2 AimTo understand large and small scale wind changes in the atmosphere
3 Objectives Review background relevant to wind Describe geostrophic wind and state its effect on the atmosphereDefine wind gradientExplain changes in surface winds with pressureDiscuss different locals windsState and explain different types of turbulence
4 1. BackgroundWindThe term wind refers to the flow of air around the earths surface.Most of the wind is horizontal flow, but a small fraction is vertical, this vertical flow is important to us as it leads to things such as the formation of cloud.Wind is a pressure difference in the atmosphere usually resulting from temperature differencesAir flows with these temperature differences from a region of higher pressure to a region of lower pressure
5 1. Background Measurement of wind Wind speed is measured using instruments such as an anemometer and the direction with a wind vaneThese instruments are usually on open ground at about 10cm above the surface, this is due to the measurements being difficult at ground level because of the roughness of the surface, presence of buildings etcTo give a more accurate reading, the wind is reported as the mean value over a 10 minute period leading up to the time of the observation
6 1. Background Reporting of wind It is important that we know where the wind is coming from and what strength it is. We need to know this for calculations such as crosswinds, drift and groundspeeds.Wind is reported on a 360 degree scale from which the wind is blowing. Eg, if the wind is 180 degrees, the wind is blowing from the southThe wind is also given a strength in knots.The format is usually the direction then the strength, eg 180/15 (the wind is blowing from the south at 15kts.Meteorologists forecast the wind from true north as the forecasts are usually widescaleHowever, at aerodromes, runways are all numbered in degrees magnetic, so it would make sense for the wind to be reported in degrees magnetic so we know if we are within crosswind limitations.
7 1. Background Variations in surface wind When the wind is moves in an anticlockwise direction, for example if it moves from 270 degrees to 240 degrees, the term used to describe it is a backing windWhen the wind moves in a clockwise direction, for example if it moves from 250 degrees to 030 degrees, the term used to describe it is a veering wind
8 1. Background Variations in surface wind A gust of wind is a momentary increase in wind strength. Gusting winds are recorded in weather forecasts and reports.A lull is the momentary decrease of wind below the reported mean.A squall is a sudden, sharp increase in wind speed usually associated with weather such as thunderstorms of heavy rain.
9 1. Background Balanced flow As we have learnt in aerodynamics, balance is the term given when the resultant of all forces is equal to zero.A balanced wind flow is when there is no acceleration in speed or direction.A balanced wind flow is one that blows in a constant direction and speed.
10 1. Background Pressure gradient force The change in pressure across a given distance is the pressure gradientThe pressure gradient is a force that is directed from a high pressure to low pressure and is called the pressure gradient forceThe pressure gradient force is responsible for the initial movement of air.Places on earth where the pressure is the same are joined by lines called isobars (iso meaning the same)Therefore the pressure gradient force will act at right angles to the isobars in the direction from high to low pressure.The greater the pressure gradient, ie the greater the temperature change over a given distance, the stronger the pressure gradient and therefore the stronger the wind.
11 1. Background Coriolis force Once air has been set in motion by the pressure gradient force, it has an apparent deflection from its path as seen by an observer on the earth.This apparent deflection is called the coriolis force and is a result of the earths rotationAs air moves from high to low pressure in the northern hemisphere, it is deflected to the right by the coriolis forceAs air moves from high to low pressure in the southern hemisphere, it is deflected to the left by the coriolis force.The amount of deflection it makes is directly related to the speed and the latitude. Slow winds will be deflected less than high moving windsWinds blowing at the poles will be deflected the greatest.The coriolis force is zero at the equator
12 2. Geostrophic wind Geostrophic wind There are 2 forces acting on a moving airstream, they are:The coriolis forceThe pressure gradientThe geostrophic wind is the theoretical wind that would result when the coriolis force balances the pressure gradientThe geostrophic wind is directed parallel to the isobarsThe strength of the geostrophic wind is directly proportional to the spacing of the isobars. (directly proportional to the pressure gradient)The closer the isobars, the stronger the wind.
13 2. Geostrophic wind Buys Ballot’s Law Buys Ballot’s Law is the relationship between pressure and wind.It states:If you stand with your back to the wind, the lower pressure is on your right hand side in the southern hemisphere.
