aviation weather for helicopter pilots

7/27/2019 Aviation Weather for Helicopter Pilots

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For Helicopter Pilots

Aviation Weather


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1 The Basics Page 11.1 The Atmosphere 1

1.2 The Atmosphere extends further above the Equator than the Poles 11.3 Subdivision of the Atmosphere 1

1.4 Composition of the Atmosphere 2

1.5 Humidity 2

1.6 Air Density 2

1.7 The Sun, Our Source Of Energy 2

1.8 Orbiting Around The Sun Effects Our Temperature 2

1.9 Heating From Solar Radiation Is Greater In The Tropics 3

1.10 Warm air rises, Cool air sinks 3

1.11 Terrestrial Radiation 3

1.12 Rotation of the earth on its axis 3

2 Temperature and Heat 42.1 Different surfaces heat differently 4

2.2 Cloud cover and its effect on surface heating and cooling 4

2.3 Transfer of heat energy 4

2.4 The Sea breeze 5

2.5 The land breeze 5

2.6 Katabatic Wind 6

2.7 The Anabatic Wind 6

2.8 Temperature is a measure of heat energy 6

2.9 Temperature Inversions 6

3 Atmospheric Pressure 7

3.1 Atmospheric Pressure 73.2 Atmospheric pressure can be measured by using: 7

3.3 Pressure Gradients 8

3.4 International Standard Atmosphere 8

4 Wind 94.1 What is wind? 9

4.2 How wind is defined 9

4.3 Veering and Backing 9

4.4 What causes a wind to blow? 9

4.5 The Pressure Gradient Force 10

4.6 Coriolis Force 10

4.7 The Geostrophic Wind 11

4.8 From High to Low Look Out Below. 11

4.9 Gradient wind blows around curved isobars 11

4.10 Surface winds 12

4.11 Diurnal Variation of the Surface Wind 12

4.12 Localised friction effects 12

4.13 Flight in turbulence 12

4.14 Windshear 13

4.15 Wind associated with Mountains 14

4.16 Wind in the Tropics 14

4.17 Microbursts 14

4.18 The Tropopause and wind 14

4.19 Jet Streams 15

4.20 Polar Front Jets 164.21 Other Jet Streams 17

4.22 Clear Air turbulence 17

Table of Contents

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5 Clouds Page 185.1 Cloud 18

5.2 Three States of Water 19

5.3 Latent Heat 19

5.4 Humidity 20

5.5 Relative Humidity 20

5.6 Wet and Dry Bulb Thermometer 20

5.7 Dew Point 205.8 Adiabatic Processes 21

5.9 Stable Air 21

5.9.1 Dry 21

5.9.2 Saturated 21

5.9.3 Absolute Stability 22

5.10 Unstable Air. 22

5.10.1 Dry 22

5.10.2 Saturated 22

5.10.3 Absolute Instability 22

5.11 Cloud formed by convection due to heating 23

5.12 Cloud formed by orographic uplift 23

5.13 Foehn wind 24

5.14 Cloud formed by Turbulence and mixing 24

5.15 Cloud formed by widespread ascent 24

5.16 Precipitation associated with cloud 24

5.17 Thunderstorms 25

5.17.1 Three Conditions Necessary For A Thunderstorm To Develop 25

5.18. Life Cycle Of A Thunderstorm 25

5.18.1 The Cumulus Stage 25

5.18.2 Mature Stage 25

5.18.3 Dissipating Stage 26

5.19 Dangers From A Thunderstorm 26

6 Air Masses and Fronts 27

6.1 Air Masses And Frontal Weather 276.2 Origin Of An Air Mass 27

6.3 Track Of An Air Mass 27

6.4 Convergence And Divergence 27

6.5 Types Of Air Masses That Affect Ireland and the British Isles 28

6.5.1 Typical Characteristics of Air Masses affecting Ireland and the British Isles 28

6.6 The Warm Front 29

6.6.1 The Warm Front As Seen By An Observer On The Ground 29

6.6.2 The General Characteristics Of A Warm Front 29

6.6.3 The Warm Front As Seen By A Pilot 30

6.7 The Cold Front 30

6.7.1 The General Characteristics Of A Cold Front 30

6.7.2 The Passage Of A Cold Front As Seen By An Observer On The Ground 316.7.3 The Cold Front As Seen By A Pilot 31

6.8 The Occluded Front 31

6.8.1 The characteristics of an Occluded Front 31

6.9 Depressions - Areas Of Low Pressure 32

6.9.1 The Three-Dimensional Pattern Of Airflow Near A Depression 32

6.9.2 Weather Associated With A Depression 32

6.9.3 Troughs Of Low Pressure 32

6.10 The Wave Or Frontal Depression 32

6.11 The Tropical Revolving Storm 32

6.12 Anticyclones Areas Of High Pressure 33

6.12.1 The three-dimensional flow of air associated with an Anticyclone 33

6.12.2 Weather Associated With A High 33

6.12.3 Ridge Of High Pressure 33

6.13 A Col. 33

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7 Icing Page 347.1 The Formation Of Ice 34

7.1.1 Icing Can Be Hazardous To Aviation 34

7.2 Supercooled water drops 34

7.3 Icing In Cloud 34

7.4 Hoar Frost 35

7.4.1 Synoptic situations which favour the formation of hoar frost 35

7.5 Rime And Glazed Or Clear Ice 357.5.1 Rime Ice 35

7.5.2 Clear Ice 36

7.6 Cloudy Or Mixed Ice 36

7.7 Intake or Impact Ice 36

7.8 Fuel Icing 36

7.9 Carburettor Icing 37

7.10 Points To Remeber 37

7.11 Icing of the pitot-static system 38

7.12 Ambient Conditions Conducive To The Formation Of Induction System Icing 38

8 Visiblity 39

8.1 Visibility 398.2 Slant Visibility 39

8.3 Runway Visual Range 40

8.4 Eye Observations By Day 40

8.5 Fog, mist and haze 40

8.6 Radiation Fog 40

8.7 Advection Fog 40

8.8 Eye Observations By Night 41

8.9 Upslope fog 41

8.10 Sea Fog 41

8.11 Smoke Pollution 41

8.12 Frontal Fog 41

8.13 Dust and sand 41

8.14 Precipitation And Visibility 428.15 Precipitation And Visual Perception 42

9 Weather Sources & Information 439.1 Aeronautical meteorological offices 43

9.2 Aerodrome meteorological offices 43

9.3 Meteorological services at aerodromes 43

9.4 Availability of periodic weather forecasts 43

9.5 Weather Information for Flight Planning 43

9.6 Special observations 43

9.7 Reports and forecasts for departure 43

9.7.1 En-route, destination and alternate(s) 43

9.8 Weather Forecasts and Reports 449.8.1 Special Forecasts 44

9.8.2 Aerodrome Forecasts (TAFs) 44

9.8.3 METARs 44

9.8.4 Trends (or Landing Forecasts) 44

9.8.5 VHF In-Flight weather Reports 44

9.9 Cloud Bases 44

9.10 CAVOK 45

9.11 Changing Weather in Forecasts 45

9.11.1 Temporary Change (TEMPO) 45

9.11.2 Lasting Changes 45

9.11.3 Probabil ity 45

9.12 Availability Of Ground Reports For Surface Conditions 45

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9.13 In Flight Meteorological Information Page 45

9.13.1 ATlS 45

9.13.2 VOLMET 46

9.13.3 Special Aerodrome Reports (SPEC) 46

9.13.4 SIGMET 46

9.13.4.1 Meteorological Abbreviations Used In Sigmets, Special Forecasts Etc 46

9.14 Weather Charts 47

9.14.1 Station Circle 47

9.14.2 Significant Weather Chart 48

9.14.3 Upper Wind Chart 49

9.15 Example of METARs and Short TAFs 50

9.16 METAR decoder 51

9.17 TAF decoder 52

Glossary of Terms 54

Index 61

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1.1 The Atmosphere.The Earth is a solid object, which a mixture of gases

surrounds. The force of gravity holds these gases to

the earth. We know this mixture of gases as AIR, and

the space that it occupies around the earth as the

Atmosphere.

1.2 The Atmosphere Extends Further

Above The Equator Than The Poles.The earth spins about its axis, carrying the

atmosphere with it and tending to throw the air to the

outside. Consequently, the atmosphere extendsfurther into space above the equator than the poles.

1.3 Subdivision of the AtmosphereThe atmosphere is divided vertically into four regions:

Troposphere;

Stratosphere;

Mesosphere, and the;

Thermosphere.Light aircraft fly in the Troposphere. High altitude jets

cruise in the Stratosphere. The boundary between the

two regions is known as the Tropopause. The

Tropopause occurs at a height of approximately20 000ft over the poles and at approximately 60 000ft

over the tropics.

