large midlatitude storms develop between cold

3i

outbreaks and carry warm air northward. The re-sult is a midlatitude band of migratory storms withever changing weather. Figure 25 is an attempt tostandardize this chaotic circulation into an averagegeneral circulation.

Since pressure differences cause wind, seasonalpressure variations determine to a great extent theareas of these cold air outbreaks and midlatitudestorms. But, seasonal pressure variations are largelydue to seasonal temperature changes. We havelearned that, at the surface, warm temperatures toa great extent determine low pressure and coldtemperatures, high pressure. We have also learnedthat seasonal temperature changes over continentsare much greater than over oceans.

During summer, warm continents tend to be

27

N. PO LE

LOW LEVEL POLAR EASTERLIES/ 1 \HIGH LEVEL SUBTROPICAL WESTERLIES

FIGURE 22. In the Northern Hemisphere, Coriolis force turns high level southerly winds to westerlies at about 30° latitude,temporarily halting further northerly progress. Low-level northerly winds from the pole are turned to easterlies, tem-porarily stopping further southward movement at about 60° latitude. Air tends to "pile up" at these two latitudes creatinga void in middle latitudes. The restless atmosphere cannot live with this void; something has to give.

FIGURE 21 Mean worldwide surface pressure distribution in July. In the warm Northern Hemisphere, warm land areastend to have low pressure, and cool oceanic areas tend to have high pressure. In the cool Southern Hemisphere, the pat-tern is reversed; cool land areas tend to have high pressure; and water surfaces, low pressure. However, the relationshipis not so evident in the Southern Hemisphere because of relatively small amounts of land. The subtropical high pressurebelts are clearly evident at about 30° latitude in both hemispheres.

28

40

I

4r. ifiBITWIL ASSwianzatinik wiumma"60.114.1.111El1 ALLT:14411MgalablagiaMME

Fu Inas 24. Mean world-wide surface pressure distribution in January. in this season, the pattern in figure 23 is reversed.in the cool Northern Hemisphere, cold continental areas are predominantly areas of high pressure while warm oceanstend to be low pressure areas. in the warm Southern Hemisphere, land areas tend to have low pressure; and oceans, highpresture. The subtropical high pressure belts are evident in both hemispheres. Note that the pressure belts shift southwardin January and northward in July with the shift in the zone of maximum heating.

areas of low pressure and the relatively cool oceans,high pressure. In winter, the reverse is truehighpressure over the cold continents and low pressureover the relatively warm oceans. Figures 23 and 24show this seasonal pressure reversal. The same pres-sure variations occur in the warm and cold seasonsof the Southern Hemisphere, although the effect isnot as pronounced because of the much largerwater areas of the Southern Hemisphere.

Cold outbreaks are strongest in the cold seasonand are predominantly from cold continental areas.Summer outbreaks are weaker and more likely tooriginate from cool water surfaces. Since these out-breaks are masses of cool, dense air, they character-istically are high pressure areas.

As the air tries to blow outward from the highpressure, it is deflected to the right by the Coriolisforce. Thus, the wind around a high blows clock-wise. The high pressure with its associated windsystem is an anticyclone.

The storms that develop between high pressuresystems are characterized by low pressure. As windstry to blow inward toward the center of low pres-sure, they also are deflected to the right. Thus, the

wind around a low is counterclockwise. The lowpressure and its wind system is a cyclone. Figure26 shows winds blowing parallel to isobars (con-tours on upper level charts). The winds are clock-wise around highs and counterclockwise aroundlows.

The high pressure belt at about 30° north lati-tude forces air outward at the surface to the northand to the south. The northbound air becomes en-trained into the midlatitude storms. The southwardmoving air is again deflected by the Coriolis forcebecoming the well-known subtropical northeasttrade winds. In midlatitudes, high level winds arepredominantly from the west and are known as theprevailing westerlies. Polar easterlies dominate low-level circulation north of about 60° latitude.

These three major wind belts are shown in fig-ure 25. Northeasterly trade winds carry tropicalstorms from east to west. The prevailing westerliesdrive midlatitude storms generally from west toeast. Few major storm systems develop in the com-paratively small Arctic region; the chief influenceof the polar easterlies is their contribution to thedevelopment of midlatitude storms.

41

29

141;16d 10'

not= 25. General average circulation in the Northern Hemisphere. Note the three belts of prevailing winds, the polareasterlies, the prevailing westerlies in middle latitudes, and the northeasterly "trade' winds. The belt of prevailing west-erlies is a mixing zone betwetzz the North Pole and the Equator characterized by migrating storms.

Our discussion so far has said nothing about frig tours where friction has little effect. We cannot,tion. Wind flow patterns aloft follow isobars or con- however, neglect friction near the surface.

FRICTION

Friction between the wind and the terrain sur-face slows the wind. The rougher the terrain, thegreater is the frictional effect. Alsb, the stronger thewind speed, the greater is the friction.One may notthink of friction as a force, but it is a very real andeffective force always acting opposite to winddirection.

As frictional force slows the windspeed, Coriolisforce decreases. However, friction does not affectpressure gradient force. Pressure gradient andCoriolis forces are no longer in balance. Thestronger pressure gradient force turns the wind atan angle across the isobars toward lower pressure

30

42

until the three forces balance as shown in figure27. Frictional and Coriolis forces combine to justbalance pressure gradient force. Figure 28 showshow surface wind spirals outward from highpressure into low pressure crossing isobars at anangle.

The angle of surface wind to isobars is about 10°over water increasing with roughness of terrain. Inmountainous regions, one often has difficulty relat-ing surface wind to pressure gradient because ofimmense friction and also because of local terraineffects on pressure.

*

Fromm 26. Air flow around pressure systems above the friction layer. Wind (black arrows) is parallel to contours andcirculates clockwise around high pressure and counterclockwise around low pressure.

THE JET STREAM

A discussion of the general circulation is incom-plete when it does r at mention the "jet stream."Winds on the average increase with height through-out the troposphere culminating in a maximumnear the level of the tropopause. These maximumwinds tend to be further concentrated in narrow

bands. A jet stream, then, is a narrow band ofstrong winds meandering through the atmosphereat a level near the tropopause. Since it is of interestprimarily to high level flight, further discussion ofthe jet stream is reserved for chapter 13, "HighAltitude Weather."

LOCAL AND SMALL SCALE WINDS

Until now, we have dealt only with the generalcirculation and major wind systems. Local terrainfeatures such as mountains and shore lines influ-ence local winds and weather.

MOUNTAIN AND VALLEY WINDS

In the daytime, air next to a mountain slope isheated by contact with the ground as it receivesradiation from the sun. This air usually becomes

43

warmer than air at the same altitude but fartherfrom the slope.

Colder, denser air in the surroundings settlesdownward and forces the wanner air near theground up the mountain slope. This wind is a "val-ley wind" so called because the air is flowing upout of the valley.

At night, the air in contact with the mountainslope is cooled by terrestrial radiation and becomesheavier than the surrounding air. It sinks along the

31

Moults 27. Surface friction slows the wind and reduces Coriolis force but does not affect pressure gradient force; windsnear the surface are deflected across the isobars toward lower pressure.

slope, producing the "mountain wind" which flowslike watcr down the mountain slope. Mountainwinds are usually stronger than valley winds, espe-cially in winter. The mountain wind often continuesdown the more gentle slopes of canyons and valleys,and in such cases takes the name "drainage wind."It can become quite strong over some terrain con-ditions and in extreme cases can become hazardouswhen flowing through canyon restrictions as dis-cussed in chapter 9.

KATABATIC WINDA katabatic wind is any wind blowing down an

incline when the incline is influential in causing thewind. Thus, thc mountain wind is a katabaticwind. Any katabatic wind originates because cold,heavy air spills down sloping terrain displacingwarmer, less dense air ahead of it. Air is heated anddried as it flows down slope as we will study in

32

later chapters. Sometimes the descending air be-comes warmer than the air it replaces.

Many katabatic winds recurring in local areashave been given colorful names to highlight theirdramatic, local effect. Some of these are the Bora,a cold northerly wind blowing from the Alps to theMediterranean coast; the Chinook, figure 29, awarm wind down the east slope of the RockyMountains often reaching hundreds of miles intothe high plains; the Taku, a cold wind in Alaskablowing off the Taku glacier; and the Santa Ana,a warm wind descending from the Sierras into theSanta Ana Valley of California.

LAND AND SEA BREEZES

As frequently stated earlier, land surfaces warmand cool more rapidly than do water surfaces;therefore, land is warmer than the sea during the

44

^..... HIGH :

.10

Datum 28. Circulation around pressure systems at the surface. Wind spirals outward from high pressure and inward tolow pressure, crossing isobars at an angle.

FIGURE 29. The "Chinook" is a katabatic (downslope) wind. Air cools as it moves upslope and warms as it blows down-slope. The Chinook occasionally produces dramatic warming over the plains just east of the Rocky Mountains.

clay; wind blows from the cool water to warmlandthe "sea breeze" so called because it blowsfrom the sea. At night, the wind reverses, blowsfrom cool land to wanner water, and creates a"land breeze." Figure 30 diagrams land and seabreezes.

/

COOLER AIR OVER WAY*

MOVING TOWARD LAND

Land and sea breezes develop only when theoverall pressure gradient is weak. Wind with astronger pressure gradient mixes the air so rapidlythat local temperature and pressure gradients donot develop along the shore line.

Fiouaz 30. Land and sea breezes. At night, cool air from the land flows toward warmer waterthe land breeze. Duringthe day, wind blows from the water to the warmer landthe sea breeze.

WIND

Rubbing two objects against each other createsfriction. If the objects are solid, no exchange ofmass occurs between the two. However, if the ob-jects are fluid currents, friction creates eddies alonga common shallow mixing zone, and a mass trans-

34

4a3

SHEAR

fer takes place in the shallow mixing layer. Thiszone of induced eddies and mixing is a shear zone.Figure 31 shows two adjacent currents of air andtheir accompanying shear zone. Chapter 9 relateswind shear to turbulence.

OP-

I

e

Fromm 31. Wind shot. Air currents of differing velocities create friction or "shear" between them. Mixing in the shearzone results in a snarl of eddies and whirls.

WIND, PRESSURE SYSTEMS, AND WEATHER

We already have shown .that wind speed is pro-portional to the spacing of isobars or contours ona weather map. However, with the same spacing,wind speed at the surface will be less than aloftbecause of surface friction.

You also can determine wind direction from aweather map. If you face along an isobar or con-tour with lower pressure on your left, wind will beblowing in the direction you are facing. On a sun.face map, wind will cross the isobar at an angletoward lower pressure; on an upper air chart, itwill be parallel to the contour.

Wind blows counterclockwise (Northern Hemi-sphere) around a low and clockwise around a high.At the surface where winds cross the isobars at anangle, you can see a transport of air from high tolow pressure. Although winds are virtually parallelto contours on an upper air chart, there still is aslow transport of air from high to low pressure.

At the surface when air converges into a low, itcannot go outward against the pressure gradient,nor can it go downward into the ground; it must

go upward.* Therefore, a low or trough is an areaof rising air.

Rising air is conducive to cloudiness and pre-cipitation; thus we have the general association oflow pressure -bad weather. Reasons for the inclem-ent weather are developed in later chapters.

By similar reasoning, air moving out of a high orridge depletes the quantity of air. Highs and ridges,therefore, are areas of descending air. Descendingair favors dissipation of cloudiness; hence the asso-ciation, high pressuregood weather.

Many times weather is more closely associatedwith an upper air pattern than with features shownby the surface map. Although features on the twocharts are related, they seldom are identical. A

*You may recall that earlier we said air "piles up" inthe vicinity of 30° latitude increasing pressure and form-ing the subtropical high pressure belt. Why, then, doesnot air flowing into a low or trough increase pressure andfill the system? Dynamic forces maintain the low ortrough; and these forces differ from the forces that main-tain the subtropical high.

47.35

weak surface system often loses its identity in theupper air pattern, while another system may bemore evident on the upper air chart than on thesurface map.

Widespread cloudiness and precipitation oftendevelop in advance of an upper trough or low. Aline of showers and thunderstorms is not uncom-mon with a trough aloft even though the surfacepressure pattern shows little or no cause for thedevelopment.

On the other hand, downward motion in a highor ridge places a "cap" on convection, preventingany upward motion. Air may become stagnant in ahigh, trap moisture and contamination in low lev-els, and restrict ceiling and visibility. Low stratus,fog, haze, and smoke are not uncommon in highpressure areas. However, a high or ridge aloft withmoderate surface winds most often produces goodflying weather.

Highs and lows tend to lean from the surfaceinto the upper atmosphere. Due to this slope, windsaloft often blow across the associated surface sys-tems. Upperwinds tend to steer surface systems inthe general direction of the upper wind flow.

An intense, cold, low pressure vortex leans lessthan does a weaker system. The intense low be-comes oriented almost vertically and is clearly evi-dent on both surface and upper air charts. Upperwinds encircle the surface low and do not blow

36

48'

across it. Thus, the storm moves very slowly andusually causes an extensive and persistent area ofclouds, precipitation, strong winds, and generallyadverse flying weather. The term cold low some-times used by the weatherman describes such asystem.

A contrasting analogy to the cold low is thethermal low. A _dry, sunny region becomes quitewarm from intense surface heating thus generatinga surface low pressure area. The warm air is car-ried to high levels by convection, but cloudiness isscant because of lack of moisture. Since in warmair, pressure decreases slowly with altitude, thewarm surface low is not evident at upper levels.Unlike the cold low, the thermal low is relativelyshallow with weak pressure gradients and no welldefined cyclonic circulation. It generally supportsgood flying weather. However, during the heat ofthe day, one must be alert for high density altitudeand convective turbulence.

We have cited three exceptions to the lowpressurebad weather, high pressuregood weatherrule: (1) cloudiness and precipitation with anupper air trough or low not evident on the surfacechart; (2) the contaminated high; and (3) thethermal low. As this book progresses, you can fur-ther relate weather systems more specifically toflight operations.

14.-

T

4

Chapter 5MOISTURE, CLOUD FORMATION,AND PRECIPITATION

Imagine, if you can, how easy flying would be if variety of hazards unmatched by any other weatherskies everywhere were clear! But, flying isn't always element. Within Earth's climatic range, water is inthat easy; moisture in the atmosphere creates a the frozen, liquid, and gaseous states.

WATER VAPOR

Water evaporates into the air and becomes an measure water vapor and express it in differentever-present but variable constituent of the atmo- ways. Two commonly used terms are (1) relativesphere. Water vapor is invisible just as oxygen and humidity, and (2) dew point.other gases are invisible. However, we can readily

4937

RELATIVE HUMIDITY

Relative humidity routinely is expressed in per-cent. As the term suggests, relative humidity is"relative." It relates the actual water vapor presentto that which could be present.

Temperature largely determines the maximumamount of water vapor air can hold. As figure 32shows, warm air can hold more water vapor thancool air. Figure 33 relates water vapor, temper-ature, and relative humidity. Actually, relative hu-midity expresses the degree of saturation. Air with100% relative humidity is saturated; less than100% is unsaturated.

If a given volume of air is cooled to some spe-cific temperature, it can hold no more water vaporthan is actually present, relative humidity be-comes 100%, and saturation occurs. What is thattemperature?

DEW POINT

Dew point is the temperature to which air mustbe cooled to become saturated by the water vaporalready present in the air. Aviation weather reportsnormally include the air temperature and dewpoint temperature. Dew point when related to airtemperature reveals qualitatively how close the airis to saturation. .

TEMPERATURE-DEW POINT SPREAD

The difference between air temperature and dewpoint temperature is popularly called the "spread."As spread becomes less, relative humidity increases,and it is 100% when temperature and dew pointare the same. Surface temperaturedew pointspread is important in anticipating fog but has lit-tle bearing on precipitation. To support precipita-tion, air must be saturated through thick layersaloft.

Fiotnts 32. Blue dots illustrate the increased water vapor capacity of warmer air. At each temperature, air can hold aspecific amount of water vaporno more.

38

50.

4

of

50

-- =I,

25

0U

AT 55°550

50%

DEW POINT

37°

oF

50

,25

p

U

AT 44°

mok.

440

50

25

0

AT 37°.

100%(SATURATED)

MAXIMUM POSSIBLE WATER VAPOR '"'... ACTUAL WATER VAPOR

,RELATIVE HUMIDITY AND DEW POINT

FIGURE 33. Relative humidity depends on both temperature and water vapor. In this figure, water vapor is constant buttemperature varies. On the left, relative humidity is 50%; the warmer air could hold twice as much water vapor as isactually present. As the air cools, center and right, relative humidity increases. As the air cools to 37° F, its capacity tohold water vapor is reduced to the amount actually present. Relative humidity is 100% and the air is now "saturated."Note that at 100% humidity, temperature and dew point are the same. The air cooled to saturation, i.e., it cooled tothe dew point.

Sometimes the spread at ground level may bequite large, yet at higher altitudes the air is sat-urated and clouds form. Some rain may reach theground or it may evaporate as it falls into the drierair. Figure 34 is a photograph of "virga"stream-

en of precipitation trailing beneath clouds butevaporating before reaching the ground. Our neverending weather cycle involves a continual reversiblechange of water from one state to another. Let'stake a closer look at change of state.

CHANGE OF STATE

Evaporation, condensation, sublimation, freez-ing, and melting are changes of state. Evaporationis the changing of liquid water to invisible water

vapor. Condensation is the reverse process. Subli-mation is the changing of ice directly to watervapor, or water vapor to ice, bypassing the liquid

39

FIGURE 34. Virga. Precipitation from the cloud evaporates in drier air below and does not retch the ground.

State in each process. Snow or ice crystals resultfrom ,the i rnavillition of, water .vapor directly tothe solid state. We are all familiar with freezingand melting processes.

LATENT HEATAny change of state involves a heat transaction

with no change in temperature. Figure 35 diagramsthe heat exchanges between the different states.Evaporation requires heat energy that comes fromthe nearest available heat source. This heat energy isknown as the "latent heat of vaporization," and itsremoval cools the source it comes from. An exam-ple is the cooling of your body by evaporation ofperspiration.

What becomes of this heat energy used by evap-oration? Energy cannot be created or destroyed, soit is bidden or stored in the invisible water vapor.When the water vapor condenses to liquid water

40

52

or sublimates directly to ice, energy originally usedin the evaporation reappears as heat and is releasedto the atmosphere. This energy is "latent heat" andis quite significamt as we learn in later chapters.Melting and freezing involve the exchange of"latent heat of fusion" in a similar manner. Thelatent heat of fusion is much less than that pf con-densation and evaporation; however, each in itsown way plays an important role in aviation weather.

As air becomes saturated, water vapor begins tocondense on the nearest available surface. Whatsurfaces are in the atmosphere on which watervapor may condense?

CONDENSATION NUCLEIThe atmosphere is never completely clean; an

abundance of microscopic solid particles suspendedin the air are condensation surfaces. These par-ticles, such as salt, dust, and combustion byproducts

GAS(VAPOR) illp LIQUID

A

AL,

SOLIDqui* MO

HEAT GIVEN OUT HEAT TAKEN IN

novas 35. Heat transactions when water changes state. Blue arrows indicate changes that absorb heat. The absorbedbeat remains bidden, or "latent" until a reverse change occurs. The red arrows show changes that release latent heat backto the surroundings. The heat exchange occurs whenever water changes state even when there is no change in temperature.These heat exchanges play important roles in suppressing temperature changes and In developing instability.

are "condensation nuclei." Some condensation nu-clei have an affinity for water and can induce con-densation or sublimation even when air is almostbut not completely saturated.

As water vapor condenses or sublimates on con-densation nuclei, liquid or ice particles begin togrow. Whether the particles are liquid or ice doesnot depend entirely on temperature. Liquid watermay be present at temperatures well below freezing.

SUPERCOOLED WATERFreezing is complex 'and liquid water droplets

often condense or persist at temperatures colderthan 0° C. Water droplets colder than 0° C aresupercooled. When they strike an exposed object,the impact induces freezing. Impact freezing ofsupercooled water can result in aircraft icing.

Supercooled water drops very often are in abun-dance in clouds at temperatures between 0° C and15° C with decreasing amounts at colder tem-peratures. Usually, at temperatures colder than15° C, sublimation is prevalent; and clouds andfog may be mostly ice crystals with a lesser amountof supercooled water. However, strong verticalcurrents may carry supercooled water to greatheights where temperatures are much colder than

5

15° C. Supercooled water has been observed attemperatures colder than 40° C.

DEW AND FROSTDuring clear nights with little or no wind, vege-

tation often cools by radiation to a temperature ator below the dew point of the adjacent air. Mois-ture then collects on the leaves just as it does on apitcher of ice water in a warm room. Heavy dewoften collects on grass and plants when none col-lects on pavements or large solid objects. Thesemore massive objects absorb abundant heat duringthe day, lose it slowly during the night, and coolbelow the dew point only in rather extreme cases.

Frost forms in much the same way as dew. Thedifference is that the dew point of surrounding airmust be colder than freezing. Water vapor thensublimates directly as ice crystals or frost ratherthan condensing as dew. Sometimes dew forms andlater freezes; however, frozen dew is easily dis-tinguished from frost. Frozen dew is hard andtransparent while frost is white and opaque.

To now, we have said little about clouds. Whatbrings about the condensation or sublimation thatresults in cloud formation?

. 41

CLOUD FORMATION

Normally, air must become saturated for con-densation or sublimation to occur. Saturation mayresult from cooling temperature, increasing dewpoint, or both. Cooling is far more predominant.

COOLING PROCESSESThree basic processes may cool air to saturation.

They are (1) air moving over a colder surface,(2) stagnant air overlying a cooling surface, and(3) expansional cooling in upward moving air.Expansional cooling is the major cause of cloud

formation. Chapter 6, "Stable and Unstable Air,"discusses expansional cooling in detail.

CLOUDS AND FOGA cloud is a visible aggregate of minute water

or ice particles suspended in air. If the cloud is onthe ground, it is fog. When entire layers of air coolto saturation, fog or sheet-like clouds result. Satura-tion of a localized updraft produces a toweringcloud. A cloud may be composed entirely of liquidwater, of ice crystals, or a mixture of the two.

PRECIPITATION

Precipitation is an all inclusive term denotingdrizzle, rain, snow, ice pellets, hail, and ice crystals.Precipitation occurs when these particles grow insize and weight until the atmosphere no longer cansuspend them and they fall. These particles growprimarily in two ways.

42

PARTICLE GROWTHOnce a water droplet or ice crystal forms, it

continues to grow by added condensation or subli-mation directly onto the particle. This is the slowerof the two methods and usually results in drizzle orvery light rain or snow.

0(Z--

0 ----t it.")

haulm 36. Growth of raindrops by collision of cloud droplets.

54

Cloud particles collide and merge into a larg-er drop in the more rapid growth process. Thisprocess produces larger precipitation particles anddoes so more rapidly than the simple condensationgrowth process. Upward currents enhance thegrowth rate and also support larger drops as shownin figure 36. Precipitation formed by mergingdrops with mild upward currents can produce lightto moderate rain and snow. Strong upward cur-rents support the largest drops and build clouds togreat heights. They can produce heavy rain, heavysnow, and hail.

LIQUID, FREEZING, AND FROZENPrecipitation forming and remaining liquid falls

as rain or drizzle. Sublimation forms snowflakes,and they reach the ground as snow if temperaturesremain below freezing.

Precipitation can change its state as the tem-perature of its environment changes. Failing snow

may melt in warmer layers of air at lower altitudesto form rain. Rain falling through colder air maybecome supercooled, freezing on impact as freezingrain; or it may freeze during its descent, falling asice pellets. Ice pellets always indicate freezing rainat higher altitude.

Sometimes strong upward currents sustain largesupercooled water drops until some freeze; sub-sequently, other drops freeze to them forminghailstones.

PRECIPITATION VERSUSCLOUD THICKNESS

To produce significant precipitation, clouds usu-ally are 4,000 feet thick or more. The heavier theprecipitation, the thicker the clouds are likely tobe. When arriving at or departing from a terminalreporting precipitation of light or greater intensity,expect clouds to be more than 4,000 feet thick.

LAND AND WATER EFFECTS

Land and water surfaces underlying the atmo-sphere greatly affect cloud and precipitation de-velopment. Large bodies of water such as oceansand large lakes add water vapor to the air. Expect

the greatest frequency of low ceilings, fog, and pre-cipitation in areas where prevailing winds have anover-water trajectory. Be especially alert for thesehazards when moist winds are blowing upslope.

BOVRIL 37. Lake effects. Air moving across a sizeable lake absorbs water vapor, Showers may appear on the leeward sideif the air is colder than the water. When the air is warmer than the water, fog often develops on the lee side.

43

5 5

In winter, cold air frequently moves over rela-tively warm lakes. The warm water adds heat andwater vapor to the air causing showers to the leeof the lakes. In other seasons, the air may be warm-er than the lakes. When this occurs, the air maybecome saturated by evaporation from the waterwhile also becoming cooler in the low levels bycontact with the cool water. Fog often becomesextensive and dense to the lee of a lake. Figure 37illustrates movement of air over both warm andcold lakes. Strong cold winds across the Great

Lakes often carry precipitation to the Appalachiansas shown in figure 38.

A lake only a few miles across can influence con-vection and cause a diurnal fluctuation in cloud-iness. During the day; cool air over the lake blowstoward the land, and convective clouds form overthe land as shown in figure 39, a photograph ofLake Okeechobee in Florida. At night, the patternreverses; clouds tend to form over the lake as coolair from the land flows over the lake creatingconvective clouds over the water.

44

COLD

MOISTURE AND

WARM AIR RISING

,-4i C

SNOW FLURRIES..... . . .

._41.1206,

Rolm 38. Strong cold winds across the Great Lakes absorb water vapor and may carry showers as far eastwardas the Appalachians.

4

a

t.

Fromm 39. A view of clouds from 27,000 feet over Lake Okeechobee in southern Florida. Note the lake effect. Duringdaytime, cool air from the lake flows toward the warmer land forming convective clouds over the land.

. IN CLOSING

Water exists in three statessolid, liquid, andgaseous. Water vapor is an invisible gas. Condensa-tion or sublimation of water vapor creates manycommon aviation weather hazards. You mayanticipate:

1. Fog when temperature-dew point spread is5° F or less and decreasing.

2. Lifting or clearing of low clouds and fogwhen temperature -dew point spread isincreasing.

3. Frost on a clear night when temperature-dew point spread is 5° F or less, is decreas-ing, and dew point is colder than 32° F.

4. More cloudiness, fog, and precipitationwhen wind blows from water than when itblows from land.

5 7

5. Cloudiness, fog, and precipitation over high-er terrain, when moist winds are blowinguphill.

6. Showers to the lee of a lake when air iscold and the lake is warm. Expect fog tothe lee of the lake when the air is warmand the lake is cold.

7. Clouds to be at least 4,000 feet thick whensignificant precipitation is reported. Theheavier the precipitation, the thicker theclouds are likely to be.

8. Icing on your aircraft when flying throughliquid clouds or precipitation with temper-ature freezing or colder.

45

AN-

11211_

Chapter 6STABLE AND UNSTABLE AIR

To a pilot, the stability of his aircraft is a vitalconcern. A stable aircraft, when disturbed fromstraight and level flight, returns by itself to asteady balanced flight. An unstable aircraft, whendisturbed, continues to move away from a normalflight attitude.

So it is with the atmosphere. A stable atmo-sphere resists any upward or downward displace-

ment. An unstable atmosphere allows an upwardor downward disturbance to grow into a vertical orconvective current.

This chapter first examines fundamental changesin upward and downward moving air and then re-lates stable and unstable air to clouds, weather,and flying.

CHANGES WITHIN UPWARD AND DOWNWARD MOVING AIR

Anytime air moves upward, it expands becauseof decreasing atmospheric pressure as shown infigure 40. Conversely, downward moving air iscompressed by increasing pressure. But as pressureand volume change, temperature also changes.

When air expands, it cools; and when com-pressed, it warms. These changes are adiabatic,meaning that no heat is removed from or added tothe air. We frequently use the terms expansionalor adiabatic cooling and compressional or adiabatic

47

Fannie 40. Decreasing atmospheric pressure causes theballoon to expand as it rises. Anytime air moves upward,it expands.

heating. The adiabatic .rate of change of temper-ature is virtually fixed in unsaturated air but variesin saturated air.

UNSATURATED AIRUnsaturated air moving upward and downward

cools and warms at about 3.0° C (5.4° F) per1,000 feet. This rate is the "dry adiabatic rate oftemperature change" and is independent of thetemperature of the mass of air through which thevertical movements occur. Figure 41 illustrates a

48

"Chinook Wind"an excellent example of dryadiabatic warming.

SATURATED AIRCondensation occurs when saturated air moves

upward. Latent heat released through condensation(chapter 5) partially offsets the expansional cool-ing. Therefore, the saturated adiabatic rate of cool-ing is slower than the dry adiabatic rate. Thesaturated rate depends on saturation temperatureor dew point of the air. Condensation of copiousmoisture in saturated warm air releases morelatent heat to offset expansional cooling than doesthe scant moisture in saturated cold air. Therefore,the saturated adiabatic rate of cooling is less inwarm air than in cold air.

When saturated air moves downward, it heatsat the same rate as it cools on ascent providedliquid water evaporates rapidly enough to main-tain saturation. Minute water droplets evaporate atvirtually this rate. Larger drops evaporate moreslowly and complicate the moist adiabatic processin downward moving air.

ADIABATIC COOLING ANDVERTICAL AIR MOVEMENT

If we force a sample of air upward into theatmosphere, we must consider two possibilities:

(1) The air may become colder than thesurrounding air, or

(2) Even though it cools, the air may remainwarmer than the surrounding air.

If the upward moving air becomes colder thansurrounding air, it sinks; but if it remains warmer,it is accelerated upward as a convective current.Whether it sinks or rises depends on the ambientor existing temperature lapse rate (chapter 2) .

Do not confuse existing lapse rate with adiabaticrates of cooling in vertically moving air.* Thedifference between the existing lapse rate of agiven mass of air and the adiabatic rates of coolingin upward moving air determines if the air is stableor unstable.

*Sometimes you will hear the dry and moist adiabaticrates of cooling called the dry adiabatic lapse rate andthe moist adiabatic lapse rate. In this book, lapse raterefers exclusively to the existing, or actual, decrease oftemperature with height in a real atmosphere. The dry ormoist adiabatic lapse rate signifies a prescribed rate ofexpansional cooling or compressional heating. An adia-batic lapse rate becomes real only when it becomes acondition brought about by vertically moving air.

"4

WIND.

CLOUDS

CHINOOK WIND-ADIABATIC HEATING

Sle PER' MOO'

CLEAR AND DRY

Flamm 41. Adiabatic warming of downward moving air produces the warm Chinook wind.

STABILITY AND INSTABILITY

Let's use a balloon to demonstrate stability andinstability. In figure 42 we have, for three situa-tions, filled a balloon at sea level with air at 31° Cthe same as the ambient temperature. We havecarried the balloon to 5,000 feet. In each situation,the air in the balloon expanded and cooled at thedry adiabatic rate of 3° C for each 1,000 feet to atemperature of 16° C at 5,000 feet.

In the first situation (left), air inside the bal-loon, even though cooling adiabatically, remainswarmer than surrounding air. Vertical motion isfavored. The colder, more dense surrounding airforces the balloon on upward. This air is unstable,and a convective current develops.

In situation two (center) the air aloft is wanner.Air inside the balloon, cooling adiabatically-nowbecomes colder than the surrounding air. The bal-loon sinks under its own weight returning to itsoriginal position when the lifting force is removed.The air is stable, and spontaneous convection isimpossible.

In the last situation, temperature of air insidethe balloon is the same as that of surrounding air.The balloon will remain at rest. This condition is

neutrally stable; that is, the air is neither stablenor unstable.

Note that, in all three situations, temperature ofair in the expanding balloon cooled at a fixed rate.The differences in the three conditions depend,therefore, on the temperature differences betweenthe surface and 5,000 feet, that is, on the ambientlapse rates.

HOW STABLE OR UNSTABLE?Stability runs the gamut from absolutely stable

to absolutely unstable, and the atmosphere usuallyis in a delicate balance somewhere in between. Achange in ambient temperature lapse rate of an airmass can tip this balance. For example, surfaceheating or cooling aloft can make the air moreunstable; on the other hand, surface cooling orwarming aloft often tips the balance toward great-er stability.

Air may be stable or unstable in layers. A stablelayer may overlie and cap unstable air; or, con-versely, air near the surface may be stable withunstable layers above.

6449

Flamm 42. Stability related to temperatures aloft and adiabatic cooling. In each situation, the balloon is filled at sea levelwith air at 31° C, carried manually to 5,000 feet, and released. In each case, air in the balloon expands and cools to 16° C(at the dry adiabatic rate of 3° C per 1,000 feet). But, the temperature of the surrounding air aloft in each situation isdifferent. The balloon on the left will rise. Even though it cooled adiabatically, the balloon remains warmer and lighterthan the surrounding cold air; when released, it will continue upward spontaneously. The air is unstable; it favors verticalmotion. In the center, the surrounding air is warmer. The cold balloon will sink. It resists our forced lifting and cannotrise spontaneously. The air is stableIt resists upward motion. On the right, surrounding air and the balloon are at thesame temperature. The balloon remains at rest since no density difference exists to displace it vertically. The air is neutral-ly stable, i.e., it neither favors nor resists vertical motion. A mass of air in which the temperature decreases rapidly withheight favors instability; but, air tends to be stable if the temperature changes little or not at all with altitude.

CLOUDSSTABLE OR UNSTABLE?

Chapter 5 states that when air is cooling and firstbecomes saturated, condensation or sublimationbegins to form clouds. Chapter 7 explains cloudtypes and their significance as "signposts in thesky." Whether the air is stable or unstable withina layer largely determines cloud structure.

Strotiform CloudsSince stable air resists convection, clouds in stable

air form in horizontal, sheet-like layers or "strata."Thus, within a stable layer, clouds are stratiforrn.Adiabatic cooling may be by upslope flow as illus-

50

61

trated in figure 43; by lifting over cold, more denseair; or by converging winds. Cooling by an under-lying cold surface is a stabilizing process and mayproduce fog. If clouds are to remain stratiform, thelayer must remain stable after condensation occurs.

Cumuliform CloudsUnstable air favors convection. A "cumulus"

cloud, meaning "heap," forms in a convective up-draft and builds upward, also shown in figure 43.Thus, within an unstable layer, clouds are cumuli-form; and the vertical extent of the cloud dependson the depth of the unstable layer.

.,

FIGURE 43. When stable air (left) is forced upward, the air tends to retain horizontal flow, and any cloudiness is flat andstratified. When unstable air is forced upward, the disturbance grows, and any resulting cloudiness shows extensive ver-tical development.

Initial lifting to trigger a cumuliform cloud maybe the same as that for lifting stable air. In addi-tion, convection may be set off by surface heating(chapter 4). Air may be unstable or slightly stablebefore condensation occurs; but for convectivecumuliform clouds to develop, it must be unstableafter saturation. Cooling in the updraft is now atthe slower moist adiabatic rate because of the re-lease of latent heat of condensation. Temperaturein the saturated updraft is warmer than ambienttemperature, and convection is spontaneous. Up-drafts accelerate until temperature within the cloud

* cools below the ambient temperature. This condi-tion occurs where the unstable layer is capped bya stable layer often marked by a temperature in-version. Vertical heights range from the shallowfair weather cumulus to the giant thunderstormcumulonimbusthe ultimate in atmospheric in-stability capped by the tropopause.

You can estimate height of cumuliform cloudbases using surface temperature-dew point spread.Unsaturated air in a convective current cools atabout 5.4° F (3.0° C) per 1,000 feet; dew pointdecreases at about 1° F (5/9° C). Thus, in a con-vective current, temperature and dew point con-

verge at about 4.4° F (2.5° C) per 1,000 feet asillustrated in figure 44. We can get a quick estimateof a convective cloud base in thousands of feet byrounding these values and dividing into the spreador by multiplying the spread by their reciprocals.When using Fahrenheit, divide by 4 or multiplyby .25; when using Celsius, divide by 2.2 or multi-ply by .45. This method of estimating is reliableonly with instability clouds and during the warmerpart of the day.

When unstable air lies above stable air, convec-tive currents aloft sometimes form middle and highlevel cumuliform clouds. In relatively shallow lay-ers they occur as altocumulus and ice crystal cirro-cumulus clouds. Altocumulus castellanus cloudsdevelop in deeper midlevel unstable layers.

Merging Stratiform and CumuliformA layer of stratiform clouds may sometimes form

in a mildly stable layer while a few ambitiops con-vective clouds penetrate the layer thus mergingstratiform with cumuliform. Convective clouds maybe almost or entirely embedded in a massive strati-form layer and pose an unseen threat to instrumentflight.

6251

WHAT DOES IT ALL MEAN?

3

FIGURE 44. Cloud base determination. Temperature anddew point in upward moving air converge at a rate ofabout 4° F or 2.2° C per 1,000 feet.

Can we fly in unstable air? Stable air? Certainlywe can and ordinarily do since air is seldom neu-trally stable. The usual convection in unstable airgives a "bumpy" ride; only at times is it violentenough to be hazardous. In stable air, flying isusually smooth but sometimes can be plagued bylow ceiling and visibility. I' behooves us in preflightplanning to take into account stability or instabilityand any associated hazards. Certain observationsyou can make on your own:

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63

1. Thunderstorms are sure signs of violentlyunstable air. Give these storms a wide berth.

2. Showers and clouds towering upward withgreat ambition indicate strong updrafts andrough (turbulent) air. Stay clear of theseclouds.

3. Fair weather cumulus clouds often indicatebumpy turbulence beneath and in the clouds.The cloud tops indicate the approximateupper limit of convection; flight above isusually smooth.

4. Dust devils are a sign of dry, unstable air,usually to considerable height. Your ridemay be fairly rough unless you can getabove the instability.

5. Stratiform clouds indicate stable air. Flightgenerally will be smooth, but low ceilingand visibility might require IFR.

6. Restricted visibility at or near the surfaceover large areas usually indicates stable air.Expect a smooth ride, but poor visibilitymay require IFR.

7. Thunderstorms may be embedded in strati-form clouds posing an unseen threat to in-strument flight

8. Even in clear weather, you have some cluesto stability, viz.:a. When temperature decreases uniformly

and rapidly as you climb (approaching3° C per 1,000 feet) you have an indi-cation of unstable air.

b. If temperature remains unchanged ordecreases only slightly with altitude, theair tends to be stable.

c. If the temperature increases with alti-tude through a layeran inversionthelayer is stable and convection is sup-pressed. Air may be unstable beneaththe inversion.

d. When air near the surface is warm andmoist, suspect instability. Surface heat-ing, cooling aloft, converging or upslopewinds, or an invading mass of colder airmay lead to instability and cumisliformclouds.

a.