14 3. Gradient Wind Gradient winds We now know that a geostrophic wind flows parallel to the isobars, but when do you see a pressure system with only straight isobars?Isobars are almost always curved and evenly spacedThis changes the geostrophic winds so they are no longer geostrophic winds but instead are in gradient wind balanceAround a high pressure in the southern hemisphere, the wind will tend to blow anticlockwise with the pressure gradient force smaller than the coriolis force, the net acceleration will be inwardsAround a low pressure in the southern hemisphere, the wind will tend to blow clockwise with the pressure gradient force greater than the coriolis force
15 4. Surface Winds Nature of the surface wind Close to the ground, the air is subject to frictional forces which change the characteristics.Gradient and geostrophic wind flow occurs above 3000ft where the effects of friction can be ignored.Close to the ground, the wind speed is decreased by friction.This decreases the coriolis effect and gives the pressure gradient more time to act on the air.The result is that the surface wind swings more in favour of the pressure gradient – ie, across the isobars out of a high pressure and into a low pressure.The surface wind tends to veer clockwise compared to the gradient wind
16 5. Local Winds Local Winds Winds can be large scale, but they can also be small scale.The name given to the small scale winds are local windsThe local winds which can affect us are:Sea breezeLand BreezeKatabatic windAnabatic windFohn windLow level Jetstream
17 5. Local Winds Sea Breezes The basic cause of a sea breeze is the differential heating rates of sea and land under conditions of strong incoming solar radiation.During the day the land heats more rapidly than the sea. This is for 2 main reasons.Land has a lower ‘specific heat’ than water, this requires less heat to raise the temperatureIncominig heat affects only a shallow layer of earth as the land is a poor conductor, whereas the water spreads the heat through a considerable depthThe warm air above the land expands and risesThis warm air moves towards the cooler column over the sea
18 5. Local Winds Land Breeze In coastal regions at night, land breezes may developLand breezes flow from the land to the seaThe earth loses heat at night due to radiation coolingThe land loses heat a lot more quickly than the waterEventually, the temperature of the land will fall below that of the water.The air in contact with the land then cools more rapidly than the air in contact with the seaThis causes a pressure difference between the land and the sea so therefore the wind will flow from land to seaLand breezes are not as strong as sea breezes due to the smaller temperature difference therefore has a smaller pressure gradient.
19 5. Local Winds Katabatic winds At night, the earth is cooled by terrestrial radiationWhen the air over a sloped terrain is cooled it becomes denser therefore wants to drain to the lower levelsThese winds are known as katabatic windsThey depend on:The degree of cooling on the slope (the greater the cooling, the greater the potential for the generation of very dense air, and therefore a greater wind speed.The roughness of the slope. (the smoother the slope, the greater potential for stronger winds as it is uninterrupted flowThe steepness of the slope (a gentle slope is more favourable than a steep slope because steep slopes cause the wind to become turbulent, therefore causing a breakdown on the movement of downward air)
20 5. Local Winds Anabatic winds An anabatic wind is the opposite of a katabatic windOn sunny days, the earth is warmed and therefore the air in contact with it also becomes warmer and less dense.This causes the air to move up the slopeThis upward movement of air up the slope is opposed by gravity, so anabatic winds are generally weaker than katabatic winds.Anabatic winds can combine with sea breezes to increase the overall strength of the wind
21 5. Local WindsFohn windWhen a moist air mass is lifted up a mountainside, the air may saturate and therefore form cloud.If precipitation occurs on the windward side, the overall moisture content passing over the hills will be reduced.Therefore the cloud base will be higher on the leeward side of the mountain compared to the windward side.The air on the leeward side will also be drier and therefore warmer.This is because the air on the windward side cools at the DALR until it reaches the base of the cloud then cools at the SALR until it warms at the SALR on the other side of the mountain to the base of the cloud on the leeward side.Below the cloud, the air warms at the DALR. This causes the temperature on the leeward side to be greater.
22 5. Local Winds Low level Jetstream The low level Jetstream occurs in the friction layers of the ridge of a high pressure system or leading edge of an anticycloneThe flow around these systems is anticlockwise in the southern hemisphere.When the pressure system is to the west of a north-south orientated mountain range, the eastward migration is obstructed.The low level jet is formed by the funnelling of air along the mountain range.The low level jet can reach maximum speeds of 70kts.