In the "average" International Standard

Atmosphere the Tropopause is assumed to occur

at 36090ft.

Most of our "weather" occurs in the Troposphere.

Significant differences exist between the Stratosphere

and the Troposphere:

In the Troposphere temperaturedecreases with height (@ 1.98oC per

1000ft, up to 36 090ft) were it is constant

at -56.5oC in the Stratosphere.

There is a mark vertical movement of airin the Troposphere. Warm air rising and

cool air descending, on both large and

small scales.

Nearly all the water vapour in theatmosphere is contained in the

Troposphere. Cloud formation rarely

extends beyond the Tropopause. However

occasionally large cumulonimbus clouds

with strong and fast vertical development

may push into the Stratosphere.

1 Walker 2000 Aviation Weather

1. The Basics

Fig 1.1


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1.4 Composition of the Atmosphere.The atmosphere, or 'AIR' is a mixture of gases that

carries water vapour. Nitrogen is the largest

component, other components are;

Nitrogen 78% By Volume.

Oxygen 21%

Other Gases 1%Water Vapour

. . . . . . . . . Total 100%

Oxygen is needed to support life and combustion and

water vapour to produce weather. All air contains

some water vapour. It is the water vapour that

condenses out to form clouds, from these we get

precipitation (rain, hail, snow, etc.) which is so vital to

life on Earth. Maritime Air (air over an ocean) will

absorb moisture from the body of water and overall will

contain more water vapour than Continental Air(air

over a continent) particularly if the continent consists

mostly of deserts.

1.5 Humidity.Water molecules are very light molecules and their

presence in large numbers in 'AIR' lowers its density,

which affects the aerodynamic performance and the

power production from the engine of an aircraft.

Performance on a damp day will be poorer than on a

dry day. Carburettor icing is more likely on a day that

has high relative humidity, this is cause by the air

expanding as it cools while mixing with vapourising

fuel. The water vapour condenses out and sticks tothe carburettor casing as ice.

1.6 Air Density.Air Density Decreases With Altitude. The force of

gravity exists between each individual air molecule

and the Earth. This causes the air molecules to draw

closer together, particularly near the Earths surface

where they become very crowded. If we look at a

cubic metre of air at the surface, it will have twice the

molecules than a cubic metre of air at 40 000ft.

The Density (or mass per unit volume) of air at sealevel is 1225 grammes per cubic metre.

Why Is Air Density So Important To Pilots?

The required lift force can be generated ata lower true air speed.

More engine power is available.

Breathing is easier and more oxygen istaken into the lungs.

1.7 The Sun, Our Source Of EnergyThe Sun radiates electromagnetic energy and we

experience this energy as light and heat. These

wavelengths (short wave) of solar radiation are such

that a large percentage penetrates the Earth's

atmosphere and is absorbed by the Earth's surface.

This causes the temperature of the Earth's surface to

rise. The ground in turn heats any part of the

atmosphere that is in contact with or very close to it,

this causes any parcel of air that is warmer than the

surrounding air to rise.

1.8 Orbiting Around The Sun EffectsOur Temperature.The Earth's axis is tilted, and as it orbits around the

Sun the earth receives differing amounts of solar

radiation, this causes our four seasons. The solar

radiation received at a place in Summer is more

intense due to the surface being presented at a less

oblique angle.

Walker 20002 Aviation Weather

Fig 1.2

Fig 1.3


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2.1 Different surfaces heat differently.The temperature and the way different surfaces heat

depend upon several things.

1. The specific heat of a surface. Water

requires more heat energy than land to

raise its temperature by 10oC. Land will

heat more quickly by day and cool faster by

night. Compared with a large body of water

the land nearby will be warmer by day and

cooler by night. Water is said to have a

higher specific heat than land.

2. Reflectivity of a surface. When solar

radiation is reflected from a surface, it

cannot be absorbed. Areas covered by

snow or water will have a high reflectivity

and will not be heated as much as an area of

land, such as a ploughed field or dense

jungle.

3. Conductivity of a surface. Currents in the

ocean transfer heat through the motion of

the water, heating it to a greater depth than

a land surface.

2.2 Cloud cover and its effect on

surface heating and cooling.Cloud cover prevents solar radiation penetrating the

Earth's surface, which results in reduced heating of

the Earth and lower temperatures. Air in contact with

the surface is subjected too much less heating. At

night cloud cover will prevent heat from escaping into

the upper atmosphere and cause the atmosphere

below the cloud to have a higher temperature.

2.3 Transfer of heat energy.

Heat energy can be redistributed in a body ortransferred to another body by several means:

1. Radiation. All bodies transmit energy as

electromagnetic radiation. The higher the

temperature of the body, the shorter the

wave length. The Sun emits short wave

radiation and the Earth long waves.

2. Absorption. Any body in the path of radiation

will absorb some of its energy. The amount

depends upon the nature of the body and the

radiation. Densely forested areas will absorbmore solar radiation than snow-covered

mountains.

2. Temperature and Heat

I really need to fix my airconditioner

Fig 2.1


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3. Conduction. Heat energy may pass within

one body or from one body to another body in

contact with it. Metals are good conductors of

heat. Wood and air are not good conductors

of heat. A parcel of air heated at the Earths

surface by conduction will not transfer this

heat energy to a neighboring parcel of air.

This is a very significant factor in the

production of weather systems.

4. Convection. A body in motion carries its heat

energy with it. A parcel of air heated at the

Earth's surface will expand, become less

dense and rise. As it rises it will carry its heat

energy higher into the atmosphere.

5. Advection. As the air heated by convection

rises cooler air will move in to replace it, this

occurs in a horizontal plane. The body of air

will bring, with it, its own heat and moisture.

The general vertical circulation pattern of air flow that

occurs on a large scale around the Earth also happens

on a much smaller scale in localised areas.

2.4 The Sea breeze.The process known as a Sea Breeze occurs on sunny

days when the land is heated more quickly than the

sea causing the air over the land mass to becomewarm, expand and lose density. This warmed air will

then start to rise (convection). The cooler air from the

sea will move in to replace the warm air that has risen

(advection). The vertical extent of a sea breeze is

approximately 1000 to 2000 feet. Sea breezes may

affect operations of airfields near the coast. This

would be as windshear, or turbulence as the aircraft

passes over one body of air to another. Cooler air

moving in over warm land may cause fog or mist,

reducing visibility.

2.5 The land breeze.At night the land cools more quickly than the sea

causing the air above it to cool and subside. The air

over the sea is warmer and will rise. The effect of a

land breeze could hold sea fog offshore during the

night, but as the land warms during daylight a sea

breeze could develop and bring the fog inshore,

causing visibility problems.Fig 2.3

Fig 2.5

Fig 2.2

Fig 2.4

Fig 2.6


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2.6 Katabatic Wind.During night time the Earth loses much heat through

terrestrial radiation and cools down. This is particularly

noticeable on clear, cloudless nights. The air in

contact with the ground loses heat to it by conduction,

and cools down, becomes more dense and sinks. In

mountainous regions the cool air will flow down the

side of the mountain and into valleys, creating aKatabatic wind. These winds can reach speeds of 30

knots down the slopes of mountains. They will die out

as the solar radiation warms the surface and it starts

to re-radiate the energy.

2.7 The Anabatic Wind.Heating of a mountain slope during the day causes the

air mass in contact with it to warm, decreasing its

density and causing it to rise along the mountainslope. The force of gravity opposes the flow up the hill,

making the Anabatic Wind weaker than the night time

Katabatic Wind.

2.8 Temperature is a measure of heat

energy.

As a body of matter absorbs heat energy, itsmolecules become agitated. This agitation is

measured as temperature, which is used to measure

heat energy. The temperature at which no molecular

agitation occurs is called absolute zero and is

measured using a scale called Kelvin:

0oKelvin = -273oCelsius.

Temperature is measured using different scales

around the world. There are several scales of

temperature measurement is use around the world.

The scale most commonly used in aviation is the

CELSIUS scale. This divides the temperature that

water boils and freezes at 100 units. Using the Celsius

scale water boils at 100oC and freezes at 0oC. Some

countries still use the FAHRENHEIT scale, where

water boils at 212oF and freezes at 32oF.

There is a requirement for the pilot to be able to

convert from one scale to the other. The easiest and

most convenient way is to use the temperature

conversion scale on the flight computer. There aremathematical formulae that the pilot should commit to

memory;

1. Celsius to Fahrenheit.

oF = 9/5 x oC + 32

2. Fahrenheit to Celsius.

o

C = 5/9 x (oF - 32 )

2.9 Temperature Inversions.The general pattern of temperature distribution in the

atmosphere is that temperature decreases with

height. The rate at which this decrease takes place is

approximately 2oC per 1000 feet climbed in a

stationary air mass. On clear nights when the Earth

loses a great deal of heat by terrestrial radiation and

cools down, the air in contact with its surface also

cools by conduction. This cooler air sinks and does not

mix with the air at higher levels. This leads to the air at

the surface being cooler than the air above, creating a

temperature inversion. The inversion may exist for

only tens of feet or maybe hundreds of feet. There areby products of a temperature inversion important to a

pilot. Windshear or a ground fog.