Chapter 7CLOUDS

Clouds, to almost everyone, have some meaning.But to you as a pilot, clouds are your weather"signposts in the sky." They give you an indicationof air motion, stability, and moisture. Clouds help

J

you visualize weather conditions and potentialweather hazards you might encounter in flight.Let's examine these "signposts" and how to identifythem.

IDENTIFICATION

For identification purposes, you need be con-cerned only with the more basic cloud types, whichare divided into four "families." The families are:high clouds, middle clouds, low clouds, and cloudswith extensive vertical development. The first threefamilies are further classified according to the waythey are formed. Clouds formed by vertical currentsin unstable air are cumulus meaning accumulation

64

or heap; they are characterized by their lumpy,billowy appearance. Clouds formed by the coolingof a stable layer are stratus meaning stratified orlayered; they are characterized by their uniform,sheet-like appearance.

In addition to the above, the prefix nimbo orthe suffix nimbus means mincloud. Thus, stratified

53

1a.

.>"

FIGURE 45. CIRRUS. Cirrus are thin, feather-like ice crystal clouds in patches or narrow bands. Larger ice crystalsoften trail downward in well-defined wisps called "mares' tails." Wispy, cirrus-like, these contain no significant king orturbulence. Dense, banded cirrus, which often are turbulent, are discussed in chapter 13.

clouds from which rain is falling are nimbostratus.A heavy, swelling cumulus type cloud which pro-duces precipitation is a cumulonimbus. Cloudsbroken into fragments are often identified by add-ing the suffix fractus; for example, fragmentarycumulus is cumulus fractus.

HIGH CLOUDSThe high cloud family is ciniforrn and includes

cirrus, cirrocumulus, and cirrostratus. They arecomposed almost entirely of ice crystals. The heightof the bases of these clouds ranges from about16,500 to 45,000 fcet in middle latitudes. Figures45 through 47 are photographs of high clouds.

MIDDLE CLOUDSIn the middle cloud family are the altostratus,

altocumulus, and nimbostratus clouds. These cloudsare primarily water, much of which may be super-cooled. The height of the bases of these cloudsranges from about 6,500 to 23,000 feet in middlelatitudes. Figures 48 through 52 are photographsof middle clouds.

54

65

LOW CLOUDSIn the low cloud family are the stratus, strato-

cumulus, and fair weather cumulus clouds. Lowclouds are almost entirely water, but at times thewater may be supercooled. Low clouds at sub-freezing temperatures can also contain snow andice particles. The bases of these clouds range fromnear the surface to about 6,500 feet in middle lati-tudes. Figures 53 through 55 are photographs oflow clouds.

CLOUDS WITH EXTENSIVEVERTICAL DEVELOPMENT

The vertically developed family of clouds includestowering cumulus and cumulonimbus. These cloudsusually contain supercooled water above the freez-ing level. But when a cumulus grows to greatheights, water in the upper part of the cloud freezesinto ice crystals forming a cumulonimbus. Theheights of cumuliform cloud bases range from1,000 feet or Iess to above 10,000 feet. Figures 56and 57 are photographs of clouds with extensivevertical development.

Ftouar 46. CIRROCUMULUS. Cirrocumulus are thin clouds, the individual elements appearing as small white flakesor patches of cotton. May contain highly supercooled water droplets. Some turbulence and icing.

Fmtnte 47. CIRROSTRATUS. Cirrostratus is a thin whitish cloud layer appearing like a sheet or veil. Cloud elementsare diffuse, sometimes partially striated or fibrous. Due to their ice crystal makeup, these clouds are associated with haloslarge luminous circles surrounding the sun or moon. No turbulence and little if any icing. The greatest problem flying inciniforrn clouds is restriction to visibility. They can make the strict use of instruments mandatory.

55

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Irtouaz 48. ALTOCUMULUS. Altocumulus are composed of white or gray colored layers or patches of solid cloud. Thecloud elements may have a waved or roll-like appearance. Some turbulence and small amounts of icing.

Itrnt te;

)4FIGURE 49. ALTOSTRATUS. Altostratus is a bluish veil er layer of clouds. It is often associated with altocumulus and

sometimes gradually merges into cirrostratus. The sun may be dimly visible through it, Little or no turbulence with mod-erate amounts of ice.

56

67

.

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FIGURE 50. ALTOCUMULUS CASTELLANUS Altocumulus castellanus are middle level convective clouds. Theyare characterized by their billowing tops and comparatively high bases. They are a good indication of mid-level'insta-

, bility. Rough turbulence with some icing.

57

68

Orr

FIGURE 51. 'STANDING LENTICULAR ALTOCUMULUS CLOUDS. Standing lenticular altocumulus clouds areformed on the crests of waves created by barriers in the wind flow. The clouds show little movement, hence the namestanding. Wind, however, can be quite strong blowing through such clouds. They are characterized by their smooth, pol-ished edges. The presence of these clouds is a good indication of very strong turbulence and should be avoided. Chapter9, "Turbulence," further explains the significance of this cloud.

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58

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Frouaa 52. NIMBOSTRATUS. Nimbostratus is a gray or dark massive cloud layer, diffused by more or less continuousrain, snow, or ice pellets. This type is classified as a middle cloud although it may merge into very low stratus or strato-cumulus. Very little turbulence, but can pose a serious icing problem if temperatures arc near or below freezing.

...

Rom 53. STRATUS. Stratus is a gray, uniform, sheet-like cloud with relatively low bases. When associated with fogor precipitation, the combination can become troublesome for visual flying. Little or no turbulence, but temperaturesnear or below freezing can create hazardous icing condition&

59

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F!OURS 54. STRATOCUMULUS. Stratocumulus bases are globular masses or rolls unlike the flat, sometimes indefinite,bases of stratus. They usually form at the top of a layer mixed by moderate surface winds. Sometimes, they form fromthe breaking up of stratus or the spreading out of cumulus. Some turbulence, and possible icing at subfreezing temperatures.Ceiling and visibility usually better than with low stratus.

jl

Rolm 55. CUMULUS. Fair weather cumulus clouds form in convective currents and are characterized by relativelyflat bases and dome-shaped tops. Pair weather cumulus do not show extensive vertical development and do not produceprecipitation. More often, fair weather cumulus indicates a shallow layer of instability. Some turbulence and no signif-leant icing.

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FIGURE 56. TOWERING CUMULUS. Towering cumulus signifies a relatively deep layer of unstable air. It showsconsiderable vertical development and has billowing cauliflower tops. Showers can result from these clouds. Very strongturbulence; some clear king above the freezing level.

4'tnouns 57. CUMULONIMBUS. Cumulonimbus are the ultimate manifestation of instability. They are vertically de-

veloped clouds of large dimensions with dense boiling tops often crowned with thick veils of dense cirrus (the anvil). Nearlythe entire spectrum of flying hazards are contained in these clouds including violent turbulence. They should be avoidedat all times 3 This cloud is the thunderstorm cloud and is discussed in detail in chapter 11, "Thunderstorms."

61

72

SIGNPOSTS

The photographs illustrate some of the basiccloud types. The caption with each photographdescribes the type and its significance to flight. Inclosing, we suggest you take a second look at the

62

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IN THE SKY

cloud photographs. Study the descriptions and po-tential hazards posed by each type and learn to usethe clouds as "signposts in the sky:'

Chapter 8AIR MASSES AND FRONTS

Why is weather today clear and cold over Okla-homa while it is warm and moist over Alabama?What caused the line of thunderstorms that youcircumnavigated over eastern Arkansas? Air massesand fronts provide the answer. You can better plan

the safety and economy of flight when you canevaluate the expected effects of air masses andfronts. This chapter explains air masses and frontsand relates them to weather and flight planning.

AIR MASSES

When a body of air comes to rest or movesslowly over an extensive area having fairly uniformproperties of temperature and moisture, the airtakes on those properties. Thus, the air over thearea becomes somewhat of an entity as illustratedin figure 58 and has fairly uniform horizontal dis-tribution of its properties. The area over which theair mass acquires its identifying distribution ofmoisture and temperature is its "source region."

7.,4

Source regions are many and varied, but the bestsource regions for air masses are large snow or ice-covered polar regions, cold northern oceans, trop-ical oceans, and large desert areas. Mid latitudesare poor source regions because transitional distur-bances dominate these latitudes giving little oppor-tunity for air masses to stagnate and take on theproperties of the underlying region..

63

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Fxruita 58. Horizontal uniformity of an air mass. (Properties of air at At, A2, etc., are about the stone as those at A;properties at Bt, B2, etc., arc about the same as those at B.)

AIR MASS MODIFICATION

Just as an air mass took on the properties of itssource region, it tends to take on properties of theunderlying surface when it moves away from its.aurce region, thus becoming modified.

The degree of modification depends on the speedwith which the air mass moves, the nature of theregion over which it moves, and the temperaturedifference between the new surface and the airmass. Some ways air masses are modified are:(1) warming from below, (2) cooling from below,(3) addition of water vapor, and (4) subtractionof water vapor:

1. Cool air moving over a warm surface isheated from below, generating instabilityand increasing the possibility of showers.

2. Warm air moving over a cool surface iscooled from below, increasing stability. Ifair is cooled to its dew point, stratus and/ orfog forms.

3. Evaporation from water surfaces and fall-ing precipitation adds water vapor to the

air. When the water is warmer than theair, evaporation can raise the dew pointsufficiently to saturate the air and formstratus or fog.

4. Water vapor is removed by condensationand precipitation.

STABILITYStability of an air mass determines its typical

weather characteristics. When one type of air massoverlies another, conditions change with height.Characteristics typical of an unstable and a stableair mass are as follows:

Unstable AirCumuliform clouds

Showery precipitation

Rough air (turbu-lence)

Good visibility, ex-cept in blowingobstructions

FRONTS

As air masses move out of their source regions,they come in contact with other air masses of dif-

64

7 5

Stable AirStratiform clouds

and fogContinuous precipi-

tationSmooth air

Fair to poor visibilityin haze and smoke

ferent properties. The zone between two differentair masses is a frontal zone or front. Across this

IP

zone, temperature, humidity and wind oftenchange rapidly over short distances.

DISCONTINUITIESWhen you pass through a front, the change from

the properties of one air mass to those of the otheris sometimes quite abrupt. Abrupt changes indicatea narrow frontal zone. At other times, the changeof properties is very gradual indicating a broadand diffuse frontal zone.

TemperatureTemperature is one of the most easily recog-

nized discontinuities across a front. At the surface,the passage of a front usually causes noticeabletemperature change. When flying through a front,you note a significant change in temperature, espe-cially at low altitudes. Remember that the temper-ature change, even when gradual, is faster andmore pronounced than a change during a flightwholly within one air mass. Thus, for safety, ob-tain a new altimeter setting after flying througha front. Chapter 3 discussed the effect of atemperature change on the aircraft altimeter.

Dew PointAs you learned in Chapter 5, dew point temper-

ature is a measure of the amount of water vaporin the air. Temperature-dew point spread is ameasure of the degree of saturation. Dew point andtemperature-dew point spread usually differ acrossa front. The difference helps identify the front andmay give a clue to differences of cloudiness and/or fog.

WindWind always changes across a front. Wind dis-

continuity may be in direction, in speed, or in both.Be alert for a wind shift when flying in the vicinityof a frontal surface; if the wind shift catches youunaware it can get you off course or even lost ina short time. The relatively sudden change in windalso creates wind shear, and you will study its sig-nificance in the next chapter, "Turbulence."

PressureA front lies in a pressure trough, and pressure

generally is higher in the cold air. Thus, when youcross a front directly into colder air, pressureusually rises abruptly. When you approach a fronttoward warm air, pressure generally falls until youcross the front and then remains steady or falls

slightly in the warm air. However, pressure pat-terns vary widely across fronts, and your coursemay not be directly across a front. The importantthing to remember is that when crossing a front,you will encounter a difference in the rate ofpressure change; be especially alert in keeping youraltimeter setting current.

TYPES OF FRONTSThe three principal types of fronts are the cold

front, the warm front, and the stationary front.-.

,..%

Cold FrontThe leading edge of an advancing cold air mass

is a cold front. At the surface, cold air is overtakingand replacing warmer air. Cold fronts move atabout the speed of the wind component perpen-dicular to the front just above the frictional layer.Figure 59 shows the vertical cross section of a coldfront and the symbol depicting it on a surfaceweather chart. A shallow cold air mass or a slowmoving cold front may have a frontal slope morelike a warm front shown in figure 60.

Warm FrontThe edge of an advancing warm air mass is a

warm frontwarmer air is overtaking and replac-ing colder air. Since the cold air is denser thanthe warm air, the cold air hugs the ground. Thewarm air slides up and over the cold air and lacksdirect push on the cold air. Thus, the cold air isslow to retreat in advance of the warm air. Thisslowness of the cold air to retreat produces afrontal slope that is more gradual than the coldfrontal slope as shown in figure 60. Consequently,warm fronts on the surface are seldom as wellmarked as cold fronts, and they usually move abouthalf as fast when the general wind flow is thesame in each case.

Stationary FrontsWhen neither air mass is replacing the other, the

front is stationary. Figure 61 shows a cross sectionof a stationary front and its symbol on a surface

-chart. The opposing forces exerted by adjacent airmasses of different densities are such that thefrontal surface between them shows little or nomovement. In such cases, the surface winds tend toblow parallel to the frontal zone. Slope of a sta-tionary front is normally shallow, although it maybe steep depending on wind distribution and densitydifference.

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FIGURE 59. Cross section of a cold front (above) with the weather map symbol (below). The symbol is a line with pointedbarbs pointing in the direction of movement. if a map is in'color, a blue line represents the cold front. The vertical scaleis expanded in the top iLustration to show the frontal slope. The frontal slope is steep near the leading edge as cold airreplaces warm air. The Sad heavy arrow shows movement of the front. Warm air may descend over the front as indicatedby the dashed arrows; but more commonly, the cold air forces warm air upward over the frontal surface as shown bythe solid arrows.

-..-,1..

FRONTAL WAVES AND OCCLUSION

Frontal waves and cyclones (areas of low pres-sure) usually form on slow-moving cold fronts oron stationary fronts. The life cycle and movementof a cyclone is dictated to a great extent by theupper wind flow.

66

In the initial condition of frontal wave develop-ment in figure 62, the winds on both sides of thefront are blowing parallel to the front (A). Smalldisturbances then may start a wavelike bend in thefront (B).

If this tendency persists and the wave increasesin size, a cyclonic (counterclockwise) circulation

71

v

FIGURE 60. Cross section of a warm front (top) with the weather map symbol (bottom). The symbol is a line with rounded- barbs pointing in the direction of movement. If a map is in color, a red line represents the warm front. Slope of a warmfront generally is more shallow than slope of a cold front. Movement of a warm front shown by the heavy black arrow isslower than the wind in the warm air represented by the light solid arrows. The warm air gradually erodes the cold air.

develops. One section of the front begins to moveas a warm front, while the section next to it beginsto move as a cold front (C). This deformation isa frontal wave.

The pressure at the peak of the frontal wavefalls, and a lOw-pressure center forms. The cycloniccirculation becomes stronger, and the surface windsare now strong enough to move the fronts; the cold

front moves faster than the warm front (D). Whenthe cold front catches up with the warm front, thetwo of them occlude (close together). The result isan occluded front or, for brevity, an occlusion (E).This is the time of maximum intensity for the wavecyclone. Note that the symbol depicting the occlu-sion is a combination of the symbols for the warmand cold fronts.

7867

FIGURE 61. Cross section of a stationary front (top) and its weather map symbol (bottom). The symbol is a line with al-ternating pointed and rounded barbs on opposite sides of the line, the pointed barbs pointing away from the cold air andthe rounded barbs away from the warm air. If a map Is in color, the symbol is a line of alternating red and blue segments.'The front has little or no movement and winds are nearly parallel to the front. The symbol in the war, lair is the tail ofa wind arrow into the page. The symbol in the cold air is the point of a wind arrow out of the page. Slope of the frontmay vary considerably depending on wind and density differences across the front.

As the occlusion continues to grow in length, thecyclonic circulation diminishes in intensity and thefrontal movement slows down (F). Sometimes anew frontal wave begins to form on the longwestward-trailing portion of the cold front (F,G),or a secondary low pressure system forms at theapex where the cold front and warm front come

68

together to form the occlusion. In the final stage,the two fronts may have become a single stationaryfront again. The low center with its remnant of theocclusion is disappearing (G).

Figure 63 indicates a warm-front occlusion invertical cross section. This type of occlusion occurswhen the air is colder in advance of the warm

I

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Daum 62. The life cycle of a frobtal wave.

8069

FIOURE 63. Cross section of a warm-front occlusion (top) and its weather map symbol (bottom). The symbol is aline with al-ternating pointed and rounded barbs on the same side of the line pointing in the direction of movement. Shown in coloron a weather map, the line is purple. In the warm front occlusion, air under the cold front is not as cold as air ahead ofthe warm front; and when the cold front overtakes the warm front, the less cold air rides over the colder air. In a warm

. front occlusion, cool air replaces cold air at the surface.

front than behind the cold front, lifting the coldfront aloft.

Figure 64 indicates a cold-front occlusion in ver-tical cross section. This type of occlusion occurswhen the air behind the cold front is colder thanthe air in advance of the warm front, lifting thewarm front aloft.-

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NON-FRONTAL LOWS

Since fronts are boundaries between air massesof different properties, fronts are not associatedwith lows lying solely in a homogeneous air mass.Nonfrontal lows are infrequent east of the RockyMountains in midlatitudes but do occur occasion-

.

Roma 64. Cross section of a cold-front occlusion. Its weather map symbol is the same as for a warm-front occlusion shownin Figure 63. In the cold-front occlusion, the coldest air Is under the cold front. When it overtakes the warm front, It liftsthe warm front aloft; and cold air replaces cool air at the surface.

ally during the warmer months. Small nonfrontallows over the western mountains are common as isthe semistationary thermal low in extreme South-western United States. Tropical lows are alsononf rontal.

FRONTOLYSIS AND FRONTOGENESISAs adjacent air masses modify and as temper-

ature and pressure differences equalize across a

front, the front dissipates. This process, frontolysis,is illustrated in figure 65. Frontogenesis is the gen-eration of a front. It occurs when a relatively sharpzone of transition develops over an area betweentwo air masses which have densities gradually be-coming more and more in contrast with each other.The necessary wind flow pattern develops at thesame time. Figure 66 shows an example of fronto-genesis with the symbol:.'

F.

Funtac 65. Frontolysis of a stationary front.

8271

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FIGURE 66. Frantogenesis of a stationary front.

FRONTAL WEATHER

In fronts, flying weather varies from virtuallyclear skies to extreme hazards including hail, tur-bulence, icing, low clouds, and poor visibility.Weather occurring with a front depends on (1) theamount of moisture available, (2) the degree ofstability of the air that is forced upward, (3) theslope of the front, (4) the speed of frontal move-ment, and (5) the upper wind flow.

Sufficient moisture must be available for cloudsto form, or there will be no clouds. As an inactivefront comes into an area of moisture, clouds andprecipitation may develop rapidly. A good exampleof this is a cold front moving eastward from thedry slopes of the Rocky Mountains into a tongueof moist air from the Gulf of Mexico over thePlains States. Thunderstorms may build rapidlyand catch a pilot unaware.

The degree of stability of the lifted air deter-mines whether cloudiness will be predominatelystratiform or cumuliform. If the warm air overrid-ing the front is stable, stratiform clouds develop. Ifthe warm air is unstable, cumuliform clouds de-velop. Precipitation from stratiform clouds is usuallysteady as illustrated in figure 67 and there is littleor no turbulence. Precipitation from cumuliformclouds is of a shower type as in figure 68, and theclouds are turbulent.

72

83

Shallow frontal surfaces tend to give extensivecloudiness with large precipitation areas (figure69). Widespread precipitation associated with agradual sloping front often causes low stratus andfog. In this case, the rain raises the humidity ofthe cold air to saturation. This and .related effectsmay produce low ceiling and poor visibility overthousands of square miles. If temperature of thecold air near the surface is below freezing but thewarmer air aloft is above freezing, precipitationfalls as freezing rain or ice pellets; however, if tem-perature of the warmer air aloft is well below freez-ing, precipitation forms as snow.

When the warm air overriding a shallow front ismoist and unstable, the usual widespread cloudmass forms; but embedded in the cloud mass arealtocumulus, cumulus, and even thunderstorms asin figures 70 and 71. These embedded storms aremore common with warm and stationary fronts butmay occur with a slow moving, shallow cold front.A good preflight briefing helps you to foresee thepresence of these hidden thunderstorms. Radar alsohelps in this situation and is discussed in chapter 11.

A fast moving, steep cold, front forces upwardmotion of the warm air along its leading edge. Ifthe warm air is moist, precipitation occurs imme-diately along the surface position of the front asshown in figure 72.

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FIGURE 67. A cold front underrunning warm, moist, stable air. Clouds are stratified and precipitation continuous.Precipitation induces stratus in the cold air.

novas 68. A cold front underrunning warm, moist, unstable air. Clouds arc cumuliform with possible showers or thun-derstorms near the surface position of the front. Convective clouds often develop in the warm air ahead of the front. Thewarm, wet ground behind the front generates low-level convection and fair weather cumulus in the cold air.

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69. A warm front with overrunning moist, stable air. Clouds are stratiform and widespread over the shallowfront. Precipitation is continuous and induces wideepread stratus in the cold air.

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Fromm 70. A slow-moving cold frcil underrunning warm, moist, unstable air. Note that the front is more shallow thanthe fast-movingfront shown in figure 68. Clouds are stratified with embedded cumulonimbus and thunderstorms. Thistype of frontal weather is especially hazardous since the infividual thunderstorms are hidden and cannot be avoidedunless the aircraft is equipped with airborne radar. .

Since an occluded front develops when a coldfront overtakes a warm front, weather with an oc-cluded front is a combination of both warm andcold frontal weather. Figures 73 and 74 show warmand cold occlusions and associated weather.

74

85

A front may have little or no cloudiness asso-ciated with it. Dry fronts occur when the warm airaloft is flowing down the frontal slope or the air isso dry that any cloudiness that occurs is at highlevels.

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FIGURE 71. A warm front with overrunning warm, moist, unstable air. Weather, clotzds, and hazards are similar to thosedescribed in figure 70 except that they generally are more widespread.

FIGURE 72. A fast moving cold front underrunning warm, moist, unstable air. Showers and thunderstorms develop alongthe surface position of the front.

The upper wind flow dictates to a great extentthe amount of cloudiness and rain accompanying afrontal system as well as movement of the frontitself. Remember in chapter 4 we said that systemstend to move with the upper winds. When windsaloft blow across a front, it tends to move with thewind. When winds aloft parallel a front, the frontmoves slowly if at all. A deep, slow moving troughaloft forms extensive cloudiness and precipitation,

83

while a rapid moving minor trough more often re-stricts weather to a rather narrow band. However,the latter often breeds severe, fast moving, turbu-lent spring weather.

INSTABILITY LINEAn instability line is a narrow, nonfrontal line or

band of convective activity. If the activity is fully'developed thunderstorms, figure 75, the line is a

75'

flouvx 73. A warm front occlusion lifting warm, moist, unstable air. Note that the associated weather is complex andencompasses all types of weather associated with both the warm and cold fronts when air is moist and unstable.

Fromm 74. A cold front occlusion lifting warm, moist, stable air. Associated weather encompasses types of weatherassociated with both warm and cold fronts whin air is moist and stable.

squall line (chapter 11, "Thunderstorms"). Insta-bility lines form in moist unstable air. An instabilityline may develop far from any front. More often,it develops ahead of a cold front, and sometimes aseries of these 'lines move out ahead of the front.A favored location for instability lines which fre-quently erupt into severe thunderstorms is a t:ewpoint front or dry line.

76

87

DEW POINT FRONT OR DRY LINE

During a considerable part of the year, dewpoint fronts are common in Western Texas andNew Mexico northward over the Plains States.Moist air flowing north from the.Gulf of Mexicoabuts the dryer and therefore slightly denser airflowing from the southwest. Except for moisture

differences, there is seldom any significant air masscontrast across this "Front"; and therefore, it iscommonly called a "dry line." Nighttime and earlymorning fog and low-level clouds often prevail onthe moist side of the line while generally clear skies

mark the dry side. In spring and early summer overTexas, Oklahoma, and Kansas, and for some dis-tance eastward, the dry line is a favored spawningarea for squall lines and tornadoes.

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FIGURE 75. An aerial view of a portion of a squall line.

8877

FRONTS AND FLIGHT PLANNING

Surface weather charts pictorially portray frontsand, in conjunction with other forecast charts andspecial analyses, aid you in determining expectedweather conditions along your proposed route.Knowing the locations of fronts and associatedweather helps you dexrmine if you can proceed asplanned. Often you can change your route to avoidadverse weather.

Frontal weather may change rapidly. For exam-ple, there may be only cloudiness associated witha cold front over northern Illinois during themorning but with a strong squall Loe forecast byafternoon. Skies may be partly cloudy during theafternoon over Atlanta in advance of a warm front,but by sunset drizzle and dense fog are forecast. Acold front in Kansas is producing turbulent thun-

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derstorms, but by midnight the upper flow is ex-pected to dissipate the thunderstorms and weakenthe front. A Pacific front is approaching Seattleand is expected to produce heavy rain by midnight.

A mental picture of what is happening and whatis forecast should greatly aid you in avoiding ad-verse weather conditions. If unexpected adverseweather develops en route, your mental picture aidsyou in planning the best diversion. If possible,always obtain a good preflight weather briefing.

We suggest you again look at figures 67 through75 and review weather conditions associated withdifferent types of fronts and stability conditions.These are only a few of many possibilities, but theyshould give some help during preflight planning orinflight diversion.

Chapter 9TURBULENCE

Everyone who flies encounters turbulence atsome time or other. A turbulent atmosphere is onein which air currents vary greatly over short dis-tances. These currents range from rather mildeddies to strong currents of relatively large dimen-sions. As an aircraft moves through these currents,it undergoes changing accelerations which jostle itfrom its smooth flight path. This jostling is turbu-lence. Turbulence ranges from bumpiness whichcan annoy crew and passengers to severe jolts whichcan structurally damage the aircraft or injure itspassengers.

Aircraft reaction to turbulence varies with thedifference in windspeed in adjacent currents, sizeof the aircraft, wing loading, airspeed, and aircraftattitude. When an aircraft travels rapidly from onecurrent to another, it undergoes abrupt changes inacceleration. Obviously, if the aircraft moved moreslowly, the changes in acceleration would be moregradual. The first rule in flying turbulence is toreduce airspeed. Your aircraft manual most likelylists recommended airspeed for penetrating tur-bulence.

Knowing where to expect turbulence helps a

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pilot avoid or minimize turbulence discomfort andhazards.- The main causes of turbulence are (1)convective currents, (2) obstructions to wind flow,and (3) wind shear. Turbulence also occurs in the

wake of moving aircraft whenever the airfoils exertliftwake turbulence. Any combination of causesmay occur at one time.

CONVECTIVE CURRENTS

Convective currents are a common cause of tur- are localized vertical air movements, both ascend-bulence, especially at low altitudes. These currents ing and descending. For every rising current, there

FtouRE 76. Effect of convective currents on final approach. Predominantly upward currents (top) tend to cause the aircraftto overshoot. Predominantly downward currents (bottom) tend to cause the craft to undershoot.

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is a compensating downward current. The down-ward currents frequently occur over broader areasthan do the upward currents, and therefore, theyhave a slower vertical speed than do the risingcurrents.

.Convective currents are most active on warmsummer afternoons when winds are light. Heatedair at the surface creates a shallow, unstable layer,and the warm air is forced upward. Convectionincreases in strength and to greater heights as sur-face heating increases. Barren surfaces such assandy or rocky wastelands and plowed fields be-come hotter than open water or ground coveredby vegetation. Thus, air at and near the surfaceheats unevenly. Because of uneven heating, thestrength of convective currents can vary consider-ably within short distances.

When cold air moves over a warm surface, itbecomes unstable in lower levels. Convective cur-rents extend several thousand feet above the sur-face resulting in rough, choppy turbulence whenflying in the cold air. This condition often occursin any season after the passage of a cold front.

Figure 76, illustrates the effect of low-level con-vective turbulence on aircraft approaching to land.Turbulence on approach can cause abrupt changesin airspeed and may even result in a stall at adangerously low altitude. To prevent the danger,

increase airspeed slightly over normal approachspeed. This procedure may appear to conflict withthe rule of reducing airspeed for turbulence pene-tration; but remember, the approach speed for youraircraft is well below the recommended turbulencepenetration speed.

As air moves upward, it cools by expansion. Aconvective current continues upward until itreaches a level where its temperature cools to thesame as that of the surrounding air. If it cools tosaturation, a cloud forms. Billowy fair weathercumulus clouds, usually seen on sunny afternoons,are signposts in the sky indicating convective tur-bulence. The cloud top usually marks the approx-imate upper limit of the convective current. A pilotcan expect to encounter turbulence beneath or inthe clouds, while above the clouds, air generally issmooth. You will find most comfortable flight abovethe cumulus as illustrated in figure 77.

When convection extends to greater heights, itdevelops larger towering cumulus clouds and cu-mulonimbus with anvil-like topss. The cumulonim-bus gives visual warning of violent convectiveturbulence discussed in more detail in chapter 11.

The pilot should also know that when air is toodry for cumulus to form, convective currents stillcan be active. He has little indication of their pres-ence until he encounters turbulence.

FIGURE 77. Avoiding turbulence by flying above convective clouds.

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81

OBSTRUCTIONS TO WIND FLOW

Obstructions such as buildings, trees, and roughterrain disrupt smooth wind flow into a complexsnarl of eddies as diagrammed in figure 78. An air-craft flying through these eddies experiences tur-bulence. This turbulence we classify as "mechan-ical" since it results from mechanical disruption ofthe ambient wind flow.

The degree of mechanical turbulence dependson wind speed and roughness of the obstructions.The higher the speed and/or the rougher the sur-

face, the greater is the turbulence. The wind car-ries theturtmlent .eddies downstreamhow fardepends on wind speed and stability of the air.Unstable air allows larger eddies to form thanthose that form in stable air; but the instabilitybreaks up the eddies quickly, while in stable airthey dissipate slowly.

Mechanical turbulence can also cause cloudinessnear the top of the mechanically disturbed layer.However, the type of cloudier :.s tells you whether

82

FIGURE 78. Eddy currents formed by wind blowing over uneven ground or over obstructions.

93

it is from mechanical or convective mixing. Me-chanical mixing produces stratocumulus clouds inrows or bands, while convective clouds form a ran-dom pattern. The cloud rows developed by me-chanical mixing may be parallel to or perpendicularto the wind depending on meteorological factorswhich we do not discuss here.

The airport area is especially vulnerable to me-chanical. turbulence which invariably causes gustysurface winds. When an aircraft is in a low-levelapproach or a climb, fluctuates in thegusts, and the aircraft !. stall. During ex-tremely gusty conditions,airspeed above normal approach or climb speed toallow for changes in airspeed. When landing witha gusty crosswind as illustrated in figure 79, bealert for mechanical turbulence and control prob-lems caused by airport structures upwind. Surfacegusts also create taxi problems.

Mechanical turbulence can affect low-level cross-country flight about anywhere. Mountainscan gen-erate turbulence to altitudes much higher than themountains themselves.

When flying over rolling hills, you may experi-ence mechanical turbulence. Generally, such tur-bulence is not hazardous, but it may be annoyingor uncomfortable. A climb to higher altitude shouldreduce the turbulence.

When flying over rugged hills or mountains,however, you may have some real turbulence prob-

lems. Again, we cannot discuss mechanical turbu-lence without considering wind speed and stability.When wind speed across mountains exceeds about40 knots, you can anticipate turbulence. Whereand to what extent depends largely on stability.

If the air crossing the mountains is unstable, tur-bulence on the windward side is almost certain. Ifsufficient moisture is present, convective cloudsform intensifying the turbulence. Convective cloudsover a mountain or along a ridge are a sure sign ofunstable air and turbulence on the windward sideand over the mountain crest.As the unstable air crosses the barrier, it spills

down the leeward slope often as a violent down-draft. Sometimes the downward speed exceeds themaximum climb rate for your aircraft and maydrive the craft into the mountainside as shown infigure 80. In the process of crossing the mountains,mixing reduces the instability to some extent.Therefore, hazardous turbulence in unstable airgenerally does not extend a great distance down-wind from the barrier.

MOUNTAIN WAVEWhen stable air crosses a mountain barrier, the

turbulent situation .is somewhat reversed. Air flow-ing up the windward side is relatively smooth.Wind flow across the barrier is laminarthat is,it tends to flow in layers. The barrier may set upwaves in these layers much as waves develop on

to

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FIGURE 79. Turbulent air in the landing area

94 83

Rolm 80. Wind flow in mountain areas. Dangerous downdrafts may be encountered on the lee side.

a disturbed water surface. The waves remainnearly stationary while the wind blows rapidlythrough them. The wave pattern, diagrammed infigure 81, is a "standing" or "mountain" wave, sonamed because it remains essentially stationary and

is associated with the mountain. The wave patternmay extend 100 miles or more downwind from thebarrier.

Wave crests extend well above the highest moun-tains, sometimes into the lower stratosphere. Under

FIOURE Si. Schematic cross section of a mountain wave. Note the standing wave pattern downwind from the mountain.Note also the rotary circulation below the wave crests. Wlien the air contains sufficient moisture, characteristic cloudsform.

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95

each wave crest is a rotary circulation also dia-grammed in figure 81. The "rotor" forms below theelevation of the mountain peaks. Turbulence canbe violent in the overturning rotor. Updrafts anddowndrafts in the waves can also create violentturbulence.

Figure 81 further illustrates clouds often asso-ciated with a mountain wave. When moisture issufficient to produce clouds on the windward side,they are stratified. Crests of the standing wavesmay be marked by stationary, lens-shaped cloudsknown as "standing lenticular" clouds. Figure 82 isa photograph of standing lenticular clouds. Theyform in the updraft and dissipate in the downdraft,so they do not move as the wind blows throughthem. The rotor may also be marked by a "rotor"cloud. Figure 8S is a photograph of a series of rotorclouds, each under the crest of a wave. But remem-ber, clouds are not always present to mark themountain wave. Sometimes, the air is too dry. Al-ways anticipate possible mountain wave turbulencewhen strong winds of 40 knots or greater blow

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across a mountain or ridge and the air is stable.You should not be surprised at any degree of

turbulence in a mountain wave. Reports of turbu-lence range from none to turbulence violent enoughto damage the aircraft, but most reports showsomething in between.

MOUNTAIN FLYINGWhen planning a flight over mountainous ter-

rain, gather as much preflight information as possi-ble on cloud reports, wind direction, wind speed,and stability of air. Satellites often help locatemountain waves. Figures 84 and 85 are photo-graphs of mountain wave clouds taken from space-craft. Adequate information may not always beavailable, so remain alert for signposts in the sky.What should you look for both during preflightplanning and during your inflight observations?

Wind at mountain top level in excess of 25 knotssuggests some turbulence. Wind in excess of 40knots across a mountain bather dictates caution.Stratified clouds mean stable air. Standing lentic-

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FIGURE 82. Standing lenticular clouds associated with a mountain wave.

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ular and/or rotor clouds suggest a mountain wave;expect turbulence many miles to the lee of moun-tains and relative smooth flight on the windwardside. Convective clouds on 'ffie windward side ofmountains mean unstable air; expect turbulence inclose proximity to and on either side of themountain.

When approaching mountains from the leewardside during strong winds, begin your climb wellaway from the mountains-100 miles in a moun-tain wave and 30 to 50 miles otherwise. Climb toan altitude 3,000 to 5,000 feet above mountain topsbefore attempting to cross. The best procedure is toapproach a ridge at a 45° angle to enable a rapidretreat to calmer air. If unable to make good onyour first attempt and you have higher altitude

WINDAs discussed in chapter 4, wind shear generates

eddies between two wind currents of differing ve-locities. The differences may be in wind speed,wind direction, or in both. Wind shear may be

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g the rotary circulation beneath mountain waves.

capabilities, you may back off and make anotherattempt at higher altitude. Sometimes you mayhave to choose between turning back or detouringthe area.

Flying mountain passes and valleys is not a safeprocedure during high winds. The mountains fun-nel the wind into passes and valleys thus increas-ing wind speed and intensifying turbulence. Ifwinds at mountain top level are strong, go high, orgo around.

Surface wind may be relatively calm in a valleysurrounded by mountains when wind aloft is strong.If taking off in the valley, climb above mountaintop level before leaving the valley. Maintain lateralclearance from the mountains sufficient to allowrecovery if caught in a downdraft.

SHEAR

associated with either a wind shift or a wind speedgradient at any level in the atmosphere. Three con-ditions are of special interest(1) wind shear witha low-level temperature inversion, (2) wind shear

Fromm 84. Mountain wave clouds over the Tibetan Plateau photographed from a manned spacecraft.

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Rolm 85. Satellite photograph of a mountain wave and the surface weather map for approximately the same time. Asingle mountain in the Aleutian chain generates the wave. Note how it spirals downwind from the source. Without thesatellite, the turbulent wave would have gone undetected unless some aircraft had flown into it.

in a frontal zone, and (3) clear air turbulence lence is discussed in detail in chapter 13, "High(CAT) at high levels associated with a jet stream Altitude Weather."or strong circulation. High-level clear air turbu-

87

9 8 .

WIND SHEAR- WITH A LOW-LEVELTEMPERATURE INVERSION

A temperature inversion forms near the surface ona clear night with calm or light surface wind as dis-cussed in chapter 2. Wind just above the inversionmay be relatively strong. As illustrated in figure 86,a wind shear zone develops between the calm andthe stronger winds above. Eddies in the shear zonecause airspeed fluctuations as an aircraft climbs ordescends through the inversion. An aircraft mostlikely is either climbing from takeoff or approach-ing to land when passing through the inversion;therefore, airspeed is slowonly a few knots greaterthan stall speed. The fluctuation in airspeed caninduce a stall precariously close to the ground.

Since surface wind is calm or very light, takeoffor landing can be in any direction. Takeoff may bein the direction of the wind above the inversion. Ifso, the aircraft encounters a sudden tailwind anda corresponding loss of airspeed when climbingthrough the inversion. Stall is possible. If approachis into thLt wind above the inversion, the headwindis suddenly lost when descending through theinver-

sion. Again, a sudden loss in airspeed may inducea stall.

