ch 4 - wind introduction introduction –the motion of air is important in many weather- producing...

Ch 4 - WindCh 4 - Wind


• IntroductionIntroduction– The motion of air is important in many The motion of air is important in many

weather-producing processes.weather-producing processes.– Moving air carries heat, moisture, and Moving air carries heat, moisture, and

pollutants from one location to another – at pollutants from one location to another – at times in a gentle breeze, occasionally in a pure times in a gentle breeze, occasionally in a pure hurricane.hurricane.

– Air movements create favorable conditions for Air movements create favorable conditions for the formation and dissipation of clouds and the formation and dissipation of clouds and precipitation: in some cases, those motions precipitation: in some cases, those motions cause the visibility to decrease to zero; in cause the visibility to decrease to zero; in others, they sweep the skies crystal clear others, they sweep the skies crystal clear (Lester, 2006).(Lester, 2006).


• IntroductionIntroduction– Winds move atmospheric mass and Winds move atmospheric mass and

therefore affect changes in atmospheric therefore affect changes in atmospheric pressures.pressures.

– As you will see, these pressure changes As you will see, these pressure changes modify the winds.modify the winds.

– All of these factors create reasons for All of these factors create reasons for the changeable nature of not only the the changeable nature of not only the wind, but also weather (Lester, 2006).wind, but also weather (Lester, 2006).


• IntroductionIntroduction– In flight, winds can have significant In flight, winds can have significant

effects on navigation.effects on navigation.– Chaotic air motions cause turbulence Chaotic air motions cause turbulence

which is, at least, uncomfortable and, at which is, at least, uncomfortable and, at worst, catastrophic.worst, catastrophic.

– Should atmospheric winds change Should atmospheric winds change suddenly over a short distance, flight suddenly over a short distance, flight may not be sustainable (Lester, 2006).may not be sustainable (Lester, 2006).


• IntroductionIntroduction– Without question, as a pilot, you must Without question, as a pilot, you must

understand air motions for efficient and safe understand air motions for efficient and safe flight.flight.

– In this chapter, we consider the causes and In this chapter, we consider the causes and characteristics of horizontal motions of the characteristics of horizontal motions of the atmosphere.atmosphere.

– The chapter material provides you with a The chapter material provides you with a practical understanding of important practical understanding of important relationships between the wind, atmospheric relationships between the wind, atmospheric pressure, and the earth’s rotation (Lester, pressure, and the earth’s rotation (Lester, 2006).2006).


• IntroductionIntroduction– You will also gain some insight into the You will also gain some insight into the

important influences of friction between important influences of friction between the moving air and the earth’s surface.the moving air and the earth’s surface.

– When you complete the chapter, you When you complete the chapter, you will not only have an understanding of will not only have an understanding of the fundamental causes of wind, but you the fundamental causes of wind, but you will also know how wind is measured will also know how wind is measured and you will be able to interpret general and you will be able to interpret general wind conditions from isobars and wind conditions from isobars and contours on weather charts (Lester, contours on weather charts (Lester, 2006).2006).


• Section A – Wind Terminology and Section A – Wind Terminology and MeasurementsMeasurements– METAR Wind InformationMETAR Wind Information

• Section B – Causes of WindSection B – Causes of Wind• Section C – Pressure Gradient ForceSection C – Pressure Gradient Force

– Causes of Pressure GradientsCauses of Pressure Gradients

• Section D – Coriolis ForceSection D – Coriolis Force• Section E – Geostrophic BalanceSection E – Geostrophic Balance

– Estimating Winds from Isobars and ContoursEstimating Winds from Isobars and Contours– D-ValuesD-Values

• Section F – FrictionSection F – Friction


• Section G – Other EffectsSection G – Other Effects– Wind Production by Vertical MotionsWind Production by Vertical Motions– Accelerated AirflowAccelerated Airflow


• Section A: Wind Terminology and Section A: Wind Terminology and MeasurementsMeasurements– Wind – Wind – horizontal air motionshorizontal air motions– Wind velocity – Wind velocity – a vector quantity a vector quantity – Vector – Vector – a vector quantity has a magnitude a vector quantity has a magnitude

and a directionand a direction– Scalar – Scalar – temperature and pressure are temperature and pressure are

examples of scalar quantities which only have examples of scalar quantities which only have magnitudemagnitude


– Wind speed – Wind speed – the magnitude of the wind the magnitude of the wind velocity usually expressed in nautical miles per velocity usually expressed in nautical miles per hour (knots), statute miles per hour (mph), hour (knots), statute miles per hour (mph), kilometers per hour, or meters per secondkilometers per hour, or meters per second