Fig 2.7 Fig 2.9

Fig 2.8


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3.1 Atmospheric Pressure.The molecules making up the air move at very high

speeds and in random directions. They bounce off any

surface that they encounter, and the force they exert

on that surface we call Atmospheric Pressure. There

are fewer air molecules at higher altitude and less

weight of molecules pressing down from above.

Therefore, atmospheric pressure decreases with

height. An aircraft flying at 25 000 feet or a town in the

mountains at 5000 feet will experience a lower

pressure than at sea level.

3.2 Atmospheric pressure can be

measured by using:

1. Mercury Barometer.Atmospheric pressure

at sea level support a column of 26 inches of

mercury by pushing it into a partial vacuum;

or an

2. Aneroid Barometer.A flexible metal

chamber that is partially evacuated is

compressed by the atmospheric pressure.

This method is used in aircraft altimeters,

where changes in atmospheric pressure are

measured and converted to read changes in

the altitude.

At sea level on a standard day the atmospheric

pressure is 1013.2 hPa. Until recently the unit of

measurement for the atmospheric pressure was the

millibar, this has changed to the hectoPascal, there is

no difference between the two units. As height is

gained above sea level in the lower levels of the

atmosphere, the pressure drops at a rate of

approximately 1hPa per 30 feet of height gained. The

altimeter in an aircraft is calibrated to show this

pressure drop in feet above sea level or feet aboveground, depending on the pressure datum being used.

3. Atmospheric Pressure

Fig 3.2

Fig 3.1


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The atmospheric pressure at a particular place is

continually changing, these changes maybe;

1. Irregular; due to the passage of pressuresystems, intensifying or weakening.

2. Regular; due to diurnal temperature variation

caused by the heating and cooling effects of

the Sun. These are known as semi-diurnal

variations of pressure.

3.3 Pressure Gradients.Readings are taken at many locations and converted

to sea level values for comparison purposes. The

places that are experiencing the same calculated sea

level pressures are then joined on a map with lines.

These lines are called isobars. These lines form

patterns on weather charts that are very meaningful.

Some will define areas of low pressure. Some will

define areas of high pressure. Others will be straight.

Variation of pressure over a horizontal distance is

called the Pressure Gradient. This occurs at right

angles to the isobars. Closely packed isobars will give

a rapid change in pressure, the pressure gradient is

said to be steep or strong then. Loosely packed or

widely spread isobars will give a flat or weak pressure

gradient. The natural tendency is for air to travel from

an area of high pressure to an area of low pressure,

the steeper the pressure gradient the stronger the flowof air. However, the flow is not directly from high to low

but somewhat modified due to the Earth's rotation.

When flying from one area or region, the pilot should

monitor the subscale setting on the altimeter. If the

aircraft is travelling from an area of high pressure

to an area of low pressure and the altimeter is not

reset for the new pressure the altimeter will over

read. The aircraft will be descending although the

altimeter still reads the correct height. The reverse

applies when flying from low pressure to high pressure

areas.

3.4 International Standard Atmosphere.A datum is needed to measure the actual atmosphere

against. This datum is called the International

Standard Atmosphere, and has been devised using

specific values against which everything relating to the

atmosphere is measured. The International Standard

Atmosphere is based on the following mean sea level

values:

i. Pressure = 1013.2 hPa;

ii. Temperature = +15 oC;

iii. Density 1225 gm / cubic metre;

iv. Lapse rate = 1.98 oC per 1000 feet up to

36 090 feet (Tropopause) where

temperature remains at -56.5oC;

v. Pressure falls at approximately 1 hPa per

30 feet.

In reality the actual atmosphere differs from ISA in

many ways. Sea level pressure varies from day to day,

even hour to hour. Temperature fluctuates betweenwide extremes at all levels. The variation vertically and

horizontally of ambient pressure affects the operation

of the altimeter.

Fig 3.6

Fig 3.5

Fig 3.3

Fig 3.4


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4.1 What is wind?The term wind refers to the flow of air over the

Earths surface. This flow is practically all horizontal

with only about 1/1000th being vertical. The vertical

flow is important however to pilots as it is this flow that

creates cumuliform clouds and thunderstorms. The

vertical flow in can be of such strength that it can

destroy aircraft. When referring to wind in aviation we

are referring to the horizontal flow of air.

4.2 How wind is defined.There are two components of wind, its strength and

direction.

i. Wind direction is the direction from which the

wind is blowing. This is expressed in degrees

measured clockwise from North.

ii. Strength, which is expressed in knots (kt).

The two, direction and strength, together describe the

wind velocity. This is usually written in the form

300/30, i.e., wind from 300o at 30 knots.

A meteorologist relates wind direction to True North,

so all winds that appear on forecasts are in os (T).

Runways are aligned in their magnetic direction.

Winds are very important to aircraft when taking off

and landing, so winds given to the pilot from the tower

will be expressed in degrees magnetic.

4.3 Veering and Backing.When a wind direction changes it is said to have

veered or backed. These terms relate to a clockwise

or counter clockwise change in direction. If wind

changes in a clockwise direction (090/10 to 120/10) it

is said to have veered. Wind that has changed in a

counter clockwise direction is said to have backed

(270/10 to 200/10).

4.4 What causes a wind to blow?A change in velocity (speed or direction or both) is

called acceleration. Acceleration is caused by a force(or forces) acting on an object. The net or resultant

force acting on an object is the combined effect of all

the forces acting on that object. If all the forces acting

on an object, balance each other so that the resultant

force equals zero then the object will not accelerate. It

will continue to move at the same speed or remain

stationary. A steady wind velocity is called a balanced

flow. The forces that cause wind to blow are;

i. The pressure gradient force;

ii. The Coriolis force.

4. Wind

Fig 4.1

Fig 4.2


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4.5 The Pressure Gradient Force.The force that is usually responsible for getting a

parcel of air moving is the pressure gradient force.

This force acts by moving air from areas of high

pressure to areas of low pressure. On meteorology

charts the places of equal pressure are joined by

isobars. The pressure gradient force acts at right

angles to these lines of equal pressure moving the airfrom the high to the low pressure. The stronger the

pressure gradient (the greater the pressure difference

over a given distance) the greater the force will be, the

stronger the wind will blow. If the Pressure Gradient

force was the only force acting on a parcel of air, it

would continue to accelerate toward the low pressure.

Getting faster and faster and eventually the high and

low pressure areas would disappear because of the

transfer of air.

4.6 Coriolis ForceThis we know is not so, so another force must exist.

The other force working here is the Coriolis Force

cause by the rotation of the Earth. It is this force that

prevents the air from rushing from the high straight

into the low pressure area. The Coriolis Force is not

a real force but an apparent force that acts on a parcel

of air moving over the rotating Earth.

Imagine a parcel of air that is stationary over the point

A on the Equator. It is in fact moving with point A as

the Earth rotates on its axis from west to east. Now

suppose that a pressure gradient exists with a high

pressure at A and a low pressure at, directly North of

A. The parcel of air starts moving toward B, but still

with its motion toward the east due to the Earth's

rotation. The further North one goes the less is the

easterly motion of the Earth and so the earth will lag

behind the easterly motion of the parcel of air. Point B

will have moved to B1, but the parcel of air will have

moved to A2. The parcel of will have appeared to

turned right. This effect is due to Coriolis Force.

If the parcel was being accelerated Southerly from a

high pressure in the north toward a low pressure nearthe Equator, the Earths rotation would appear to get

away from the parcel of air as it travels south. The

parcel of air would again appear to have turn right,

having moved from B to B2 west of A1. The faster the

airflow the greater the Coriolis effect, no air flow

means no Coriolis effect. The Coriolis effect is greater

in higher latitudes toward the poles, where changes in

latitude cause more significant changes in speed at

which each point is moving toward the East. In the

Northern hemisphere the Coriolis effect deflects the

wind to the right and the reverse occurs in the

Southern hemisphere.

Coriolis force acts to the right in the Northernhemisphere.


Fig 4.3Fig 4.4

Fig 4.5


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4.8 From High to Low Look Out Below.If an aircraft is experiencing starboard drift when

flying, in the Northern hemisphere, the wind is from the

left and therefore, according to Buys Ballotss Law, the

aircraft is flying toward an area of low pressure. Low

pressure often has poor weather associated with it,such as low cloud, rain and poor visibility. Unless the

pilot resets the altimeter to the lower QNH the

altimeter is going to over read, not good.