When taking off or landing in calm wind underdear skies within a few hours before or after sun-rise, be prepared for a temperature inversion nearthe ground. You can be relatively certain of ashear zone in the inversion if you know the wind at2,000 to 4,000 feet is 25 knots or more. Allow amargin of airspeed above normal climb or approachspeed to alleviate danger of stall in event of turbu-lence or sudden change in wind velocity.

WIND SHEAR IN A FRONTAL ZONE

As you have learned in chapter 8, a front cancontain many hazards. However, a front can bebetween two dry stable airmasses and can be de-void of clouds. Even so, wind changes abruptly inthe frontal zone and can induce wind shear turbu-lence. The degree of turbulence depends on themagnitude of the wind shear. When turbulence isexpected in a frontal zone, follow turbulence pene-tration procedures recommended in your aircraftmanual.

FIOURE 86. Wind shear in a zone between relatively calm wind below an inversion and strong wind above the inversion.This condition is most common at night or in early morning. It can cause an abrupt turbulence encounter at low altitude.

WAKE TURBULENCE

An aircraft receives its Lift by accelerating amass of air downward. Thus, whenever the wingsare providing lift, air is forced downward underthe wings generating rotary motions or vortices offthe wing tips. When the landing gear bears the en-tire weight of the aircraft, no wing tip vortices

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develop. But the instant the pilot "hauls back" onthe controls, these vortices begin. Figure 87 illus-trates how they might appear if visible behind theplane as it breaks ground. These vortices continuethroughout the flight and until the craft again set-tles firmly on its landing gear.

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FIGURE 87. Wake turbulence wing tip vortices developing as aircraft breaks ground. These vortices develop when theaircraft is rotated into a flying attitude and the wings begin developing lift.

These vortices spread downward and outwardfrom the flight path. They also drift with the wind.Strength of the vortices is proportional to theweight of the aircraft as well as other factors.Therefore, wake turbulence is more intense behindlarge, transport category aircraft than behind smallaircraft. Generally, it is a problem only when foI-

.lowing the larger aircraft.

The turbulence persists several minutes and maylinger after the aircraft is out of sight. At con-trolled airports, the controller generally warns pi-lots in the vicinity of possible wake turbulence.When left to your own resources, you could use afew pointers. Most jets when taking off Iift the nosewheel about midpoint in the takeoff roll; therefore,vortices begin about the middle of the takeoff roIl.Vortices behind propeller aircraft begin only ashort distance behind lift-off: Following a landingof either type of aircraft, vortices end at about thepoint where the nose wheel touches down. Avoidflying through these vortices. More specifically,when using the same runway as a heavier aircraft:

(1) if landing behind another aircraft, keepyour approach above his approach and keep yourtouchdown beyond the point where his nose wheeltouched the runway (figure 88 (A) ) ;

(2) if landing behind a departing. aircraft,land only if you can complete your landing roll

before reaching the midpoint of his, takeoff roll(figure 88 (B) ) ;

(3) if departing behind another departingaircraft, take off only if you can become airbornebefore reaching the midpoint of his takeoff roll andonly if you can climb fast enough to stay above hisflight path (figure 88 (C)) ; and

(4) if departing behind a landing aircraft,don't unless you can taxi onto the runway beyondthe point at which his nose wheel touched downand have sufficient runway left for safe takeoff(figure 88 (D) ).

If parallel runways are available and the heavieraircraft takes off with a crosswind on the down-wind runway, you may safely use the upwind run-way. Never land or take off downwind from theheavier aircraft. When using a runway crossing hisrunway, you may safely use the upwind portion ofyour runway. You may cross behind a departingaircraft behind the midpoint of his takeoff roll.You may cross ahead of a landing aircraft aheadof the point at which his nose wheel touches down.

_If none of these procedures is possible, wait 5 min-utes or so for the vortices to dissipate or to blowoff the runway.

The foregoing procedures are elementary. Theproblem of wake turbulence is more operationalthan meteorological. The FAA issues periodic ad-

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visory circulars of operational problems. If you planto operate out of airports used routinely by air car-riers, we highly recommend you read the latest

advisory circulars on wake turbulence. Titles ofthese circulars are listed in the FAA "Advisory Cir-cular Checklist and Status of Regulations."

IN CLOSING

We have discussed causes of turbulence, claisitfiedit into types, and offered some flight procedureh toavoid it or minimize its hazards. Occurrences ofturbulence, however, are local in extent and tran-sient in character. A forecast of turbulence specifiesa volume of airspace that is small when comparedto useable airspace but relatively large comparedto the localized extent of the hazard. Although gen-eral forecasts of turbulence are quite good, fore-casting precise locations is at present impossible.

Generally, when a pilot receives a forecast, heplans his flight to avoid areas of most probableturbulence. Yet the best laid plans can go astray

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1 0 1'

and he may encounter turbulence. Since no instru-ments are currently available for directly observingturbulence, the man on the ground can only con-firm its existence or absence via pilot reports.HELP YOUR YELLOW PILOT AND THEWEATHER SERVICESEND PILOT RE-PORTS.

To make reports and forecasts meaningful, tur-bulence is classified into intensities based on theeffects it has on the aircraft and passengers.Section 16 of AVIATION WEATHER SERVICES(AC 00-45) lists and describes these intensities. Usethis guide in reporting your turbulence encounters.

,

Chapter 10ICING

Aircraft icing is one of the major weather haz-ards to aviation. Icing is a cumulative 1- Izard. Itreduces aircraft efficiency by increasing weight, re-ducing lift, decreasing thrust, and increasing drag.As shown in figure 89, each effect tends to eitherslow the aircraft or force it downward. Icing alsoseriously impairs aircraft engine performance. Othericing effects include false indications on flight in-struments, loss of radio communications, and loss of

operation of control surfaces, brakes, and landinggear.

In this chapter we discuss the principles of struc-tural, induction system, and instrument icing andrelate icing to cloud types and other factors. Al-though ground icing and frost are structural icing,we discuss them separately because of their differ-ent effect on an aircraft. And we wind up thechapter with a few operational pointers.

91

1L)2

DRAG INCREASES

Effects of Icing are Cumulative

LIFT LESSENS

WEIGHT GROWS

Stalling Speed increases

THRUST FALLS OFF

FIGURE 89. Effects of structural icing.

STRUCTURAL ICING

Two conditions are necessary for structural icingin Might: (1) the aircraft must be flying throughvisible water such as rain or cloud droplets, and(2) the temperature at the point where the mois-ture strikes the aircraft must be 0° C or colder.Aerodynamic cooling can lower temperature of anairfoil to 0° C even though the ambient tempera-ture is a few degrees warmer.

Supercooled water increases the rate of icing andis essential to rapid accretion. Supercooled wateris in an unstable liquid state; when an aircraftstrikes a supercooled drop, part of the drop freezesinstantaneously. The late..: heat of fusion releasedby the freezing portion raises the temperature ofthe remaining portion to the melting point. Aero-dynamic effects may cause the remaining portionto freeze. The way in which the remaining portionfreezes determines the type of king. The types ofstructural icing are clear, rime, and a mixture ofthe two. Each type has its identifying features.

92

CLEAR ICE

Clear ice forms when, after initial impact, theremaining liquid portion of the drop flows out overthe aircraft surface, gradually freezing as a smoothsheet of solid ice. This type forms when drops arelarge as in rain or in cumuliform clouds.

Figure 90 illustrates ice on the cross section of anairfoil, clear ice shown at the top. Figures 91 and92 are photographs of clear structural icing. Clearice is hard, heavy, and tenacious. Its removal bydeicing equipment is especially difficult.

RIME ICERime ice forms when drops are small, such as

those in stratified clouds or light drizzle. The liquidportion remaining after initial impact freezes rap-idly before the drop has time to spread over theaircraft surface. The small frozen droplets trap airbetween them giving the ice a white appearance as

1 o 3

CLEAR - HARD AND GLOSSY

RIME - BRITTLE AND FROST-LIKE

MIXED - HARD ROUGHCONGLOMERATE

FIGURE 90. Clear, rime, and mixed icing on airfoils.

104

shown at the center of figure 90. Figure 93 is aphotograph of rime.

Rime ice is lighter in weight than clear ice andits weight is of little significance. However, itsirregular shape and rough surface make it veryeffective in decreasing aerodynamic efficiency ofairfoils, thus reducing lift and increasing drag.Rime ice is brittle and more easily removed thanclear ice.

MIXED CLEAR AND RIME ICINGMixed ice forms when drops vary in size or when

liquid drops are intermingled with snow or ice par-ticles. It can form rapidly. Ice particles becomeimbedded in clear ice, building a very rough ac-cumulation sometimes in a mushroom shape onleading edges as shown at the bottom of figure 90.Figure 94 is a photo of mixed icing built up on apitot tube.

ICING INTENSITIESBy mutual agreement and for standardization

the FAA, National Weather Service, the militaryaviation weather services, and aircraft operatingorganizations have classified aircraft structural icinginto intensity categories. Section 16 of AvoatoNWEATHER SERVICES (AC 00-45) has a table listingthese intensities. The table is your guide in esti-mating bow ice of a specific intensity will affectyour aircraft. Use the table also in reporting icewhen you encounter it.

93

0.0

. :%Pv;*...!

Fasuaz 91. Clear wing icing (leading edge and underside). (Courtesy Dean T. Bowden, General Dynamics /Convair.)

94

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Floune 92. Propeller icing. Ice may form on propellers just as on any airfoil. It reduces propeller efficiency and mayinduce severe vibrations.

95

106

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FIGURE 93. Rime Icing on the nose of a Mooney "Mark 21" aircraft. (Photo by Norman Hoffman, Mooney Aircraft, Inc.,courtesy the A.O.P.A. Pilot Magazine.)

96

107

Ast

it R

tFIGURE 94. External icing on a pitot tube.

INDUCTION SYSTEM ICING

Ice frequently forms in the air intake of an en-gine robbing the engine of air to support combus-tion. This type icing occurs with both piston andjet engines, and almost everyone in the aviationcommunity is familiar with carburetor icing. Thedownward moving piston in a piston engine or thecompressor in a jet engine forms a partial vacuumin the intake. Adiabatic expansion in the partialvacuum cools the air. Ice forms when the temper-ature drops below freezing and sufficient moistureis present for sublimation. In piston engines, fuel

To 8

evaporation produces additional cooling. Inductionicing always lowers engine performance and caneven reduce intake flow below that necessary forthe engine to operate. Figure 95 illustrates car-buretor icing.

Induction icing potential varies greatly amongdifferent aircraft and occurs under a wide rangeof meteorological conditions. It is primarily anengineering and operating problem rather thanmeteorological.

?""

FIGURE 95. Carburetor icing. Expansional cooling of air and vaporization of fuel can induce freezing and cause ice toclog the carburetor intake.

INSTRUMENT ICING

Icing of the pitot tube as seen in figure 96 re-duces ram air pressure on the airspeed indicatorand renders the instrument unreliable; 'Most mod-ern aircraft also have an outside static pressureport as part of the pitot-static system. Icing of thestatic pressure port reduces reliability of all instal-ments on the systemthe airspeed, rate-of-climb,and the altimeter.

98

1 9

Ice forming on the radio antenna distorts itsshape, increases drag, and imposes vibrations thatmay result in failure in the communications systemof the aircraft. The severity of this icing dependsupon the shape, location, and orientation of theantenna. Figure 97 is a photograph of clear ice onan antenna mast.

Flamm 96. Internal pitot tube icing. It renders airspeed indicator unreliable.

ICING AND CLOUD TYPES

Basically, all clouds at subfreezing temperatureshave icing potential. However, drop size, drop dis-tribution, and aerodynamic effects of. the aircraftinfluence ice formation. Ice may not form eventhough the potential exists.

The condition most favorable for very hazardousking is the presence of many large, supercooledwater drops. Conversely, an equal or lesser num-ber of smaller droplets favors a slower rate of icing.

Small water droplets occur most often in fog andkw-level clouds. Drizzle or very light rain is evi-dence of the presence of small drops in such clouds;but in many cases there is no precipitation at all.The most common type of king found in lower-level stratus clouds is rime.

On the other hand, thick extensive stratified

clouds that produce continuous rain such as alto-stratus and nimbostratus usually have an abun-dance of liquid water because of the relativelylarger drop size and number. Such cloud systemsin winter may cover thousands of square miles andpresent very serious icing conditions for protractedflights. Particularly in thick stratified clouds, con-cenn.:ktions of liquid water normally are greaterwith Ntarmer temperatures. Thus, heaviest icingusually will be found at or slightly above the freez-ing level where temperature is never more than afew degrees below freezing. In layer type clouds,continuous icing conditions are rarely found to bemore- than 5,000 feet above the freezing level, andusually are two or three thousand feet thick.

The upward currents in cumuliform clouds are

99

1 i 0 .

FIGURE 97. Clear ice on an aircraft antenna mast.

favorable for the formation and support of manylarge water drops. The size of raindrops and rain-fall intensity normally experienced from showersand thunderstorms confirm this. When an aircraftenters the heavy water concentrations found incumuliform clouds, the large drops break andspread rapidly over the leading edge of the airfoilforming a film of water. If temperatures are freez-ing or colder, the water freezes quickly to form asolid sheet of clear ice. Pilots usually avoid cumuli-form clouds when possible. Consequently, icing re-ports from such clouds are rare and do not indicatethe frequency with which it can occur.

The updrafts in cumuliform clouds carry largeamounts of liquid water far above the freezinglevel. On rare occasions icing has been encounteredin thunderstorm clouds at altitudes of 30,000 to40,000 feet where the free air temperature wascolder than minus 40° C.

While an upper limit of critical icing potentialcannot be specified in cumuliform clouds, the cellu-lar distribution of such clouds usually limits thehorizontal extent of icing conditions. An exception,of course, may be found in a protracted flightthrough a broad zone of thunderstorms or heavyshowers.

OTHER FACTORS IN ICING

In addition to the above, other factors also enterinto icing. Some of the more important ones arediscussed below.

FRONTS

A condition favorable for rapid accumulation ofclear icing is freezing rain below a frontal surface.

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1 1 1

Rain forms above the frontal surface at temper-atures warmer than freezing. Subsequently, it fallsthrough air at temperatures below freezing andbecomes supercooled. The supercooled drops freezeon impact with an aircraft surface. Figure 98 dia-grams this type of icing. It may occur with eithera warm front (top) or a cold front. The icing can

le

be critical because of the large amount of super-cooled water. Icing can also become serious incumulonimbus clouds along a surface cold front,along a squall line, or embedded in the cloud shieldof a warm front.

TERRAIN

Air blowing upslope is cooled adiabatically.When the air is cooled below the freezing point,

BELOW FREEZING TEMPERATURE

0°C

STRATIFIED CLOUDS '

the water becomes supercooled. In stable air blow-ing up a gradual slope, the cloud drops generallyremain comparatively small since larger drops failout as rain. Ice accumulation is rather slow andyou should have ample time to get out of it beforethe accumulation becomes extremely dangerous.When air is unstable, convective clouds develop amore serious hazard as described in "Icing andCloud Types."

BELOW FREEZING TEMPERATURE

0°C _-m-4101...J

. _Fioui 98. Freezing rain with a warm front (top) and a cold front (bottom). Rainfall through warm air aloft

into subfreezing cold air near the ground. The rain becomes supercooled and freezes on impact.

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101

Icing is more probable and more hazardous inmountainous regions than over otner terrain. Moun-tain ranges cause rapid upward air motions on thewindward side, and these vertical currents supportlarge water drops. The movement of a frontal sys-tem across a mountain range often combines thenormal frontal lift with the upsiope effect of themountains to create extremely hazardous icingzones.

Each mountainous region has preferred areas oficing depending upon the orientation of mountainranges to the wind flow. The most dangerous kingtakes place above the crests and to the windwardside of the ridges. This zone usually extends about5,000 feet above the tops of the mountains; butwhen clouds are cumuliform, the zone may extendmuch higher.

SEASONS

Icing may occur during any season of the year;but in temperate climates such as cover most of thecontiguous United States, icing is more frequent inwinter. The freezing level is nearer the ground inwinter than in summer leaving a smaller low-levellayer of airspace free of icing conditions. Cyclonicstorms also are more frequent in winter, and theresulting cloud systems are more extensive. Polarregions have the most dangerous icing conditions inspring and fall. During the winter the air is nor-mally too cold in the polar regions to contain heavyconcentrations of moisture necessary for icing,' andmost cloud systems are stratiform and are com-posed of ice crystals.

GROUND ICING

Frost, ice pellets, frozen rain, or snow may accu-mulate on parked aircraft. You should remove allice prior to takeoff, for it reduces flying efficiency ofthe aircraft. Water blown by propellers or splashedby wheels of an airplane as it taxis or runs through

pools of water or mud may result in serious aircrafticing. Ice may form in wheel wells, brake mech-anisms, flap hinges, etc., and prevent proper oper-ation of these paits. Ice on runways and taxiwayscreate traction and braking problems.

FROST

Frost is a hazard to flying long recognized inthe aviation community. Experienced pilots havelearned to remove all frost from airfoils prior totakeoff. Frost forms near the surface primarily inclear, stable air and with light windsconditionswhich in all other respects make weather ideal forflying. Because of this, the real hazard is often min-imized. Thin metal airfoils are especially vulner-able surfaces on which frost will form. Figure 99is a photograph of frost on an airfoil.

Frost does not change the basic aerodynamicshape of the wing, but the roughness of its surfacespoils the smooth flow of air 'thus causing a slow-ing of the airflow. This slowing of the air causes

early air flow separation over the affected airfoilresulting in a loss of lift. A heavy coat of hardfrost will cause a 5 to 10 percent increase in stallspeed. Even a small amount of frost on airfoilsmay prevent an aircraft from becoming airborneat normal takeoff speed. Also possible is that, onceairborne, an aircraft could have insufficient marginof airspeed above stall so that moderate gusts orturning flight could, produce incipient or completestalling.

Frost formation in flight offers a more compli-cated problem. The extent to which it will form isstill a matter of conjecture. At most, it is compar-atively rare.

IN CLOSING

Icing is where you find it. As with turbulence,icing may be local in extent and transient in char-acter. Forecasters can identify regions in whichicing is possible. However, they cannot define theprecise small pockets in which it occurs. You should

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plan your flight to avoid those areas where icingprobably will be heavier than your aircraft canhandle. And you must be prepared to avoid or toescape the hazard when encountered en route.

Drams 99. Frost on an aircraft. Always remove ice or frost before attempting takeoff.

Here are a few specific points to remember:I. Before takeoff, check weather for possible

icing areas along your planned route. Checkfor pilot reports, and if possible talk toother pilots who have flown along yourproposed route.

2. If your aircraft is not equipped with de-icing or anti-icing equipment, avoid areasof icing. Water (clouds or precipitation)must be visible and outside air temperaturemust be near 0° C or colder for structuralice to form.

3. Always remove ice or frost from airfoils be-fore attempting takeoff.

4. In cold weather, avoid, when possible, taxi-ing or taking off through mud, water, orslush. If you have taxied through any ofthese, make a preflight check to ensure free-dom of controls.

5. When climbing out through an icing layer,climb at an airspeed a little faster than nor-mal to avoid a stall.

114.7.m.r.makt

6. Use deicing or anti-icing equipment whenaccumulations of ice are not too great.When such equipment becomes less thantotally effective, change course or altitudeto get out of the icing as rapidly as possible.

7. If your aircraft is not equipped with apitot-static system deicer, be alert for erro-neous readings from your airspeed indicator,rate-of-climb indicator, and altimeter.

8. In stratiform clouds, you can likely alleviateicing by changing to a flight level andabove-freezing temperatures or to one cold-er than 10° C. An altitude change alsomay take you out of clouds. Rime icingin stratiform clouds can be very extensivehorizontally.

9. In frontal freezing rain, you may be able toclimb or descend to a layer warmer thanfreezing. Temperature is always warmerthan freezing at some higher altitude. Ifyou are going to climb, move quickly; pro-crastination may leave you with too much

103

ice. If you are going to descend, you mustknow the temperature and terrain below.

10. Avoid cumuliform clouds if at all possible.Clear ice may be encountered anywhereabove the freezing level. Most rapid accum-ulations are usually at temperatures from0 °C to 15° C.

11. Avoid abrupt maneuvers when your air-craft is heavily coated with ice since the air-craft has lost some of its aerodynamicefficiency.

104115

12. When "iced up," fly your landing approachwith power.

The man on the ground has no way of observingactual icing conditions. His only confirmation ofthe existence or absence of icing comes from pilots.Help your fellow pilot and the weather service bysending pilot reports when you encounter icingor when icing is forecast but none encountered.Use the table in Section 16 of AviATioN WEATHERSERVICES as a guide in reporting intensities.

Chapter 11THUNDERSTORMS

Many times you have to make decisions involv-ing thunderstorms and flying. This chapter looksat where and when thunderstorms occur most fre-quently, explains what creates a storm, and looks

inside the storm at what goes on and what it an-do to an aircraft. The chapter also describes howyou can use radar and suggests some do's and don'tsof thunderstorm flying.

WHERE AND WHEN?

In some tropical regions, thunderstorms occuryear-round. In midlatitudes, they develop mostfrequently in spring, summer, and fall. Arctic re-gions occasionally experience thunderstorms duringsummer.

Figure 100 shows the average number of thun-derstorms each year in the adjoining 48 States. Note

116

the frequent occurrences in the south-central andsoutheastern States. The number of days on whichthunderstorms occur varies widely from season toseason as shown in figures 101 through 104. Ingeneral, thunderstorms are most frequent duringJuly and August and least frequent in Decemberand January.

105

106

FIGURE 100. The average number of thunderstorms each year.

117

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Fromm 102. The average number of days with thunderstorms during summer.

1 1 9

FIGURE 103. The average number of days with thunderstorms during fall.

109

120

110

Ftouaz 104. The average number of days with thunderstorms during winter.

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s.

1

THEY DON'T JUST HAPPEN

For a thunderstorm to form, the air must have(1) sufficient water vapor, (2) an unstable lapserate, and (3) an initial upward boost (lifting) tostart the storm process in motion. We discussedwater vapor in chapter 5 and stability in chapter6; but, what about lifting? Surface heating, con-

verging winds, sloping terrain, a frontal surface, orany combination of these can provide the lift.

Thunderstorms have been a subject of consider-able investigation for many years as they are today.Figuratively speaking, let's look inside a thunder-storm.

THE INSIDE STORY

Forced upward motion creates an initial updraft.Cooling in the updraft results in condensation andthe beginning of a cumulus cloud. Condensation_releases latent heat which partially offsets cooling inthe saturated updraft and increases buoyancy withinthe cloud. This increased buoyancy drives the up-draft still faster drawing more water vapor intothe cloud; and, for awhile, the updraft becomesself-sustaining. All thunderstorms progress througha life cycle from their initial development throughmaturity and into degeneration.

LIFE CYCLEA thunderstorm cell during its life cycle pro-

gresses through three stages(1) the cumulus, (2)the mature, and (3) the dissipating. It is virtuallyimpossible to visually detect the transition from onestage to another; the transition is subtle and by nomeans abrupt. Furthermore, a thunderstorm maybe a cluster of cells in different stages of the lifecycle.

The Cumulus StageAlthough most cumulus clouds do not grow into

thunderstorms, every thunderstorm begins as acumulus. The key feature of the cumulus stage isan updraft as illustrated in figure 105(A). Theupdraft varies in strength .and extends from verynear the surface to the cloud top. Growth rate ofthe cloud may exceed 3,000 feet per minute, so itis inadvisable to attempt to climb over rapidlybuilding cumulus clouds.

Early during the cumulus stage, water dropletsare quite small but grow to raindrop size as thecloud grows. The upwelling air carries the liquidwater above the freezing level creating an icinghazard. As the raindrops grow still heavier, theyfall. The cold rain drags air with it creating a

cold downdraft coexisting with the updraft; thecell has reached the mature stage.

The Mature StagePrecipitation beginning to fall from the cloud

base is your signal that a downdraft has developedand a cell has entered the mature stage. Cold rainin the downdraft retards compressional heating,and the downdraft remains cooler than surround-ing air. Therefore, its downward speed is acceler-ated and may exceed 2,500 feet per minute. Thedownrushing air spreads outward at the surface asshown in figure 105(B) producing strong, gustysurface winds, a sharp temperature drop, and arapid rise in pressure. The surface wind surge is a"plow wind" and its leading edge is the "first gust."

Meanwhile, updrafts reach a maximum withspeeds possibly exceeding 6,000 feet per minute.Updrafts and downdrafts in close proximity createstrong vertical shear and a very turbulent environ-ment. All thunderstorm hazards reach their great-est intensity during the mature stage.

Tice Dissipating Stage .

Downdrafts characterize the dissipating stage ofthe thundeistorm cell as shown in figure 105(C)and the storm dies rapidly. When rain has endedand downdrafts have abated, the dissipating stage'is complete. When all cells of the thunderstormhave completed this stage, only harmless cloud rem-nants remain.

HOW BIG?Individual thunderstorms measure from less than

5 miles to more than 30 miles in diameter. Cloudbases range from a few hundred feet in very moistclimates to 10,000 feet or higher in drier regions.Tops generally range from 25,000 to 45,000 feetbut occasionally extend above 65,000 feet.

111

1 2 2

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,`'..fr :-..

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hot= 105. The st.,.les of a thunderstorm. (A) is the cumulus stage; (B), the mature stage; and (C), the dissipating stage.Arrows depict air flow.

ROUGH AND ROUGHER

Duration of the mature stage is closely related toseverity of the thunderstorm. Some storms occur atrandom in unstable air, last for only an hour ortwo, and produce only moderate gusts and rainfall.These are the "air mass" type, but even they aredangerously rough to fly through. Other thunder-storms form in lines, last for several hours, dumpheavy rain and possibly hail, and produce strong,gusty winds and possibly tornadoes. These stormsare the "steady state" type, usually are rougherthan air mass storms, and virtually defy flightthrough them.

AIR MASS THUNDERSTORMSAir mass thunderstorms most often result from

surface heating. When the storm reaches the ma-ture stage, rain falls through or immediately besidethe updraft. Falling precipitation induces frictionaldrag, retards the updraft and reverses it to a down-draft. The storm is self-destructive. The downdraftand cool precipitation cool the lower portion of thestorm and the underlying surface. Thus, it cuts offthe inflow of water vapor; the storm runs out of

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energy and dies. A self-destructive cell usually hasa life cycle of 20 minutes to 1%2 hours.

Since air mass thunderstorms generally resultfrom surface heating, they reach maximum inten-sity and frequency over land during middle andlate afternoon. Off shore, they reach a maximumduring late hours of, darkness when land tempera-ture is coolest anti cool air flows off the land overthe relatively warm water.

STEADY STATE THUNDERSTORMSSteady state thunderstorms usually are associ-

ated with weather systems. Fronts, convergingwinds, and troughs aloft force upward motionspawning these storms which often form into squalllines. Afternoon heating intensifies them.

In a steady state storm, precipitation falls outsidethe updraft as shown in figure 106 allowing the up-draft to continue unabated. Thus, the mature stageupdrafts become stronger and last much longerthan in air mass stormshence, the name, "steadystate." A steady state cell may persist for severalhours.

FIOURE 106. Schematic of the mature stage of a steady state thunderstorm cell showing a sloping updraft with the down-draft and precipitation outside the updraft not impeding it. The steady state mature cell may continue for many hoursand deliver the most violent thunderstorm hazards.

HAZARDS

A thunderstorm packs just about every weatherhazard known to aviation into one vicious bundle.Although the hazards occur in numerous combi-nations, let's separate them and examine eachindividually.

TORNADOESThe most violent thunderstorms draw air into

their cloud bases with great vigor. If the incomingair has any initial rotating motion, it often formsan extremely concentrated vortex from the surfacewell into the cloud. Meteorologists have estimated

124

that wind in such a vortex can exceed 200 knots;pressure inside the vortex is quite low. The strongwinds gather dust and debris, and the low pressuregenerates a funnel- shaped cloud extending down-ward from the cumulonimbus base. If the cloud ,does not reach the surface, it is a "funnel cloud,"figure 109; if it touches a land surface, it is a "tor-nado," figure 107; if it touches water, it is a "waterspout," figure 108.

Tornadoes occur with isolated thunderstorms attimes, but much more frequently, they form withsteady state thunderstorms associated with cold .

113

Fromm 107. A tornado.

fronts or squall lines. Reports or forecasts of tor-nadoes indicate that atmospheric conditions arefavorable for violent turbulence.

An aircraft entering a tornado vortex is almostcertain to suffer structural damage. Since thevortex extends well into the cloud, any pilot in-advertently caught on instruments in a severethunderstorm could encounter a hidden vortex.

Families of tornadoes have been observed asappendages of the main cloud extending severalmiles outward from the area of lightning and pre -cipitatkn. Thus, any cloud connected to a severethunderstorm carries a threat of violence.

Frequently, cumulonimbus mamma clouds occurin connection with violent thunderstorms and tor-nadoes. The cloud displays rounded, irregular pock-ets or festoons from its base and is a signpost ofviolent turbulence. Figure 110 is a photograph ofa cumulonimbus mamma cloud. Surface aviationreports specifically mention this and other especiallyhazardous clouds.

Tornadoes occur most frequently in the GreatPlains States east of the Rocky Mountains. Figure111 shows, however, that they have occurred inevery State.

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Fan= 108. A waterspout.

SQUALL LINES

A squall line is a non-frontal, narrow band ofactive thunderstorms. Often it develops ahead ofa cold front in moist, unstable air, but it may de-velop in unstable air far removed frOm any front.The line may be too long to easily detour and toowide and severe to penetrate. It often containssevere steady-state thunderstorms and presents thesingle most intense weather hazard to aircraft. Itusually forms rapidly, generally reaching maximumintensity during the late afternoon and the firstfew hours of darkness. Figure 112 is a photographof an advancing squall line.

TURBULENCE

Hazardous turbulence is present in an thunder-storms; and in a severe thunderstorm, it can dam-age an airframe. Strongest turbulence within thecloud occurs with shear between updrafts and

r

FIGURE 109. Funnel clouds.(Photograph by Paul Herter, NWS.)

downdrafts. Outside the cloud, shear turbulencehas been encountered several thousand feet aboveand 20 miles laterally from a severe storm. A lowlevel turbulent area is the shear zone between theplow wind and surrounding air. Often, a "rollcloud" on the leading edge of a storm marks theeddies in this shear. The roll cloud is most preva-lent with cold frontal or squall line thunderstormsand signifies an extremely turbulent zone. The firstgust causes a rapid and sometimes drastic changein surface wind ahead of an approaching storm.Figure 113 shows a schematic cross section of athunderstorm with areas outside the cloud whereturbulence may be encountered.

It is almost impossible to hold a constant alti-tude in a thunderstorm, and maneuvering in anattempt to do so greatly increases stresses on theaircraft. Stresses will be least if the aircraft is heldin a constant attitude and allowed to "ride thewaves." To date, we have no sure way to pick "softspots" in a thunderstorm.

ICING

Updrafts in a thunderstorm support abundantliquid water; and when carried above the freezinglevel, the water becomes supercooled. When tem-perature in the upward current cools to about15° C, much of the remaining water vaporsublimates as ice crystals; and above this level, theamount of supercooled water decreases.

Supercooled water freezes on impact with anaircraft (see chapter 10). Clear icing can occurat any altitude above the freezing level; but athigh levels, icing may be rime or mixed rime andclear. The abundance of supercooled water makesclear icing very rapid between 0° C and 15° C,and encounters can be frequent in a cluster of cells.Thunderstorm icing can be extremely hazardous.

HAIL

Hail competes with turbulence as the greatestthunderstorm hazard to aircraft. Supercooled dropsabove the freezing level begin to freeze. Once adrop has frozen, other drops latch on and freezeto it, so the hailstone growssometimes into a hugeiceball. Large hail occurs with severe thunder-storms usually built to great heights. Eventually thehailstones fall, possibly some distance from thestorm core. Hail has been observed in clear airseveral miles from the parent thunderstorm.

As hailstones fall through the melting level, theybegin to melt, and precipitation may reach theground as either hail or rain. Rain at the surfacedoes not mean the absence of hail aloft. You shouldanticipate possible hail with any thunderstorm,especially beneath the anvil of a large cumulonim-bus. Hailstones larger than one-half inch in diem-eter can significantly damage an aircraft in a fewseconds. Figure 114 is a photograph of an aircraftflown through a "hail" of a thunderstorm.

LOW CEILING AND VISIBILITYVisibility generally is near zero within a thunder-

storm cloud. Ceiling and visibility also can becomerestricted in precipitation and dust between thecloud base and the ground. The restrictions createthe same problem as all ceiling and visibility re-strictions; but the hazards are increased many foldwhen associated with the other thunderstorm haz-ards of turbulence, hail, and lightning which makeprecision instrument flying virtually impossible.

1 2 6115

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Fromm 110. Cumulonimbus Mamma clouds, associated with cumulonimbus clouds, indicate extreme instabilitiO0

EFFECT ON ON ALTIMETERS

Pressure usually falls rapidly with the approachof a thunderstorm, then rises sharply with the on-set of the first gust and arrival of the cold down-draft and heavy rain showers, falling back to nor-mal as the storm moves on. This cycle of pressurechange may occur in 15 minutes. If the altimetersetting is not corrected, the indicated altitude maybe in error by over 100 feet.

THUNDERSTORM ELECTRICITY

Electricity generated by thunderstorms is rarelya great hazard to aircraft, but it may cause damageand is annoying to flight crews. Lightning is themost spectacular of the electrical discharges.

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127.

Lightning

A lightning strike can puncture the skin of anaircraft and can damage communication and elec-tronic navigational equipment. Lightning has beensuspected of igniting fuel vapors causing explosion;however, serious accidents due to lightning strikesare extremely rare. Nearby lightning can blind thepilot rendering him momentarily unable to navi-gate either by instrument or by visual reference.Nearby lightning can also induce permanent errorsin the magnetic compass. Lightning discharges,even distant ones, can disrupt radio communica-tions on low and medium frequencies.

A few pointers on lightning:1. The more frequent the lightning, the more

severe the thunderstorm.

TORNADO INCIDENCE BY STATE AND AREA, 1953-1973

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UPPER FIGURE li NWASEN OF TORNADOES

LOWER FIGURE IS MEAN ANNUAL MOAK" OF

TORNADOES PEN 10000 MARE MUIR

r1OURE 111. Tornado incidence by State and area.

2. Increasing frevency of lightning indicatesa growing thunderstorm.

3. Decreasing lightning indicates a stormnearing the dissipating stage.

4. At night, frequent distant flashes playingalong a large sector of the horizon suggesta probable squall line.

Precipitation StaticPrecipitation static, a steady, high mtvel of noise

in radio receivers is caused by intereim. corona dis-charges from sharp metallic points e nd edges of

flying aircraft. It is encountered often in the vicin-ity of thunderstorms. When an aircraft flies throughclouds, precipitation, or a concentration of solidparticles (ice, sand, dust, etc.)., it accumulates acharge of static electricity. The electricity dis-charges onto a nearby surface or into the air caus-ing a noisy disturbance at lower frequencies.

The corona discharge is weakly luminous andmay be seen at night. Although it has a rather eerieappearance, it is harmless. It was named "St.Elmo's Fire" by Mediterranean sailors, who sawthe brushy discharge at the top of ship masts.

117

128

118

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Pm= 112. Squall line thunderstorms.

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WIND SHEARTURBULENCE

TURBULENCE

------ANVIL

STORMMOVEMENT

ROLL CLOUD

,90." WIND SHEARW. TURBULENCE

FIRST GUST

Fmuitz 113. Schematic cross section of a thunderstorm. Note areas outside the main cloud whereturbulence may be encountered.

119

130

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FouRE 114. Hail damage to an aircraft.

THUNDERSTORMS AND RADARWeather radar detects droplets of precipitation

size. Strength of the radar return (echo) dependson drop size and number. The greater the numberof drops, the stronger is the echo; and the largerthe drops, the stronger is the echo. Drop size de-termines echo intensity to a much greater extentthan does drop number.

Meteorologists have shown that drop size is al-most directly proportional to rainfall rate; and thegreatest rainfall rate is in thunderstorms. There-fore, the strongest echoes are thunderstorms. Hail-stones usually are covered with a film of water and,therefore, act as huge water droplets giving thestrongest of all echoes. Showers show less intenseechoes; and gentle rain and snow return the weak-est of all echoes. Figure 115 is a photograph of aground based radar scope.

120

Since the strongest echoes identify thunderstorms,they also mark the areas of greatest hazards. Radarinformation can be valuable both from groundbased radar for preflight planning and from air-borne radar for severe weather avoidance.

Thunderstorms build and dissipate rapidly, andthey also may move rapidly. Therefore, do not at-tempt to preflight plan a course between echoes.The best use of ground radar information is to iso-late general areas and coverage of echoes. Youmust evade individual storms from inflight observa-tions either by visual sighting or by airborne radar.

Airborne weather avoidance radar is, as its nameimplies, for avoiding severe weathernot for pene-trating it. Whether to fly into an area of radarechoes depends on echo intensity, spacing betweenthe echoes, and the capabilities of you and your

P.

Al.

Fromm 115. Radar ph-otograph of a line of thunderstorms.

aircraft. Remember that weather radar detectsonly precipitation drops; it does not detect minutecloud droplets. Therefore, the radar scope providesno assurance of avoiding instrument weather inclouds and fog. Your scope may be clear betweenintense echoes; this clear area does not necessarilymean you can fly between the storms and maintainvisual sighting of them.

The most intense echoes are severe thunder-

storms. Remember that hail may fall several milesfrom the cloud, and hazardous turbulence may ex-tend as much as 20 miles from the cloud. Avoidthe most intense echoes by at least 20 miles;that is, echoes should be separated by at least 40miles. before you fly between them. As echoesdiminish in intensity, you can reduce the distanceby which you avoid them. Figure 116 illustratesuse of airborne radar in avoiding thunderstorms.

DO'S AND DON'TS OF THUNDERSTORM FLYING

Above all, remember this: never regard anythunderstorm as "light" even when radar observersreport the echoes are of light intensity. Avoidingthunderstormi is the best policy. Following aresome Do's and Don'ts of thunderstorm avoidance:

I. Don't land or take off in the face of anapproaching thunderstorm. A sudden windshift or low level turbulence could causeloss of control.

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2. Don't attempt to fly under a thunderstormeven if you can see through to the otherside. Turbulence under the storm could bedisastrous.