– Wind direction – Wind direction – the direction from which the the direction from which the wind is blowing, measured in degrees, or to wind is blowing, measured in degrees, or to eight or sixteen points of the compass, eight or sixteen points of the compass, clockwise from true north (360 degrees)clockwise from true north (360 degrees)


• METAR Wind InformationMETAR Wind Information– Sustained speed – Sustained speed – reported wind speeds and reported wind speeds and

directions are usually one or two-minute directions are usually one or two-minute averages; this average wind speed is also averages; this average wind speed is also referred to as the sustained speedreferred to as the sustained speed

– Peak wind – Peak wind – the maximum instantaneous the maximum instantaneous wind speed greater than 25 knots since the last wind speed greater than 25 knots since the last hourly observationhourly observation


• Section B: Causes of Wind – Section B: Causes of Wind – the most the most important forces that affect air motions are important forces that affect air motions are pressure gradient force, coriolis force and pressure gradient force, coriolis force and frictional force frictional force


• Section C: Pressure Gradient Force – Section C: Pressure Gradient Force – the difference in the difference in pressure between two points divided by the distance pressure between two points divided by the distance between the pointsbetween the points

• the larger the pressure difference, the greater the the larger the pressure difference, the greater the acceleration through the openingacceleration through the opening

•magnitude of the pressure gradient = (P1 – P2 / magnitude of the pressure gradient = (P1 – P2 / distance)distance)

– Horizontal pressure gradient – Horizontal pressure gradient – the atmosphere causes the atmosphere causes air parcels to be accelerated across the surface of the air parcels to be accelerated across the surface of the earth toward low pressure when a horizontal pressure earth toward low pressure when a horizontal pressure gradient force existsgradient force exists

– Causes of Pressure GradientsCauses of Pressure Gradients•Differential heating – Differential heating – creation of a horizontal creation of a horizontal

temperature gradient by temperature gradient by


• Thermal circulation – Thermal circulation – in general, the movement in general, the movement of air which results from differential heatingof air which results from differential heating– thermal circulations have two horizontal thermal circulations have two horizontal

branchesbranches

•an upper branch which is called the return an upper branch which is called the return flow and a lower branch; figure 4-6 sea flow and a lower branch; figure 4-6 sea breezebreeze


• Section D: Coriolis Force – Section D: Coriolis Force – a deflective force a deflective force resulting from earth’s rotationresulting from earth’s rotation– it acts 90 degrees to the right of wind direction it acts 90 degrees to the right of wind direction

in the Northern Hemisphere and 90 degrees to in the Northern Hemisphere and 90 degrees to the left of the wind in the Southern the left of the wind in the Southern HemisphereHemisphere


• Section E: Geostrophic Balance – Section E: Geostrophic Balance – Coriolis and Coriolis and pressure gradient forces tend to be equal in pressure gradient forces tend to be equal in magnitude but opposite in directionmagnitude but opposite in direction– Geostrophic winds – Geostrophic winds – the related wind is the the related wind is the

geostrophic windgeostrophic wind

• it is quite helpful in understanding the it is quite helpful in understanding the characteristics of wind and it provides a characteristics of wind and it provides a good approximation to the actual wind good approximation to the actual wind


• ***When the isobars on the surface analysis ***When the isobars on the surface analysis chart are close together, the pressure chart are close together, the pressure gradient force is large and wind speeds are gradient force is large and wind speeds are strongstrong


• ***Wind directions can be inferred from ***Wind directions can be inferred from isobaric patternsisobaric patterns


• ***Winds do not blow directly from large ***Winds do not blow directly from large scale high pressure areas to low-pressure scale high pressure areas to low-pressure areas because of Coriolis forceareas because of Coriolis force


• Estimating Winds from Isobars and ContoursEstimating Winds from Isobars and Contours – ***The 500 mb constant pressure chart is ***The 500 mb constant pressure chart is

suitable for flight planning at FL 180suitable for flight planning at FL 180– observed temperature and wind observed temperature and wind

information give approximate conditions information give approximate conditions along the proposed routealong the proposed route


• D-Values – D-Values – the difference between the two (True the difference between the two (True Altitude (TA) – Pressure Altitude (PA))Altitude (TA) – Pressure Altitude (PA))– the cross track geostrophic wind is proportional the cross track geostrophic wind is proportional

to the gradient in D-values along the flight to the gradient in D-values along the flight tracktrack


• Section F: Friction – Section F: Friction – the force that resists the relative the force that resists the relative motion of two bodies in contactmotion of two bodies in contact– Skin friction – Skin friction – friction also occurs within fluids, such friction also occurs within fluids, such

as the atmosphere and at the interface between fluids as the atmosphere and at the interface between fluids and solidsand solids