When the aircraft experiences left drift this means that

it is heading into an area of higher pressure (applying

Buys Ballotss Law), higher pressure often suggests

more stable air and generally better weather (although

fog may occur). If the pilot does not reset the altimeter

to the higher regional QNH the altimeter is going to

under read.

4.9 Gradient wind blows around curved

isobars.Isobars are usually curved, for a wind to flow parallel

to these isobars it must be accelerated, in the sense

that its direction is being changed. In order for the air

to curve into the turn it must have a force acting on itto pull it into the turn. For a wind that is blowing around

a LOW (counter clockwise) in the Northern

hemisphere, the net force results from the Pressure

Gradient being greater than the Coriolis force, thereby

pulling the air flow into the LOW. For a wind blowing

around a HIGH (clockwise) in the Northern

hemisphere, the net results from the Coriolis force

being greater than the Pressure Gradient. Since the

Coriolis force increases with wind speed, it follows that

the wind around a HIGH will be faster than those

around a LOW with the same isobaric spacing.

4.7 The Geostrophic Wind.Two forces act on a moving airstream;

i. Pressure gradient;

ii. Coriolis force.

The pressure gradient force gets air moving and the Coriolis effect turns it right. This curving of the airflow over the

Earth's surface will continue until the pressure gradient force is balanced by the Coriolis force. Resulting in a wind

flow that is steady and blowing in a direction parallel to the isobars, this balanced flow is called the GeostrophicWind.

The Geostrophic wind flows in a direction parallel to the isobars with the low pressure on the left, and at a strength

directly proportional to the spacing of the isobars (proportional to the pressure gradient). The closer the isobars the

stronger the wind.

Fig 4.6

Fig 4.8


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In the Northern Hemisphere the result is a wind

flowing parallel to the isobars, clockwise around a high

(anticyclonic motion) and counter clockwise (cyclonic

motion) around a low. Balanced wind flow around

curved isobars is called the Gradient wind.

4.10 Surface winds.

Surface wind is the horizontal movement of air closeto the ground. It is measure by placing wind indicators

or wind socks at 30ft above the surface in flat open

spaces. Surface winds are important to pilots as they

directly affect takeoffs and landings.

Wind near the surface is usually less in strength than

winds at higher levels. The Gradient wind that flows

parallel to the isobars at higher levels is slowed by the

friction that exists between the lower level air and the

surface of the Earth. Coriolis effect is decreased due

to the slower wind speed, and the wind will back as a

result. The rougher the surface the greater the wind

will slow. Flat areas, like, deserts or oceans will affect

the wind less than hilly or city areas with many

obstructions.

A reduced wind speed results in a reduced Coriolis

force (since it depends upon speed). Therefore the

Pressure Gradient force will have more of an effect in

the lower levels, causing the wind to flow toward the

area of Low pressure and out toward the area of High

pressure. Instead of flowing parallel to the isobars.

The surface wind tends to back compared to the

Gradient wind. Over oceans the surface wind may

slow by one-third of the gradient wind and back by100. Over land the surface wind may slow by two-

thirds and back by 300. Frictional forces due to the

Earths surface decreases rapidly with height and are

negligible above 2000 feet above the ground level

(agl). The turbulence created by rough surfaces also

fades out at approximately the same

level.

4.11 Diurnal Variation of the Surface

Wind.Heating of the lower level air during the day will

promote vertical movement of this air. This causesmixing of the various level layers of air and the effect

of the Gradient wind will be brought closer the ground.

The surface wind by the day will resemble the

Gradient wind more closely than by night. The day

surface wind will be seen as a stronger wind that has

veered as compared to the night surface wind.

At night the mixing of the layers is reduced. The

Gradient wind will continue to blow at altitude, but its

effect will not be mixed with the air flow at the surface

to such an extent as during the day. The night wind at

surface level will drop in strength and the Corioliseffect will weaken. Compared with the day wind the

night wind will drop in strength and back in direction.

4.12 Localised friction effects.The surface wind will bear no resemblance to the

Gradient wind at 2000 feet agl and above if it has to

blow over and around obstacles such as hills, trees,

buildings, etc. The wind will form turbulent eddies.

The size and strength will depend upon both the size

of the obstacle and the wind strength.


Fig 4.10

Fig 4.11

Fig 4.9


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4.13 Flight in turbulence.To a small degree some turbulence is always present

in the atmosphere and as pilots, one quickly becomes

used to its effect. Moderate to severe turbulence,

however, is uncomfortable and can be dangerous to

the aircraft and its occupants. Severe turbulence can

cause condition that may over stress the aircraft.

Vertical gusts will increase the angle of attack on arotor blade, causing an increase in the Lift generated

for that particular airspeed. If the angle of attack is

increased beyond the critical angle, the rotor blade will

stall. Load factor (or g-force) is a measure of the

stress on an aircraft and each category of aircraft is

built to take only a certain Load Factor. It is important

that these Load Factors are not exceeded. One

means of achieving this is to fly the aircraft at

"turbulence penetration speed" which is much slower

than the normal cruise speed.

When turbulence is encountered:

i. Slow, reduce air speed.

ii. Manipulate the controls to maintain a

steady altitude.

iii. Avoid flying close to hills or objects that

will create more turbulence.

iv. If turbulence is strong land as soon as

possible.

Avoiding turbulence is better, and to some extent

this is possible:

i. Do not fly if moderate to severe turbulence

is forecast.

ii. Avoid flying underneath, in or near

thunderstorms.

iii. Avoid flying under large cumulus cloud.

Large up drafts produce them.

iv. Do not fly in the lee of hills when strong

winds are blowing, they tumble over ridges

and create turbulence that your aircraft will

not be able to out perform.

v. Do not fly low over rough ground when

strong winds are blowing.

4.14 Windshear.Windshear is the variation of wind speed and/or

direction from place to place. Windshear is generally

present to some extent when an aircraft is

approaching the ground for landing, because of the

different speed and direction of the surface wind

compared with the Gradient wind aloft. Low level

windshear can be quite marked at night or in early

morning when there is little mixing of the lower layers,

for instance when an inversion exists. Windshear can

be expected when a Sea Breeze or a Land Breeze is

blowing, or near a Thunderstorm. Cumulonimbus

clouds have enormous updrafts associated with them.The effects of these can be felt up to 10 to 20 NM

away from the actual cloud. Windshear and

turbulence associated with a Thunderstorm can

destroy aircrafts.

Fig 4.12

Fig 4.13


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4.15 Wind associated with Mountains.Wind that flows over mountains and down the lee side

can dangerous to aviation, not only because of the

turbulence, but because the aircraft must climb into it

to maintain the altitude. Wherever possible maintain

several thousand feet clearance when flying over

mountainous regions during strong winds.

Other winds that can be dangerous are, Katabatic

winds that flow down cool mountain slopes at night

and early morning, Valley winds. Valley winds can

change direction by as much as 180o to flow down a

valley. The valley acts as a venturi, and increases the

speed at which the wind flows. Large mountains or

mountain ranges cause an effect on wind that may

extend well above ground level, resulting in Mountain

Waves (Standing waves) possibly with associated

lenticular clouds.

The up-currents and down-currents associated with

mountain waves can be quite strong and may extend

for 30 to 40 NM downwind of the mountains. Rotor

areas may form beneath the crests of the nearer lee

waves, and are often characterised by Roll Cloud.

There may be severe turbulence in the rotor zone.

4.16 Wind in the Tropics.In tropical areas, Pressure Gradients are generally

weak and so will not cause the air to flow at high

speeds. Local effects, such as Land and Sea Breezes,

may have a stronger influence than the Pressure

Gradient. The Coriolis force that causes the air to flow

parallel to the isobars is very weak in the tropics since

the distance from the Earths axis remains constant.

The Pressure Gradient force, though relatively weak,

will dominate and the air will flow more from the high

pressure areas to the low pressure areas than parallel

to the isobars. Instead of using isobars (which join

areas of equal pressure) on tropical charts, it is more

common to use:

i. Streamlines to indicate wind direction,

which will be out-drafts from high

pressure, and in-drafts to low pressure.

ii. Isotachs, which are dotted lines joining

places of equal wind strength.

4.17 Microbursts.These are sudden local downdraughts from the base

of a thunderstorm; they hit the ground and spread out.

They are of particular concern when taking off or

landing. The speed out of the cloud is approximately

70 - 80 knots vertically down, this then spreads out in

all directions at a speed of approximately 60 knots

horizontally. Let us consider the take off case. The

pilot initiates the climb and is suddenly subjected to a

strong head wind; the aircraft's indicated airspeed

increases, the pilot slows the aircraft. The head wind

suddenly changes to a downdraught and the airspeed

is now low, also the aircraft is in a column of

descending air, the rate of climb could be zero. The

aircraft now experiences a tail wind. This could quite

easily take the aircraft below the stall speed and it

crashes. A similar outcome could be the result of

approaching to land through a microburst.