3. Don't try to circumnavigate thunderstormscovering 6/10 of an area or more eithervisually or by airborne radar.

4. Don't fly without airborne radar into acloud mass containing scattered embedded

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Rot= 116. the of airborne radar to avoid heairy precipitation and turbulence. When echoes are extremely intense, avoidthe most intense echoes by at least 20 miles. You should avoid flying between these very intense echoes unless they areseparated by at least 40 miles. Hazardous turbulence and hail often extend several miles from the storm centers.

thunderstorms. Scattered thunderstorms notembedded usually can be visually circum-navigated.

5. Do avoid by at least 20 miles any thunder-storm identified as severe or giving an in-tense radar echo. This is especially trueunder the anvil of a large cumulonimbus.

6. Do clear the top of a known or suspectedsevere thunderstorm by at least 1,000 feetaltitude for each 10 knots of wind speed atthe cloud top. This would exceed thealtitude capability of most aircraft.

7. Do remember that vivid and frequent light-ning indicates a severe thunderstorm.

8. Do regard as severe any thunderstorm withtops 35,000 feet or higher whether the topis visually sighted or determined by radar.

If you cannot avoid penetrating a thunderstorm,following are some Do's Before entering the storm:

1. Tighten your safety belt, put on yourshoulder harness if you have one, andsecure all loose objects.

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2. Plan your course to take you through thestorm in a minimum time and hold it.

3. To avoid the most critical icing, establisha penetration altitude below the freezinglevel or above the level of 15° C.

4. Turn on pitot heat and carburetor or jetinlet heat. Icing can be rapid at any alti-tude and cause almost instantaneous powerfailure or loss of airspeed indication.

5. Establish power settings for reduced tur-bulence penetration airspeed recommendedin your aircraft manual. Reduced airspeedlessens the structural stresses on the aircraft.

6. Turn up cockpit lights to highest intensityto lessen danger of temporary blindnessfrom lightning.

7. If using automatic pilot, disengage altitudehold mode and speed hold mode. The auto-matic altitude and speed controls will in-crease maneuvers of the aircraft thus in-creasirf structural stresses.

8.- If using airborne radar, tilt your antenna...

.,

up and down occasionally. Tilting it upmay detect a hail shaft that will reach apoint on your course by the time you do.Tilting it down may detect a growingthunderstorm cell that may reach youraltitude.

Following are some Do's and Don'ts Duringthunderstorm penetration:

I. Do keep your eyes on your instruments.Looking outside the cockpit can increasedanger of temporary blindness from light-ning.

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2. Don't change power settings; maintain set-tings for reduced airspeed.

3. Do maintain a constant attitude; let theaircraft "ride the waves." Maneuvers intrying to maintain constant altitude increasestresses on the aircraft.

4. Don't turn back once you are in the thun-derstorm, A straight course through thestorm most likely will get you out of thehtzarcls most quickly. In addition, turningmaneuvers increase stresses on the aircraft.

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Chapter 12COMMON IFR PRODUCERS

Most aircraft accidents related to low ceilingsand visibilities involve pilots who are not instru-ment qualified. These pilots attempt flight by visualreference into weather that is suitable at best only

'for instrument flight. When you Iose sight of thevisual horizon, your senses deceive you; you losesense of directionyou can't tell up from down.You may doubt that you will Iose your sense ofdirection, but one good scare has changed thethinking of many a pilot. "Continued VFR into

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adverse weather" is the cause of about 25 per-cent of all fatal general aviation accidents.

Minimum values of ceiling and visibility deter-mine Visual Flight Rules. Lower ceiling and/orvisibility require instrument flight. Ceiling is themaximum height from which a pilot can maintainVFR in reference to the ground. Visibility is howfar he can see Amnon WEATHER SERVICES (AC00-45) contains details of ceiling and visibilityreports.

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Don't let yourself be caught in the statistics of"continued VER into adverse weather." IrR pro-ducers are fog, low clouds, haze, smoke, blowing

obstructions to vision, and precipitation. Fog andlow stratus restrict navigation by visual referencemore often than all other weather parameters.

FOG

Fog is a surface based cloud composed of eitherwater droplets or ice crystals. Fog is the most fre-quent cause of surface visibility below 3 miles, andis one of the most common and persistent weatherhazards encountered in aviation. The rapidity withwhich fog can form makes it especially hazardous.It is not unusual for visibility to drop from VFR toless than a mile in a few minutes. It is primarily ahazard during takeoff" and landing, but it is alsoimportant to VFR pilots who must maintain visualreference to the ground.

Small temperature-dew point spread is essentialfor fog to form. Therefore, fog is prevalent incoastal areas where moisture is abundant. How-ever, fog can occur anywhere. Abundant conden-sation nuclei enhances the formation of fog. Thus,fog is prevalent in industrial areas where by-products of combustion provide a high concentra-tion of these nuclei. Fog occurs most frequently inthe colder months, but the season and frequency ofoccurrence vary from one area to another.

Fog may form (1) by cooling air to its dewpoint, or (2) by adding moisture to air near the

ground. Fog is classified by the way it forms. For-mation may involve more than one process.

RADIATION FOGRadiation fog is relatively shallow fog. It may be

dense enough to hide the entire sky or may concealonly part of the sky. "Ground fog" is a form ofradiation fog. As viewed by a pilot in flight, denseradiation fog may obliterate the entire surface be-low him; a less dense fog may permit his observa-tion of a small portion of the surface directly belowhim. Tall objects such as buildings, hills, and towersmay protrude upward through ground fog givingthe pilot fixed references for VFR flight. Figure117 illustrates ground fog as seen from the air.

Conditions favorable for radiation fog are clearsky, little or no wind, and small temperature-dewpoint spread (high relative humidity). The fogforms almost exclusively at night or near daybreak.Terrestrial radiation cools the ground; in turn, thecool ground cools the air in contact with it. Whenthe air is cooled to its dew point, fog forms. Whenrain soaks the ground, followed by clearing skies,

126

FIGURE 117. Ground fog as seen from the air.

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radiation fog is not uncommon the followingmorning.

Radiation fog is restricted to land because watersurfaces cool little from nighttime radiation. It isshallow when wind is calm. Winds up to about 5knots mix the air slightly and tend to deepen thefog by spreading the cooling through a deeper layer.Stronger winds disperse the fog or mix the airthrough a still deeper layer with stratus cloudsforming at the top of the mixing layer.

Ground fog usually "burns off" rather rapidlyafter sunrise. Other radiation fog generally clearsbefore noon unless clouds move in over the fog.

ADVECTION FOGAdvection fog forms when moist air moves over

colder ground or water. It is most common alongcoastal areas but often develops deep in continentalareas. At sea it is called "sea fog." Advection fogdeepens as wind speed increases up to about 15knots. Wind much stronger than 15 knots lifts thefog into a layer of low stratus or stratocumulus.

The west coast of the United States is quite vul-nerable to advection fog: This fog frequently forms

COLDWATER CURRENT

offshore as a result of cold water as shown in figure118 and then is carried inland by the wind. Duringthe winter, advection fog over the central and east-ern United States results when moist air from theGulf of Mexico spreads northward over cold groundas shown in figure 119. The fog may extend as farnorth as the Great Lakes. Water areas in northernlatitudes have frequent dense sea fog in summer asa result of warm, moist, tropical air flowing north-ward over colder Arctic waters.

A pilot will notice little difference between flyingover advection fog and over radiation fog exceptthat skies may be cloudy above the advection fog.Also, advection fog is usually more extensive andmuch more persistent than radiation fog. Advec-tion fog can move in rapidly regardless of the timeof day or night.

UPSLOPE FOGUps lope fog forms as a result of moist, stable air

being cooled adiabatically as it moves up slopingterrain. Once the upslope wind ceases, the fog dis-sipates. Unlike radiation fog, it can form undercloudy skies. Ups lope fog is common along the

Flame 118. Advection fog off the coast of California.

13V127

Fromm 119. Advection fog over the southeastern United States and Gulf Coast The fog often may spread to the GreatLakes and northern Appalachians.

eastern slopes of the Rockies and somewhat lessfrequent east of the Appalachians. Upsiope fogoften is quite dense and extends to high altitudes.

PRECIPITATION-INDUCED FOGWhen relatively warm rain or drizzle falls through

cool air, evaporation from the precipitation sat-urates the cool air and forms fog. Precipitation-induced fog can become quite dense and continuefor an extended period of time. This fog may ex-tend over large areas, completely suspending airoperations. It is most commonly associated withwarm fronts, but can occur with slow moving coldfronts and with stationary fronts.

Fog induced by precipitation is in itself hazard-

ous as is any fog. It is especially critical, however,because it occurs in the proximity of precipitationand other possible hazards such as icing, turbulence,and thunderstorms.

ICE FOGIce fog occurs in cold weather when the tem-

perature is much below freezing and water vaporsublimates directly as ice crystals. Conditions favor-able for its formation are the same as for radiationfog except for cold temperature, usually 25° For colder, It occurs mostly in the Arctic regions, butis not unknown in middle latitudes during the coldseason. Ice fog can be quite blinding to someoneflying into the sun.

LOW STRATUS CLOUDS

Stratus clouds, like fog, are composed of ex-tremely small water droplets or ice crystals sus-pended in air. An observer on a mountain in astratus layer would call it fog. Stratus and fog fre-quently exist together. In many cases there is noreal line of distinction between the fog and stratus;rather, one gradually merges into the other. Flight

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visibility may approach zero in stratus clouds. Stra-tus tends to be lowest during night and early morn-ing, lifting or dissipating due to solar heating duringthe late morning or afternoon. Low stratus cloudsoften occur when moist air mixes with a colder airmass or in any situation where temperaturedewpoint spread is small.

4

HAZE AND SMOKE

Haze is a concentration of salt particles or otherdry particles not readily classified as dust or otherphenomenon. It occurs in stable air, is usually onlya few thousand feet thick, but sometimes may ex-tend as high as 15,000 feet. Haze layers often havedefinite tops above which horizontal visibility is

good. However, downward visibility from above ahaze layer is poor, especially on a slant. Visibility,in haze varies greatly depending upon whether thepilot is facing the sun. Landing an aircraft into thesun is often hazardous if haze is present.

Smoke concentrations form primarily in indus-trial areas when air is stable. It is most prevalent atnight or early morning under a temperature inver-sion but it can persist throughout the day. Figure120 illustrates smoke trapped under a temperatureinversion.

When skies are clear above haze or smoke, vis-ibility generally improves during the day; however,the improvement is slower than the clearing of fog.Fog evaporates, but 'haze or smoke must be dis-persed by movement of air. Haze or smoke may beblown away; or heating during the day may causeconvective mixing spreading the smoke or haze toa higher altitude, decreasing the concentration nearthe surface. At night or early morning, radiationfog or stratus clouds often combine with haze orsmoke. The fog and stratus may clear rather rap-idly during the day but the haze and smoke willlinger. A heavy cloud cover above haze or smokemay block sunlight preventing dissipation; visibilitywill improve little, if any, during the day.

BLOWING RESTRICTIONS TO VISIBILITY

Strong wind lifts blowing dust in both stable andunstable air. When air is unstable, dust is lifted togreat heights (as much as 15,000 feet) and may bespread over wide areas by upper winds. Visibility isrestricted both at the surface and aloft. When air isstable, dust does not extend to as great a height asin unstable air and usually is not as widespread.

Dust, once airborne, may remain suspended andrestrict visibility for several hours after the windsubsides. Figure 121 is a photograph of a duststorm moving in with an approaching cold front.

Blowing sand is more local than blowing dust;the sand is seldom lifted above 50 feet. However,visibilities within it may be near zero. Blowing sand

::.

Flamm 120. Smoke trapped in stagnant air under an inversion.

139129

may occur in any dry area where loose sand is ex-posed to strong wind.

Blowing snow can be troublesome. Visibility atground level often will be near zero and the skymay become obscured when the particles are raisedto great heights.

Fount 121. Aerial photograph of blowing dust approach-ing with a cold front. The dust cloud outlines the leadingsurface of the advancing cold air.

PRECIPITATION

Rain, drizzle, and snow are the forms of precip-itation which most commonly present ceiling and/orvisibility problems. Drizzle or snow restricts vis-ibility to a greater degree than rain. Drizzle falls instable air and, therefore, often accompanies fog,haze, or smoke, frequently resulting in extremelypoor visibility. Visibility may be reduced to zero in

heavy snow. Rain seldom reduces surface visibilitybelow 1 mile except in brief, heavy showers, butrain does limitcockpit visibility. When rain streamsover the aircraft windshield, freezes on it, or fogsover the inside surface, the pilot's visibility to theoutside is greatly reduced.

OBSCURED OR PARTIALLY OBSCURED SKY

To be classified as obscuring phenomena, smoke,haze, fog, precipitation, or other visibility restrict-ing phenomena must extend upward from the sur-face. When the sky is totally hidden by the surfacebased phenomena, the ceiling is the vertical visibil-ity from the ground upward into the obscuration.If clouds or part of the sky can be seen above theobscuring phenomena, the condition is defined asa partial obscuration; a partial obscuration doesnot define a ceiling. However, a cloud layer abovea partial obscuration may constitute a ceiling.

An obscured ceiling differs from a cloud ceiling.With a cloud ceiling you normally can see theground and runway once you descend below thecloud base. However, with an obscured ceiling,

the obscuring phenomena restricts visibility betweenyour altitude and the ground, and you have re-stricted slant visibility. Thus, you cannot alwaysclearly see the runway or approach lights even afterpenetrating the level of the obscuration ceiling asshown in figure 122.

Partial obscurations also present a visibility prob-lem for the pilot approaching to land but usuallyto a lesser degree than the total obscuration. How-ever, be especially aware of erratic visibility reduc-tion in the partial obscuration. Visibility along therunway or on the approach can instantaneously be-come zero. This abrupt and unexpected reductionin visibility can be extremely hazardous especiallyon touchdown.

IN CLOSING

In your preflight preparation, be aware of oralert for phenomena that may produce IVR ormarginal VFR (flight conditions. Current chartsand special analyie's along with forecast and prog-nostic charts are your best sources of information.

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You may get your preflight weather from a briefer;or, you may rely on recorded briefings; and youalways have your own inflight observations. Noweather observation is more current or more accu-rate than the one you make through your cockpit

-1'

EFFECTIVE

PILOTCEILING

500'

REPORTEDCEILING RV

GOOD SLANT RANGEVISIBILITY A

...etuator

-4111Lemeihicieisdrall.r....4Zw.tipmobtgr/LAtall

FIGURE 122. Difference between the ceiling caused by a surface-based obscuration (B) and the ceiling caused by a layeraloft (A). When visibility is not restricted, slant range vision is good upon breaking out of the base of a layer aloft.

window. In any event, your understanding of IFRproducers will help you make better preflight andinflight decisions.

Do not fly VFR in weather suitable only forIFR. If you do, you endanger not only your ownlife but the lives of others both in the air andon the ground. Remember, the single cause of thegreatest number of general aviation fatal accidentsis "continued VFR into adverse weather." Themost common cause is vertigo, but you also run therisk of flying into unseen obstructions. Furthermore,pilots who attempt to fly VFR under conditions be-low VFR minimums are violating Federal AviationRegulations.

The threat of flying VFR into adverse weatheris far greater than many pilots might realize.A pilot may press onward into lowering ceiling andvisibility complacent in thinking that better weatherstill lies behind him. Eventually, conditions are toolow to proceed; he no longer can see a horizonahead. But when he attempts to turn around, hefinds so little difference in conditions that he cannot

re-establish a visual horizon. He continued too farinto adverse weather; he is a prime candidate forvertigo.

Don't let an overwhelming desire to reach yourdestination entice you into taking the chance offlying too far into adverse weather. The IFR pilotmay think it easier to "sneak" through rather thango through the rigors of getting an IFR clearance.The VFR pilot may think, "if I can only make ita little farther." If you can go IFR, get a clearancebefore you lose your horizon. If you must. stayVFR, do a 180 while you still have a horizon. The180 is not the maneuver of cowards. Any pilotknows how to make a 180; a good pilot knowswhen.

Be especially alert for development of t

1. Fog the following morning when at dusktemperaturedew point spread is 15° F orless, skies are clear, and winds are light.

2. Fog when moist air is flowing from a rel-atively warm surface to a colder surface.

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3. Fog when temperature-dew point spread is5° F or less and decreasing.

4. Fog or low stratus when a moderate orstronger moist wind is blowing over an ex-tended upslope. (Temperature and dewpoint converge at about 4° F for every1,000 feet the air is lifted.)

5. Steam fog when air is blowing from a coldsurface (either land or water) over warmerwater.

6. Fog when rain or drizzle falls through coolair. This is especially prevalent during win-ter ahead of a warm front and behind astationary front or stagnating cold front.

7.' Low stratus clouds whenever there is an in-flux of low level moisture overriding a shal-low cold air mass.

8. Low visibilities from haze and smoke whena high pressure area stagnates over an in-dustrial area.

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9. Low visibilities due to blowing dust or sandover semiarid or arid regions when windsare strong and the atmosphere is unstable.This is especially prevalent in spring. If thedust extends upward to moderate or greaterheights, it can be carried many miles be-yond its source.

10. Low visibility due to snow or drizzle.11. An undercast when you must make a VFIZ.

desr:ent.Expect little if any improvement in visibility

when:1. Fog exists below heavily overcast skies.2. Fog occurs with rain or drizzle and precip-

itation is forecast to continue.3. Dust extends to high levels and no frontal

passage or precipitation is forecast.4. Smoke or haze exists under heavily overcast

skies.5. A stationary high persists over industrial

areas.

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la

Part TWO

OVER. AND BEYOND

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i

Chapter 13HIGH ALTITUDE WEATHER

Many general aviation as well as air carrier andmilitary aircraft routinely fly the upper troposphereand lower stratosphere. Weather phenomena ofthese higher altitudes include the tropopause, thejet stream, cirrus clouds, clear air turbulence, con-

141

densation trails, high altitude "haze" layers, andcanopy static. This chapter explains these phenom-ena along with the high altitude aspects of themore common icing and thunderstorm hazards.

135

THE TROPOPAUSE

Why is the high altitude pilot interested in thetropopause? Temperature and wind vary greatly inthe vicinity of the tropopause affecting efficiency,comfort, and safety of flight. Maximum winds gen-erally occur at levels near the tropopause. Thesestrong winds create narrow zones of wind shearwhich often generate hazardous turbulence. Pre-flight knowledge of temperature, wind, and windshear is important to flight planning.

In chapter 1, we learned that the tropopause isa thin layer forming the boundary between thetroposphere and stratosphere. Height of the tropo.pause varies from about 65,000 feet over the Equa-for to 20,000 feet or, lower over the poles. The

tropopause is not continuous but generally descendsstep-wise from the Equator to the poles. Thesesteps occur as "breaks." Figure 123 is a crosssection of the troposphere and lower stratosphereshowing the tropopause and associated features.Note the break between the tropical and the polartropopauses.

An abrupt change in temperature lapse ratecharacterizes the tropopause. Note in figure 123how temperature above the tropical tropopause in-creases with height and how over the polar tropo-pause, temperature remains almost constant withheight.

450

60.000'

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Rolm 123. A cross section of the upper troposphere and lower stratosphere showing the tropopause and associated fea-tures. Note the "break" between the high tropical and the lower polar tropopause. Maximum winds occur in the vicinityof this break.

THE JET STREAM

Diagrammed in figure 124, the jet stream is anarrow, shallow, meandering river of maximumwinds extending around the globe in a wavelikepattern. A second jet stream is not uncommon, andthree at one time are not unknown. A jet may beas far south as the northern Tropics. A jet in mid-latitudes generally is stronger than one in or near

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1 4 t)

the Tropics. The jet stream typically occurs in abreak in the tropopause as shown in figure 123.Therefore, a jet stream occurs in an area of inten-sified temperature gradients characteristic of thebreak.

The concentrated winds, by arbitrary definition,must be 50 knots or greater to classify as a jet

ALA.

e 4ANA A"4414'

Ftovaz 124. Artist's concept of the jet stream. Broad arrow shows direction of wind.

stream. The jet maximum is not constant; rather,it is broken into segments, shaped something likea boomerang as diagrammed in figure 125.

Jet stream segments move with pressure ridgesand troughs in the upper atmosphere. In generalthey travel faster than pressure systems, and max-

imum wind speed varies as the segments progressthrough the systems. In midlatitude, wind speed inthe jet stream averages considerably stronger inwinter than in summer. Also the jet shifts farthersouth in winter than in summer.

1000 - 3000 mi

3000 7000 ft

A

100. 400 rni

Fianna 125. A jet stream segment.

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14

In figure 123 note how wind speed decreases out-ward from the jet core. Note also that the rate ofdecrease of wind speed is considerably greater onthe polar side than on the equatorial side; hence,the magnitude of wind shear is greater on the polarside than on the equatorial side.

Figure 126 shows a map with two jet streams.The paths of the jets approximately conform to theshape of the contours. The northerly jet has threesegments of maximum wind, and the southerly onehas two. Note how spacing of the height contoursis closer and wind speeds higher in the vicinity ofthe jets than outward on either side. Thus hori-

zontal wind shear is evident on both sides of the jetand is greatest near the maximum wind segments.

Strong, long-trajectory jet streams usually areassociated with well-developed surface lows andfrontal systems beneath deep upper troughs or lows.Cyclogenesis is usually south of the jet stream andmoves nearer as the low deepens. The occludinglow moves north of the jet, and the jet crosses thefrontal system near the point of occlusion. Figure127 diagrams mean jet positions relative to surfacesystems. These long jets mark high level boundariesbetween warm and cold air and are favored placesfor cirriform cloudiness.

houaz 126. Multiple jet streams. Note the "segments" of maximum winds embedded in the general pattern. Turbulenceusually is greatest on the polar sides of these maxima.

FIGURE 127. Mean jet positions relative to surface systems. Cyclogenesis (development) of a surface low usually is south ofthe jet as shown on the left. The deepening low moves nearer the jet, center. As it occludes, the low moves north of thejet, right; the jet crosses the frontal system near the point of occlusion.

CIRRUS CLOUDS

Air travels in a "corkscrew" path around the jetcore with upward motion on the equatorial side.Therefore, when high level moisture is available,cirriform clouds form on the equatorial side of thejet. Jet stream cloudiness can form independentlyof well-defined pressure systems. Such cloudinessranges primarily from scattered to broken coveragein shallow layers or streaks. Their sometimes fishhook and streamlined, wind-swept appearance al-ways indicates very strong upper wind usually quitefar from developing or intense weather systems.

The most dense cirriform clouds occur with well-defined systems. They appear in broad bands.Cloudiness is rather dense in an upper trough,thickens downstream, and becomes most dense atthe crest of the downwind ridge. The clouds taperoff after passing the ridge crest into the area of de-scending air. The poleward boundary of the cirrusband often is quite abrupt and frequently casts ashadow on lower clouds, especially in an occludedfrontal system. Figure 128a is a satellite photographshowing a cirrus band casting a shadow on lower

clouds. Figure 128b is an infrared photo of thesame system; the light shade of the cirrus band in-dicates cold temperatures while warmer low cloudsare the darker shades.

The upper limit of dense, banded cirrus is nearthe tropopause; a band may be either a single layeror multiple layers 10,000 to 12,000 feet thick.Dense, jet stream cirriform cloudiness is most prev-alent along midlatitude and polar jets. However, acirrus band usually forms along the subtropical jetin winter when a deep upper trough plunges south-ward into the Tropics.

Cirrus clouds, in themselves, have little effect onaircraft. However, dense, continuous coverage re-quires a pilot's constant reference to instruments;most pilots find this more tiring than flying with avisual horizon even though IFR.

A more important aspect of the jet stream cirrusshield is its association with turbulence. Extensivecirrus cloudiness often occurs with deepening sur-face and upper lows; and these deepening systemsproduce the greatest turbulence.

148 139

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Fromm 128a. Satellite photograph of an occluded system centered at about 44° N and 137° W. Here, the jet extendssouth-southwest to north-northeast along the polar (more westerly) boundary of the cirrus band from 35° N, 141° Wthrough 43° N, 135' W to 51° N, 130° W. Shadow of the cirrus band is clearly evident as a narrow dark line from 45° N,134,5° W to 49° N, 132° W.

140

149

Ronan *128b. Infrared photograph of the system shown in figure 128a. The warmer the radiating surface, the darker theshade; the cold cirrus appears nearly white. Infrared dearly distinguishes the banded jet stream cirrus from other cirrusand lower clouds.

141

150

CLEAR AIR TURBULENCE

Clear air turbulence (CAT) implies turbulencedevoid of clouds. However, we -commonly reservethe term for high level wind shear turbulence, evenwhen in cirrus clouds.

Cold outbreaks colliding with warm air from thesouth intensify weather systems in the vicinity ofthe jet stream along the boundary between thecold and warm air. CAT develops in the turbulentenergy exchange between the contrasting air masses.Cold and warm advection along with strong windshears develop near the jet stream, especially wherecurvature of the jet stream sharply increases indeepening upper troughs. CAT is most pronouncedin winter when temperature contrast is greatest be-tween cold and warm air.

A preferred location of CAT is in an uppertrough on the cold (polar) side of the jet stream.Another frequent CAT location, shown in figure129, is along the jet stream north and northeast ofa rapidly deepening surface low.

Even in the absence of a. '.vell-defined jet stream,

CAT often is experienced in wind shears associatedwith sharply curved contours of strong lows, troughs,and ridges aloft, and in areas of strong, cold orwarm air advection. Also mountain waves can cre-ate CAT. Mountain wave CAT may extend fromthe mountain crests to as high as 5,000 feet abovethe tropopause, and can range 100 miles or moredownstream from the mountains.

CAT can be encountered where there seems tobe no reason for its occurrence. Strong winds maycarry a turbulent volume of air away from its sourceregion. Turbulence intensity diminishes down-stream, but some turbulence still may be encoun-tered where it normally would not be expected.CAT forecast areas are sometimes elongated to in-dicate probable turbulence drifting.downwind fromthe main source region.

A forecast of turbulence specifies a volume ofairspace which is quite small when compared to thetotal volume of airspace used by aviation, but isrelatively large compared to the localized extent of

FIGURE 129. A frequent CAT location is along the jet stream north and northeast of a rapidly deepening surface low.

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151

the hazard. Since turbulence in the forecast vol-ume is patchy, you can expect to encounter it onlyintermittently and possibly not at all. A flightthrough forecast turbulence, on the average, en-counters only light and annoying turbulence 10to 15 percent of the time; about 2 to 3 percent ofthe time there is a need to have all objects secured;the pilot experiences control problems only abouttwo-tenths of 1 percent of the timeodds of thisgenuinely hazardous turbulence are about 1 in 500.

Look again at figure 126. Where are the mostprobable areas of CAT? Turbulence would begreatest near the windspeed maxima, usually onthe polar sides where there is a combination ofstrong wind shear, curvature in the flow, and coldair advection. These areas would be to the north-west of Vancouver Island, from north of the GreatLakes to east of James Bay and over the Atlanticeast of Newfoundland. Also, turbulence in the formof mountain waves is probable in the vicinity ofthe jet stream from southern California across theRockies into the Central Plains.

In flight planning, use upper air charts and fore-casts to locate the jet stream, wind shears, and areasof most probable turbulence. AVIATION WEATHERSERVICES (AC 00-45) explains in detail how to

obtain these parameters. If impractical to avoidcompletely an area of forecast turbulence, proceedwith caution. You will do well to avoid areas wherevertical shear exceeds 6 knots per 1,000 feet or hor-izontal shear exceeds 40 knots per 150 miles.

What can you do if you get into CAT rougherthan you care to fly? If near the jet core, you couldclimb or descend a few thousand feet or youcould move farther from the jet core. If caught inCAT not associated with the jet stream, your bestbet is to change altitude since you have no positiveway of knowing in which direction the strongestshear lies. Pilot reports from other flights, whenavailable, are helpful.

Flight maneuvers increase stresses on the aircraftas does turbulence. The increased stresses are cu-mulative when the aircraft maneuvers in turbb-lence. Maneuver gently when in turbulence tominimize stress. The patchy nature of CAT makescurrent pilot reports extremely helpful to observers,briefers, forecasters, air traffic controllers, and,most important, to your fellow pilots. Always, if atall possible, make inflight weather reports of CATor other turbulence encounters; negative reportsalso help when no CAT is experienced where itnormally might be expected.

CONDENSATION! TRAILS

A condensation trail, popularly contracted to"contrail," is generally defined as a cloud-likestreamer which frequently is generated in the wakeof aircraft flying in clear, cold, humid air, figure130. Two distinct types are observedexhaust trailsand aerodynamic trails. "Distrails," contracted fromdissipation trails, are produced differently from ex-haust and aerodynamic trails.

EXHAUST CONTRAILSThe exhaust contrail is formed by the addition

to the atmosphere of sufficient water vapor fromaircraft exhaust gases to cause saturation or super-saturation of the air. Since heat is also added to theatmosphere in the wake of an aircraft, the additionof water vapor must be of such magnitude that itsaturates or supersaturates the atmosphere in spiteof the added heat. There is evidence to support theidea that the nuclei which are necessary for con-densation or sublimation may also be donated tothe atmosphere in the exhaust gases of aircraftengines, further aiding contrail formation. These

nuclei are relatively large. Recent experiments,however, have revealed that visible exhaust con-trails may be prevented by adding very minutenuclei material (dust, for example) to the exhaust.Condensation and sublimation on these smaller nu-clei result in contrail particles too small to be visible.

AERODYNAMIC CONTRAILSIn air that is almost saturated, aerodynamic

pressure reduction around airfoils, engine nacelles,and propellers cools the air to saturation leavingcondensation trails from these components. Thistype of trail usually is neither as dense nor as per-sistent as exhaust trails. However, under criticalatmospheric conditions, an aerodynamic contrailmay trigger the formation and spreading of a deckof cirrus clouds.

Contrails create one problem unique to militaryoperations in that they reveal the location of anaircraft attempting to fly undetected. A more gen-eral operational problem is a cirrus layer some-times induced by the contrail. The induced layer

152143

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FiouRE 130. "'outtalk The thin contrail is freshly formed by an aircraft (not visible) in the lower rightcenter of the photograph.

may make necessary the strict use of instrumentsby a subsequent flight at that altitude.

DISSIPATION TRAILS ID1STRAILS)The term dissipation trail applies to a rift in

clouds caused by the heat of exhaust gases from an

aircraft flying in a thin cloud layer. The exhaustgases sometimes warm the air to the extent that itis no longer saturated, and the affected part of thecloud evaporates. The cloud must be both thin andrelatively warm for a distrail to exist; therefore,they are not common.

HAZE LAYERS

Haze layers not visible from the ground are, attimes, of concern at high altitude. These layers arereally cirrus clouds with a very low density of icecrystals. Tops of these layers generally are verydefinite and are at the tropopause. High level hazeoccurs in stagnant air; it is rare in fresh outbreaksof cold polar air. Cirrus haze is common in Arctic

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winter. Sometimes ice crystals restrict visibilityfrom the surface to the tropopause.

Visibility in the haze sometimes may be nearzero, especially when one is facing the sun. Toavoid the poor visibility, climb into the lower strato-sphere or descend below the haze. This change maybe several thousand feet.

CANOPY STATIC

Canopy static, similar to the precipitation staticsometimes encountered at lower levels, is producedby particles brushing against plastic-covered air-craft surfaces. The discharge of static electricityresults in a noisy disturbance that interferes with

radio reception. Discharges can occur in such rapidsuccession that interference seems to be continuous.Since dust and ice crystals in cirrus clouds are theprimary producers of canopy static, usually youmay eliminate it by changing altitude.

ICING

Although icing at high altitude is not as commou,,,,r#

or extreme as at low altitudes, it can occur. It canform quiCkli on airfoils and exposed parts of jetengines. Structural icing at high altitudes usuallyis rime, although clear ice is possible.

High altitude icing generally forms in tops oftall cumulus buildup; anvils, and even in detachedcirrus. Clouds over mountains are more likely tocontain liquid water than those over more gentlysloping terrain because of the added lift of the

mountains. Therefore, icing is more likely to occurand to be more hazardous over mountainous areas.

Because ice generally accumulates slowly at highaltitudes, anti-icing equipment usually eliminatesany serious problems. However, anti-icing systemscurrently in use are not always adequate. If such isthe case, avoid the icing problem by changing alti-tude or by varying course to remain clear of theclouds. Chapter 10 discusses aircraft icing in moredetail.

THUNDERSTORMS

A well-developed thunderstorm may extend up-ward through the troposphere and penetrate thelower stratosphere. Sometimes the main updraft ina thunderstorm may toss hail out the top or theupper portions of the storm. An aircraft may en-counter hail in clear air at a considerable distancefrom the thunderstorm, especially under the anvilcloud. Turbulence may be encountered in clearair for a considerable distance both above andaround a growing thunderstorm.

Thunderstorm avoidance rules given in chapter11 apply equally at high altitude. When flying inthe clear, visually avoid all thunderstorm tops. In asevere thunderstorm situation, avoid tops by atleast 20 miles. When you are on instruments,weather avoidance radar assures you of avoidingthunderstorm hazards. If in an area of severe thun-derstorms, avoid the most intense echoes by at least20 miles. Most air carriers now use this distance asthe minimum for thunderstorm avoidance.

75.1 145'

Chapter 14ARCTIC WEATHER

The Arctic, strictly speaking, is the region shownin figure 131 which lies north of the Arctic Circle(661/2° latitude). However, this chapter includesAlaskan weather even though much of Alaska liessouth of the Arctic Circle.

Because of the lack of roads over most Arcticareas, aviation is the backbone of transportationbetween communities. As the economy expands, sowill air transportation.

Your most valuable source of information con-cerning flying the Arctic is the experienced Arcticflyer. To introduce you to Arctic flying weather,this chapter surveys climate, air masses, and frontsof the Arctic; introduces you to some Arctic weatherpeculiarities; discusses weather hazards in the Arc-tic; and comments on Arctic flying.

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Fmuaa 131. The Arctic. The Aictic Circle is at 66H° N latitude.

CLIMATE, AIR MASSES, AND FRONTS

Climate of any region is largely determined bythe amount of energy received from the sun; butlocal characteristics of the area also influenceclimate.

LONG DAYS AND NIGHTSA profound seasonal change in length of day and

night occurs in the Arctic because of the Earth'stilt and its revolution around the sun. Figure 132shows that any point north of the Arctic Circle has

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1 5 3

autumn and winter days when the sun stays all daybelow the horizon and days in spring and summerwith 24 hours of sunshine. The number of thesedays increases toward the North Pole; there the sunstays below the horizon for 6 months and shinescontinuously during the other 6 months.

Twilight in the Arctic is prolonged because ofthe shallow angle of the sun below the horizon. Inmore northern latitudes, it persists for days whenthe sun remains just below the horizon. This

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hot= 132. Sunshine in the Northern Hembphere. The sun shines a full 24 hours on the entire area north of the ArcticCircle (top) on June 21; the amount of sunshine decreases until none falls anywhere in the area on December 22. Graphs(below) show duration of sunshine and naudcal twilight per day at two points north of the Arctic Circle and for Anchorage,Alaska, at a latitude about' 54° south of the circle.

157149

abundance of twilight often makes visual referencepossible at night.

LAND AND WATERFigure 131 shows the water and land distribution

in the Arctic. Arctic mountain ranges are effectivebarriers to air movement. Large masses of air stag-nate over the inland continental areas. Thus, theArctic continental areas are air mass source regions.

A large portion of the Arctic Ocean is coveredthroughout the year by a deep layer of icethepermanent ice pack as shown in figure 133. Eventhough the ocean is ice-covered through much of

the year, the ice and the water below contain moreheat than the surrounding cold land, thus moderat-ing the climate to some extent. Oceanic and coastalareas have a milder climate during winter thanwould be expected and a cool climate in summer.As opposed to large water bodies, large land areasshow a more significant seasonal temperaturevariation.

TEMPERATURE

As one would expect, the Arctic is very cold inwinter; but due to local terrain and the movementof pressure systems, occasionally some areas are sur-

150

haulm 133. The permanent Arctic ice pack.

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41%

prisingly warm. During winter, coastal areas aver-age about 20 degrees warmer than the interior.During summer, interior areas are pleasantly warmwith many hours of sunshine. Coastal areas haverelatively cool short summers due to their proximityto water.

CLOUDS AND PRECIPITATIONCloudiness over the Arctic is at a minimum dur-

ing winter reaching a maximum in summer andfall, figure 134. Spring also brings many cloudy

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days. During summer afternoons, scattered cumulusclouds forming over the interior occasionally growinto thundershowers. These thundershowers, usuallycircumnavigable, move generally from northeast tosouthwest in the polar easterlies which is oppositethe general movement in midlatitudes.

Precipitation in the Arctic is generally light. An-nual amounts over the ice pack and along thecoastal areas are only 3 to 7 inches. The interior issomewhat wetter, with annual amounts of 5 to 15inches. Precipitation falls mostly in the form of

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FIGURE 134. Average number of cloudy days per month. Note that most stations show the greatest number of cloudy daysin the warmer season.

159 .

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snow over ice caps and oceanic areas and mostly assummer rain over interior areas.

WINDStrong winds occur more often along the coasts

than elsewhere. The frequenCy of high winds incoastal areas is greatest in fall and winter. Windspeeds are generally light in the continental interiorduring the entire year, but are normally at theirstrongest during summer and fall.

AIR MASSESWINTERIn winter, air masses form over the expanded ice

pack and adjoining snow-covered land areas. Theseair masses are characterized by very cold surfaceair, very low humidity, and strong low-level tem-perature inversions. Occasionally, air from unfrozenocean areas flows northward over the Arctic. Theseintrusions of moist, cold air account for most of the

infrequent wintertime cloudiness and precipitationin the Arctic.

AIR MASSESSUMMERDuring the summer, the top layer of the Arctic

permafrost layer melts leaving very moist ground,and the open watet areas of the Polar Basin in-crease markedly. Thus, the entire area becomesmore humid, relatively mild, and semimaritime incharacter. The largest amount of cloudiness andprecipitation occurs inland during the summermonths.

FRONTS

Occluded fronts are the rule. Weather conditionswith occluded fronts are much the same in theArctic as elsewherelow clouds, precipitation, poorvisibility, and sudden fog formation. Fronts aremuch more frequent over coastal areas than overthe interior. ,

ARCTIC PECULIARITIES

Several Arctic phenomena are peculiar to thatregion. At times, they have a direct bearing onArctic flying.

EFFECTS OF TEMPERATURE INVERSIONThe intense low-level inversion over the Arctic

during much of the winter causes soundincludingpeople's voicesto carry over extremely long dis-tances. Light rays are bent as they pass at low an-gles through the inversion. This bending creates aneffect known as loominga form of mirage thatcauses objects beyond the horizon to appear abovethe horizon. Mirages distorting the shape of thesun, moon, and other objects are common withthese low level inversions.