– Form drag – Form drag – caused by turbulence induced by the caused by turbulence induced by the shape of the aircraftshape of the aircraft

– Surface friction – Surface friction – describes the resistive force that describes the resistive force that arises from a combination of skin friction and arises from a combination of skin friction and turbulence near the earth’s surfaceturbulence near the earth’s surface

– Boundary layer – Boundary layer – a transition zone between large a transition zone between large surface frictional effects near the ground and negligible surface frictional effects near the ground and negligible effects above the boundary layer; figure 4-12effects above the boundary layer; figure 4-12


• ***Because of the decrease of surface ***Because of the decrease of surface frictional effects with height, the winds at frictional effects with height, the winds at 2,000 feet AGL tend to parallel the isobars2,000 feet AGL tend to parallel the isobars– At the surface, winds cross the isobars at At the surface, winds cross the isobars at

an angle toward lower pressure and are an angle toward lower pressure and are weaker than winds aloftweaker than winds aloft


• ***Wind is caused by pressure differences ***Wind is caused by pressure differences and modified by the earth’s rotation and and modified by the earth’s rotation and surface frictionsurface friction


• Section G: Other EffectsSection G: Other Effects– Wind Production by Vertical Motions – Wind Production by Vertical Motions – see Figure 4-see Figure 4-

1313– Accelerated AirflowAccelerated Airflow

•Centrifugal force – Centrifugal force – when air moves along a curved when air moves along a curved path, even if it is traveling at a constant speed, it is path, even if it is traveling at a constant speed, it is subjected to an accelerationsubjected to an acceleration

– the direction of motion is constantly changing the direction of motion is constantly changing along the pathalong the path

– this is known as centripetal accelerationthis is known as centripetal acceleration– it is due to an imbalance in forcesit is due to an imbalance in forces

when discussing this effect, some find it more when discussing this effect, some find it more convenient to refer to a force that produces the convenient to refer to a force that produces the centripetal acceleration – the centrifugal centripetal acceleration – the centrifugal acceleration acceleration


• Cyclostrophic balance – Cyclostrophic balance – coriolis force, the coriolis force, the pressure gradient and centrifugal forces may be pressure gradient and centrifugal forces may be in cyclostrophic balance and produce in cyclostrophic balance and produce cyclostrophic windscyclostrophic winds– the most dramatic examples of these are dust the most dramatic examples of these are dust

devils and tornadoes; figure 4-14devils and tornadoes; figure 4-14

SummarySummary

• The basic properties of horizontal motions of The basic properties of horizontal motions of the atmosphere have been examined in this the atmosphere have been examined in this chapter.chapter.

• You should now understand that air responds You should now understand that air responds to pressure gradients by being accelerated to pressure gradients by being accelerated toward lower pressure.toward lower pressure.

• Furthermore, pressure gradients are caused Furthermore, pressure gradients are caused by temperature gradients and the movement by temperature gradients and the movement of atmospheric mass by the winds.of atmospheric mass by the winds.

• Once the air is in motion, Coriolis force Once the air is in motion, Coriolis force becomes important, especially in large scale becomes important, especially in large scale atmospheric circulations (Lester, 2006). atmospheric circulations (Lester, 2006).

SummarySummary

• The wind that results when Coriolis force is The wind that results when Coriolis force is exactly in balance with the pressure exactly in balance with the pressure gradient force is the geostrophic wind.gradient force is the geostrophic wind.

• Because the near balance of these two Because the near balance of these two forces is common, the geostrophic wind has forces is common, the geostrophic wind has proven to be a very useful estimate of actual proven to be a very useful estimate of actual wind in a variety of applications ranging wind in a variety of applications ranging from the interpretation of isobars and from the interpretation of isobars and contours on weather charts, to navigation. contours on weather charts, to navigation. (Lester, 2006). (Lester, 2006).

SummarySummary

• Friction modifies the geostrophic balance, Friction modifies the geostrophic balance, especially in the atmospheric boundary especially in the atmospheric boundary layer where its effect is apparent in cross-layer where its effect is apparent in cross-isobar airflow, turbulence, and gusty winds.isobar airflow, turbulence, and gusty winds.

• Your knowledge of the basic causes and Your knowledge of the basic causes and characteristics of wind will be of great value characteristics of wind will be of great value as you examine vertical motions, clouds, as you examine vertical motions, clouds, and weather in the next two chapters and, and weather in the next two chapters and, subsequently, specific atmospheric subsequently, specific atmospheric circulations (Lester, 2006). circulations (Lester, 2006).

ch 4 - wind introduction introduction –the motion of air is important in many weather- producing...

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