4.18 The Tropopause and windWhy is would a be pilot interested in the tropopause?

Temperature and wind vary greatly in the vicinity of the

tropopause affecting efficiency, comfort, and safety of

flight. Maximum winds generally occur at levels near

the tropopause. These strong winds create narrow

zones of wind shear which often generate hazardous

turbulence. Pre-flight knowledge of temperature, wind,

and wind shear is important to flight planning. The

tropopause is a thin layer forming the boundary

between the troposphere and stratosphere. Height ofthe tropopause varies from about 65,000 feet over the

Equator to 20,000 feet or lower over the poles. The

Fig 4.14

Fig 4.15


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tropopause is not continuous but generally descends

step-wise from the Equator to the poles. These steps

occur as "breaks." An abrupt change in temperature

lapse rate characterises the tropopause. Note that the

temperature above the tropical tropopause increases

with height and over the polar tropopause,

temperature remains almost constant with height.

4.19 Jet StreamsWhen thermal effects are very strong jet streams can

form. These can be likened to a hollow flat tube

through which air passes at high speed. One open end

of the tube being an entry and the other an exit for the

passing air. The World Meteorology Organisation

defines a jet stream thus:

A strong narrow current concentrated along a

quasi-horizontal axis in the upper troposphere orstratosphere characterised by strong vertical and

lateral wind shears and featuring one or more

velocity maxima. The windspeed must be greater

than 60 kt.

Typical dimensions for a jet are 1500 nm long, 200 nm

wide and 12,000 ft deep.

The general shape together with typical isotach values

in a cross section diagram are shown in Fig 4.16. The

isotachs show that there are very strong windshears

on the cold or polar side of the jet and above the jetcore too. In these areas therefore and particularly on

the cold side there is strong clear air turbulence. Jet

streams in the troposphere have a general westerly

direction and speeds well above 100 kt are common.

In the region of east Asia and Japan speeds can be up

to 300 kt. There are two main locations for

tropospheric jet streams. These are the subtropical

jets and the polar front jets. In both cases the jet

streams form in the warm air below the tropical

tropopause.

4.20 Polar Front JetsAt the polar front there is the meeting between polar

and tropical air and therefore a strong north/south

thermal gradient. This produces a westerly jet stream

in both hemispheres in accord with Buys Ballot's Law.

The polar front is frequently separated into different

segments and contained within polar front

depressions. A cross section of a polar front low is at

Fig 17. The basic skeleton of this cross section is

shown at Fig 18 and it will be noted that there is adistinct increase in the mean temperature of a column

of air in the warm sector compared with the two

columns in the polar maritime air ahead of the warm

front and behind the cold front respectively. The

resulting strong thermal components cause jet

streams to form in the warm air below the tropical

tropopause. It will further be apparent that the jet is

more likely, or, likely to give a stronger wind, in

association with the cold front because of the shorter

horizontal distance between the warm and cold

columns.

Fig 4.16


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The plan diagram of a polar front low at Fig 19 shows

that the two separate jets are in fact one jet stream

lying roughly parallel to the frontal surfaces. The

portion of the jet in association with the warm front is

some 400 nm ahead of the surface warm front position

and is parallel to the front. This results in a general

northwesterly jet. The jet behind the cold front surfaceposition by some 200 nm, is again parallel to the front

and usually from a generally southwesterly direction.

The isopleths of thickness are indicated to show the

thermal winds. Care should be taken when viewing

this plan diagram which can give the impression that

the jets are in the cold air. This is not the case as the

slope of the fronts allows the plan position to be

transposed to the warm air at height. With passage of

a polar front low from the west, the surface winds veer

from southerly through to northwesterly in the northern

hemisphere. The upper winds however back with

passage of a polar front low from the west, changingfrom northwesterly through westerly to southwesterly

behind the cold front. This can be appreciated from Fig

4.19. Polar front jets move with lows and are therefore

not as permanent as the subtropical jets. They are

more numerous and tend to be stronger in the wintermonths. The reasons are that there are more fronts in

winter and also during this season there are greater

mean temperature differences between the continents

and the oceans.

4.21 Other Jet StreamsBesides the main jet stream locations, there are upper

level winds well in excess of 60 kt in association with

increasing wind strength with increase of height, when

mountain waves are formed and these can spread into

the stratosphere. The zonal easterly winds in low

latitudes can sometimes produce tropospheric

easterly jets in the summer hemisphere, although

these tend to be fragmented in location. In the

stratosphere the easterly winds become jets with

speeds of 75 to 100 kn. These are positioned in bothhemispheres but are more pronounced in the summer

hemisphere.

Fig 4.19


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4.22 Clear Air TurbulenceClear air turbulence is turbulence out of cloud which

does not include the turbulence in the friction layer.

Hence this is turbulence at all heights above a few

thousand feet. At the higher levels, CAT can cause

loss of control, stalling and airframe damage when it is

severe.

CAT is common in association with jet streams, for

around the boundaries of a jet, vertically and

horizontally, there are strong windshears in terms of

wind speed. The turbulence is more severe on top of

the jet and more particularly on the cold or polar side.

It is also more severe with stronger winds, with jets

which are curved and with those which occur above

and to the lee of mountain ranges. In this latter

instance the vertical movements caused by mountains

can speed up the jets and also enhance the shear in

speed.

Frontal jets can produce more severe turbulence than

the subtropical type because they move with the

movement of the front. This movement is roughly at

right angles to the direction of wind flow. The diagram

Fig 4.20 shows the different features of the turbulence

in association with jet streams and fronts.

Sharp directional windshears with upper level troughs

and sometimes with upper ridges can cause

turbulence. In these instances flight along the trough

or ridge line should be avoided if possible. The areas

of CAT are shown in Fig 4.21.

The same figure also shows turbulence in association

with CB cloud. The instability lifting inside the cloud

causes air from the sides to enter the uplift area

thereby causing turbulence all around the cloud.

There is often CAT above a CB cloud. This more

frequently occurs where the cloud tops have been

restricted due to the dryness of the air above.

Therefore the lifting is still present although the waterdroplets at the cloud top have evaporated. It has been

mentioned earlier in this chapter that clear air

turbulence must occur with mountain waves if the air

is dry and there is thus no cloud. A similar situation

applies with rotor streaming. Additionally there will be

CAT in association with the upper level jet stream

which occurs with mountain waves.

Where CAT occurs at high level (nominally above FL

150) and is not associated with cumuliform cloud or

thunderstorms it is reported as TURB. To reduce CAT

effects it is recommended that aircraft are flown at the'rough' air speed for the aircraft type and if possible

that areas where the terrain drops abruptly be

avoided. For the CAT associated with jet streams, with

a direct 'head-on' or 'tail-on' jet a change of flight level

or heading can be efficacious. For a cross track jet a

change of flight level only is worthwhile.

Fig 4.20

Fig 4.20


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5.1 Cloud.A cloud is the discernible assemblage of tiny water

droplets and/or ice crystals in the free air.

Classification of cloud types and individual clouds is

not straightforward. Clouds take on various forms,

many of which continuously change. It is important to

have an understanding of cloud classification as

meteorological forecasts and reports use this system

to give a picture of the weather for the Pilot.

Clouds are defined by four main groups:

1. Cirriform (fibrous)

2. Cumuliform (heaped)

3. Stratiform (layered)

4. Nimbus (rain bearing)

Clouds are further divided according to the level of

their bases above mean sea level, resulting in ten

basic types.

High Level Cloud. These clouds have a base above20 000ft and look fine and spidery. They are in a very

cold region, and are composed of ice crystals rather

than water particles.

1. Cirrus (Ci): Detached cloud in the form of

white delicate filaments. White patches or

narrow bands. These clouds have a fibrous

or silky appearance. They have little moisture

or turbulence and move across the sky with

little change of shape or form.

2. Cirrocumulus (Cc): Cirrus indicates high andcumulus indicates heaped or lumpy. Thin,

white patch, sheet or layer of cloud with no

shading, composed of very small elements in

the form of grain or ripples, joined together or

separate, and more or less regularly

arranged. These clouds are often referred to

as mackerel sky.

3. Cirrostratus (Cs): Cirrus indicates high and

stratus indicates sheets. Transparent veil of

fibrous or smooth appearance, totally or partly

covering the sky and generally producing a

halo effect around the Sun or the Moon.

Middle Level Cloud. These clouds have a base

above approximately 6500ft.