AURORA BOREALISIn theory, certain energy particles from the sun

strike the Earth's magnetic field and are carriedalong the lines of force where they tend to lowerand converge near the geomagnetic poles. The en-ergy particles then pass through rarefied gases ofthe outer atmosphere, illuminating them in muchthe same way as an electrical charge illuminatesneon gas in neon signs.

The Aurora Borealis takes place at high altitudesibii_ove the Earth's surface and thus has been ob-served as far south as Florida. However, the highest

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frequency of observations is over the northernUnited States and northward. Displays of auroravary from a faint glow to an illumination of the-Earth's surface equal to a full moon. They fre-quently change shape and form and are also calleddancing lights or northern lights.

LIGHT REFLECTION BYSNOW-COVERED SURFACES

Much more light is reflected by 'snow- coveredsurfaces than by darker surfaces. Snow often re-flects Arctic sunlight sufficiently to blot out shad-ows, thus markedly decreasing the contrast betweenobjects. Dark distant mountains may be easily rec-ognized, but a crevasse normally directly in viewmay be undetected due to lack of contrasts.

LIGHT FROM CELESTIAL BODIESIllumination from the moon and stars is much

more intense in the Arctic than in lower latitudes.Pilots have found that light from a half-moon overa snow-covered field may be sufficient for landing.Even illumination from the stars creates visibilityfar beyond that found elsewhere. Only under heavyovercast skies does the night darkness in the Arcticbegin to' approach the degree of darkness in, lowerlatitudes.

WEATHER HAZARDS

Weather hazards include visibility restricting phe-nomena, blowing snow, icing, frost, and lack ofcontrastwhiteout.

FOGFog limits landing and takeoff in the Arctic more

than any other visibility restriction. Water-dropletfog is the main hazard to aircraft operations incoastal areas during the summer. Ice fog is themajor restriction in winter.

Ice FogIce fog is common in the Arctic. It forms in

moist air during extremely cold, calm conditions inwinter, occurring often and tending to persist. Ef-fective visibility is reduced much more in ice fogwhen one is looking toward the sun. Ice fog maybe produced both naturally and artificially. Ice fogaffecting aviation operations most frequently is pro-duced by the combustion of aircraft fuel in coldair. When the wind is very light and the temper-ature is about 30° F or colder, ice fog oftenforms instantaneously in the exhaust gases of auto-mobiles and aircraft. It lasts from as little as a fewminutes to days.

Steam FogSteam fog, often called "sea smoke," forms in

winter when cold, dry air passes from land areasover comparatively warm ocean waters. Moistureevaporates rapidly from the water surface; butsince the cold air can hold only a small amount ofwater vapor, condensation takes place just abovethe surface of the water and appears as "steam"rising from the ocean. This fog is composed entirelyof water droplets that often freeze quickly and fallback into the water as ice particles. Low level tur-bulence can occur and icing can become hazardous.

Advection FogAdvection fog, which may be composed either of

water droplets or of ice crystals, is most common inwinter and is often persistent. Advection fog formsalong coastal areas when comparatively warm,moist, oceanic air moves over cold land. If the landareas are hilly or mountainous, lifting of the airresults in a combination of low stratus and fog. Thestratus and fog quickly diminish inland. Lee sidesof islands and mountains usually are free of advec-tion fog because of drying due to compressional

heating as the air descends downslope. Icing in ad-vection fog is in the form of rime and may becomequite severe.

BLOWING SNOWOver the frozen Arctic Ocean and along the

coastal areas, blowing snow and strong winds arecommon hazards during autumn and winter. Blow-ing snow is a greater hazard to flying operations inthe Arctic than in midlatitudes because the snow is"dry" and fine and can be picked up easily by lightwinds. Winds in excess of 8 knots may raise thesnow several feet off the ground obliterating ob-jects such as runway markers as illustrated in figure135. A sudden increase in surface wind may causean unlimited visibility to drop to near zero in a fewminutes. This sudden loss of visibility occurs frejquently without warning in the Arctic. Strongerwinds sometimes lift blowing snow to heights above1,000 feet and produce drifts over 30 feet deep.

ICINGIcing is most likely in spring and fall, but is also

encountered in winter. During spring and fall, icingmay extend to upper levels along frontal zones.While icing is mostly a problem over water andcoastal areas, it does exist inland. It occurs typicallyas rime, but a combination of clear and rime is notunusual in coastal mountains.

FROST

In coastal areas during spring, fall, and winter,heavy frost and rime may form on aircraft parkedoutside, especially when fog or ice fog is present.This frost should be removed; it reduces lift and isespecially hazardous if surrounding terrain requiresa rapid rate of climb.

WHITEOUT"Whiteout" is a visibility restricting phenomenon

that occurs in the Arctic when a layer of cloudinessof uniform thickness overlies a snow or ice-coveredsurface. Parallel rays of the sun are broken up anddiffused when passing through the cloud layer sothat they strike the snow surface from many angles.The diffused light then reflects back- and forthcountless times between the snow and the cloudeliminating all shadows. The result is a loss ofdepth perception. Buildings, people, and dark-colored objects appear to float in the air, and the

161153

holm 135. Visibility reduced by blowing snow. Common in Arctic regions since wind easily picks up the dry,powder-like snow.

horizon disappears. Low level flight over icecap gerous. Disastrous accidents have occurred as aterrain or landing on snow surfaces becomes dan- result of whiteouts.

ARCTIC FLYING WEATHER

A great number of pilots who fly Alaska and theArctic are well seasoned. They are eager to be ofhelp and are your best sources of information.Alaska and the Arctic are sparsely settled with

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mostly natural landmarks to guide you as illustratedin figure 136. Before flying in the Arctic, be sure tolearn all you can about your proposed route.

Generally, flying conditions in the Arctic are

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Puma 136. A typical frozen landscape of the Arctic.

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good when averaged over the entire year; how-ever, areas of Greenland compete with the Aleu-tians for the world's worst weather. These areasare exceptions.

Whiteout; in conjunction with overcast skies,often present a serious hazard especially for visualflight. Many mountain peaks are treeless androunded rather than ragged, making them un-usually difficult to distinguish under poor visibilityconditions.

OCEANIC AND COASTAL AREASIn oceanic and coastal areas, predominant haz-

ards change with the seasons. In summer, the mainhazard is fog in coastal areas.

In winter, ice fog is the major restriction to air-craft operation. Blowing and drifting snow oftenrestrict visibility also. Storms and well-defined

frontal passages frequeng the.toastal areas accom-panied by turbulence, especially in the coastalmountains. '

Icing is most frequent in spring and fall ttn4may extend to high levels in active, turbulentfrontal zones. Fog is also a source of icing whentemperature is colder than freezing.

CONTINiNTAL ARF4SOver the continental interior, good flying weather

prevails much of the year; although during winter,ice fog often restricts aircraft operations. In termsof ceiling and visibility, the summer months pro-vide the best flying weather. However, the numberof cloudy days during the summer exceeds those inwinter. Thunderstorms develop on occasion duringthe summer, but they usually can be circumnav-igated without much interference with flight plans.

IN CLOSING

If one were to summarize general weather condi-tions and flight precautions over Alaska, northernCanada, and the Arctic, he would say:

1. Interior areas generally have good flyingweather, but coastal areas and Arctic slopesoften are plagued by low ceiling, poor vis-ibility, and icing.

2. "Whiteout" conditions over ice and snowcovered areas often cause pilot disorienta-tion.

3. Flying conditions are usually worse in moun-tain passes than at reporting stations alongthe route.

4. Routes through the mountains are subjectto strong turbulence, especially in and nearpasses. ,

5. Beware of a false mountain pass that maylead to a dead-end.

6. Thundershowers sometimes occur in the in-terior during May through August. They

are usually circumnavigable and generallymove from northeast to southwest.

7. Always file a flight plan. Stay on regularlytraversed routes, and if downed, stay withyour plane.

8. If lost during summer, fly down-drainage,that is, downstream. Most airports are lo-cated near rivers, and chances are you canreach a landing strip by flying downstream.If forced down, you will be close to wateron which a rescue plane can land. In sum-mer, the tundra is usually too soggy forlanding.

9. Weather stations are few and far between.Adverse weather between stations may goundetected unless reported by a pilot inflight. A report confirming good weatherbetween stations is also just as important.Help yourself and your fellow pilot byreporting weather en route.

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Chapter 15TROPICAL WEATHER

Technically, the Tropics lie between latitudes231/2° N and 231/2° S. However, weather typical ofthis region sometimes extends as much as 45° fromthe Equator. One may think of the Tropics as uni-formly rainy, warm, and humid. The facts are,however, that the Tropics contain both the wettest

and driest regions of the world. This chapter de-scribes the circulation basic to the Tropics, terraininfluences that determine arid and wet regions, andtransitory systems that invade or disturb the basictropical circulation.

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CIRCULATION

In chapter 4, we learned that wind blowing outof the subtropical high pressure belts toward theEquator form the northeast and southeast tradewinds of the two hemispheres. These trade windsconverge in the vicinity of the Equator where airrises. This convergence zone is the "intertropicalconvergence zone" (ITCZ). In some areas of theworld, seasonal temperature differences betweenland and water areas generate rather large circula-tion patterns that overpower the trade wind circu-lation; these areas are "monsoon" regions. Tropicalweather discussed here includes the subtropicalhigh pressure belts, the trade wind belts, the inter-tropical convergence zone, and monsoon regions.

SUBTROPICAL HIGH PRESSURE BELTSIf the surface under the subtropical high pres-

sure belts were all water of uniform temperature,the high pressure belts would be continuous highsaround the globe. The belts would be areas of de-scending or subsiding air and would be character-ized by strong temperature inversions and very littleprecipitation. However, land surfaces at the lati-tudes of the high pressure belts are generally warm-er throughout the year than are water surfaces.Thus, the high pressure belts are broken into semi-permanent high pressure anticyclones over oceanswith troughs or lows over continents as shown infigures 23 and 24, chapter 4. The subtropical highsshift southward during the Northern Hemispherewinter and northward during summer. The sea-sonal shift, the height and strength of the inversion,and terrain features determine weather in diesubtropical high pressure belts.

Continental WeatherAlong the west coasts of continents under a sub-

tropical high, the air is stable. The inversion isstrongest and lowest where the east side of an anti-cyclone overlies the west side of a continent. Mois-ture is trapped under the inversion; fog and lowstratus occur frequently. However, precipitation israre since the moist layer is shallow and the air isstable. Heavily populated areas also add contami-nants to the air which, when trapped under theinversion, create an air pollution problem.

The extreme southwestern United States, forexample, is dominated in summer by a subtropicalhigh. We are all familiar with the semi-arid sum-mer climate of southern California. Rainfall is

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infrequent but fog-is common along the coast. Con-taminants trapped along with fog under the stronginversion may persist for days creating "smog."

In winter, the subtropical high pressure beltsshift southward. Again, let's consider southernCalifornia as an example.. In winter, the areacomes under the influence of midlatitude circu-lation which increases frequency of rain. Also, anoccasional wintertime outbreak of polar air bringsclear skies with excellent visibility.

The situation on eastern continental coasts is justthe opposite. The inversion is weakest and highestwhere the west side of an anticyclone overlies theeastern coast of a continent. Convection can pene-trate the inversion, and showers and thunderstormsoften develop. Precipitation is generally sufficientto support considerable vegetation. For example, inthe United States, Atlantic coastal areas at thesame latitude as southern California are far fromarid in summer.

Low ceiling and fog often prevent landing at awest coast destination, but a suitable alternate gen-erally is available a few miles inland. Alternateselection may be more critical for an eastern coastdestination because of widespread instability andassociated hazards.

Weather over Open SeaUnder a subtropical high over the open sea,

cloudiness is scant. The few clouds that do develophave tops from 3,000 to 6,000 feet depending onheight of the inversion. Ceiling and visibility aregenerally quite ample for VFR flight.

Island WeatherAn island under a subtropical high receives very

little rainfall because of the persistent temperatureinversion. Surface heating over some larger islandscauses light convective showers. Cloud tops areonly slightly higher than over open water. Temper-atures are mild, showing small seasonal and diurnalchanges. A good example is the pleasant, balmyclimate of Bermuda.

TRADE WIND BELTSFigures 138 and 139 show prevailing winds

throughout the Tropics for July and January. Notethat trade winds blowing out of the subtropicalhighs over ocean areas are predominantly north-easterly in the Northern Hemisphere anei south-

easterly in the Southern Hemisphere. The inversionfrom the subtropical highs is carried into the tradewinds and is known as the "trade wind inversion."As in a subtropical high, the inversion is strongestwhere the trades blow away from the west coastof a continent and weakest where they blow ontoan eastern continental shore. Daily variations fromthese prevailing directions are small except duringtropical storms. As a result, weather at any specificlocation in a trade wind belt varies little from dayto day.

Weather over Open SeaIn the trade wind belt, skies over open water are

about one-half covered by clouds on the average.Tops range from 3,000 to 8,000 feet depending onheight of the inversion. Showers, although morecommon than under a subtropical high, are stilllight with comparatively little rainfall. Flyingweather generally is quite good.

Continental WeatherWhere trade winds blow offshore along the west

coasts of continents, skies are generally clear andthe area is quite arid. The Baja Peninsula of LowerCalifornia is a well-known example. Where tradewinds blow onshore on the east sides of continents,rainfall is generally abundant in showers and oc-casional thunderstorms. The east coast of Mexicois a good example. Rainfall may be carried a con-siderable distance inland where the winds are notblocked by a mountain barrier. Inland areas blockedby a mountain barrier are deserts; examples arethe Sahara Desert and the arid regions of south-western United States. Afternoon convective cur-rents are common over arid regions due to strongsurface heating. Cumulus and cumulonimbusclouds can develop, but cloud bases are high andrainfall is scant because of the low moisturecontent. '

Flying weather along eastern coasts and moun-tains is subject to the usual hazards of showers andthunderstorms. Flying over arid regions is goodmost of the time but can be turbulent in afternoonconvective currents; be especially aware of dustdevils. Blowing sand or dust sometimes restrictsvisibility.

Island WeatherMountainous islands have the most dramatic ef-

fect on trade wind weather. Since trade winds areconsistently from approximately the same direction,

they always strike the same side of the island; thisside is the windward side. The opposite side is theleeward side. Winds blowing up the windward sideproduce copious and frequent rainfall, althoughcloud tops rarely exceed 10,000 feet. Thunderstormsare rare. Downslope winds on the leeward slopesdry the air leaving relatively dear skies and muchless rainfall. Many islands in the trade wind belthave lush vegetation and even rain forests on thewindward side while the leeward is semiarid. Forexample, the island of Oahu, Hawaii, is about 24miles wide in the direction of the trade winds. An-nual rainfall averages from about 60 inches on thewindward coast to 200 inches at the mountain tops,decreasing to 10 inches on the leeward shore.

The greatest flying hazard near these islands isobscured mountain tops. Ceiling and visibility oc-casionally restrict VFR flight on the windward sidein showers. IFR weather is virtually nonexistenton leeward slopes.

Islands without mountains have little effect oncloudiness and rainfall. Afternoon surface heatingincreases convective cloudiness slightly, but showeractivity is light. However, any island in either thesubtropical high pressure belt or trade wind beltenhances cumulus development even though topsdo not reach great heights. Therefore, a cumulustop higher than the average tops of surroundingcumulus usually marks the approximate location ofan island. If it becomes necessary to "ditch" in theocean, look for a tall cumulus. If you see one, headfor it. It probably marks a land surface, increasingyour chances of survival.

THE INTERTROPICAL CONVERGENCEZONE (ITCZ)

Converging winds in the intertropical conver-gence zone (ITCZ) force air upward. The inver-sion typical of the subtropical high and trade windbelts disappears. Figures 138 and 139 show theITCZ and its seasonal shift. The ITCZ is wellmarked over tropical oceans but is weak and ill-defined over large continental areas.

Weather over.Islands and Open WaterConvection in the ITCZ Parries huge quantities

of moisture to great heights. Showers and thunder-storms frequent the ITCZ and tops to 40,000 feetor higher are common as shown in figure 137. Pre-cipitation is copious. Since convection dominatesthe ITCZ, there is little difference in weather overislands and open sea under the ITCZ.

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Flying through the ITCZ usually presents nogreat problem if one follows the usual practice ofavoiding thunderstorms. He usually can find a safecorridor between storms.

Since the ITCZ is ill-defined over continents, wewill not attempt to describe ITCZ continentalweather as such. Continental weather ranges fromarid to rain forests and is more closely related tothe monsoon than to the ITCZ.

MONSOONIf you refer again to figures 23 and 24 in chapter

4, you can see that over the large land mass ofAsia, the subtropical high pressure breaks downcompletely. Asia is covered by an intense high dur-ing the winter and a well-developed low during thesummer. You can also see the same over Australiaand central Africa, although the seasons are reversedin the Southern Hemisphere.

The cold, high pressures in winter cause wind toblow from the deep interior outward and offshore.In summer, wind direction reverses and warmmoist air is carried far inland into the low pres-sure area. This large scale seasonal wind shift isthe "monsoon." The most notable monsoon is thatof southern and southeastern Asia.

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Summer or Wet Monsoon Weather

During the summer, the low over central Asiadraws warm, moist, unstable maritime air fromthe southwest over the continent. Strong surfaceheating coupled with rising of air flowing up thehigher terrain produces extensive cloudiness, copi-ous rain, and numerous thunderstorms. Rainfall atsome stations in India exceeds 400 inches per yearwith highest amounts between June and October.

The monsoon is so pronounced that it influencescirculation many miles out over the ocean. Note infigure 138 that in summer, prevailing winds fromthe Er:stator to the south Asian coast are southerlyand southeasterly; without the monsoon influence,these areas would be dominated by northeasterlytrades. Islands within the monsoon influence receivefrequent showers.

Winter Monsoon WeatherNote in figure 139 how the winter flow has re-

versed from that shown in figure 138. Cold, dry airfrom the high plateau deep in the interior warmsadiabatically as it flows down the southern slopesof the Himalayan Mountains. Virtually no rain fallsin the interior in the dry winter monsoon, As the

Flame 138. Prevailing winds throughout the Tropics in July. Remember that in the Southern Hemisphere, circulationaround pressure centers is opposite that in the Northern Hemisphere.

Ftnuez 139. Prevailing winds In the Tropics in January.

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dry air moves off shore over warmer water, itrapidly takes in more moisture, becomes warmer inlow levels and therefore, unstable. Rain is frequentover off-shore islands and even along coastal areasafter the air has had a significant over-watertrajectory.

The Philippine Islands are in an area of specialinterest. During the summer, they are definitely insoutherly monsoon flow and are subjected to abun-dant rainfall. In the winter, wind over the Phil-ippines is northeasterlyin the transition zone be-tween the northeasterly trades and the monsoonflow. It is academic whether we call the phenom-enon the trade winds or monsoon; in either case,it produces abundant rainfall. The Philippineshave a year-round humid, tropical climate.

Other Monsoon AreasAustralia in July (Southern Hemisphere winter)

is an area of high pressure with predominantly off-shore winds as shown in figure 138. Most of thecontinent is dry during the winter. In January,figure 139, winds are onshore into the continentallow pressure. However, most of Australia is rimmedby mountains, coastal regions are wet where theonshore winds blow up the mountain slopes. Theinterior is arid where down-slope winds are warmedand dried.

Central Africa is known for its humid climate

and jungles. Note in figures 138 and 139 that pre-vailing wind is onshore much of the year overthese regions. Some regions are wet the year round;others have the seasonal monsoon shift and havea summer wet season and a winter dry season.Climate of Africa is so varied that only a detailedarea-by-area study can explain the climate typicalof each area.

In the Amazon Valley of South America duringthe Southern Hemisphere winter (July) , southeasttrades, as shown in figure 138, penetrate deep intothe valley bringing abundant rainfall which con-tributes to the jungle climate. In January, theITCZ moves south of the valley as shown in figure139. The northeast trades are caught up in themonsoon, cross the Equator, and also penetrate theAmazon Valley. The jungles of the Amazon resultlargely from monsoon winds.

Flying Weather in MonsoonsDuring the winter monsoon, excellent flying

weather prevails over dry interior regions. Overwater, one must pick his way around showers andthunderstorms. In the summer monsoon, VFRflight over land is often restricted by low ceilingsand heavy rain. Int flight must cope with thehazards of thunderstorms. Freezing level in theTropics is quite high-14,000 feet or highersoicing is restricted to high levels.

TRANSITORY SYSTEMS

So far, we have concentrated on prevailing cir-culations. Now let's turn to migrating tropicalweather producersthe shear line, trough aloft,tropical wave, and tropical cyclone.

SHEAR LINEA wind shear line found in the Tropics mainly

results from midlatitude influences. In chapter 8we stated that an air mass becomes modified whenit flows from its source region. By the time a coldair mass originating in high latitudes reaches theTropics, temperature and moisture are virtuallythe same on both sides of the front. A shear line,or wind shift, is all that remains. A shear line alsoresults when a semi-permanent high splits into twocells inducing a trough as shown in figure 140.

These shear lines are zones of convergence creat-ing forced upward motion. Consequently, consider-

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able thunderstorm and rain shower activity occursalong a shear line.

TROUGH ALOFTTroughs in the atmosphere, generally at or above

10,000 feet, move through the Tropics, especiallyalong the poleward fringes. Figure 141 shows sucha' trough across the Hawaiian Island chain. As atrough moves to the southeast or east, it spreadsmiddle and high cloudiness over extensive areas tothe east of the trough line. Occasionally, a well-developed trough will extend deep into the Tropics,and a closed low forms at the equatorial end of thetrough. The low then may separate from the troughand move westward producing a large amount ofcloudiness and precipitation. If this occurs in thevicinity of a strong subtropical jet stream, extensive

J.

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FlOURS 140. A shear line and an induced trough caused by a polar high pushing into the subtropics.

and sometimes dense cirrus and some convectiveand clear air turbulence often develop.

Troughs and lows aloft produce considerableamounts of rainfall in the Tropics, especially overland areas where mountains and surface heatinglift air to saturation. Low pressure systems aloftcontribute significantly to the record 460 inchesaverage annual rainfall on Mt. Waialeale onKauai, Hawaii. Other mountainous areas of theTropics are also among the wettest spots on earth.

TROPICAL WAVETropical waves (also called easterly waves) are

common tropical weather disturbances, normallyoccurring in the trade wind belt. In the NorthernHemisphere, they usually develop in the southeast-

em perimeter of the subtropical high pressure sys-tems. They travel from east to west around thesouthern fringes of these highs in the prevailingeasterly circulation of the Tropics. Surface windsin advance of a wave are somewhat more northerlythan the usual trade wind direction. As the waveapproaches, as shown in figure 142, pressure falls;as it passes, surface wind shifts to the east-southeastor southeast. The typical wave is preceded by verygood weather but followed by extensive cloudiness,as shown, in figure 143, and often by rain andthunderstorms. The weather activity is roughly ina north-south line.

Tropical waves occur in all seasons, but are morefrequent and stronger during summer and earlyfall. Pacific waves frequently affect Hawaii; Atlan-

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FIGURE 141. A trough aloft across the Hawaiian Islands. Extensive cloudiness develops east of the trough.

tic waves occasionally move into the Gulf of Mex-ico, reaching the U.S. coast.

TROPICAL CYCLONETropical cyclone is a general term for any low

that originates over tropical oceans. Tropical cy-clones are classified according to their intensitybased on average one-minute wind speeds. Windgusts in these storms may be as much as 50 percenthigher than the average one-minute wind speeds.Tropical cyclone international classifications are:

(1) Tropical Depressionhighest sustainedwinds up to 34 knots (64 km/h),

(2) Tropical Stormhighest sustained windsof 35 through 64 knots (65 to 119 km/h), andHurricane or Typhoonhighest sustainedwinds 65 knots (120 km/h) or more.

Strong tropical cyclones are known by differentnames in different regions of the world. A tropical

(3)

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cyclone in the Atlantic and eastern Pacific -is a"hurricane"; in the western' Pacific, "typhoon";near Australia, "willy-willy"; and in the IndianOcean, simply "cyclone." Regardless of the name,these tropical cyclones produce serious aviationhazards. Before we delve into these aspects, leeslook at the development, movement, and decay ofthese cyclones.

DevelopmentPrerequisite to tropical cyclone development are

optimum sea surface temperature under weathersystems that produce low-level convergence andcyclonic wind shear. Favored breeding grounds aretropical (easterly) waves, troughs aloft, and areasof converging northeast and southeast trade windsalong the intertropical convergence zone.

The low level convergence associated with thesesystems, by itself, will not support development ofa tropical cyclone. The system must also have

A

30°

20'

10°LOW

FIGURE 142. A Northern Hemisphere easterly wave. Progressing from (A) to (B), note that winds shift generally fromnortheasterly to southeasterly. The wave moves toward the west and is often preceded by good weather and followed byextensive cloudiness and precipitation.

300 150 0 150 300

MILES

ROME 143. Vertical cross section along line AB in figure 142.

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horizontal outflowdivergenceat high tropo-spheric levels. This combination creates a "chim-ney," in which air is forced upward causing cloudsand precipitation. Condensation releases large quan-tities of latent heat which raises the temperature ofthe system and accelerates the upward motion. Therise in temperature lowers the surface pressurewhich increases tow-level convergence. This drawsmore moisture-laden air into the system. Whenthese chain-reaction events continue, a huge vortexis generated which may culminate in hurricaheforce winds.

Figure 144 shows regions of the world wheretropical cyclones frequently develop. Notice thatthey usually originate between latitudes 5° and20°. Tropical cyclones are unlikely within 5° of theEquator because the Coriolis force is so small nearthe Equator that it will not turn the winds enoughfor them to flow around a low pressure area. Windsflow directly into an equatorial low and rapidlyfill it.

MovementTropical cyclones in the Northern Hemisphere

usually move in a direction between west and north-west while in low latitudes. As these storms movetoward the midlatitudes, they come under the in-fluence of the prevailing westerlies. At this time thestorms are under the influence of two wind systems,i.e., the trade winds at low levels and prevailingwesterlies aloft. Thus a storm may move very errat-

ically and may even reverse course, or circle. Final-ly, the prevailing westerlies gain control and thestorm recurves toward the north, then to the north-east, and finally to the east-northeast. By this timethe storm is well into midlatitudes.

DecayAs the storm curves toward the north or east, it

usually begins to lose its tropical characteristics andacquires characteristics of lows in middle latitudes.Cooler air flowing into the storm gradually weak-ens it. If the storm tracks along a coast line or overthe open sea, it gives up slowly, carrying its furyto areas far removed from the Tropics. However,if the storm moves well inland, it loses its moisturesource and weakens from starvation and increasedsurface friction, usually after leaving a trail ofdestruction and flooding.

When a storm takes on middle latitude charac-teristics, it is said to be "extratropical" meaning"outside the Tropics." Tropical cyclones produceweather conditions that differ somewhat from thoseproduced by their higher latitude cousins and inviteour investigation.

Weather in a Tropical DepressionWhile in its initial developing stage, the cyclone

is characterized by a circular area of broken toovercast clouds in multiple layers. Embedded inthese clouds are numerous showers and thunder-storms. Rain shower and thunderstorm coverage

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FIGURE 144. Principal regions where tropical cyclones form and their favored directions of movement.

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varies from scattered to almost solid. Diameter ofthe cloud pattern varies from less than 100 milesin small systems to well over 200 miles in large ones.

Weather in Tropical Storms and HurricanesAs cyclonic flow increases, the thunderstorms and

rain showers form into broken or solid lines paral-leling the wind flow that is spiraling into the centerof the storm. These lines are the spiral rain bandsfrequently seen on radar. These rain bands con-tinually change as they rotate around the storm.Rainfall in the rain bands is very heavy, reducingceiling and visibility to near zero. Winds are usual-ly very strong and gusty and, consequently, gen-erate violent turbulence. Between the rain bands,ceilings and visibilities are somewhat better, andturbulence generally is less intense.

The "eye" usually forms in the tropical stormstage and continues through the hurricane stage. Inthe eye, skies are free of turbulent cloudiness, andwind is comparatively light. The average diameterof th6 eye is between 15 and 20 miles, but some-times is as small as 7 miles and rarely is more than30 miles in diameter. Surrounding the eye is awall of cloud that may extend above 50,000 feet.This "wall cloud" contains deluging rain and thestrongest winds of the storm. Maximum windspeeds of 175 knots have been recorded in somestorms. Figure 145 is a radar display and 146, asatellite photograph of a mature hurricane. Notethe spiral rain bands and the circular eye. Noticethe similarity between these two figures.

Detection and WarningThe National Weather Service has a specialized

hurricane forecast and warning service center atMiami, Florida, which maintains constant watchfor the formation and development of tropical cy-clones. Weather information from land stations,ships at sea, reconnaissance aircraft, long range

radars, and weather satellites is fed into the center.The center forecasts the development, movement,and intensity of tropical cyclones. Forecasts andwarnings are issued to the public and aviation in-terests by field offices of the National WeatherService.

FlyingAll pilots except those especially trained to

explore tropical storms and hurricanes shouldAVOID THESE DANGEROUS STORMS. Oc-casionally, jet aircraft have been able to fly oversmall and less intense storms, but the experienceof weather research aircraft shows hazards at alllevels within them.

Tops of thunderstorms associated With tropicalcyclones frequently exceed 50,000 feet. Winds ina typical hurricane are strongest at low levels, de-creasing with altitude. However, research aircrafthave frequently encountered winds in excess of 100knots at 18,000 feet. Aircraft at low levels are ex-posed to sustained, pounding turbulence due to thesurface friction of the fast-moving air. Turbulenceincreases in intensity in spiral rain bands and be-comes most violent in the wall cloud surroundingthe eye.

An additional hazard encountered in hurricanesis erroneous altitude readings from pressure altim-eters. These errors are caused by the large pressuredifference between the periphery of the storm andits center. One research aircraft lost almost 2,000feet true altitude traversing a storm while the pres-sure altimeter indicated a constant altitude of5,000 feet.

In short, tropical cyclones are very hazardous,so avoid them! To bypass the storm in a min-imum of time, fly to the right of the storm to takeadvantage of the tailwind. If you fly to the left ofthe storm, you will encounter strong headwindswhich may exhaust your fuel supply before youreach a safe landing area.

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Ftounz 145. Radar photograph of hurricane "Donna" observed at Key West, Florida.

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Boum 146. A hurricane observed by satellite.

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Chapter 16SOARING WEATHER

While horse racing may be the "Sport of Kings,"soaring may be considered the "King of Sports."Soaring bears the relationship to flying that sailingbears to power boating. Soaring has made notablecontributions to meteorology. For example, soar-ing pilots have probed' thunderstorms and moun-tain waves with findings that have made flyingsafer for all pilots. However, soaring is primarilyrecreational.

A sailplane must have auxiliary power to be-come airborne such as a winch, a ground tow, ora tow by a powered aircraft. Once the sailcraft isairborne and the tow cable released, performance

of the craft depends on the weather and the skillof the pilot. Forward thrust comes from glidingdownward relative to the air the same as thrustis developed in a power-off glide by a conven-tional aircraft. Therefore, to gain or maintainaltitude, the soaring pilot must rely on upwardmotionof the air.

To a sailplane pilot, "lift" means the rate ofclimb he can achieve in an up-current, while "sink"denotes his rate of descent in a downdraft or inneutral air. "Zero sink" means that upward cur-rents are just strong enough to enable him to holdaltitude but not to climb. Sailplanes are highly

1 7i171

efficient machines; a sink rate of a mere 2 feet persecond provides an airspeed of about 40 knots, anda sink rate of 6 feet per second gives an airspeedof about 70 knots. Some two-place training crafthave somewhat higher sink rates.

in lift, a sailplane pilot usually flies 35 to 40knots with a sink rate of about 2 feet per second.Therefore, if he is to remain airborne, he musthave an upward air current of at least 2 feet per

second. There is no point in trying to soar untilweather conditions favor vertical speeds greaterthan the minimum sink rate of the aircraft. Thesevertical currents develop from several sources, andthese sources categorize soaring into five classes:(I) Thermal Soaring, (2) Frontal Soaring, (3) SeaBreeze Soaring, (4) Ridge or Hill Soaring, and(5) Mountain Wave Soaring.

THERMAL SOARING

Peter Dixon estimates that about 80 percent ofall soaring in the U.S. depends on thermal lift.*What is a thermal? A thermal is simply the updraftin a small-scale convective current. Chapter 4 inthe section "Convection," and chapter 9 in thesection, "Convective Currents," explain the basicprinciple of convective circulation. The explana-tions are adequate for the pilot of a powered air-craft; but to the soaring pilot, they are only abeginning.

All pilots scan the weather pattern for convectiveactivity. Remember that turbulence is proportionalto the speed at which the aircraft penetrates ad-

*Peter L. Dixon. SomuNG, page.129; 1970; Balla alineBooks, New York City.

jacent updrafts and downdrafts. The fast movingpowered aircraft experiences "pounding" and triesto avoid convective turbulence. The slower movingsoaring pilot enjoys a gradual change from ther-mals to areas of sink. He chases after localconvective cells using the thermals for lift.

A soaring aircraft is always sinking relative tothe air. To maintain or gain altitude, therefore, thesoaring pilot must spend sufficient time in thermalsto overcome the normal sink of the aircraft as wellas to regain altitude lost in downdrafts. He usuallycircles at a slow airspeed in a thermal and thendarts on a beeline to the next thermal as shown infigure 147.

Low-level heating is prerequisite to thermals;and this heating is mostly from the sun, although

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Fromm 147. Thermals generally occur over a small portion of an area while downdrafts predominate. Updrafts in thethermals usually are considerably stronger than the downdrafts. Sailplane pilots gain altitude in thermals and hold al-titude loss in downdrafts to a minimum.

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it may be augmented by man-made heat sourcessuch as chimneys, factories, and cities. Cool airmust sink to force the warm air upward in ther-mals. Therefore, in small-scale convection, ther-mals and downdrafts coexist side by side. The netupward displacement of air must equal the netdownward displacement. Fast rising thermals gen-erally cover a small percentage of a convective areawhile slower downdrafts predominate over theremaining greater portion as diagrammed in figure147..,' Since thermals depend on solar heating,' thermalsoaring is restricted virtually to daylight hours withconsiderable sunshine. Air tends to become stableat night due to low-level cooling by terrestrial radia-tion, often resulting in an inversion at or near thesurface (see chs. 3 and 6). Stable air suppressesconvection, and thermals do not form until theinversion "bums off" or lifts sufficiently to allowsoaring beneath the inversion. The earliest thatsoaring may begin varies from early forenoon toearly afternoon, the time depending on the strengthof the inversion and the amount of solar heating.Paramount to a pilot's soaring achievement is hisskill in diagnosing and locating thermals.

LOCATING THERMALSSince convective thermals develop from uneven

heating at the surface, the most likely place for athermal is above a surface that heats readily.

Types of Terrain SurfacesWhen the sky is cloudless, the soaring pilot must

look for those surfaces that heat most rapidly andseek thermals above those areas. Barren sandy orrocky surfaces, plowed fields, stubble fields sur-rounded by green vegetation, cities, factories, chim-neys, etc., are good thermal sources. A pilot learnsthrough experience the most favorable spots in hislocal area. But terrain features are only part of thestory; time of day influences not only when thermalsform but also where.

Sun AngleAngle of the sun profoundly affects location of

thermals over hilly landscapes. During early fore-noon, the sun strikes eastern slopes more directlythan other slopes; therefore, the most favorableareas for thermals are eastern slopes. The favor-able areas move to southern slopes during midday.In the afternoon, they move to western slopes be.fore they begin to weaken as the evening sun sinks

toward the western horizon. For example, if arocky knob protrudes above a grassy plain, themost likely area of thermals is over the easternslope in the forenoon and the western slope in theafternoon. Once a pilot has sighted a likely surface,he may look for other visual cues.

Dust and SmokeSurface winds must converge to feed a rising

thermal; so when you sight a likely spot for athermal, look for dust or smoke movement near thesurface. If you can see dust or smoke "streamers"from two or more sources converging on the spotas shown in figure 148(A), you have chosen wisely.If, however, the streamers diverge as shown infigure 148(B), a downdraft most likely hovers overthe spot and it's time to move on.

Rising columns of smoke from chimneys andfactories mark thermals augmented by man-made

- sources. These rising columns are positive indica-tion of thermals. They are good sources of lift ifupward speed is great enough to support the air-craft and if they are broad enough to permit cir-cling. Towns or cities may provide thermals; but touse a thermal over a populated area, the pilot musthave sufficient altitude to glide clear of the area inevent the thermal subsides.

Dust DevilsDint devils occur under sunny skies over sandy

or dusty, dry surfaces and are sure signs of strongthermals with lots of lift. To tackle this excellentsource of lift, you must use caution. The thermalsare strong and turbulent and are surrounded byareas of little lift or possibly of sink.

If approaching the dust devil at too low an alti-tude, an aircraft may sink to an altitude too lowfor recovery. A recommended procedure is''to al-ways approach the whirling vortex at an altitude500 feet or more above theground. At this altitude,you have enough airspace for maneuvering in theevent you get into a downdraft or turbulence toogreat for comfort.

A dust devil may rotate either clockwise orcounterclockwise. Before approaching the dustycolumn, determine its direction of rotation by ob-serving dust and debris near the surface. PhilipWills* quotes R. H. Swinn, Chief Instructor of theEgyptian Gliding School, on approaching and en-

*Philip Wills. ON Bono A Bute, page 79; 1953;Max Parrish and Co., Ltd.

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CONVERGING

DUST AND SMOKE

DUST SMOKE DUST

DIVERGING

DUST AND SMOKE

SMOKE

FIGURE 148. Using surface dust and smoke movement as indications of a thermal. When you have sighted an area whichyou think will heat rapidly (the red area), look for dust or smoke movement at the surface as an indicator of surface wind.Converging dust or smoke streamers (left) enhance the probability of a thermal. Diverging streamers reduce the likeli-hood of a thermal.

tering a dust devil: "... at around 500 feet; thepilot turns towards the dust devil and cuts hisspeed as he approaches it to the minimum con-sistent with the control of the glider. As he nearsthe whirling column of sand he makes a circle onthe outside of the dust devil against the directionof rotation, care being taken to give it a widerberth on the downwind side. In light of the vari-ometer reading on the initial circle, closer contactis made with the column or a hasty retreat is beatto a safer orbit."