4. Altocumulus (Ac):Alto means middle level

and cumulus means heaped or lumpy. A layer

of cloud composed of flattened globular

masses or rolls. They are arranged in groups

or lines or waves which may be joined to form

a continuous layer or appear in broken

patches and are shaded either white or grey.

Forms a Corona around the Sun or Moon.

Vertical development of Ac may be sufficient

to produce precipitation in the form of Virga orslight showers.

5. Altostratus (As):Alto means middle level

and stratus means layer. Greyish or bluish

cloud sheet of fibrous or uniform appearance

totally or partly covering the sky and having

parts thin enough to reveal the Sun through

vaguely, possibly as though through ground

glass.


5. Clouds


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Low Level Cloud. These clouds have a base below

approximately 6500ft.

6. Nimbostratus (Ns): Nimbo means rain

bearing and stratus means layer. Sometimes

confused with As but is darker grey and has a

lack of a distinct lower surface. Dark grey

cloud layer generally covering the whole sky

and thick enough throughout to block the Sun

or Moon. The base is diffuse due to more or

less continuously falling rain or snow.

7. Stratocumulus (Sc.): Stratus means layered

and cumulus means heaped. Grey or whitish

patch or sheet of cloud which has dark parts

composed of rounded masses or rolls which

may be joined or show breaks between the

thicker areas. Associated weather if any, is

very light rain, drizzle or snow.

8. Stratus (St.): Stratus means layer. Greycloud layer with fairly uniform base. May give

precipitation in the form of drizzle. When the

Sun is visible through the cloud its outline is

clearly discernible.

9. Cumulus (Cu): Cumulus means heaped.

Detached clouds, generally dense and with

sharp outlines. Developing vertically in the

form of rising mounds, domes or towers, of

which the upper part often resembles a

cauliflower. The sunlit parts of these clouds

are mostly brilliant white while the base isrelatively dark as sunlight may not reach it.

Precipitation in the form of snow or rain may

occur with large Cumulus.

10. Cumulonimbus (Cb): Cumulo means

heaped and nimbus means rain bearing.

Heavy and dense cloud with considerable

vertical extent in the form of a mountain or

huge tower. At least part of the upper portion

is usually fibrous or striated, often appearing

as an anvil or vast plume. The base appears

dark and stormy. Low ragged cloud clouds

are frequently observed below the base and

generally other varieties of low cloud such as,

Cu, Sc are joined to or in close proximity to the

Cb. Lightning, thunder and hail are

characteristic of this type of cloud, while

associated weather with this type of cloud

may be moderate to heavy showers of rain,

snow or hail.

Above are the ten main cloud classifications, there are

certain variations that may be mentioned.

Stratus fractus and cumulus fractus observed asshreds or fragments below nimbostratus or

altostratus.

Castellanus, a number of small cumuliform clouds

sharing a common base and indicating the growth of

middle level clouds in an unstable atmosphere.

Lenticularis, lens-shaped clouds formed in standing

waves over mountains caused by strong winds aloft

and often associated with cumuliform cloud.

Noting the type of precipitation will help in determining

a particular type of cloud. Showers that start and stop

suddenly followed by clear skies only occur with

convective clouds such as Cumulus and

Cumulonimbus.

precipitation which usually starts and finishes

gradually over a long period is associated with

stratiform cloud.

Drizzle from Stratus and Stratocumulus, heavy

continuous rain or snow from Nimbostratus and rain

from Altostratus.

Cloud is formed from the water vapour contained in

the atmosphere. This water vapour is taken up into

the atmosphere by evaporation from oceans and other

bodies where water is present.

5.2 Three States of Water.Water can exist in three states, gas (vapour), liquid

(water) and solid (ice). Water as a vapour (gas) is not

visible, but when this vapour condenses out (liquid) it

forms water droplets which we see as cloud, fog, mist,

rain or dew. When water exists in its solid form (ice)we see it as snow, hail, frost and ice.

5.3 Latent Heat.

Any change of state involves a heat transaction withno change in temperature. The amount of heat energy

required to raise one gram of water one degree

Fig 5.1


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centigrade is one calorie. If 10 calories of heat energy

are applied to one gram of ice at -10oC the

temperature of the ice would rise to 0oC. A further 80

calories of heat energy are now required to change the

state of one gram of water from its solid to its liquid

form without changing the temperature. Having

changed the state of the water from its solid to its

liquid form, a further 100 calories are required to raise

the temperature to 100oC. To change the state of one

gram of water from liquid to vapour form, without

changing the temperature, will now require a further

540 calories if heat energy. The heat energy

required to change the state of the water from a

solid to a liquid, and then from a liquid to a gas,

without change of temperature, is known as latent

heat. This energy is store in the water and is released

as the water vapour changes back to liquid and then

to ice.

5.4 Humidity.The amount of water vapour present in the air is called

humidity, but the actual amount is not as important as

whether the air can support that water vapour or not.

5.5 Relative Humidity.When a parcel of air is supporting as much water

vapour as it can, it is said to be saturated, and has a

relative humidity of 100%. If it is supporting less water

vapour than its full capacity it is said to be

unsaturated, and its relative humidity will be less than

100%. Air that is supporting only 50% of its capacity

is said to have a relative humidity of 50%. There are

many ranges of relative humidity from 0% to 100%. Incloud and fog it is 100%, over a desert it may be 20%.

Relative Humidity is defined as the ratio of water

vapour actually in a parcel of air relative to what it

can hold at a particular temperature and pressure.

Temperature largely determines the maximum amountof water vapour air can hold. Warm air can hold more

water vapour than cold air.

5.6 Wet and Dry Bulb Thermometer.A wet and dry bulb system is used to determine the

surface air temperature, relative humidity and the dew

point. The wet bulb temperature is not the dew point

temperature, except when the air is saturated. The

dry bulb thermometer measures the temperature of

the free air. A wet bulb thermometer is a normal

thermometer, the bulb of which is wrapped in a singlelayer of muslin. It is kept continuously moist by

distilled water through a short wick. Any evaporation

is shown by a lower wet bulb temperature, due to the

extraction of latent heat of evaporation, from the bulb.

The drier the air, the greater the evaporation, and the

larger the amount of heat removed. A large difference

between dry and wet bulb temperatures therefore

indicates dry air, or low relative humidity. Identical

temperatures indicate no evaporation and therefore

saturated air or 100% relative humidity. Wet bulb

temperature may be defined as the lowest

temperature to which air may be cooled by the

evaporation of water.

5.7 Dew Point.Dew point is the temperature to which air must be

cooled to become saturated by the water vapour

already present in the air. Aviation weather reports

normally include the air temperature and dew pointtemperature. Dew point when related to air

temperature reveals qualitatively how close the air is

to saturation. The difference between air temperature

and dew point temperature is called the spread. As

spread becomes less, relative humidity increases, and

it is 100% when temperature and dew point are the

same. Surface temperature-dew point spread is

important for determining fog, but has little bearing on

precipitation. To support precipitation, air must be

saturated through thick layers aloft. Sometimes the

spread at ground level may be quite large, but at

higher altitudes the air is saturated and clouds form.Some rain may reach the ground or it may evaporate

as it falls into the dryer air.


Fig 5.3

Fig 5.4


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Relative humidity depends on both temperature and

water vapour. In this figure, water vapour is constant

but temperature varies. On the left, relative humidity

is 50%, the warmer air could hold twice as much water

vapour than is actually present. As the air cools,

centre and right, relative humidity increases. As the

air cools to the dew point, its capacity to hold water is

reduced to the amount actually present. Relative

humidity is 100% and the air is saturated.

Note at 100% humidity. Temperature and dew

point are the same. When the air cooled to

saturation, it cooled to the dew point.

5.8 Adiabatic Processes.

An adiabatic process is one in which heat is neitheradded nor removed from the system. The expansion

and compression of gases are adiabatic processes

where, although heat is neither added nor removed,

the temperature of the system may change, e.g.,

placing your finger over a bicycle pump will illustrate

that compressing air increases its temperature.

Conversely, air that is compressed and stored at room

temperature will feel cool if released to the

atmosphere and allowed to expand. Reducing

pressure will lower the temperature. A very common

adiabatic process that involves the expansion of a gas

and its cooling is when a parcel of air rises in theatmosphere. This can be initiated by the heating of a

parcel of air, causing it to expand and become less

dense than the surrounding air, hence it will rise. A

parcel of air can also be forced aloft as it blows over a

mountain range.

The change of temperature which occurs solely

because of change of pressure is known as

adiabatic heating or cooling as appropriate.

When considering adiabatic lapse rates it is assumed

that no heat energy will flow between the parcel of air

and the surrounding environment. Unsaturated air willcool adiabatically at about 3oC / 1000 ft as it rises.

This is known as the Dry Adiabatic Lapse Rate

(DALR). Cooler air can support less water vapour, so,

as a parcel of air rises and cools, its relative humidity

will increase. At the height where its temperature is

reduced to the dew point (100% relative humidity)

water will start to condense out and form cloud.

Above this height the now saturated air will continue to

cool as it rises, but, because latent heat is now given-

off as the water vapour condenses into the lower

energy state, the cooling will not be as great. The rate

at which saturated air cools as it rises is known as the

Saturated Adiabatic Lapse Rate (SALR) and may be

assumed to have a value of approximately half the

DALR, i.e., 1.5oC/1000 ft.

The rate of temperature change in the surrounding

atmosphere is called the Environmental Lapse Rate

(ELR) and its relationship to the DALR and SALR is a

main factor in determining the levels of the bases and

tops of clouds that form. The ISA assumes an ELR of

2oC / 1000ft.

5.9 Stable Air.Stable air is air which when displaced vertically

will tend to return to its original level.

5.9.1 Dry.

Consider the following simplified tephigram (Fig 5.7).

Dry air is lifted manually to 5000 ft. It will cool at the

DALR (3oC / 1000 ft). The parcel of air will therefore

cool from +15oC to 0oC in this example. The ELR (2oC

/ 1000 ft), the actual temperature of the surrounding air

is to the right of the DALR, this shows a temperature

of +5oC at 5000 ft. The parcel of air is cooler and moredense than its surrounding air and will sink back to its

original level when the lifting force is removed. This

air is STABLE.

5.9.2 Saturated.

The parcel of air (Fig 5.8) is saturated throughout the

ascent to 5000 ft and has cooled at the SALR

(1.5oC/1000 ft) from +10oC at mean sea level to 2.5oC

at 5000 ft. The ELR, is to the right of the SALR. The

temperature of the surrounding air is greater than the

parcel. The parcel of air is cooler and more dense

than its surrounding air and will sink back to its originallevel when the lifting force is removed. This air is

STABLE.

Fig 5.6


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5.9.3 Absolute Stability.

Fig 5.9 shows the ELR lying to the right of both the

DALR and the SALR lines. This means that the

atmosphere is stable regardless of whether or not the

parcel of air is dry or saturated when lifted. The parcel

of air will be cooler and more dense at the top of the

lifting force and will sink.

5.10 Unstable Air.Unstable air is air which when displaced vertically

will continue to ascend until it reaches air with an

equally low density.

5.10.1 Dry.

Fig 5.10 shows dry unstable air. The ELR line lies to

the left of the DALR line. The air will remain dry

throughout its ascent within the lifting layer. At the

upper limit of the lifting layer (5000 ft) the air is warmerand less dense than the surrounding air and it will

continue to rise seeking air of the same equally low

density.

5.10.2 Saturated.

Fig 5.11 shows saturated air. The ELR line lies to the

left of the SALR line. The air will remain saturated

throughout its ascent within the lifting layer. At the

upper limit of the lifting layer (5000 ft) the air is warmer

and less dense than the surrounding air and it will

continue to rise seeking air of the same equally low

density.

5.10.3 Absolute Instability.

Fig 5.12 shows absolute instability. The ELR line lies

to the left of both the DALR and SALR lines. The air

will continue to rise regardless of whether it is dry,

saturated or starts dry and becomes saturated during

its ascent within the lifting layer.

The type of cloud which forms depends upon the

Atmospheric Environment. The nature and extent of

any cloud which forms depends upon the nature of the

surrounding atmosphere through which it is

ascending. As long as the parcel of air is warmer than

its surroundings, it will continue to rise. An

atmosphere in which a parcel of air, when given

vertical motion, continues to move away from its

original level is called Unstable.

Cumiliform clouds may form in such an atmospheric

situation, the more moisture in the air, the higher its

dew point temperature. If the surrounding air is

warmer than the parcel of air it will stop rising as its

density is greater than its surroundings. Anatmosphere in which air tends to remain at the one

level is called Stable.



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5.11 Cloud formed by convection due to

heating.Suppose that a parcel of air overlying a large ploughed

field heats up to +17oC whereas the air in the

surrounding area is only 12oC. The heated parcel of

air will start to rise, due to lower density, and cool at

the DALR of 3oC/1000 ft. If the ELR happens to be

1o

C/1000 ft, then the local area air through which theheated parcel is rising will cool at 1oC/1000 ft. The

moisture content of the parcel of air is such that it will

reach dew point when it cools to 11oC. By 2000 ft agl

the rising parcel of air will have reached saturation and

the water vapour will start to condense out and form

cloud. At 2000 ft agl the local area air will have cooled

to 10oC, the parcel of rising air is still warmer 11oC

(less dense) and will continue to rise. As the air

continues to rise above the level at which cloud first

started to form, latent heat will be given-off as more

and more vapour condenses into liquid water. The

rising air now cools at the reduced SALR of

1.5oC/1000 ft. Cooling at the new rate, the parcel of air

will have cooled to the same temperature 8oC as the

surrounding air at 4000 ft agl and will stop rising. This

produces a Cumulus cloud with base at 2000 ft and

tops of 4000 ft.

5.12 Cloud formed by orographic uplift.Air flowing over mountains rises and cools

adiabatically. If it cools below its dew point

temperature, water vapour will condense out and

clouds will form. Descending on the other side the air

warms adiabatically, once the air temperature exceeds

the dew point temperature the water vapour will no

longer condense out. The liquid water drops now start

to vapourise, and the cloud will cease to exist below

this level. A cloud that forms as a cap over the top of

a mountain is known as lenticular cloud. It will remain

more or less stationary whilst the air flows through it.

Sometimes when an air stream flows over a mountain

range and there is a stable layer of air above standing

waves occur. This is a wavy pattern as the air flow

settles back into more steady flow and, if the air is

moist, lenticular clouds may form in the crest of the lee

waves, and rotor clouds may form at a low level. The

level at which the cloud base forms depends upon the

moisture content of the parcel of air and its dew point

temperature. The cloud base may be below the

mountain tops or well above depending upon the

situation. Once having started to form the cloud may

sit low over the mountain as stratiform cloud (if the air

is stable) or as (if the air is unstable)the cloud will becumiliform and may rise to high levels.

Fig 5.13

Fig 5.14


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5.13 Foehn wind.If air rising up a mountain slope is moist enough to

have a dewpoint temperature and is cooled to it before

reaching the summit of the mountain, cloud will form

on the windward side, the air will now cool at the

SALR. If precipitaion occurs, moisture will be lost.

This will raise the dewpoint temperature causing the

cloud base to be higher on the lee side of the

mountain. As the air descends on the lee side it will

warm at the DALR which is at a greater rate than the

air cooling in the cloud. This results in a warmer, drier

wind on the lee side of the mountain known as a

Foehn Wind.

5.14 Cloud formed by Turbulence and

mixing.As air flows over the Earths surface friction causes

eddies to set up which causes mixing in the lower

levels. A very strong wind and rough surface will give

strong eddy currents a much deeper mixing layer. Theair in the rising currents will cool and if the turbulence

extends to a sufficient height and cooling to the airs

dew point may occur. Water vapour will condense out

into water droplets and clouds will form. The

descending air currents in the turbulent layer will

warm, if it warms above the dew point temperature the

liquid water droplets will return to vapour and the cloud

will not exist below this level. With turbulent mixing,

stratiform cloud may form over very large areas,

possibly with an undulating base. It may be

continuous Stratus or broken Stratocumulus.

5.15 Cloud formed by widespread

ascent.When two large air masses of differing temperature

meet, the warmer and less dense air will flow over or

be undercut by the cooler more dense air. As the

warmer air mass is forced aloft it will cool and if thedewpoint is reached cloud will form. The boundery

layer between the two air masses is called the Front

(see 6 Air Masses and Fronts).

5.16 Precipitation associated with cloud.Precipitation is falling water that reaches the ground.

This includes;

Rain consisting of liquid water drops;

Drizzle consisting of fine water drops;

Snow consisting of branched and star shapedice crystals;

Hail consisting of small balls of ice;

Freezing rain or drizzle which freezes oncontact with a cold surface (may be the

ground or an aircraft in flight).

Continuous rain or snow is often associated with

Nimbostratus and Altostratus clouds. Intermittent rain

or snow with Altostratus or Stratocumulus. Rain or

snow showers are associated with cumuliform clouds,

such as, Cumulonimbus, Cumulus and Altocumulus,

extremely heavy showers and/or hail coming from

Foehn Wind

Fig 5.15

Fig 5.16


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Cumulonimbus. Fine drizzle or snow is associated

with Stratus or Stratocumulus. It is possible to identify

cloud types by their precipitation. Showery

precipitation generally falls from cumuliform clouds

and non-showery precipitation from stratiform clouds,

mainly Altostratus and Nimbostratus. Rain that falls

from the base of a cloud but evaporates before

reaching the ground is called Virga.