Ftouftz 149. Horizontal cross section of a dust devil ro-tating clockwise. If the aircraft approaches the dust devilwith the direction of rotation as on the left, increasingtailwind reduces airspeed and may result in loss of altitudeor even a stall. When the pilot regains equilibrium, hiscircling speed is the sum of his airspeed and the tangentialspeed of the vortex; his radius of turn may be too greatto remain in the thermal. When approaching against therotation, the aircrakgaing. airspeed; circling speed isslowed as the tangential speed of the vortex is subtractedfrom airspeed. The pilot has much more freedom andlatitude for maneuvering. At the center is a core provid-ing little or no lift. Immediately surrounding the core is aturbulent wall.

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Why should 9 ciuM iifEir against thedifeition 'of.-

rotation? Figure 149 diagrams a horizontal crosssection of a clockwise rotating dust devil and waysof entering it. If you enter with the direction ofrotation as on the left, the wind speed is added toyour airspeed giving you a fast circling speed prob-ably too great to remain in the thermal. Againstthe rotation as on the right, wind speed is sub-tracted from airspeed giving you a slow circlingspeed.

Why slow your airspeed to a minimum? As youapproach the increasing headwinds, the inertia ofthe aircraft causes a surge in airspeed. If your ap-proach is too fast, the surge could push the airspeedabove the redline.

Stay out of the "eye" of the vortex. Centrifugalforce in the center throws air outward, greatly re-ducing pressure within the hollow center. The rari-fied air in the center provides very little lift, andthe wall of the hollow center is very turbulent.Further quoting Mr. Swinn,* "A too tight turn onthe downwind side put a part of my inside winginto the vortex; the shock threw me into the strapsand the wing bent in an alarming manner. Thiscentral area of greatly reduced pressure is some-thing to be experienced to be believed. Closelyfollowing on this was the shock of hitting the areaof greatest uplift just outside the central core. Thenet result was that the machine was thrown com-pletely out of the column."

If you are 500 feet or more above the ground buthaving trouble finding lift, the dust devil is wellworth a try. If the thermal is sufficiently broad topermit circling within it, you have it made. Thedust column may be quite narrow, but this fact doesnot necessarily mean the thermal is narrow; thethermal may extend beyond the outer limits ofvisible dust. The way to find out is to try it. Ap-proach the dusty column against the direction ofrotation at minimum airspeed. Enter the columnnear the outer edge of the dust and stay away fromthe hollow vortex core. Remain alert; you arecircling little more than a wing span away fromviolent turbulence.

Birds and SailplanesSoaring birds have an uncanny ability to locate

thermals. When birds remain airborne without

'Ibid., page 80. Mr. Wills' book discusses at lengththe splendors and perils of dust devil flying by an ex-perienced soaring pilot. It is recommended reading for agreater insight into this special aspect of soaring.

ifil;ping their wings, they are riding a thermal. Aclimbing sailplane also shows the pilot's skill inlocating thermals. When fishermen are scatteredalong a river bank or lake shore, the best place tocast your line is near the fisherman who is catchingfish. So it is with soaring. Slip in below the success-fully soaring aircraft and catch the thermal he isriding or cut in among or below soaring birds.

Wind causes a thermal to lean with altitude.When seeking the thermal supporting soaring birdsor aircraft, you must make allowance for the wind.The thermal at lower levels usually is upwind fromyour high-level Visual cue. A thermal may not becontinuous from the surface upward to the soaringbirds or sailplane; rather it may be in segments orbubbles. If you are unsuccessful in finding thethermal where you expect it, seek elsewhere,

Cumulus CloudsWhen convective clouds develop, thermal soaring

usually is at its best and the problem" of locatingthermals is greatly simplified. In chapter 6 welearned that upward moving air expands and coolsas it rises. If the air is moist enough, expansionalcooling lowers temperature to the dew point; aconvective, or cumulus, cloud forms atop the ther-mal. Cumulus clouds are positive signs of thermals,but thermals grow and die. A cloud grows with arising thermal; but when the thermal dies, thecloud slowly evaporates. Because the cloud dis-sipates after the thermal ceases, the pilot who canspot the difference between a growing and dyingcumulus has enhanced his soaring skill.

The wannest and most rapidly rising air is in thecenter of the thermal. Therefore, the cloud basewill be highest in the center giving a concave shapeto the cloud base as shown in the left and center offigure 150. When the thermal ceases, the baseassumes'a convex shape as shown on the right. An-other cue to look for is the outline of the cloudsides and top. Outline of the growing cumulus isfirm and sharp. The dying cumulus has frag-mentary sides and lacks the definite outline. Theseoutlines are diagrammed alsc in figure 150. Figure151 is a photograph of a dying ...umulus.

You can expect to find a thermal beneath eitherof the growing cumuli in figure 150. On the aver-age, the infant cumulus on the left would be thebetter choice because of its longer life expectancy.This is of course playing the probabilities since allcumuli do not grow to the same size.

As a cumulus cloud grows, it may shade the

181175

riouRz 150. Cumulus clouds grow only with active thermals as shown left and center. On the right, the thermal has sub-sided and the cloud is decaying. Look for a thermal only under a cumulus with a concave base and sharp upper outlines.A cumulus with a convex base or fragmentary outline is dissipating; the thermal under it has subsided. Most often, acloud just beginning to grow as on the left is the better choice because of its longer life expectancy.

surface that generated it. The surface cools, tem-porarily arresting the thermal. As the cloud dis-sipates or drifts away- with the wind, the surface

. again warms and regenerates the thermal. Thisintermittent heating is one way in which thermalsoccur as segments or bubbles.

Cloud cover sometimes increases as surface heat-ing increases until much of the sky is covered.Again, surface heating is cut off causing the ther-mals to weaken or cease entirely. The cloudinessmay then decrease. If it is not too late in the day,thermals will regeue:at... In the interim period ofextensive cloud tx.ver, you may have no choice butto land and w it for the. clouds to move on ordecrease in coverage

The clouds rosy build upward to a high-level in-version and spread out at the base of the inversionto cover much of the sky. Solar heating is cut offand thermals weaken or die. This type of cloudinesscan be persistent, often remaining until nearsunset, and can halt thermal soaring until anotherday.

Although abundant convective cloud cover re-

duces thermal activity, we cannot quote a definiteamount that renders thermals too weak for soaring.About 5/10 cover seems to be a good average ap-proximation. RestrictiOn of thermals by cumuluscloudiness first becomes noticeable at low levels.A sailplane may be unable to climb more than afew hundred feet at a low altitude while pilots athigher levels are maintaining height in or justbeneath 6/10 to 8/10 convective cloud cover.

Towering Cumulus and CumulonimbusWhen air is highly unstable, the cumulus cloud

can grow into a more ambitious towering cumulusor cumulonimbus. These clouds are a differentbreed. The energy released by copious condensa-tion can increase buoyancy until the thermals be-come violent (see chs. 6, 7, and 11). Toweringcumulus can produce showers. The cumulonimbusis the thunderstorm cloud producing heavy rain,hail, and icing. Well-developed towering cumulusand cumulonimbus are for the experienced pilotonly. Some pilots find strong lift in or near con-vective precipitation, but they avoid hail which

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FIGURE 151. Photograph of a dying cumulus. Note the indistinct edges and cloud fragments. The base appears to be con-vex. One would expect little or no lift beneath this cloud. In contrast, note the top of the cumulus in the lower left corner.Edges are more defined, and a thermal is more likely under this cloud.

can seriously batter the aircraft and ultimatelydeplete the wallet.

Violent thermals just beneath and within thesehighly developed clouds often are so strong thatthey will continue to carry a sailplane upwardeven with nose down and airspeed at the redline.The unwary pilot may find himself sucked into thecloud. The soaring pilot who inadvertently entereda thunderstorm and returned to tell about it neverhankers for a repeat performance.

Middle and High CloudinessDense, broken or overcast middle and high

cloudiness shade the surface cutting off surfaceheating and convective thermak. On a generallywarm bright day but with thin or patchy middleor high cloudiness, cumulus may develop, but the

thermals are few and weak. The high-level cloudi-ness may drift by in patches. Thermals may surgeand wane as the cloudiness decreases and increases.Never anticipate optimum thermal soaring whenplagued by these mid- and high-level clouds.

Altocumulus castellanus clouds, middle-level con-vective clouds shown in figure 152, develop in up-drafts at and just below the cloud levels. They donot extend upward from the surface. If a sailplanecan reach levels near the cloud bases, the updraftswith altocumulus castellanus can be used in thesame fashion as thermals formed by surface convec-tion. The problem is reaching the convective level.

Wet GroundWet ground favors eiermals less than dry ground

since wet ground heats more slowly (see ch. 2,

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noun 152. Altocumulus castellanus clouds are middle level convective clouds. Most often, they develop in an unstablelayer aloft, and thermals do not extend from the ground upward to these clouds. Convection with these clouds may beused for Iift U the pilot is able to attain altitude to the base of the unstable layer. Smoke lying near the ground indicatesstability in the lower levels.

"Heat and Temperature"). Some flat areas withwet soil such as swamps and tidewater areas havereputations for being poor thermal soaring areas.Convective clouds may be abundant but thermalsgenerally are weak.

Showery precipitation from scattered cumulus orcumulonimbus is a sure sign of unstable air favor-able for thermals. But when showers have soakedthe ground in localized areas, downdrafts are al-most certain over these wet surfaces. Avoid showersoaked areas when looking for lift.

So much for locating thermals. A pilot can alsoenhance his soaring skill by knowing what goes onwithin a thermal.

THERMAL STRUCTUREThermals are as varied as trees in a forest. No

two are exactly alike. When surface heating is in-tense and continuous, a thermal, once begun, con-tinues for a prolonged period in a steady column

17$

as in figure 153. Sometimes called the "chimneythermal," this type seems from experience to bemost prevalent. In the chimney thermal, lift isavailable at any altitudf below a climbing sail-plane or soaring.kftrds.

When heating is slow or intermittent, a "bubble"may be pinched off and forced upward; after aninterval ranging from a few minutes to an hour ormore, another bubble forms and rises as in figure154. As explained earlier, intermittent shading bycumulus clouds forming atop a thermal is onereason for the bubble thermal. A sailplane or birdsmay be climbing in a bubble, but an aircraft at-tempting to enter the thermal at a lower altitudemay find no lift.

A favored theoretical structure of some bubblethermals is the vortex shell which is much like asmoke ring blown upward as diagrammed in figure155. Lift is strongest in the center of the ring;downdrafts may occur in the edges of the ring or

184

FIGURE 153. Experience indicates that the "chimney"thermal, which is continuous from the ground upward,is the most prevalent type. A sailplane can find lift insuch a thermal beneath soaring birds or other soaringaircraft.

shell; and outside the shell, one would expect weakdownnrafts.

Wind and Wind ShearThermals develop with a calm condition or with

light, variable wind. However, it seem: that a sur-face wind of 5 to 10 knots favors more organizedthermals.

A surface wind in excess of 10 knots usuallymeans stronger winds aloft resulting in verticalwind shear. This shear causes thermals to leannoticeably. When seeking a thermal under a climb-ing sailplane and you know or suspect that ther-mals are leaning in shear, look for lift upwind fromthe higher aircraft as shown in figure 156.

FIGURE 154. Thermals may be intermittent "bubbles."Frequency of the bubbles ranges from a few minutes toan hour or more. A soaring pilot will be disappointedwhen he seeks lift beneath birds or sailplanes soaring inthis type thermal.

Effect of shear on thermals depends on the rela-tive strength of the two. Strong thermals can re-main fairly well organized with strong vertical windshear; surface wind may even be at the maximumthat will allow a safe launch. Weak thermals aredisorganized and ripped to shreds by strong verticalwind shear; individual thermal elements becomehard to find and often are too small to use for lift.A shear in excess of 3 knots per thousand feet dis-torts thermals to the extent that they are difficultto use.

No critical surface wind" speed can tell us when.to expect such a shear. However, shearing actionoften is visible in cumulus clouds. A cloud some-times leans but shows a continuous chimney. At

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FIGURE 155. It is believed that a bubble thermal some-times develops a vortex ring resembling a smoke ringblown straight upward. The center of the ring providesexcellent lift. A pilot finds only weak lift or possibly sinkin the fringes of the ring.

other times, the clouds are completely severed intosegments by the shear as in figure 157. Remember,however, that this shearing action is at cloud level;thermals below the clouds may be well organized.

We must not overlook one other vital effect ofthe low-level wind shear. On final approach forlanding, the aircraft is descending into decreasingheadwind. Inertia of the aircraft into the decreas-ing wind causes a drop in airspeed. The decreasein airspeed may result in loss of control and per-haps a stall. The result can be an inelegant land-ing with possible injury and aircraft damage. Agood rule is to add one knot airspeed to normalapproach speed for each knot of surface wind.

Thermal StreetsNot infrequently, thermals become organized in-

to "thermal streets." Generally, these streets areparallel to the wind; but on occasion they have

180

been observed at right angles to the wind. Theyform when wind direction changes little throughoutthe convective layer and the layer is capped byvery stable air. The formation of a broad system ofevenly spaced streets is enhanced when wind speedreaches a maximum within the convective layer;that is, wind increases with height from the surfaceupward to a maximum and then decreases withheight to the top of the convective layer. Figure158 diagrams conditions favorable for thermalstreeting. Thermal streeting may occur either inclear air or with convective clouds.

The distance between streets in such a system istwo to three times the general depth of the con-vective layer. If convective clouds ate present, thisdistance is two to three times the height of thecloud tops. Downdrafts between these thermalstreets are usually at least moderate and sometimesstrong. Cumulus cloud streets frequently form inthe United States behind cold fronts in the coldair of polar outbreaks in which relatively flatcumuli develop. A pilot can soar under a cloudstreet maintaining generally continuous flight andseldom, if ever, have to circle. Figure 159 is aphotograph of bands of cumulus clouds markingthermal streets.

HEIGHT AND STRENGTH OF THERMALSSince thermals are a product of instability, height

of thermals depends on the depth of the unstablelayer, and their strength depends on the degreeof instability. If the idea of instability is not clearto you, now is the time to review chapter 6.

Most Jikely you will be soaring from an airportwith considerable soaring activitypossibly thehome base of a soaring club and you are inter-ested in a soaring forecast. Your airport may havean established source of a daily soaring weatherforecast from the National Weather Service. Ifconditions are at all favorable for soaring, you willbe specifically interested in the earliest time soaringcan begin, how high the thermals will he, strengthof the thermals, cloud amountsboth convectiveand higher cloudinessvisibility at the surface andat soaring altitudes, probability of showers, andwinds both at the surface and aloft. The forecastmay include such items as the thermal index (TI),the maximum temperature forecast, and the depthof the convective layer.

Many of these parameters the forecaster deter-mines from upper air soundings plotted on a

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187181

STABLE AIR

MAXIMUM WIND CONVECTIVE LAYER

WIND SPEED

Flamm 158. Conditions favorable for thermal 3treeting. A very stable layer caps the convective layer, and wind reachesa maximum within the convective layer. If cumulus clouds mark thermal streets, the top of the convective layer is aboutthe height of the cloud tops.

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189183

pseudo-adiabatic chart. If you become familiarwith this chart, you can better grasp the meaningsof some of these forecast parameters; and you maytry a little forecasting on your own.

The Pseudo-Adiabatic ChartThe pseudo-adiabatic chart is used to graphically

compute adiabatic changes in vertically moving airand to determine stability. it has five sets of linesshown in figure 160. These lines are:

1. Pressure in millibars (horizontal lines),2. Temperature in degree:. Celsius (vertical

lines),3. Dry adiabats (sloping black lines),

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4. Lines of constant water vapor or mixingratio* (solid red lines), and

5. Moist adiabats (dashed red lines).The chart also has an altitude scale in thousandsof feet along the right margin and a Fahrenheittemperature scale across the bottom.

You might like to get one of these charts from aNational Weather Service Office. The chart used inactual practice has a much finer grid than the oneshown in figure 160. You can cover the chart withacetate anti :heck examples given here along withothers y' c .develop yourself. This procedure cangreatly e1.1.;,:ice your feel for processes occurringin a vertically moving atmosphere.

*Ratio of water vapor to dry air.

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Examples shown here deal with dry thermals;and since the red lines in figure 160 concern- moistadiabatic changes, they are omitted from the ex-amples. If you care to delve deeper into use ofthe chart, you will find moist adiabatic processeseven more intriguing than dry processes.

Plotted SoundingAn upper air observation, or sounding, is plotted

on the pseudo-adiabatic chart as shown by theheavy, solid black line in figure 161. This plotting

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is the vertical temperature profile at the time theradiosonde observation was taken. It is the actualor existing lapse rate (see ch. 6). Blue lines areadded to the illustration showing appropriate alti-tudes to aid you in interpreting the chart.

Depth of Convective Layer(Height of Thermals)

We know that for air to be unstable, the existinglapse rate must be equal to or greater than the dryadiabatic rate of cooling. In other words, in figure

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FIGURE 161. An early morning upper air observation plotted on the pseudo-adiabatic chart. The solid black line is thevertical temperature profile or existing lapse rate from the surface to about 15,000 feet ASL. Blue altitude lines are pro-jected across the chart from the altitudz scale on the right to aid in interpretation. If thermals are to develop, the lapserate must become equal to or greater than the dry adiabatic rate of cooling that is, the line representing the lapse ratemust slope parallel to or slope more than the dry adiabats. Since it does not, the air in the early morning was stable. Bythe time the surface temperature reached 80° F, convection occurred to 5,000 feet; the existing lapse rate then was parallelto the dry adiabat following the dashed line from the surface to 5,000 feet; the air was unstable in the lower levels. By thetime the temperature reached the afternoon maximum of 90° F, the air was unstable to 13,000 feet; the existing lapse ratein the heat of the day was dry adiabatic and the air unstable to 13,000 feet ASL. This is the maximum height you couldexpect thermals on this particular day.

191 185

161, the solid black line representing the plottedexisting lapse rate would slope parallel to or slopemore than the dry adiabats. Obviously it does not.Therefore, at the time the sounding was taken, theair was stable; there was no convective or unstablelayer, and thermals were nonexistent. Thermalsoaring was impossible.

Now assume that the sounding was made aboutthe time of sunrise. Surface temperature was 59° F(15° C) . As temperature rises near the surfaceduring the day, air in the lower levels is warmedand forced upward, cooling at the dr, adiabaticrate. Convection begins in the lowest levels. By thetime the surface temperature reaches 80° F (about27° C), convection lifts the air to the level at whichit cools adiabatically to the temperature of thesurrounding air at 5,000 feet. The existing lapserate now becomes dry adiabatic from the surface to5,000 feet and follows the dashed line from thesurface to that level. Surface elevation is 2,000 feetASL; so the convective layer is now 3,000 feet deep.Thermals exist to 3,000 feet above the surface, andlow-level soaring is now possible. Above 5,000 feetthe lapse rate still is essentially unchanged fromthe initial lapse rate.

Maximum Height of ThermalsLet's further assume that maximum temperature

forecast for the day is 90° F (about 30° C). Plot90° F at the surface elevation and draw a line (thedashed black line) parallel to the dry adiabats tothe level at which it intersects the early morningsounding. This level is 13,000 feet ASL. The con-vective layer at time of maximum heating wouldbe 11,000 feet deep and soaring should be possibleto 13,000 feet ASL. The existing lapse rate in theheat of the day would follow the dashed line fromthe surface to 13,000 feet; above 13,000, the lapserate would remain essentially unchanged.

Remember that we are talking about dry ther-mals. If convective clouds form below the indicatedmaximum thermal height, they will greatly distortthe picture. However, if cumulus clouds do de-velop, thermals below the cloud base should bestrengthened. If more higher clouds develop thanwere forecast, they will curtail surface heating, andmost likely the maximum temperature will be cool-er than forecast. Thermals will be weaker and willnot reach as high an altitude.

Thermal Index (TI)Since thermals depend on sinking cold air forc-

ing warm air upward, strength of thermals depends

186

on the temperature difference between the sinkingair and the rising airthe greater the temperaturedifference the Stronger the thermals. To arrive atan approximation of this difference, the forecastercomputes a thermal index (TI).

A thermal index may be computed for any level;but ordinarily, indices are computed for the 850 -and 700-millibar levels, or about 5,000 and 10,000feet respectively. These levels are selected becausethey are in the altitude domain of routine soaringand because temperature data are routinely avail-able for these two levels.

Three temperature values are neededthe ob-served 850-millibar and 700-millibar temperaturesand the forecast maximum temperature. Let's as-sume a sounding as in figure 162 with an 850 -millibar temperature of 15° C, a 700-millibar tem-perature of 10° C, and forecast maximum of 86° F(30° C). Plot the three temperatures using careto place the maximum temperature plot at fieldelevation (2,000 feet in figure 162). Now drawa line (the black dashed line) through the maxi-mum temperature parallel to the dry adiabats. Notethat the dashed line intersects the 850-millibarlevel at 20° C and the 700-millibar level at 4° C.Algebraically subtract these temperatures from ac-tual sounding temperatures at corresponding levels.Note the difference is 5° C at 850 millibars(15 20 = 5) and +6 at 700 millibars (10 --4 = +6). These values are the TI's at the twolevels.

Strength of thermals is proportional to the mag-nitude of the negative value of the TI. A TI of 8or 10 predicts very good lift and a long soaringday. Thermals with this high a negative value willbe strong enough to hold together even on a windyday. A TI of 3 indicates a very good chance ofsailplanes reaching the altitude of this temperaturedifference. A TI of 2 to zero leaves much doubt;and a positive TI offers even less hope of thermalsreaching the altitude. Remember that the TI is aforecast value. A miss in the forecast maximum ora change in temperature aloft can alter the pictureconsiderably. The example in figure 162 shouldpromise fairly strong thermals to above 5,000 feetbut no therinals to 10,000.

Figure 163 is another example showing an earlymorning sounding with a 3,000-foot surface tem-perature of 10° C (50° F), an 850-millibar tern-perature of 15° C, a 700-millibar temperature of3° C, and a forecast maximum of 86° F (30° C).What are the TI's at 850 and 700 millibars? Wouldyou expect thermals to 850 millibars? Would they

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FIGURE 162. Computing the thermal index (TI). From an early morning upper air observation, obtain the 850-millibarand 700-millibar temperatures-15° C and 10° C respectively, in this example. Obtain a forecast maximum temperature,86° F, and plot it at the surface elevation. Draw a dry adiabat, the dashed line, upward through the 700-millibar level.This dry adiabat is the temperature profile of a rising column of air. To find the TI at any level, subtract the temperatureof the rising column at that level from the temperature of the original sounding at the same level. The TI at 850 millibarsis 5 (15 20 = 5). At 700 millibars, the TI is +6 (10 4 = +6).

be moderate, strong, or weak? How about at 700millibars? What is the maximum altitude youwould expect thermals to reach? Answers: 850 -millibar TI, 8; 700-millibar TI, 5; thermalswould reach both level; strong at 850, moderateat 700; maximum altitude of thermals; about16,000 feet ASL.

Often the National Weather Service will have noupper air sounding taken near a soaring base. Fore-casts must be based on a simulated sounding de-rived from distant observations. At other times, forsome reason a forecast may not be available. Fur-thermore, you can often augment the forecast with

local observations. You are never at a complete lossto apply some of the techniques just described.

Do It YourselfThe first step in determining height and strength

of thermals is to obtain a local sounding. How doyou get a local sounding? Send your tow aircraftaloft about sunrise and simply read outside air tem-peratures from the aircraft thermometer and alti-tudes from the altimeter. Read temperature's at500-foot intervals for about the first 2,000 feet andat 1,000-foot intervals at higher altitudes. The in-formation may be radioed back to the ground, or

193187

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FIGURE 163. Another example of computing TI's and maximum height of thermals. See discussion in caption of figure162. By the time of maximum heating, excellent lift should be available in lower levels and moderate lift above 10,000feet. Although thermals should continue to 16,000 feet, you could expect weak lift above 12,000 or 13,000 feet becauseof the small difference between temperatures in the thermal and in the surrounding air.

may be recorded in flight and analyzed after land-ing. When using the latter method, read temper-atures on both ascent and descent and average thetemperatures at each level. This type of soundingis an airplane observation or APOB. Plot thesounding on the pseudo-adiabatic chart using thealtitude scale rather than the pressure scale.

Next we need a forecast maximum temperature.Perhaps you can pick up this forecast temperaturefrom the local forecast. If not, you can use yourbest judgment comparing today's weather withyesterday's.

Following is an APOB as taken by the tow air-craft from an airport elevation of 1,000 feet ASL:.

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Alt.

Temperatures

Ascent Descent Avg.

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Plot the APOB on the pseudo-adiabatic chart usingthe average temperatures from the last column.Figure 164 shows the plotted APOB.

Next we need a forecast maximum temperature.Let's assume that a local forecast is not availableand that weather today is essentially the same asit was yesterday. Yesterday's maximum was 95° F(35° C), so let's use the same maximum for today.We should not be too far wrong. Plot the maxi-mum as shown and proceed to compute TI's andmaximum height of thermals. Since our temper-ature data are for indicated altitudes rather thanpressure levels, let's compute TI's for 5,000 feetand 10,000 feet rather than for pressure levels.What do you get for a TI at 5,000 feet? At 10,000

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feet? What is the anticipated maximum altitudeof thermals? Answers: TI at 5,000 feet, 4; TI at10,000 feet, 3; maximum altitude of thermals,14,000 feet.

Although these procedures are primarily for drythermals, they work reasonably well for thermalsbelow the bases of convective clouds.

Convective Cloud BasesSoaring experience suggests a shallow, stable

layer immediately below the general level of con-vective cloud bases through which it is difficult tosoar. This layer is 200 to 600 feet thick and isknown as the sub-cloud layer. The layer appears toact as a filter allowing only the strongest ther-

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mals to penetrate it and form convective clouds.Strongest thermals are beneath developing cumulusclouds.

Thermals intensify within a convective cloud;but evaporation cools the outer edges of the cloudcausing a downdraft immediately surrounding it.Add to this the fact that downd rafts predominatebetween cumulus clouds, and you can see the slimchance of finding lift between clouds above thelevel of the cloud base. in general, thermal soaringduring convective cloud activity is practical onlyat levels below the cloud base.

In chapter 6, we learned to estimate height inthousands of feet of a convective cloud base bydividing the surface temperaturedew point spreadby 4. If the rising column were self-containedthatis, if no air were drawn into the sides of the ther-mal the method 'would give a fairly accurateheight of the base. However, this is not the case.Air is entrained or drawn into the sides of thethermal; and this entrained air lowers the watervapor content of the thermal allowing it to reacha somewhat higher level before condensation oc-curs. Bases of the clouds are generally 10 to 15percent higher than the computed height.

Entrainment is a sticky problem; observers andforecasters can only estimate its effect. Until apositive technique is developed, heights of cumulusbases will tend to be reported and forecast too low.Currently, in the eastern United States, cumulusbases are seldom reported above 6,000 feet whenthe base may actually be 7,000 or 8,000 feet. Inthe western part of the country, cumulus baseshave been observed by aircraft at 12,000 to 14,000feet above the ground but seldom are reportedabove 10,000 feet.

CROSS-COUNTRY THERMAL SOARINGA pilot can soar cross-country using either iso-

lated thermals or thermal streets. When usingisolated thermals, he gains altitude circling in ther-mals and then proceeds toward the next thermalin the general direction of his cross-country. Undera thermal street, he may be able to proceed withlittle if any circling if his chosen course parallelsthe thermal streets. It goes without saying that hecan obtain the greatest distance by flying in thedirection of the wind.

190196

In the central and eastern United States, themost favorable weather for cross-country soaringoccurs behind a cold' front. Lindsay* has foundthat about 82 percent of thermal cross-countrys inthese areas were made after a cold front had passedand ahead of the following high pressure center.Four factors contribute to making this patternideal. (1) The cold polar air is usually dry, andthermals can build to relatively high altitudes. (2)The polar air is colder than the ground; and thus,the warm ground aids solar radiation in heatingthe air. Thermals begin earlier in the morning andlast later in the evening. On occasions, soarablelift has been found at night. (3) Quite often,colder air at high altitudes moves over the cold,low-level outbreak intensifying the instability andstrengthening the thermals. (4) The wind profilefrequently favors thermal streetinga real boonto speed and distance.

The same four factors may occur with cold fron-tal passages over mountainous regions in the west-ern United States. However, rugged mountainsbreak up the circulation; and homogeneous condi-tions extend over smaller, areas than over the east-ern parts of the country. The western mountainregions and particularly the desert southwest haveone decided advantage. Air is predominantly drywith more abundant daytime thermal activity fa-voring cross-country soaring although it may befor shorter distances.

Among the world's most favorable tracks forlong distance soaring is a high plains corridor alongthe east slope of the Rocky Mountains stretchingfrom southwest Texas to Canada.** Many cross-country records have been set in this corridor.Southwest Texas is the chosen site for many na-tional and international soaring meets. Terrain inthe corridor is relatively flat and high with fewtrees; terrain surface ranges from barren to shortgrass. These surface features favor strong thermalactivity. Prevailing wind is southerly and mod-erately strong giving an added boost to northboundcross-countrys.

*Charles V. Lindsay. "Types of Weather FavoringCross-Country Soaring." Soaring, December 1964, pp.6-9.

** For an in-depth discussion of this area, see "ThermalSoaringSouthwest Style," by David H. Owens, Soar-ing, May 1966, pp. 10-12.

,

FRONTAL SOARING

Warm air forced upward over cold air above afrontal surface can provide lift for soaring. How-ever, good frontal lift is transitory, and it accountsfor a very small portion of powerless flight. Seldomwill you find a front parallel to your desired cross-country route, and seldom will it stay in positionlong enough to complete a flight. A slowly movingfront provides only weak lift. A fast moving frontoften plagues the soaring pilot with cloudiness andturbulence.

A front can on occasion provide excellent lift fora short period. You may on a cross-country be rid-ing wave or ridge lift and need to move over aflat area to take advantage of thermals. A frontmay offer lift during your transition.

Fronts often are marked by a change in cloudtype or amount. However, the very presence ofclouds may deter you from the front. Spotting adry front is difficult. Knowing that a front isin the vicinity and studying your aircraft reactioncan tell you when you are in the frontal lift. Stay-ing in the lift is another problem. Observingground indicators of surface wind helps.

An approaching front may enhance thermal orhill soaring. An approaching front or a frontalpassage most likely will disrupt a sea breeze ormountain wave. Post frontal thermals in cold airwere discussed earlier.

SEA BREEZE SOARING

In many coastal areas during the warm seasons,a pleasant breeze from the sea occurs almost daily.Caused by the heating of land on warm, sunnydays, the sea breeze usually begins during earlyforenoon, reaches a maximum during the after-noon, and subsides around dusk after the land hascooled. The leading edge of the cool sea breezeforces warmer air inland to rise as shown in figure165. Rising air from over land returns seaward athigher altitude to complete the convective cell.

A sailplane pilot operating in or near coastalareas often can find lift generated by this con-vective circulation. The transition zone between thecool, moist air from the sea and the warm, drier airinland is often narrow and is a shallow, ephemeralkind of pseudo-cold front.

SEA BREEZE FRONTSometimes the wedge of cool air is called a sea

breeze front. If sufficient moisture is present, aline of cumuliform clouds just inland may markthe front. Whether marked by clouds or not, theupward moving air at the sea breeze front occasion-ally is strong enough to support soaring flight.Within the sea breeze, i.e., between the sea breezefront and the ocean, the air is usually stable, andnormally, no lift may be expected at lower levels.However, once airborne, pilots occasionally have

found lift at higher levels in the return flow aloft.A visual indication of this lift is cumulus extendingseaward from the sea breeze front.

The properties of a sea breeze front and theextent of its penetration inland depend on factorssuch as the difference in land and sea watertemperatures, general wind flow, moisture, andterrain.

Land vs Sea Water TemperatureA large difference in land and sea water tem-

perature intensifies the convective 'cell generatinga sea breeze. Where coastal waters are quite cool,such as along the California coast, and land tem-peratures warm rapidly in the daytime, the seabreeze becomes pronounced, penetrating perhaps50 to 75, miles inland at times. Copious sunshineand cool sea waters favor a weft-developed seabreeze front.

Strength and Direction of General WindThe sea breeze is a local effect. Strong pressure

gradients with a well-developed pressure systemcan overpower the sea breeze effect. Winds willfollow the direction and speed dictated by thestrong pressure gradient. Therefore, a sea breezefront is most likely when pressure gradient is weakand wind is light.

197191

SEAI BREEZE

FRONT

WARM (""

haurus 165. Schematic cross section through a sea breeze front. If the air inland is moist, cumulus often marks the front.

MoistureWhen convection is very deep, the frontal effect

of a sea breeze may sometimes trigger cumulo-nimbus clouds provided the lifted air over landcontains sufficient moisture. More often, the cumu-lus are of limited vertical extent. Over vegetationwhere air is usually moist, sea breeze cumulus arethe rule. Over arid regions, little or no cumulusdevelopment may be anticipated with a sea breezefront.

Terrain

Irregular or rough terrain in a coastal area mayamplify the sea breeze front and cause convergencelines of sea breezes originating from different areas.Southern California and parts of the HawaiianIslands are favorable for sea breeze soaring becauseorographic lift is added to the frontal convection.Sea breezes occasionally may extend to the leewardsides of hills and mountains unless the ranges arehigh and long without abrupt breaks. In eithercase, the sea breeze front converges on the wind-ward slopes, and upslope winds augment the con-vection. Where terrain is fairly flat, sea breezes may

192

penetrate inland for surprising distances but withweaker lift along the sea breeze front. In theTropics, sea breezes sometimes penetrate as muchas 150 miles inland, while an average of closer to50 miles inland is more usual in middle latitudes.Sea breezes reaching speeds of 15 to 25 knots arenot uncommon.

VISUAL CLUES

When a sea breeze front develops, visual obser-vations may provide clues to the extent of lift thatyou may anticipate, viz.:

1. Expect little or no lift on the seaward side ofthe front when the sea air is markedly voidof convective clouds or when the sea breezespreads low stratus inland. However, somelift may be present along the leading edgeof the sea breeze or just ahead of it.

2. Expect little or no lift on the seaward sideof the front when visibility decreases mark-edly in the sea breeze air. This is an in-dicator of stable air within the sea breeze.

3. A favorable visual indication of lift alongthe sea breeze front is a line of cumulusclouds marking the front; cumuli between

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the sea breeze front and the ocean also indi-cate possible lift within fhe sea breeze air,especially at higher levels. Cumulus bases inthe moist sea air often are lower than alongthe front.

4. When a sea breeze front is void of cumulusbut converging streamers of dust or smokeare observed, expect convection and liftalong the sea breeze front.

5. Probably the best combination to be sightedis cumuli and converging dust or smokeplumes along the sea breeze front as itmoves upslope over hills or mountains. Theupward motion is amplified by the upslopewinds.

6. A difference in visibility between the sea airand the inland air often is a visual clue tothe leading edge of the sea breeze. Visibilityin the sea air may be restricted by hazewhile visibility inland is unrestricted. Onthe other hand, the sea air may be quiteclear while visibility inland is restricted bydust or smoke.

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LOCAL SEA BREEZE EXPLORATIONS

Unfortunately, a sea breeze front is not alwayseasy to find, and it is likely that many an oppor-tunity for sea breeze soaring goes unnoticed. Asyet, little experience has been accrued in locatinga belt of sea breeze lift without visual clues such asclouds, haze, or converging smoke or dust plumes..As the sport of soaring grows, so will the knowledgeof sea breeze soaring expand and the peculiaritiesof more local areas come to light. In the UnitedStates, the area where the most experience prob-ably has been gained is over the southern Cali-fornia high desert where the sea breeze moves east-ward over the Los Angeles Coastal Plain into theMojave Desert.

Los Angeles "Smoke Front"The sea breeze front moving from the Los An-

geles coastal plain into the Mojave Desert has beendubbed the "Smoke Front." It has intense thermalactivity and offers excellent lift along the leadingedge of the front. Associated with the sea breeze

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0 10 20 30 40 Tr*I i i 1 -I I

STATUTE MILES

FtouRz 166. Sea breeze flow into the San Fernando Valley. Note the San Fernando convergence zone, upper left, andthe Elsinore convergence zone, lower right.

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that moves inland over the Los Angeles coastalplain are two important zones of convergence,shown in figure 166. Sea breezes of different originmeet in the convergence zones producing verticalcurrents capable of supporting sailplanes. Oneconvergence line is the "San Fernando Conver-gence Zone;" a larger scale zone is in the Elsinorearea, also shown in figure 166. This convergencezone apparently generates strong vertical currentssince soaring pilots fly back and forth across thevalley along the line separating smoky air to thenorth from relatively clear air to the south. Alti-tudes reached depend upon the stability, but usual-ly fall wi thin the 6,000 feet to 12,000 feet ASLrange for the usual dry thermal type lift. Seaward,little or no lift is experienced in the sea breeze airmarked by poor visibility.

Cape Cod PeninsulaFigure 167 shows converging air between sea

breezes flowing inland from opposite coasts of the

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Cape Cod Peninsula. Later in the development ofthe converging sea breezes, the onset of convectionis indicated by cumulus over the peninsula. Sail-plane pilots flying over this area as well as overLong Island, New York, have found good lift inthe convergence lines caused by sea breezes blow-ing inland from both coasts of the narrow landstrips.

Great takes AreaSea breeze fronts have been observed along the

shore lines of the Great Lakes. Weather satelliteshave also photographed this sea breeze effect onthe western shore of Lake Michigan. It is quitelikely that conditions favorable for soaring occurat times.

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14 POSSIBLE LOCATION OFif.SEA BREEZE -FRONT

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7000 FT.

6000 FT.

5000 FT.

4000 FT.

3000 FT.

2000 FT.

1000 FT.

Rome 167. Sea breeze convergence zone, Cape Cod, Massachusetts. Sea breezes from opposite coastsconverge over the cape.

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RIDGE OR HILL SOARING

Wind blowing toward hills or ridges flows up-ward, over and around the abrupt rises in terrain.The upward moving air creates lift which is some-times excellent for soaring. Figure 168 is a schematicshowing area of best lift. Ridge or hill soaringoffers great sport to the sailplane pilot who acceptsthe challenge and can wait for proper wind andstability combinations.

WINDTo create lift over hills or ridges,. wind direction

should be within about 30 to 40 degrees norm21to the ridge line. A sustained speed of 15 knots ormore usually generates enough lift to support asailplane. Height-of-the lift usually is two or threetimes the height of the rise from the valley floorto the ridge crest. Strong winds tend to increaseturbulence and low-level eddies without an appre-ciable increase in the height of the lift.

STABILIT'Stability affects the continuity and extent of lift

over hills or ridges. Stable air allows relatively

streamlined upslope flow. A pilot experiences littleor no turbulence in the steady, uniform area ofbest lift shown in figure 168. Since stable air tendsto return to its original level, air spilling over thecrest and downslope is churned into a snarl ofleeside eddies, also shown in. .6gure 168. Thus,stable air favors smooth lilt but troublesome leesidelow-altitude turbulence.

When the airstream is moist and unstable, up.slope lift may release the instability generatingstrong convective currents and cumulus clouds overwindward slopes and hill crests. The initially lam-inar flow is broken up into convective cells. Whilethe updrafts produce good lift, strong downdraftsmay compromise low altitude flight over roughterrain. As with thermals, the lift will be transitoryrather than smooth and uniform.

STEEPNESS OF SLOPEVery, gentle slopes provide little or no lift. Most

favorable for soaring is a smooth, moderate slope.An ideal slope is about 1 to 4 which with an up-slope wind of 15 knots creates lift of about 6 feet

PO...,..011.....'

FIGURE 168. Schematic cross section of airflow aver a ridge. Note the area of best lift. Downdrafts predominate leewardin the "wind shadow."

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per second. With the same slope, a high-perfor- potential lift. The turbulent eddies also enhancemance sailcraft with a sinking speed of 2 feet per the possibility. of a low-altitude upset.second presumably could remain airborne withonly a 5-knot wind!

Very steep escarpments or rugged slopes induce CONTINUITY OF RIDGESturbulent eddies. Strong winds extend these eddies Ridges extending for several miles withoutto a considerable height usually disrupting any abrupt breaks tend to provide uniform lift through-

FlOURV, 169. Strong winds flowing around an isolated peak produce little lift. During light winds, sunlit slopes may bea favored location for thermals.

196

IL 401.1)'Ir .14.4

out their length. In contrast, a single peak divertswind flow around the peak as well as over it andthus is less favorable for soaring. Figure 169 showswind flow around an isolated peak.

Some wind flow patterns over ridges and hillsare illustrated in figure 170. Deviations from thesepatterns depend on wind direction and speed, onstability, on slope profile, and on general terrainroughness.

SOARING IN UPSLOPE LIFTThe soaring pilot, always alert, must remain

especially so in seeking or riding hill lift. You maybe able to spot indicators of good lift. Other cluesmay mark areas to avoid..

When air is unstable, do not venture too near theslope. You can identify unstable air either by theupdrafts and downdrafts in dry thermals or bycumulus building over hills or ridges. Approachingat too low an altitude may suddenly put you in adowndraft, forcing an inadvertent landing.

When winds are strong, surface friction maycreate low-level eddies even over relatively smoothslopes. Also, friction may drastically reduce theeffective wind speed near the surface. When climb-ing at low altitude toward a slope under theseconditions, be prepared to turn quickly toward thevalley in event you lose lift. Renew your attemptto climb farther from the hill.

If winds are weak, you may find lift only very nearthe sloping surface. Then you must "hug" the slopeto find needed lift. However, avoid this procedureif there are indications of up and down drafts. Ingeneral, for any given slope, keep your distancefrom the slope proportional to wind speed.

Leeward of hills and ridges is an area wherewind is blocked by the obstruction. Among soaringcircles this area is called the "wind shadow." Inthe wind shadow, downdrafts predominate asshown in figure 168. If you stray into the windshadow at an altitude near or below the altitudeof the ridge crest, you may be embarrassed by anunscheduled and possibly rough landing. Try tostay within the area of best lift shown in figure 168.

Fromm 170. Windflow over various types of terrain. Themany deviations from these patterns depend on windspeed, slope profile, and terrain roughness.

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MOUNTAIN WAVE SOARING

The great attraction of soaring in mountainwaves stems from the continuous lift to greatheights. Soaring flights to above 35,000 feet havefrequently been made in mountain waves. Once asoaring pilot has reached the rising air of a moun-tain wave, he has every prospect of maintainingflight for several hours. While mountain wavesoaring is related to ridge or hill soaring, the lift ina mountain wave is on a larger scale and is lesstransitory than Iift over smaller rises in terrain.Figure 171 is a cross section of a typical mountainwave.

FORMATIONWhen strong winds blow across a mountain

range, large "standing" waves occur downwindfrom the mountains and upward to the tropopause.The waves may develop singly; but more often,they occur as a series of waves downstream fromthe mountains. While the waves remain aboutstationary, strong winds are blowing through them.

KILOMETERS 5

You may compare a mountain wave to a series ofwaves formed downstream from a submerged rockyridge in a fast flowing creek or river. Air dipssharply immediately to the lee of a ridge, thenrises and falls in a wave motion downstream.

A strong mountain wave requires:

1. Marked stability in the airstream disturbedby the mountains. Rapidly building cumu-lus over the mountains visually marks theair unstable; convection, evidenced by thecumulus, tends to deter wave formation.

2. Wind speed at the level of the summitshould exceed a minimum which variesfrom 15 to 25 knots depending on the heightof the range. Upper winds should increaseor at least remain constant with height upto the tropopause.

3. Wind direction should be within 30 degreesnormal to the range. Lift diminishes aswinds more nearly parallel the range.

10 15 20 25

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=MAIN DOWNDRAFT (MAX. $,000 FEET/MINUTE)

=00 = MAIN UPDRAFT (MAX. 5000 FEET/MINUTE)

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FIGURE 171. Schematic cross section of a mountain wave. Best lift is upwind from each wave crept for about one-third thedistance to the preceding wave crest

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WAVE LENGTH AND AMPLITUDE

Wave length is the horizontal distance betweencrests of successive waves and is usually between2 and 25 miles. In general, wave length is con-trolled by wind component perpendicular to theridge and by stability of the upstream flow. Wavelength is directly proportional to wind speed andinversely proportional to stability. Figure 172 illus-trates wave length and also amplitude.

Amplitude of a wave is the vertical dimensionand is half the altitude difference between thewave trough and crest. In a typical wave, ampli-tude varies with height above the ground. It isleast near the surface and near the tropopause.Greatest amplitude is roughly 3,000 to 6,000 feetabove the ridge crest. Wave amplitude is controlledby size and shape of the ridge as well as wind andstability. A shallow layer of great stability andmoderate wind produces a- greater wave amplitudethan does a deep layer of moderate stability andstrong winds. Also, the greater the amplitude, theshorter is the wave length. Waves offering thestrongest and most consistent lift are those withgreat amplitude and short wave length.

VISUAL INDICATORS

If the air has sufficient moisture, lenticular (lens-shaped) clouds mark wave crests. Cooling of air

ascending toward the wave crest saturates the airforming clouds. Warming of air descending beyondthe wave crest evaporates the cloud. Thus, by con-tinuous condensation windward of the wave crestand evaporation leeward, the cloud appears sta-tionary although wind may be blowing through thewave at 50 knots or more. Lenticular clouds insuccessive bands downstream from the mountainmark a series of wave crests.

Spacing of lenticulars Mav+.5 the wave length.Clearly identifiable lenticulars also suggest largerwave amplitude than clouds which barely exhibitlenticular form. These cloud types along withstratiform clouds on the windward slopes and alongthe mountain crest indicate the stability favorableto mountain wave soaring.

Thunderstorms or rapidly building cumulus overmountains mark the air unstable. As they reachmaturity, the thunderstorms often drift downwindacross leeward valleys and plains. Strong convectivecurrents in the unstable air deter wave formation.If you sight numerous instability clouds, wait untilanother day for mountain wave soaring.

SOARING TURBULENCEA mountain wave, in a manner similar to that

in a thermal, means turbulence to powered aircraft,but to a slowly moving sailcraft, it produces lift

Flamm 172. Wave length and amplitude.

2 0 5199

and sink above the level of the mountain crest.But as air spills over the Crest like a waterfall, itcauses strong downdrafts. The violent overturningforms a series of "rotors" in the wind shadow ofthe mountain which are hazardous even to asailplane (see ch. 9, figs. 81 through 84). Cloudsresembling long bands of stratocumulus sometimesmark the area of overturning air. These "rotorclouds" appear to remain stationary, parallel therange, and stand a few miles leeward of the moun-tains. Turbulence is most frequent and most severein the standing rotors just beneath the wave crestsat or below mountain-top levels. This rotor turbu-lence is especially violent in waves generated bylarge mountains such as the Rockies. Rotor turbu-lence with lesser mountains is much less severe butis always present to some extent. The turbulenceis greatest in well-developed waves.

FAVORED AREASMountain waves occur most frequently along

the central and northern Rockies and the northernAppalachians. Occasionally, waves form to the leeof mountains in Arkansas, Oklahoma, and south-western Texas. Weather satellites have observedwaves extending great distances downwind fromthe Rocky Mountains; one series extended fornearly 700 miles. The more usual distance is 150to 300 miles. White Appalachian waves are not asstrong as those over the Rockies, they occur fre-quently; and satellites have observed them at an

average of 115 miles downwind. Wave length ofthese waves averages about 10 nautical miles.

RIDING THE WAVESYou often can detect a wave by the uncanny

smoothness of your climb. On first locating a wave,turn into the wind and attempt to climb directlyover the spot where you first detected lift providedyou can remain at an altitude above the level ofthe mountain crest. The lee side turbulent areais for the experienced pilot only. After cautiouslyclimbing well up into the wave, attempt to deter-mine dimensions of the zone of lift. If the wave isover rugged terrain, it may be impossible and un-necessary to determine the wave length. Lift oversuch terrain is likely to be in patchy bands. Overmore even terrain, the wave length may be easyto determine and use in planning the next stage offlight.

Wave clouds are a visual clue in your search forlift. The wave-like shape of lenticulars is usuallymore obvious from above than from below. Liftshould prevail from the crest of the lenticulars up-wind about one-third the wave length. When yourcourse takes you across the waves, climb on thewindward side of the wave and fly as quickly aspossible to the windward side of the next wave.Wave lift of 300 to 1,200 feet per minute is notuncommon. Soaring pilots have encountered ver-tical currents exceeding 3,000 feet per minute, thestrongest ever reported being 8,000 feet per minute.

IN CLOSING

Records are made to be broken. Altitude anddistance records are a prime target of many sail-plane enthusiasts. Distance records may be possibleby flying a combination of lift sources such as ther-mal, frontal, ridge, or wave. Altitude records areset in mountain waves. Altitudes above 46,000 feethave been attained over the Rocky Mountains;soaring flights to more than 24,000 feet have beenmade in Appalachian waves; and flights to as high

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as 20,000 feet have been recorded, from NewEngland to North Carolina.

We sincerely hope that this chapter has givenyou an insight into the minute variations in wea-ther that profoundly affect a soaring aircraft. Whenyou have remained airborne for hours withoutpower; you have met a unique challenge and ex-perienced a singular thrill of flying.

GLOSSARY OF WEATHER TERMS

A

absolute instabilityA state of a layer within the atmo-sphere in which the vertical distribution of - temperatureis such that an air parcel, if given an upward or down-ward push, will move away from its initial level withoutfurther outside force being applied.

absolute temperature scaleSee Kelvin TemperatureScale.

absolute voracitySee voracity.

adiabatic processThe process by which fixed relation-ships are maintained during changes in temperature,volume, and pressure in a body of air without heat beingadded or removed from the body.

advectionThe horizontal transport of air or atmosphericproperties. In meteorology, sometimes referred to as thehorizontal component of convection.

advection fogFog resulting from the transport of warm,humid air over a cold surface.

air densityThe mass density of the air in terms of weightper unit volume.

air mass-1n meteorology, an extensive body of air withinwhich the conditions of temperature and moisture in ahorizontal plane are essentially uniform.

air mass classificationA system used to identify and tocharacterize the different air masses according to a basicscheme. The system most commonly used classifies airmasses primarily according to the thermal propertiPs ftheir source regions: "tropical" (T); "polar" (P); sthd"Arctic" or "Antarctic" (A). They are further classifiedaccording to moisture characteristics as "continental"(c) or "maritime" (m).

air parcelSee,parcel.

albedoThe ratio of the amount of electromagnetic radio.Lion reflected by a body to the amount incident upon it,commonly expressed in percentage; in meteorology, usu-ally used in reference to Insolation (solar radiation); i.e.,the albedo of wet sand is 9, meaning that about 9% ofthe incident Insolation is reflected; albedocs of other sur-faces range upward to 8045 for fresh snow cover; aver-age albedo for the earth and its atmosphere has beencalculated to range from 35 to 43.

altimeterAn instrument which determines the altitude ofan object with respect to a fixed level. See pressurealtimeter.

altimeter settingThe value to which the scale of a pres-sure altimeter is set so as to read true altitude at fieldelevation.

altimeter setting indicator A precision aneroid barometercalibrated to indicate directly the altimeter setting.

altitudeHeight expressed in units of distance above areference plane, usually above mean sea level or aboveground.

(1) corrected altitudeIndicated altitude of an air-craft altimeter corrected for the temperature of thecolumn of air below the aircraft, the correction beingbased on the estimated departure of existing tem-perature from standard atmospheric temperature;an approximation of true altitude.

(2) density altitudeThe altitude in the standard at-mosphere at which the air has the same density asthe air at the point in questioh. An aircraft will havethe same performance characteristics as it wouldhave in a standard atmosphere at this altitude.

indicated altitudeThe altitude above mean sealevel indicated on a pressure altimeter set at currentlocal altimeter sett*.

(4) pressure altitudeThe altitude in the standard at-mosphere at which the pressure is the same as at thepoint in question. Since an altimeter Operates solelyon pressure, this is the uncorrected altitude indicatedby an altimeter set at standard sea level pressure of29.92 inches or 1013 millibars.

radar altitudeThe altitude of an aircraft deter-mined by radar-type radio altimeter; thus the actualdistance from the nearest terrain or water featureencompassed by the downward directed radar beam.For all practical purposes, it is the "actual" distanceabove a ground or inland water surface or the truealtitude above an ocean surface.

(6) true altitudeThe exact distance above mean sealevel.

(3)

(5)

altocumulusWhite or gray layers or patches of cloud,often with a waved appearance; cloud elements appearas rounded masses or rolls; composed mostly of liquidwater droplets which may be supercooled; may containice crystals at subfreezing temperatures.

altocumulus castellanusA species of middle cloud ofwhith at least a fraction of its upper part presents somevertically developed, cumuliform protuberances (someof which are taller than they are wide, as castles) andwhich give the cloud a crenelated or turreted appear-ance; especially evident when seen from the side; ele-ments usually have a common base arranged in fines.

. This cloud indicates instability and turbulence at thealtitudes of occurrence.

anemometerAn instrument for measuring wind speed.

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aneroid barometerA barometer which operates on theprinciple of having changing atmospheric pressure benda metallic surface which, in turn, moves a pointer acrossa scale graduated in units of pressure.

angel-1n radar meteorology, an echo caused by physicalphenomena not discernible to the eye; they have beenobserved when abnormally strong temperature and/ormoisture gradients were known to exist; sometimes at-tributed to insects or birds flying in the radar beam.

anomalous propagation (sometimes called AP)In radarmeteorology, the greater than normal bending of theradar beam such that echoes are received from groundtargets at distances greater than normal ground clutter.

anticycloneAn area of high atmospheric pressure whichhas a closed circulation that is anticyclonic, i.e., as viewedfrom above, the circulation is clockwise in the NorthernHemisphere, counterclockwise in the Southern Hemi-sphere, undefined at the Equator.

anvil eloudPoputar name given to the top portion of acumulonimbus cloud having an anvil-like form.

APOIIA sounding made by an aircraft.

Arctic .-iir=An air mass with characteristics developedmostly in winter over Arctic surfaces of ice and snow.Arctic air extends to great heights, and the surface tem-peratures are basically, but not always, lower than thoseof polar air.

Arctic front The surface of discontinuity between verycold (Arctic) air flowing directly from the Arctic regionand another less cold and, consequently, less dense airmass.

astronomical twilightSee twilight.

atmosphereThe mass of air surrounding the Earth.

atmospheric pressure (also called barometric pressure)The pressure exerted by the atmosphere as a consequenceof gravitational attraction exerted upon the "column" ofair lying directly above the point in question.

atmospherics -- Disturbing effects produced in radio re-ceiving apparatus by atmospheric electrical phenomenasuch as an electrical storm. Static.

auroraA luminous, rad!ztt emission over middle andhigh latitudes confined to the thin air of high altitudesand centered over the earth's magnetic poles. Called"aurora borealis" (northern lights) or "aurora australis"according to its occurrence in the Northern or SouthernHemisphere, respectively.

attenuationIn radar meteorology, any process whichreduces power density in radar signals.(I) precipitation attenuationReduction of power

density because of absorption or reflection of energyby precipitation.

(2) range attenuationReduction of radar powerdensity because of distance from the antenna. Itoccurs in the outgoing beam at a rate proportionalto 1 /range=. The return signal is also attenuated atthe same rate.

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BbackingShifting of the wind in a counterclockwise di-

rection with respect to either space or time; opposite ofveering. Commonly used by meteorologists to refer to acyclonic shift (counterclockwise in the Northern Hemi-sphere and clockwise in the Southern Hemisphere).

baascatterPertaining to radar, the energy reflected orscattered by a target; an echo.

banner cloud (also called cloud banner)A banner-likecloud streaming of from a mountain peak.

barographA continuous-recording barometer.

barometerAn instrument for measuring the pressure ofthe atmosphere; the two principle types are mercurial andaneroid.

barometric altimeter Sec pressure altimeter.

barometric pressureSame as atntospheric)iressure.

barometric tendencyThe change of barometric pressurewithin a specified period of time. In'aviation weather ob-servations, routinely determined periodically, usually fora 3-hour period.

beam resolutionSee resolution.

Beaufort scaleA scale of wind speeds.

black blizzardSame as duststorm.

blizzardA severe weather condition characterized by lowtemperatures and strong winds bearing a great amount ofsnow, either falling or picked up from the ground.

blowing dustA type of lithometeor composed of dust par-ticles picked up locally from the surface and blown aboutin clouds or sheets.

blowing sandA type of lithometeor composed of sandpicked up locally from the surface and blown about inclouds or sheets.

blowing snowA type of hydrometeor composed of snowpicked up from the surface by the wind and carried to aheight of 6 feet or more.

blowing sprayA type of hydrorneteor composed of waterpartides picked up by the wind from the surface of alarge body of water.

bright bandIn radar meteorology, a narrow, intense echoon the range-height indicator scope resulting from water-covered ice particles of high reflectivity at the meltinglevel.

Buys Ballot's law If an observer in the Northern Hemi-sphere stands with his back to the wind, lower pressure isto his Ieft.

Ccalm 'The absence of wind or of apparent motion of the

air.

cap cloud (also called cloud eap)A standing or station-ary cap-like cloud crowning a mountain summit.

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ceilingIn meteorology in the U.S., (1) the height abovethe surface of the base of the lowest layer of clouds orobscuring phenomena. ..aloft that hides more than half of thesky, or (2) -the vertical visibility into an obscuration. Seesummation principle.

ceiling balloonA small balloon used to determine theheight of a cloud base or the extent of vertical visibility.

ceiling lightAn instrument which projects a verticallight beam onto the base of a cloud or into surface-based obscuring phenomena; used at night in conjunc-don with a clinometer to determine the height of the cloudbase or as an aid in estimating the vertical visibility.

ceilometerA cloud-height measuring system. It projects.light on the cloud, detects the reflection by a photo-electric cell, and determines height by triangulation.

Celsius temperature scale (abbreviated C)A tempera-ture scale with zero degrees as the melting point of pureice and 100 degrees as the boiling point of pure water atstandard sea level atmospheric pressure.

Centigrade temperature scaleSame as Celsius tempera-ture scale.

chaffPertaining to radar, (1) short, finc strips of metallicfoil dropped from aircraft, usually by military forces,specifically for the purpose of jamming radar; (2) appliedloosely to echoer resulting from chaff.

change of stateIn meteorology, the transformation ofwater from one form, i.e., solid (ice), liquid, or gaseous(water vapor), to any other form. There are six possibletransformations designated by the five terms following:(1) condensationThe change of water vapor to liquid

water.(2) evaporation--The change of liquid water to water

vapor.(3) freezingThe change of liquid water to ice.(4) meltingThe change of ice to liquid water.(5) sublimationThe change of (a) ice to water vapor

or (b) water vapor to ice. See latent heat.

ChinookA warm, dryfochn wind blowing down the east-ern slopes of the Rocky Mountains over the adjacentplains in the U.S. and Canada.

cirriformAll species and varieties of cirrus, cirrocumulus,and cirrostratus clouds; descriptive of clouds composedmostly or entirely of small ice crystals, usually transparentand white; often producing halo phenomena not observedwith other cloud forms. Average height ranges upwardfrom 20,000 feet in middle latitudes.

cirrocumulusA cirriform cloud appearing as a thin sheetof small white puffs resembling flakes or patches of cottonwithout shadows; sometimes confused with altocumulus.

cirrostratusA cirriform cloud appearing as a whitish veil,usually fibrous, sometimes smooth; often produces halophenomena; may totally cover the sky.

cirrusA cirriform cloud in the form of thin, white feather-like clouds in patches or narrow bands; have a fibrousand/or silky sheen; large ice crystals often trail down-

ward a considerable vertical distance in fibrous, slanted,or irregularly curved wisps called mares' tails.

civil twilightSee twilight.

clear air turbulence (abbreviated CAT)Turbulenceencountered in air where no clouds are present; morepopularly applied to high level turbulence associatedwith wind shear.

clear icing (or clear ice)Generally, the formation of alayer or mass of ice which is relatively transparent be-cause of its homogeneous structure and small numberand size of air spaces; wed commonly as synonymouswith glaze, particularly with respect to aircraft icing.Compare with rime icing. Factors which favor clear icingare large drop size, such as those found in cumuliformclouds, rapid accretion of supercooled water, and slowdissipation of latent heat of fusion.

climateThe statistical collective of the weather condi-tions of a point or area during a specified interval of time(usually several decades); may be expressed in a varietyof ways.

climatologyThe study of climate.

clinometerAn instrument used in weather observing formeasuring angles of inclination; it is used in conjunctionwith a ceiling tight to determine cloud height at night.

cloud bankGenerally, a fairly well-defined mass of cloudobserved at a distance; it covers an appreciable portionof the horizon sky, but does not extend overhead.

cloudburstIn popular teminology, any sudden and heavyfall of rain, almost always of the shower type.

cloud capSee cap cloud.

cloud detection radarA vertically directed radar to de-tect cloud bases and tops.

cold frontAny non-occluded front which moves in such away that colder air replaces warmer air.

condensationSee change of state.

condensation levelThe height at which a rising parcel orlayer of air would become saturated if lifted adiabatically.

condensation nucleiSmall particles in the air on whichwater vapor condenses or sublimates.

condensation trail (or contrail) (also called vapor trail)A cloud-like streamer frequently observed to form behindaircraft flying in clear, cold, humid air.

conditionally unstable airUnsaturated air that will be-come unstable on the condition it becomes saturated. Seeinstability.

conductionThe transfer of heat by molecular actionthrough a substance or from one substance in contactwith another; transfer is always from warmer to coldertemperature.

constant pressure chartA chareof a constant pressuresurface; may contain analyses of height, wind, tempera-ture, humidity, and/or other elements.

2 ti 9203

continental polar airSee polar air.

continental tropical airSee tropical air.

contourIn meteorology, (I) a line of equal height on aconstant pressure chart; analogous to contours on a re-lief map; (2) in radar meteorology, a line on a radarscope of equal echo intensity.

contouring circuitOn weather radar, a circuit whichdisplays multiple contours of echo intensity simulta-neously on the plan position indicator or range-height indicatorscopc..See contour (2).

contrail Contraction for condensation trail.

convection(1) in general, mass motions within a fluidresulting in transport and mixing of the properties ofthat fluid. (2) In meteorology, atmospheric motions thatare predominantly vertical, resulting in vertical transportand mixing of atmospheric properties; distinguished fromadvection.

convective cloudSee cumuliforin.

convective condensation level (abbreviated CCL)Thelowest level at which condensation will occur as a resultof convection due to surface heating. When condensationoccurs at this level, the layer between the surface and theCCL will be thoroughly mixed, temperature lapse ratewill be dry adiabatic, and mixing ratio will be constant.

convective instabilityThe state of an unsaturated layerof alr whose lapse rates of temperature and moisture aresuch that when lifted adiabatically until the layer be-comes saturated, convection is spontaneous.

convergenceThe condition that exists when the distri-bution of winds within a given area is such that there isa net horizontal inflow of air into the area. In conver-gence at lower levels, the removal of the tesulting excessis accomplished by an upward movement of air; conse-quently, areas of low-level convergent winds are regionsfavorable to the occurrence of clouds and precipitation.Compare with divergence.

Coriolis forceA deflective force resulting from earth's ro-tation; it acts to the right of wind direction in the North-ern Hemisphere and to the left in the Southern Hemi-sphere.

coronaA prismatically colored circle or arcs of a circlewith the sun or moon at its center; coloration is fromblue inside to red outside (opposite that of a halo); variesin size (much smaller) as opposed to the fixed diameterof the halo; characteristic of clouds composed of waterdroplets and valuable in differentiating between middleand cirriform clouds.

corpossnt See St. Elmo's Fire.

corrected altitude (approximation of true altitude) Seealtitude.

cumuliform --A term descriptive of all convective cloudsexhibiting vertical development in contrast to the hori-zontally extended stratifonn types.

cumulonimbus A cumuliform cloud type; it Is heavy anddense, with considerable vertical extent in the form of

204

massive towers; often with tops in the shape of an anvilor massive plume; under the base of cumulonimbus,which often is very dark, there frequently exists virga,precipitation and low ragged clouds (scud), either mergedwith it or not; frequently accompanied by lightning,thunder, and sometimes hail; occasionally produces atornado or a waterspout; the ultimate manifestation ofthe growth of a cumulus cloud, occasionally extendingwell into the stratosphere.

cumulonimbus mammaA cumulonimbus cloud havinghanging protuberances, like pouches, festoons, or udders,on the under side of the cloud; usually indicative ofsevere turbulence.

cumulusA cloud in the form of individual detacheddomes or towers which are usually dense and well de-fined; develops vertically in the form of rising mounds ofwhich the bulging upper part often resembles a cauli-flower; the sunlit parts of these clouds are mostly brilliantwhite; their bases are relatively dark and nearly hori-zontal.

cumulus fractusSee fractus.

eyeIogenesisAny development or strengthening of cy-clonic circulation in the atmosphere.

cyclone(1) An area of low atmospheric pressure whichhas a closed circulation that is cyclonic, i.e., as viewedfrom above, the circulation is counterclockwise in theNorthern Hemisphere, clockwise in the Southern Hemi-sphere, undefined at the Equator. Because cyclonic circu-lation and relatively low atmospheric pressure usually co-exist, in common practice the terms cyclone and low areused interchangeably. Also, because cyclones often areaccompanied by inclement (sometimes destructive)weather, they are frequently referred to simply as storms.(2) Frequently misused to denote a tornado. (3) In theIndian Ocean, a tropical cyclone of hurricane or typhoonforce.

DdeepeningA decrease in the central pressure of a pres-

sure system; usually applied to a low rather than to ahigh, although technically, it is acceptable in either sense.

density(l) The ratio of the mass of any substance to thevolume ft occupiesweight per unit volume. (2) Theratio of any quantity to the volume or area it occupies,i.e., population per unit area, power density.

density altitudeSee altitude.

depressionIn meteorology, an area of low pressure; alow or trough. This is usually applied to a certain stage inthe development of a tropical cyclone, to migratory lowsand troughs, and to upper-level lows and troughs thatare only weakly developed.

dewWater condensed onto grass and other olijects nearthe ground, the temperatures of which have fallen belowthe initial dew point temperature of the surface air, butis still above freezing. Compare with frost.

dew point (or dew-point tempeeature)The tempera-ture to which a sample of air must be cooled, while the

.-

mixing ratio and barometric pressure remain constant, inorder to attain saturation with respect to water.

discontinuityA zone with comparatively rapid transitionof one or more meteorologi.cal elements.

disturbanceIn meteorology, applied rather loosely: (1)any low pressure or cyclone, but usually one that is rela-tively small in size; (2) an area where weather, wind,pressure, etc., show signs of cyclonic development; (3)any deviation in flow or pressure that is associated witha disturbed state of the weather, i.e., cloudiness and pre-cipitation; and (4) any individual circulatory systemwithin the primary circulation of the atmosphere.

diurnalDaily, especially pertaining to a cycle completedwithin a 24-hour period, and which recurs every 24 hours.

divergenceThe condition that exists when the distribu-tion of winds within a given area is such that there is anet horizontal flow of air outward from the region. Indivergence at lower levels, the resulting deficit is com-pensated for by subsidence of air from aloft; consequentlythe air is heated and the relative humidity lowered mak-ing divergence a warming and drying process. Low-leveldivergent regions are areas unfavorable to the occurrenceof clouds and precipitation. The opposite of convergence.

doldrumsThe equatorial belt of calm or light and vari-able winds between the two tradewind belts. Compare

intertropical convergence Lose.

downdraftA relative small scale downward current ofair; often observed on the lee side of large objects re-stridng the smooth flow of the air or in precipitationareas in or near aunuliform clouds.

drifting snowA type of hydrometeor composed of snowparticles picked up from the surface, but carried to aheight of less than 6 feet.

drizzlelA form of precipitation. Very small water dropsthat appear to float with tke air currents while falling inan irregular path (unlike rain, which falls in a compara-tively straight path, and unlike fog droplets which remainsuspeqed in the air).

dropson0eA radiosonde dropped by parachute from anaircraft to obtain soundings (measurements) of the atmo-sphere tbelow.

dry adiabatic lapse rateThe rate of decrease of tem-perature. with height when unsaturated air is lifted adia-batically (due to expansion as it is lifted to lower pressure).Set adiabatic process.

dry bulb--A name given to an ordinary thermometer usedto determine temperature of the air; also used as a con-traction for dry -bulb temperature. Compare wet bulb,

dry-bulb temperatureThe temperature of the air.

dustA type of lithometeor composed of small earthen par-ticles suspended in the atmosphere.

dust devilA small, vigorous whirlwind, usually of shortduration, tendered visible by dust, sand, and debris pickedup from the ground.

duster - -Sande as duststorm.

dustatorm (also called duster, black blizzard)An un-usual, frequently severe weather condition characterizedby strong winds and dust-filled air over an extensive area.

D-valueDeparture of true altitude from pressure alti-tude (see altitude); obtained by algebraically subtractingtrue altitude from pressure altitude; thus it may be plusor minus. On a constant pressure chart, the differencebetween actual height and standard atmospheric height of aconstant pressure surface.

.-..

BeehoIn radar, (1) the energy reflected or scattered by a

target; (2) the radar scope presentation of the return froma target.

eddyA local irregularity of wind in a larger scale windflow. Small scale eddies produce turbulent conditions.

estimated ceilingA ceiling classification applied whenthe ceiling height has been estimated by the observer orhas been determined by some other method; but, becauseof the specified limits of time, distance, or precipitationconditions, a more descriptive classification cannot beapplied.

evaporationSee change of state.

extratropical low (sometimes called extratropical cy-clone, extratropical storm)Any cyclone that is not atropical 9slone, usually referring to the migratory frontalcyclones of middle and high latitudes.

eyeThe roughly circular area of calm or relatively lightwinds and comparatively fair weather at the center of awell-developed tropical cyclone. A wall cloud marks theouter boundary of the eye.

FFahrenheit temperature scale (abbreviated F)A tem-

perature scale with 32 degrees as the melting point ofpure ice and 212 degrees as the boiling point of purewater at standard sea level atmospheric pressure (29.92inches or 1013.2 millibars).

Fall windA cold wind blowing downsIope. Fall winddiffers from foehn in that the air is initially cold enoughto remain relatively cold despite compressional heatingduring descent.

fillingAn increase in the central press ;re of a pressuresystem; opposite of deepening; more commonly applied toa low rather than a high.

first gustThe leading edge of the spreading downdraft,plow wind, from an approaching thunderstorm.

flow lineA streamline.

foehnA warm, dry downstope wind; the warmness anddryness being due to adiabatic compression upon de-scent; characteristic of mountainous regions. See adia-batic process, Chinook, Santa Ana.

fog A 19*omettor consisting of numerous minute waterdroplets and based at the surface; droplets are smallenough to be suspended in the earth's atmosphere in-

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definitely. (Unlike drizzle, it does not fall to the surface;differs from cloud only in that a cloud is not based at thesurface; distinguished from haze by its wetness and graycolor.)

fractesClouds in the form of irregular shreds, appearingas if torn; have a clearly ragged appearance; applies onlyto stratus and cumulus, i.e., cumulus fractus and stratusfractus.

freezingSee change of state.

freezing levelA level in the atmosphere at which thetemperature is 0° C (32° F).

frontA surface, interface, or transition zone of discon-tinuity between two adjacent air masses of different den-sities; more simply the boundary between two differentair masses. See frontal zone.

frontal zoneA front or zone with a marked increase ofdensity gradient; used to denote that fronts are not trulya "surface" of discontinuity but rather a "zone" of rapidtransition of meteorological dements.

frontogenesisThe initial formation of a front or frontalOne.

frontolysisThe dissipation of a front.

frost (also boarfrost)lee crystal deposits formed by sub-limation when temperature and dew point are belowfreezing.

funnel cloudA tornado cloud or vortex cloud extendingdownward from the parent cloud but not reaching theground.

G

glazeA coating of ice, generally clear and smooth, formedby freezing of supercooled water on a surface. See clearicing.

gradientIn meteorology, a horizontal decrease in valuePer unit distance of a parameter in the direction of maxi-mum decrease; most commonly used with pressure, tem-perature, and moisture.

ground clutter Pertaining to radar, a duster of echoes,generally at short range, reflected from ground targets.

ground fogIn the United States, a fog that conceals teasthan 0.6 of the sky and is not contiguous with the base ofclouds.

gustA sudden brief increase in wind; according to U.S.weather observing practice, gusts are reported when thevariation in wind speed between peaks and lulls is atleast 10 knots.

HhailA form ofprecipitation composed of balls or irregular

lumps of ice, always produced by eonvective cloudswhich are nearly always cumulonimbus.

haloA prismatically colored or whitish circle or arcs of acircle with the sun or moon at its center; coloration, ifnot white, is from red inside to blue outside (opposite

206

that of a corona); fixed in size with an angular diameter of22° (common) or 46° (rare); characteristic of cloudscomposed of ice crystals; valuable in differentiating be-tween cimform and forms of lower clouds.

hazeA type of lithometeor composed of fine dust or saltparticles dispersed through a portion of the atmosphere;particles are so small they cannot be felt or individuallyseen with the naked eye (as compared with the largerparticles of dust), but diminish the visibility; distinguishedfrom fog by its bluish or yellowish tinge.

highAn area of high barometric pressure, with its at-tendant system of winds; an anticyclone. Also high pressuresystem.

hoar frostSee frost.

humidityWater vapor content of the air; may be ex-pressed as specific humidity, relative humidity, or mixing ratio.

hurricaneA tropical cyclone in the Western Hemispherewith winds in excess of 65 knots or 120 km/h.

bydrometeorA general term for particles of liquid wateror ice such as rain, fog, frost, etc., formed by modificationof water vapor in the atmosphere; also water or tee par-ticles lifted from the earth by the wind such as sea sprayor blowing snow.

hygrograph The record produced by a continuous-recording hygrometer.

hygrometerAn instrument for measuring the water vaporcontent of the air.

Iice crystalsA type of precipitation composed of unbranched

crystals in the form of needles, columns, or plates; usuallyhaving a very Alight downward motion, may fall from acloudless sky.

ice fogA type of fog composed of minute suspended par-ticles of icc; occurs at very tow temperatures and maycause halo phenomena.

ice needlesA form of ice crystals.

ice pelletsSmall, transparent or translucent, round orirregularly shaped pellets of ice. They may be (1) hardgrains that rebound on striking a hard surface or (2)pellets of snow encased in ice.

icing In general, any deposit of ice forming on an object.See clear icing, rime king, glaze.

indefinite ceilingA ceiling classification denoting verticalvisibility into a surface based obscuration.

indicated altitudeSee altitude.

insolationIncoming solar radiation falling upon the earthand its atmosphere.

instabilityA general term to indicate various states of theatmosphere in which spontaneous convection will occurwhen prescribed criteria are met; indicative of turbu-lence. See absolute instability, conditionally unstable air,convective instability.

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intertropical convergence zou The boundary zonebetween the trade wind srfem of the Northern andSouthern Hemispheres; ies characterized in maritimeclimates by showery pr cipitation with cumulonimbusclouds sometimes exte ng to great heights.

inversionAn ine e in temperature with heighta re-versal of the no decrease with height in the tropo-sphere; may be applied to other meteorologicalproperties.

isobarA line oft equal or constant barometric pressure.

iso echoIn ratar circuitry, a circuit that reverses signalstrength abo4 a specified intensity level, thus causing avoid on the ,Scope in the most intense portion of an echowhen maximum intensity is greater than the specifiedlevel.

boheightOn a weather chart, a line of equal height;same as contour (1).

isolineA line of equal value of a variable quantity, i.e.,an Udine of temperature is an isotherm, etc. See isobar,isotach, etc.

isoshearA line of equal wind shear.

biotechA line of equal or constant wind speed.

isothermA line of equal or constant temperature.

isothermalOf equal or constant temperature, with re-spect to either space or time; more commonly, tempera-ture with height; a zero lapse rate.

Ijet streamA quasi-horizontal stream of winds 50 knots or

more concentrated within a narrow band embedded inthe westerlies in the high troposphere.

Kkatabatic windAny wind blowing downslope. See fall

wind, foehn.

Kelvin temperature scale (abbreviated K)A tempera-ture scale with zero degrees equal to the temperature atwhich all molecular motion ceases, i.e., absolute zero(0° K = 273° C); the Kelvin degree is identical tothe Celsius degree; hence at standard sea level pressure,the melting point is 273° K and the boiling point 373° K.

knotA unit of speed equal to one nautical mile per hour.

Lland breezeA coastal breeze blowing from land to sea,

caused by temperature difference when the sea surface iswarmer than the adjacent land. Therefore, it usuallyblows at night and alternates with a sea breeze, whichblows in the opposite direction by day.

lapse rateThe rate of decrease of an atmospheric variablewith height; commonly refers to decrease of temperaturewith height.

latent heatThe amount of heat absorbed (converted tokinetic energy) during the processes of change of liquidwater to water vapor, ice to water vapor, or ice to liquid

water; or the amount released during the reverse pro.cases. Four basic classifications are:

(1) latent heat' of condensationHeat released duringchange of water vapor to water.

(2) latent heat of fusionHeat released during changeof water to ice or the amount absorbed in change ofice to water.

(3) latent heat Of sublimationHeat released duringchange of water vapor to ice or the amount absorbedin the change of ice to water vapor.