5.17 Thunderstorms.Thunderstorms are a very severe weather hazard to

aviation, they generate spectacular weather which is

usually accompanied with, lightning, thunder, heavy

rain showers, hail, squalls and possibly tornadoes.

Thunderstorms are associated with Cumulonimbus

clouds and there may be several thunderstorm cells in

the one cloud.

5.17.1 Three Conditions Necessary For A

Thunderstorm To Develop.

Deep instability in the atmosphere, once airstarts to rise it will continue rising. A steep

lapse rate with warm air in the lower level and

cold air in the upper levels.

A moisture content.

A trigger action to start the air rising; a frontforcing air aloft; a mountain forcing air aloft;

strong heating action of the air in contact with

the Earth; heating of the lower layers of a

polar air mass as it moves to lower latitudes.

5.18. Life Cycle Of A Thunderstorm.

5.18.1 The Cumulus Stage.

Moist air heated from below begins to rise, as it rises

it cools at the DALR to its dewpoint temperature.

Water vapour will condense out as liquid drops and

cloud forms. The air continues to rise releasing latent

but now cools at the SALR. In the early stage of

development of a Thunderstorm, there are strong

updrafts over an area of one or two miles in diameter

with no significant down drafts. Air being drawn into

the cloud at all levels causes the updrafts to becomeeven stronger with height. The updrafts frequently

occur at a rate that an aircraft cannot out perform.

Temperature inside the cloud is higher than the

surrounding air causing the cloud to continue to build

higher and higher. The strong warm updrafts carry

water droplets higher and higher to levels often well

above the freezing level. They may freeze or continue

as super cooled water droplets. The water droplets

will coalesce to form bigger drops. This Cumulus

stage lasts approximately 10 to 20 minutes.

5.18.2 Mature Stage.

The water droplets will become too large and heavy

for the updrafts to support and will start to fall. As they

fall in great numbers, they will drag air inside the cloud

down with them causing down drafts. The first

lightning flashes and rain from the bottom of the cloud

usually occur at this stage. The descending air will

warm adiabatically, but the cold water drops will slow

down the rate at which it warms. Resulting in very

cool down drafts compared to the updrafts. Heavy

rain and/or hail will fall from the base of the cloud,

being heaviest in the first five minutes.

Fig 5.18

Fig 5.17


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The top of the cloud may reach the Tropopause and in

temperate latitudes may reach 20 000 ft and in the

tropics as high as 50 000 ft. The top may spread out

into an anvil shape, caused by the upper winds. The

updrafts and down drafts are of such a magnitude

(more than 5000 ft per minute) that they can out

perform a jet aircraft and cause structural damage.

The rapidly changing direction of the air flow can

cause an aircraft wing to stall. When the cold down

drafts flows out from the bottom of the cloud they

change direction and flow along the ground

horizontally. This produces a strong windshear. The

out flowing cold air will undercut the inflowing warm

air, which causes a mini cold front. A gusty wind and

sudden drop in air temperature may precede the

storm. A roll cloud may form slightly ahead of the

base of the cloud where the cold down drafts and the

warm updrafts pass. The mature stage lasts typically

20 to 40 minutes.

5.18.3 Dissipating Stage.The cold down drafts gradually causes the warm

updrafts to weaken and the supply of warm moist air

to the upper levels is reduced. The cold down drafts

continues and spread out over the whole cloud which

starts to collapse from the top. Eventually the

temperature of the cloud will warm to reach that of its

surroundings and the cloud will collapse into a

stratiform cloud.

Thunderstorms are HAZARDOUS to aviation. The

danger from a Thunderstorm does not exist just in orunder the cloud, but for some distance around it.

Thunderstorms should be avoided by at least 10nm

and in severe situations 20nm or more. Large aircraft

are equipped with weather radar to enable pilots to do

this. The VFR pilot must use their eyes and common

sense.

5.19 Dangers From A Thunderstorm Severe windshear (causes flight path

deviations, handling problems, loss of

airspeed and possibly structural damage);

Severe turbulence (causing loss of controland possibly structural damage);

Severe icing; Hail damage (airframe andcockpit windows);

Poor visibility;

Lightning strikes (causing damage toelectrical equipment and/or airframes);

Static (causing interference to radios andradio navigation equipment).

Fig 5.19


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6.1 Air Masses And Frontal Weather.An air mass is a large parcel of air with consistent

properties (such as temperature and moisture

content) throughout. Air masses are usually classed

by origin, its path over the Earths surface and whether

it is diverging or converging.

6.2 Origin Of An Air Mass.Maritime air flowing over oceans will absorb moisture

and tend to become saturated, in its lower levels.

Continental air flowing over a land mass will remain

relatively dry since little water is available forevaporation.

6.3 Track Of An Air Mass.Polar air flowing towards the lower latitudes will be

warmed from below and become unstable. Tropical

air flowing to the higher latitudes will be cooled from

below and become more stable.

6.4 Convergence And Divergence.An air mass influenced by the divergence of air flowing

out of a HIGH pressure system at the Earths surface

will slowly sink (known as subsidence) and become

warmer, drier and more stable. An air mass influenced

by convergence as air flows into a LOW pressure

system at the surface will be forced to rise slowly,

becoming cooler, moister and less stable.

6. Air Masses and Fronts

When you said watch the front I thought

you meant the weather.

Fig 6.1Fig 6.2


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6.5 Types Of Air Masses That Affect Ireland and the British Isles.Frontal Weather Air Masses have different characteristics, depending upon their origin and the type of surface over

which they have been passing. Because of these differences there is usually a distinct division between adjacent

air masses. These divisions are known as 'Fronts', and there are two basic types:- Cold Fronts and Warm Fronts.

'Frontal Activity' describes the interaction between the air masses, as one mass replaces the other.

6.5.1 Typical Characteristics of Air Masses affecting Ireland and the British Isles.

Air Mass Summer Winter

Warm & Humid. Fog or low ST & perhaps drizzle, may

persist to SW & over sea but break to lee of hills & inland

by day, giving moderate visibil ity.

S to SW, can be strong with risk of gales

Mild, moist & stable. Poor visiblity, extensive fog, ST &

drizzle; a few breaks to E & N of hills. Strong winds lift fog

to ST.

Tropical

Maritime (Tm)

Winds

Tropical

Continental

Tc

Winds

Hot, dry & hazy. Often clear skies. Risk of fog or ST on

S coast. Perhaps high level CB & Thunder if mixed with

moist Tm from Biscay.

Mild, dry & hazy. Some ST. Not common in winter.

Mostly S to SE, usually light, occaisionally moderate

Polar

Continental

Pc

Winds

Warm or very warm except near E coast where some ST

or fog may occur. Rather hazy but mostly clear skies

inland.

Mostly between NNE and SE

Very cold. Wintry showers from CU/SC near E coast if sea

track long enough. Risk of freezing PPN if mild Westerlies

aloft. Frost, perhaps severe, may last all day.

Artic Maritime

(Am)

Winds

Cold & Unstable; CU, perhaps CB; showers. Often clear

skies to S of high ground. May be frost in places.

Exceptionally good visibility.

Mainly NNW to NNE

Very cold. Vigorous instability of limited depth. Sleet, snow

or hail showers in the North & on E Coast. Clear inland.

Servere frost, perhaps all day.

Polar Maritime

(Pm)

Winds

Returning

Polar Maritime

(rPm)

Winds

Mild in winter, cool in summer. PM tracking south of Irish lattitude over the Atlantic becoming unstable, then tracking

north over cooler sea, becoming stable at low levels. Stability variable depending on length of track. Some fog or ST,

especially in the SW, but SC inland with risk of CU, CB, showers or even thunder developing. Snow is rare. Visibility

moderate by day but variable at night.

Mostly between S and W.

Cool & unstable; moderate humidity. CU, CB over sea &

NW facing coasts; forming inland by day. Showery, some

hail & thunder. Good visibility; bumoy flying. Servere

icing & turbulence in CB.

WSW to N, often blustery and strong.

Cold; unstable, sea temperatures giving CU, CB &

showers (snow over hills & in North). Few showers well

inland. Overnight frost, and perhaps fog if light winds.

Note: Wind directions are those most likely with the stated airmass. the type of airmass cannot be infered

soley from the wind direction.

Fig 6.3


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6.6 The Warm FrontIf two air masses meet so that the warmer air replaces

the cooler air at the surface, a Warm Front is said to

exist. The boundary at the Earth's surface between

the two air masses is represented on a weather chart

as a line with semi-circles pointed in the direction of

movement. The slope formed in a Warm Front as the

warm air slides up over the cold air is fairly shallowand so the cloud that forms in the usually quite stable

rising warm air is likely to be stratifo

aviation weather for helicopter pilots

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