(4) latent heat of vaporiratioaHeat absorbed in thechange of water to water vapor; the negative oflatent heat of condensation.

layerIn reference to sky cover, clouds or other obscuringphenomena whose bases are approximately at the samelevel. The layer may be continuous or composed of de-tached elements. The term "layer" does not imply thata dear s; ace exists between the layers or that the cloudsor obscuring phenomena composing them are of the sametype.

lee waveny stationary wave disturbance caused by abarrier in a fluid flow. In the atmosphere when sufficientmoisture is present, this wave will be evidenced by len-ticular clouds to the lee of mountain barriers; also calledmountain wave or standing wave.

lenticular cloud (or lenticnlaria)A species of cloudwhose elements have the form of more or less isolated,generally smooth lenses or almonds. These clouds appearmost often in formations of orographic origin, the resultof lee waves, in which case they remain nearly stationarywith respect to the terrain (standing cloud), but theyalso occur in regions without marked orograOhy.

level of free convection (abbreviated isqThe levelat which a Parcel of air lifted dry-adiabatically until satu-rated and moist-adiabatically thereafter would becomewarmer than its surroundings in a conditionally unstableatmosphere._ Stu conditional instability and adiabaticprocess.

lifting condensation level (abbreviated LCL)Thelevel at which a parcel of unsaturated air lifted dry-adiabatically would become saturated. Compare level of

fret convection and convective condensation level.

ligbtningGenerally, any and all forms of visible electricaldischarge produced by a thunderstorm.

lithometeorThe general term for dry particles suspendedin the atmosphere such as dust, haze, smoke, and sand.

lowAn area of low barometric pressure, with its attend-ant system of winds. Also called a barometric depressionor gelato.

Mmammato cumulusObsolete. See cumulonimbus mamma.

mare'slailSec cirrus.

maritime polar air (abbreviated ml) See polar air.

maritime tropical air (abbreviated mT)See tropical air.

213 207*

maximum wind axisOn a constant pressure chart, a linedenoting the axis of maximum wind speeds at that con-stant pressure surface.

mean sea levelThe average height of the surface of thesea for all stages of tide; used as reference for elevationsthroughout the U.S.

measured ceilingA ceiling classification applied whenthe ceiling value has been determined by instruments orthe known heights of unobecured portions of objects,other than natural landmarks.

meltingSee change of state.

mercurial barometerA barometer in which pressure is de-termined by balancing air pressure against the weight ofa column of mercury in an evacuated glass tube.

meteorological visibilityln U.S. observing practice, amain category of visibility which includes the subcate-gories of prevailing visibility and runway visibility. Mete-orological visibility is a measure of horizontal visibilitynear the earth's surface, based on sighting of objects inthe daytime or unfocused lights of moderate intensity atnight. Compare slant visibility, runway visual range, verticalvisibility. See surface visibility, tower risibility, and sectorvisibility.

meteorologyThe science of the atmosphere.

nsicrobarographAn aneroid barograph designed to recordatmospheric pressure changes of very small magnitudes.

millibar (abbreviated mb.)An internationally used unitof pressure equal to 1,000 dynes per square centimeter.It is convenient for reporting atmospheric pressure.

mistA popular expression for drizzle or heavy fog.

mixing ratioThe ratio by weight of the amount of watervapor in a volume of air to the amount of dry air; usuallyexpressed as grams per kilogram (g/kg).

moist-adiAbatic lapse rate---See saturated-adiabatic lapserate.

moistureAn all-inclusive term denoting water in any orall of its three states.

nionsocasA wind that in summer blows from sea to a con-tinental interior, bringing copious rain, and in winterblows rrom the interior to the sea, resulting in sustained.dry weather.

mountain waveA standing wave or lee wave to the lee of amountain barrier.

Nnautical twilightSee twilight.

negative vorticitySee voracity.

nimbostratusA principal cloud type, gray colored, oftendark, the appearance of which Is rendered diffuse bymore or less continuously falling rain or snow, which inmost eases reaches the ground. It is thick enough through-out to blot out the sun.

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noctilueent cloudsClouds of unknown composition whichoccur at great heights, probably around 75 to 90 kilome-ters. They resemble thin cirrus, but usually with a bluishor silverish color, although sometimes orange to red,standing out against a dark night sky. Rarely observed.

normalIn meteorology, the value of an element averagedfor a given location over a period of years and recognizedas a standard.

numerical forecastingSee numerical weather prediction.

numerical weather predictionForecasting by digitalcomputers solving mathematical equations; used exten-sively in weather services throughout the world.

0ObscurationDenotes sky hidden by surface -based obscur-

ing phenomena and vertical visibility restricted overhead.

obscuring phenomenaAny hydromekor or &home:tor otherthan clouds; may be surface based or aloft.

occlusionSame as occluded front.

occluded front (commonly called occlusion, also calledfrontal occlusion)A composite of two fronts as a coldfront overtakes a warm front or quasi-stationary front.

orographic---Of, pertaining to, or caused by mountains asin orographic clouds, orographic lift, or orographic pre-cipitation.

ozoneAn unstable form of oxygen; heaviest concentra-tions are in the stratosphere; corrosive to some metals;absorbs most ultraviolet solar radiation.

..1m',0..;.., t P

parcelA small volume of air, small enough to containuniform distribution of its meteorological properties, andlarge enough to remain relatively self-contained and re-spond to all meteorological processes. No specific dimen-sions have been defined, however, the order of magnitudeof 1 cubic foot has been suggested.

partial obscurationA designation of sky cover when partof the sky is hidden by surface based obscuring phenomena.

pilot balloonA small free-lift balloon used to determinethe speed and direction of winds in the upper air.

pilot balloon observation (commonly called PIBAL)Amethod of winds-aloft observation by visually tracking aPilot balloon.

plan position indicator (PP1) scopeA radar indicatorscope displaying range and azimuth of targets in poi&coordinates.

plow windThe spreading downdraft of a thunderstorm; astrong, straight-line wind in advance of the storm. Seefirst gust.

Polar airAn air mass with characteristics developed overhigh latitudes, especially within the subpolar highs. Con-tinental polar air (eP) has cold surface temperatures, lowmoisture content, and, especially in its source regions, hasgreat stability in the lower layers. It is shallow in cora-

214

parison with Arctic air. Maritime polar (mP) initiallypossesses similar properties to those of continental polarair, but in passing over warmer water it becomes un-stable with a higher moisture content. Compare tropicalair.

polar frontThe semipermanent, semicontinuous frontseparating Mr masses of tropical and polar origins.

positive vorticitySee vorticity.

power densityIn radar meteorology the amount of ra-diated energy per unit cross sectional area in the radarbeam.

precipitationAny or all forms of water particles, whetherliquid or solid, that fall from the atmosphere and reachthe surface. It is a major class of hydrometeor, distinguishedfrom cloud and virga in that it must reach the surface.

precipitation attenuationSee attenuation.

pressureSee atmospheric pressure.

pressure altimeterAn aneroid barometer with a scale grad-uated in altitude instead of pressure using standard atmo-spheric pressure-height relationships; shows indicatedaltitude (not necessarily true altitude); may be set tomeasure altitude (indicated) from any arbitrarily chosenlevel. Ste altimeter setting, altitude.

pressure altitude --See altitude.

pressure gradientThe rate of decrease of pressure perunit distance at a fixed time.

pressure jumpA sudden, significant increase in stationPressure.

pressure tendencySee barometric tendency.

prevailing easterliesThe broad current or pattern ofpersistent easterly winds in the Tropics and in polarregions.

prevailing visibilityIn the U.S., the greatest horizontalvisibility which is equaled or exceeded throughout halfof the horizon circle; it need not be a continuous half.

prevailing westerliesThe dominant west-to-east motionof the atmosphere, centered over middle latitudes of bothhemispheres.

prevailing windDirection from which the wind blowsmost frequently.

prognostic chart (contracted PROG)A chart of ex-pected or forecast conditions.

pseudo - adiabatic lapse sateSee saturated-adiabatic lapserate.

psychrometerAn instrument consistit , of a wet-bulb anda dry-bulb thermometer for measuring wet-bulb and dry-bulb temperature; used to determine water vapor contentof the air.

pulsePertaining to radar, a brief burst of electromagneticradiation emitted by the radar; of very short time dura-tion. See puke length.

pulse lengthPertaining to radar, the dimension of aradar pulse; may be expressed as the time duration or thelength in linear units. Linear dimension is equal to timeduration multiplied by the speed of propagation (ap-proximately the speed of light).

Qquasi-stationary front (commonly called stationary

front)A front which is stationary or nearly so; conven-tionally, a front which is moving at a speed of less than 5knots is generally considered to be quasi-stationary.

RRADAR (contraction for radio detection and ranging)

An electronic instrument used for the detection and rang-ing of distant objects of such composition that they scatteror reflect radio energy. Since hydrometeors can scatter radioenergy, weather radars, operating on certain frequencybands, can detect the presence of precipitation, clouds, orboth.

radar altitudeSee altitude.

radar beamThe focused energy radiated by radar similarto a flashlight or searchlight beam.

radar echoSee echo.

radarsonde obiervationA rawiluonde observation in whichwinds are determined by radar tracking a balloon - bornetarget.

radiationThe emission of energy by a medium and trans-ferred, either through free space or another medium, inthe form of electromagnetic waves.

radiation fogFog characteristically resulting when radia-tional cooling of the earth's surface lowers the air tem-perature near the ground to or below its initial dew pointon calm, clear nights.

radiosondeA balloon-borne instrument for measuringpressure, temperature, and humidity aloft. Radiosondeobservationa sounding made by the instrument.

rainA form of precipitation; drops are larger than drizzleand fall in relatively straight, although not necessarilyvertical, paths as compared to drizzle which fails in ir-regular paths.

rain showerSee shower.

range attenuationSee attenuation.

range-height indicator (KM) scopeA radar indicatorscope displaying a vertical cross section of targets along aselected azimuth.

range resolutionSee resolution.

RAOBA radiosonde observation.

rawinA rawinsoede observation.

rawinsonde observationA combined winds aloft andradiosonde observation. Winds are determined by track-ing the radiosonde by radio direction finder or radar,

215209

refractionIn radar, bending of the radar beam by varia-tions in atmospheric density, water vapor content, andtemperature.

normal refractionRefraction of the rada beamunder normal atmospheric conditions; normal radiusof curvature of the beam is about 4 times the radiusof curvature of the Earth.

(2) superrefractionMore than normal bending of theradar beam resulting from abnormal vertical gra-clients of temperature and/or water vapor.

(3) subrefractionLess than normal bending of theradar beam resulting from abnormal vertical gra-dients of temperature and/or water vapor.

relative humidityThe ratio of the existing amount ofwater vapor in the air at a given temperature to the maxi-mum amount that could exist at that temperature; usually=premed in percent.

relative vortidtySee vordcity.remote scopeIn radar meteorology a "slave" scope re-

moted from weather radar.

resolutionPertaining to radar, the ability of radar toshow discrete targets separately, i.e., the better the resolu-don, the closer two targets can be to each other, and stiltbe detected as separate targets.(1) beam resolutionThe ability of radar to distinguish

between targets at approximately the same range butat different azimuths.

(2) range resolutionThe ability of radar to distinguishbetween targets on the same azimuth but at differentranges.

ridge (also called ridge line)In meteorology, an don-gated area of relatively high atmospheric pressure; usuallyassociated with and most clearly identified as an area ofmaximum anticyclonic curvature of the wind flow (leo-barr, contours, or streamlines).

rime icing (or rime ice) The formation of a white ormilky and opaque granular deposit of ice formed by therapid freezing of supercooled water droplets as they im-pinge upon an exposed aircraft.

rocketsondeA type of radiosonde launched by a rocket andmaking its measurements during a parachute descent;capable of obtaining soundings to a much greater heightthan possible by balloon or aircraft.

roll cloud (sometimes improperly called rotor cloud)A dense and horizontal roll-shaped accessory cloud lo-cated on the lower leading edge of a cumulonimbus or lessoften, a rapidly developing cumulus; indicative of turbu-lence.

rotor cloud (sometimes improperly called roll cloud)-- -A turbulent doud formation found in the lee of somelarge mountain barriers, the air in the. cloud rotatesaround an axis parallel to the range; indicative of pos-sible violent turbulence.

runway temperatureThe temperature of the air justabove a runway, 'ideally at engine and/or wing height,used in the determination of density altitude; useful atairports when critical values of density altitude prevail.

(1)

210

216

runway visibility The meteorological visibility along anidentified runway determined from a specified point onthe runway; may be determined by a transmissometv orby an observer.

runway visual rangeAn instrumentally derived hori-zontal distance a pilot should see down the runway fromthe approach end; based on either the sighting of highintensity runway lights or on the visud contrast of otherobjects, whichever yields the greatest visual range.

S

St. Elmo's Fire (also called corposant)A luminous brushdischarge of electricity from protruding objects, such asmasts and yardarms of ships, aircraft, lightning rods,steeples, etc., occurring in stormy weather.

Sarta. AnaA hot, dry, foehn wind, generally from thenortheast or east, occurring west of the Sierra NevadaMountains especially in the pass and river valley nearSanta Ana, California.

saturated adiabatic lapse rateThe rate of decrease oftemperature with, height as saturated air is lifted with nogain or loss of heat from outside sources; varies with tem-perature, being greatest at low temperatures. See adia-batic process and dry-adiabatic lapse rate.

saturationThe condition of the atmosphere when actualwater vapor present is the maximum possible at existingtemperature.

scudSmall detached masses of stratus }cactus clouds helowa layer of higher clouds, usually nimbostratus.

sea breezeA coastal breeze blowing from sea to land,caused by the temperature difference when the land sur-face is warmer than the sea surface. Compare land breeze.

sea fogA type of advectWn fog formed when air that hasbeen lying over a warm surface is transported over acolder water surface.

sea level pressureThe atmospheric pressure at mean sea level,either directly measured by stations at sea level or em-pirically determined from the station Pressure and tem-perature by stations not at sea level; used as a commonreference for analyses of surface pressure patterns.

sea smokeSame as steam fog.

sector visibility Meteorological visibility within a specifiedsector of the horizon circle.

sensitivity time controlA radar circuit designed to cor-rect for range attenuation so that echo intensity on thescope is proportional to reflectivity of the target regardlessof range.

sbearSee wind shear.

sbowerPreciPaation from a annulform cloud; characterizedby the suddenness of beginning and ending, by the rapidchange of intensity, and usually by rapid change in theappearance of the sky; showery precipitation may be inthe form of rain, ice pellets, or snow.

slant visibilityFor an airborne observer, the distance atwhich he can see and distinguish objects on the ground.

(A.

sleetSee ice pellets.

smogA mixture of smoke and fog.

smokeA restriction to visibility resulting from combustion.

enowPrecipitation composed of white or translucent icecrystals, chiefly in complex branched hexagonal form.

snow flurryPopular term for snow shower, particularly ofa very light and brief nature.

snow grainsPraipitation of very small, white opaquegrains of ice, similar in structure to snow crystals. Thegrains are fairly flat or elongeed, with diameters gener-ally less than 0.04 inch (1 mm.).

snow pelletsPraipitesion consisting of white, opaque ap-proximately round (sometimes conical) ice particles hav-ing a snow-like structure, and about 0.08 to 0.2 inch indiameter; crisp and easily crushed, differing in this re-spect from snow grains; rebound from a hard surface andoften break up.

snow showerSee shower.

solar radiationThe total electromagnetic radiation emittedby the sun. See insolation.

soundingIn meteorology, an upper-air observation; aradiosonde observation.

source regionAn extensive area of the earth's surfacecharacterized by relatively uniform surface conditionswhere large masses of air remain long enough to take oncharacteristic temperature and moisture properties im-parted by that surface.

specific bumidityThe ratio by weight of water vapor in asample of air to the combined weight of water vapor anddry air. Compare mixing ratio.

squallA sudden increase in wind speed by at least 15knots to a peak of 20 knots or more and lasting for atle'ast one minute. Essential difference between a gust anda squall is the duration of the peak speed.

squall lineAny nonfrontal line or narrow band of activethunderstorms (with or without squalls).

stabilityA state of the atmosphere in which the verticaldistribution of temperature ts such that a parcel will resistdisplacement from its initial level. (See also instability.)

standard atmosphere A hypothetical atmosphere basedon climatological averages comprised of numerous phys-ical constants of which the most important are:(1) A surface temperature of 59° F (15° C) and a surface

pressure of 29.92 inches of mercury (1013.2 millibars)at sea level;

(2) A lapse rate, in the troposphere of 6.5° C per kilometer(approximately 2° C per 1,000 feet);

(3) A trope/vase of 11 kilometers (approximately 36,000feet) with a temperature of --56.5° C; and

(4) An isothermal lapse rate in the stratosphere to an alti-tude of 24 kilometers (approximately 80,000 feet).

standing cloud (standing lenticular altocumulus)Seelenticular cloud.

'teaseling waveA wave that remains stationary in a mov-ing fluid. In aviation operations it Is used most commonlyto refer to a lee wave or mountain wave.

stationary frontSame as quasi - stationary front.

station pressureThe actual atmospheric pressure at the ob-serving station.

steam fogFog formed when cold air moves over rela-tively warm water or wet ground.

storm detection radarA weather radar designed to de-tect hydrometeors of precipitation size; used primarily todetect storms with large drops or hailstones as opposedto clouds and light precipitation of small drop size.

stratiformDescriptive of clouds of extensive horizontaldevelopment, as contrasted to vertically developed cumu-Worm clouds; characteristic of stable air and, therefore,composed of small water droplets.

stratocumulusA low cloud, predominantly stratiform ingray and/or whitish patches or layers, may or may notmerge; elements arc tessellated, rounded, or roll-shapedwith relatively flat tops.

stratosphereThe atmospheric layer above the trope-pause, average altitude' of base and top, 7 and 22 milesrespectively; characterized by a slight average increaseof temperature-front base to top and is very stable; alsocharacterized by low moisture content and absence ofclouds.

stratusA low, gray cloud layer or sheet with a fairly uni-form base; sometimes appears in ragged patches; seldomproduces precipitation but may produce drizzle or snowgrains. A stratiform cloud.

stratus fractusSee fractus.

streamlineIn meteorology, a line whose tangent is thewind direction at any point along the line. A Bowline.

sublimationSee change of state.

subrefractionSee refraction.

subsidenceA descending motion of air in the atmosphereover a rather broad area; usually associated with diver-gence.

summation principleThe principle states that the coverassigned to a layer is equal to the summation of the skycover of the lowest layer plus the additional coverage atall successively higher layers up to and including thelayer in question. Thus, no layer can be assigned a skycover less than a lower layer, and no sky cover can begreater than 1.0 (10/10).

superadiabatic lapse rateA lapse rate greater than the--shy-adiabatic lapse rate. See absolute instability.

supercooled waterLiquid water at temperatures colderthan freezing.

superrefractionSee refraction.

surface inversionAn inversion with its base at the surface,often caused by cooling of the air near the surface as aresult of terrestrial radiation, especially at night.

... '0....

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2i7

surface visibilityVisibility observed from eye-level abovethe ground.

synoptic chartA chart, such as the familiar weather map,which depicts the distribution of meteorological condi-tions over an area at a given time.

TtargetIn radar, any of the many types of objects detected

by radar.

temperatureIn general, the degree of hotness or coldnessas measured on some definite temperature scale by meansof any of various types of thermometers.

temperature inversionSee inversion.

terrestrial radiationThe total infrared radiation emittedby the Earth and its atmosphere.

thermographA continuous-recording thermometer.

thermometerAu instrument for measuring temperature.

theodoliteAn optical instrument which, in meteorology,is used principally to observe the motion of a pilot balloon.

thunderstormIn general, a local storm invariably pro-duced by a cumulonimbus cloud, and always accompaniedby lightning and thunder.

tornado (sometimes called cyclone, twister)A violentlyrotating column of air, pendant from a cumulonimbuscloud, and nearly always observable as "funnel-shaped."It le the most destructive of all small-scale atmosphericphenomena.

towering cumulus -A rapidly growing cumulus in whichheight exceeds width.

tower visibilityPrevailing vesibility determined from thecontrol tower.

Wade windsPrevailing, almost continuous winds blowingwith an easterly component from the subtropical highpressure belts toward the intertropical convergence ;one;northeast in the Northern Hemisphere, southeast in theSouthern Hemisphere.

transmissometerAn instrument system which shows thetransmissivity of light through the atmosphere. Trans-nrissivity may be translated either automatically or man-ually into visibility and/or runway visual range.

tropical airAn air mass with characteristics developedover low latitudes. Maritime tropical air (mT), the prin-cipal type, is produced over the tropical and subtropicalseas; very warm and humid. Continental tropical (cT) isproduced over subtropicid arid regions and is hot andvery dry. Compare polar air.

tropical cycloneA general term for a game that origi-nates over tropical oceans. By international agreement,tropical cyclones have been classified according to theirintensity, as follows:(1) tropical depressionwinds up to 34 knots (64

km/h);(2) tropical storm winds of 35 to 64 knots (65 to 119

km/h);

212

(3) hurricane or typhoonwinds of 65 knots or higher(120 km/h).

tropical depressionSee tropical cyclone.

tropical stormSee tropical cyclone.

tropoparueThe transition zone between the troposphereand stratosphere, usually characterized by an abrupt changeof lapse rate.

troposphereThat portion of the atmosphere from theearth's surface to the tropopause; that is, the lowest 10 to20 kilometers of the atmosphere. The troposphere ischaracterized by decreasing temperature with height.and by appreciable water vapor.

trough (also called trough line)In meteorology, anelongated area of relatively low atmospheric pressure;usually associated with and most clearly identified as anarea of maximum cyclonic curvature of the wind flow(isobars, onions:, or 'itreamliees); compare with ridge.

true altitudeSee altitude.

true wind directionThe direction, with respect to truenorth, from which the wind is blowing.

turbulence In meteorology, any irregular or disturbedflow in the atmosphere.

twilightThe intervals of incomplete darkness followingsunset and preceding sunrise. The time at which eveningtwilight ends or morning twilight begins is determinedby arbitrary convention, and several kinds of twilighthave been defined and used; most commonly civil, nau-tical, and astronomical twilight.(1) Civil TwilightThe period of time before sunrise

and after sunset when the sun is not more than 6°below the horizon.

(2) Nautical TwilightThe period of time before sun-rise and after sunset when the sun is not more than12° below the horizon.

(3) Astronomical TwilightThe period of time beforesunrise and after sunset when the sun is not morethan 18° below the horizon.

twisterIn the United States, a colloquial term for tornado.

typhoonA tropical cyclone in the Eastern Hemisphere withwinds in excess of 65 knots (120 km/h).

UundereastA cloud layer of ten-tenths (1.0) coverage (to

the nearest tenth) as viewed from an observation pointabove the layer.

unlimited ceilingA clear sky or a sky cover that doesnot meet the criteria for a ceiling.

unstableSee instability.

updraftA localized upward current of air.

upper frontA front aloft not extending to the earth'ssurface.

upslope fogFog formed when air flows upward over ris-ing terrain and is, consequently, adiabatically cooled toor below its initial dew point.

218

Vvapor pressureIn meteorology, the pressure of water

vapor in the atmosphere. Vapor pressure is that part ofthe total atmospheric pressure due to water vapor and isindependent of the other atmospheric gases or vapors.

vapor trailSame as condensation trail.

veeringShifting of the wind in a clockwise direction withrespect to either space or time; opposite of backing.Commonly wed by meteorologists to refer to an anticy-clonic shift (clockwise in the Northern Hemisphere andcounterclockwise in the Southern Hemisphere).

vertical visibility The distance one can see upward Intoa surface based obscuration; or the maximum height fromwhich a pilot in flight can recognize the ground througha surface based obscuration.

virgaWater or ice particles falling from a cloud, usuallyin wisps or streaks, and evaporating before reaching theground.

visibilityThe greatest distance one can see and identifyprominent objects.

visual rangeSee runway visual range.

vortexIn meteorology, any rotary flow in the atmosphere.

vortickyTurning of the atmosphere. Vorticity may beimbedded in the total flow and not readily identified bya flow pattern.

(a) absolute voracitythe rotation of tbe Earth impartsvorticity to the atmosphere; absolute vorticity is thecombined vorticity due to this rotation and voracity,due to circulation relative to the Earth (relativevorticity).negative voracityvoracity caused by anticyclonicturning; it is associated with downward motion ofthe air.

(c) positive vorticity--vorticity caused by cyclonicturning; it is associated with upward motion of theair.

(d) relative veaticityvorticity of the air relative to theEarth, disregarding the component of voracity re-sulting from Earth's rotation.

(b)

wwake turbulenceTurbulence found to the rear of a solid

body in motion relative to a fluid. In aviation terminol-ogy, the turbulence caused by a moving aircraft.

walLeloudThe well-defined bank of vertically developedclouds having a wall-like appearance which form theouter boundary of the eye of a well-developed tropicalcyclone.

warm front Any non-occluded front which moves in sucha way that warmer air replaces colder air.

warm sectorThe area covered by warm air at the surfaceand bounded by the warm front and cold front of a wavecyclone.

water equivalentThe depth of water that would resultfrom the melting of snow or ice.

waterspoutSes tornado.

water vaporWater in the invisible gaseous form.

wave cyclone --A cyclone which forms and moves along afront. The circulation about the cyclone center tends toproduce a wavelike deformation of the front.

weatherThe state of the atmosphere, mainly with respect.to its effects on life and human activities; refers to in-stantaneous conditions or short term changes as opposedto climate.

weather radarRadar specifically designed for observingweather. See cloud detection radar and storm detectionradar.

weather vaneA wind vans.

wedgeSame as ridge.

wet bulbContraction of either wet-bulb temperature or wet-bulb thermometer.

wet-bulb temperature The lowest temperature that can beobtained on a wet-bulb thermometer in any given sample ofair, by evaporation of water (or ice) from the muslinwick; used in computing dew point and relative humicliO.

wet-bulb thermometerA thermometer with a muslin-covered bulb used to measure wet-bulb temperature.

whirlwindA small, rotating column of air; may be vis-ible as a dust devil.

willy-willyA tropical cyclone of hurricane strength nearAustralia.

windAir in motion relative to tbe surface of tbe earth;generally wed to denote horizontal movement.

wind directionThe direction from which wind is blow-ing.

wind speedRate of wind .movement in distance per unittime.

wind vaneAn instrument to indicate wind direction.

wind velocityA vector term to Include both wind direc-tion and wind speed.

wind shear The rate of change of wind velocity (directionand/or speed) per unit distance; conventionally ex-pressed as vertical or horizontal wind shear.

xYZzonal windA west wind; the westerly component of a

wind. Conventionally used to describe large-scale flowthat b neither cyclonic nor anticyclonic.

219 213

A

Adiabatic Process, 47Adiabatic Rate of Cooling

Dry, 48Moist, 48

Advection, 24Advection Fog, 127, 153Aerodynamic Coattails, 143Airborne Weather Radar, 120Aircraft Accidents, 125Aircraft Observation (APOB), 187Air Density. See Atmosphere.Air Mass, 63, 148

Modification, 64Source Region, 63Thunderstorms. See Thunderstorms.

Altimeter, 17Altimeter Setting, 18Ak.u.,nry, 17Altitude, 17

Corrected, 18Density. See Density Altitude.Indicated, 17Pressure, 19True, 17

Altocumulus Clouds. See Clouds.Altostratus Clouds. See Clouds.Aneroid Barometer, 12, 17Anticyclone, 29Arctic, 147

Air Masses, 148Aurora Borealis, 152Blowing Snow, 153Clouds, 151Flying Weather, 154Fog, 153Icing, 153Light from Celestial Bodies, 152Precipitation, 151Reflection by Snow Cover, 152Whiteout, 153

Atmosphere, 1Composition of, 2Density of, 3Standard, 2, 13, 17Structure of, 2

Atmospheric Pressure, 11Measurement of, 11

Aurora Borealis, 152

Barometer, 17Aneroid, 12, 17

INDEXMercurial, 12

Blowing Restrictions to Visibility, 129, 153Bora, 32

C

Canopy Static, 145Carburetor Icing, 97Ceiling, 115Celsius (Centigrade) Temperature Scale, 6Change of State, 39Changes in Upward and Downward Moving Air, 47Chinook, 32Cirrocumulus Clouds. See Clouds.Cirrostratus Clouds. See Clouds.Cirrus Clouds. See Clouds.Cirrus Haze, 144Clear Air Turbulence (CAT), 87, 142Clouds, 53

Altocumulus, 56Altocumulus Castellanus, 57Altostratus, 56Cirriform, 54Cirrocumulus, 55Cirrostratus, 55Cirrus, 54, 139Classification, 53Composition, 42Cumuliform, 50Cumulonimbus, 54, 61, 81Cumulonimbus Mamma, 114Cumulus, 53, 60Families, 53Formation, 37, 42Funael, 113High, 54Identification, 53Lenticular, 58, 85, 199Low, 54Middle, 54Nimbostratus, 54, 59Roll, 115Rotor, 85Standing Lenticular, 58, 85, 199Standing Wave, 58, 84,199Stratiform, 50Stratocumulus, 60, 83Stratus, 53, 59,128Towering Cumulus, 61Vertically Developed, 54

Col, 15Cold Front. See Front.Cold Low, 36Common IFR Producers, 125

220215

Compressional Warming, 47Condensation, 39Condensation Nuclei, 40Condensation Trails, 143Constant Pressure Analysis, 16Constant Pressure Chart, 16Constant Pressure Surface, 17Contour, 16Contour Analysis. See Constant Pressure Analysis.Convection, 23Convective Currents, 24, 80Convective Turbulence, 80Cooling by Expansion, 47Coriolis Force, 25, 29Corrected Altitude, 18Cross-Couniry Soaring, 190, 200Cumuliform Clouds. See Clouds.Cumulonimbus Clouds. See Clouds.Cumulus Clouds. See Clouds.Cyclone, 29

Tropical, 164

D

Deflective Force. See Coriolis P"rce.Density Altitude, 19

Computing, 21Density of Air, 3Dew, 41Dew Point, 38 -

Change with Altitude, 51Dew Point Front, 76Dissipation Trails, 144Diurnal Temperature Range, 7Do's and Don'ts of Thunderstorm Flying, 121Downdraft, 83, 111Drainage Wind, 32Drizzle. See Precipitation.Dry Adiabatic Rate of Cooling, 48Dry Line, 76Dust, 129Dust Devil, 52, 173

E

Easterly Wive, 163Embedded Thunderstorms. See Thunderstorms.Evaporation, 39Exhaust Trails, 143Expansional Cooling, 47

F

Fahrenheit Temperature Scale, 6First Gust, 111Flight Planning, 21, 45, 52, 78, 85, 90, 102, 121, 130,

143,155Tog, 42, 126

Advection, 127, 153Ground, 126

216

221

Ice, 128, 153Precipitation Induced, 128Radiation, 126Sea, 127Steam, 153Upslope, 127

Freezing Precipitation. See Precipitation.Frictional Force, 29 ss

Front, 64,148Cold, 65Dew Point, 76Occluded, 67Stationary, 65Warm, 65

Frontal Discontinuities, 65Frontal Soaring, 191Frontal Waves and Occlusion, 66Frontal Weather, 72, 100, 112Frontogenesis, 71Frontolysis, 71Frost, 41, 102, 153Frozen Precipitation. See Precipitation.Funnel Cloud, 113

General Circulation, 26Ground Fog, 126

G

H

Hail, 43, 115Haze, 129Heat and Temperature, 6

Latent, 40Heating by Compression, 47Height Contour, 16High Altitude Weather, 135High Clouds. See Clouds.High Level Haze, 144High Pressure System, 15Hill Soaring, 195Humidity, 38Hurricane, 164Hurricane Reconnaissance, 167Hypoxia, 3

IIce Fog, 128,153Icing- -

Aircraft, 41, 91,153Carburetor, 97Clear, 92Effects of, 91Ground, 102High Altitude, 145Icing and Cloud Types, 99Induction System, 97Instrument, 98

Intensities, 93, 104Mixed, 93Rime, 92Structural, 92Thunderstorm, 101, 114

Inactive Front, 74Indicated Altitude, 17Induced Trough, 162Induction System Icing, 97Instability, 49, 64Instability Line, 75Instrument Icing, 98Intertropical Convergence Zone, 159Inversion, 9Isobar, 15Isobaric Analysis, 15

JJet Stream, 31, 136

Cirrus, 139Related to Surface Systems, 138Related to Tropopause, 31, 136Turbulence, 142

Katabatic Wind, 32

K

L

Lake Effect, 43Land and Sea Breezes, 32Lapse Rate, 9Latent Heat, 40Lenticular Cloud. See Clouds.Lift, 171Lightning, 116Local Winds, 31Low Clouds. See Clouds.Low Pressure Systems, 15, 35

Non-Frontal, 36, 70Low Stratus Clouds. See Clouds.

M

Mechanical Turbulence, 82Melting, 39Melting Point, 6Mercurial Barometer, 12Middle Clouds. See Clouds.Moist Adiabatic Rate of Cooling, 48Moisture, 37Monsoon, 160Mountain Flying, 85Mountain Wave, 58, 83,198Mountain Wave Soaring, 198Mountain Wind, 31

N

Nimbostratus Clouds. See Clouds.NonFrontal Low, 36, 70Northern Lights (Aurora Borealis), 152

0Obscured Sky, 130Obstructions to Wind Flow, 82Occluded Front. See Front.Oxygen Pressure, 3Oxygen, Use of, 4

P

Pilot Reports, 90, 104, 143, 155Pitot Tube Icing, 98Plow Wind, 111Polar Easterliis, 29Polar Outbreak, 29Precipitation, 37, 42

Drizzle, 43, 130Formation, 42Freezing, 43Frozen, 43Hail, 43, 115Ice Pellets, 43Rain, 39, 43, 130Snow, 43, 130

Precipitation Induced Fog, 128Precipitation Static, 117Pressure Altitude, 19Pressure Analysis, 15Pressure, Atmospheric, 11

Measurement of, 11Sea Level, 13Station, 12Units of, 12Variation with Altitude, 12, 36Variation with Seasons, 27Variation with Temperature, 13, 27Variation with Topography, 29

Pressure Gradient, 24Pressure Gradient Force, 24Pressure Patterns, 15, 33Pressure Surface, Constant, 17Pressure Systems, 15, 35Pressure Variation. See Pressure.Pressurization, 4Prevailing Westerlies, 29Propeller Icing, 95Pseudo-Adiabatic Chart, 184

Quasi-Stationary Front, 65

222

Q

217

Radiation, 6Radiation Fog, 126Radiosonde, 16Rain. See Precipitation.Relative Humidity, 38Restrictions to Visibility, 36, 115, 126, 129Ridge, 15, 35Ridge or Hill Soaring, 195Rime. See Icing.Roll Cloud.See Clouds.Rotor, 85Rotor Cloud. See Clouds.

S

Saint Elmo's lire, 117Santa Ana Wind, 32Saturated Adiabatic Rate of C_ ooling, 48Saturated Air, 41Sea Breeze, 32Sea Breeze Front, 191Sea Breeze Soaring, 191Sea Fog, 127Sea Level Pressure, 13Sea Smoke, 153Shear Line, 162Shear, Wind. See Wind Shear.Signposts in the Sky, 50, 62Sink, 171Sink Rate, 171Slant Visibility, 130Smoke, 129Smoke Front, 193Snow. See Precipitation.Soaring

Cross-Country, 190, 191, 200Frontal, 191Mountain Wave, 198Ridge or Hill, 195Sea Breeze, 191Thermal, 172Turbulence, 173, 176, 191, 195, 199

Soaring Weather, 171Solar Radiation, 6Source Regions of Air Masses, 63Spatial Disorientation, 125Spread, TemperatureDew Point, 38Squall Line, 114Stability, 49, 64Stable Air, 47, 64Standard Atmosphere, 2, 13, 17Standard Atmospheric Pressure, 13Standing Wave. See Mountain Wave.Static--

Canopy, 145Precipitation, 117

Station Pressure, 12Stationary Front. See Front.Steam Fog, 153Stratiform Clouds. See Clouds.

21$

Stratocumulus Clouds. See Clouds.Stratosphere, 2Stratus Clouds. See Clouds.Sublimation, 39, 41Subtropical High Pressure Belt, 27, 158Supercooled Water, 41Surface Weather Chart, 15

223

TTaku, 32Temperature, 6

Diurnal Range, 7Inversion, 9Lapse Rate, 9Measurement, 6Scales, 6Seasonal Range, 7Variation with Height, 9Variation with Latitude, 7Variation with Topography, 7World, 7

TemperatureDew Point Spread, 38Terrain Effects, 43, 101, 150Terrestrial Radiation, 6Thermal, 172

Height, 180Index, 186Lift, 172Locating, 173Streets, 180Strength, 180Structure, 178Terrain Effects on, 173Types of, 178

Thermal Low, 36, 71Thermal Soaring, 172Thunderstorm, 36, 61, 105, 145

Air Mass, 112Avoidance, 120Climatology, 105Effect on Altimeters, 116Electricity, 116Embedded, 51, 72, 101, 121Flying, 121Formation, 111Frontal, 72, 112Hazards, 113Life Cycle, 111Penetration, 123Radar Observations of, 120Seasonal Distribution, 105Severe, 112Stages of, 111Steady State, 112Structure, 111

Tornado, 113Trade Winds, 29, 158Tropical--

Circulation, 158Cyclone, 164Depression, 164

- ,

Easterly Wave, 163High Pressure Belts, 27, 158Intertropical Convergence Zone, 159Monsoon, 160Shear Line, 162Storm, 164Trade Wind Belts, 27, 158Trough Aloft, 162Wave, 163

Tropical Weather, 157Tropopause, 2, 31, 136Troposphere, 2Trough, 15, 35

Aloft, 36, 162True Altitude, 17Turbulence, 79

Clear Air, 87, 142Convective, 80In Thunderstorms, 81, 114Mechanical, 82Wake, 88Wind Shear, 65, 86,142

Typhoon, 164

Unstable Air, 47, 6+Updrafts, 50, 111

I

U

224

Upper Air Observations, 16, 187Ups lope Fog, 127

Valley Wind, 31Virga, 39

V

W

Wake Turbulence, 88Warm Front. See Front.Warming by Compression, 47Water Surface, Effect of, 7, 27, 32, 43, 64,150Water Vapor, 37Waterspout, 113Weather and Aircraft Accidents, 125Whiteout, 153 ...Willy-Willy, 164Wind, 23Wind Gusts, 111Wind Shadow, 197, 200Wind Shear, 34, 65, 86'Wind Shear Turbulence, 65, 86, 142Wind Systems, 35

*U.S. 00VIERNMIZIV MONTANO 01.14C71, I ris 0.471-011

219