23 6. Turbulence Turbulence Turbulence can take on many different forms, have different strengths and be formed by different means.It is important we know about turbulence because we need to know about passenger comfort, structural integrity and the actual flying in turbulenceThe different types of turbulence are:Mechanical turbulenceThermal convectionInversion turbulenceFrontal turbulenceClear Air TurbulenceMountain Waves
24 6. Turbulence Mechanical turbulence When wind blows over obstacles such as hills, trees, buildings etc, the wind will cause turbulent eddies.The size of these eddies depends on the size of the obstacle and wind strength.This turbulence is known as mechanical turbulenceThe severity of the turbulence depends on:Stability of the airStrength of the windNature of the obstructionsMechanical turbulence is greatest below 500’AGL
25 6. Turbulence Thermal convection As we know by now, when samples of air are heated, they become less dense and rise.These rising thermals are mostly gentle and rise at 100fpm or so, but sometimes large thermals can be up to 2000fpm.The presence of cumulus or cumulonimbus cloud can be an indication of convective turbulence, but the absence of cloud does not mean that there may be no turbulence
26 6. Turbulence Thermal convection On a warm summers day, convection can cause very strong turbulence.When close to the ground, convective turbulence can be very variable and is affected by the heat source. Convective turbulence often has the following forms:Thermal streetDrifting thermals
27 6. Turbulence Thermal convection – thermal street Thermal street is as it sounds, a street of thermalsWhen thermals are in a moving air mass, they can be carried downwind causing a line of thermalsThermal streets are often characterized or identified by a line of cumulus clouds that lie parallel to the wind
28 6. Turbulence Thermal convection – drifting thermals A drifting thermal often takes on the form of either a dust devil or willy willies.These are hazardous to aircraft operating in close proximity to the ground and can lift dust up to 9000’AGL.Dust devils usually last for 30 minutes but may last up to an hour depending on the strengthThe danger of flying in a dust devil is the loss of control of an aircraft, for this reason they need to be avoided.Conditions necessary for formation are:Unstable airHeating sourceLight winds
29 6. Turbulence Inversion turbulence As we have previously discussed, an inversion is caused due to a temperature change in the lower levels of the atmosphereWith this sharp temperature change there will be varying degrees of turbulenceThe degree of turbulence that can be encountered when flying through an inversion are:The depth of the inversionThe amount of temperature rise throughout the inversionThe value of any vertical windshearThe IAS of the aircraft
30 6. Turbulence Frontal turbulence A front has the ability to create strong turbulenceThis is due to the horizontal windshear throughout the front.Not only is it the horizontal windshear, whenever there is a front there is usually an inversion, so this turbulence would be magnified with the inversion turbulence.The magnitude of the frontal turbulence depends on:The width of the frontal zoneThe relative movement of the air massesTemperature differential between the air massesSpeed of the frontDegree of instabilityThe presence of any associated thunderstorms
31 6. Turbulence Clear air turbulence (CAT) Turbulence does not only happen in the lower layers of the atmosphere, it happens in the upper atmosphere also, usually above 15000’Turbulence in this region not associated with cloud is known as Clear Air Turbulence. (CAT)CAT is often found in the vicinity of Jetstream's at their bordersIn Australia, often airlines like to fly their eastward legs in Jetstream'sFlying in Jetstream's increases groundspeed significantly therefore burning less fuel.There is however a trade-off, if an airline was to use a Jetstream, they could potentially encounter CAT on entry and exit to the Jetstream
32 6. Turbulence Mountain waves or standing waves Airflow over a ridge or mountain may disturb flow up to a great heightWhen the air flows up a mountain and reaches the ridge it may be smoothWhen it reaches the ridge and flows to the other side a dramatic change in flow can take place.The flow does not return to horizontal flow but continues as a wave that may be smooth but also may contain dangerous turbulent zonesConditions necessary for formation of mountain waves are:Wind strength 25-30ktsWind speed increase with heightStable layer.If the airstream is sufficiently moist, cloud may form in the ascending sections producing lens-shaped clouds called lenticular clouds
33 6. Turbulence Pilot actions on encountering turbulence If unexpected turbulence is encountered, the following should be adopted:Decrease speed to below Vb (turbulence penetration speed)Fasten seatbeltsHold the flight attitude, minimise large elevator inputs and accept changes in altitudeThe following can be adopted to avoid turbulenceDon’t fly underneath or near thunderstormsAvoid flying underneath large cumulus cloudsAvoid flying in the lee of hills and mountainsAvoid flying low level over rough ground with strong winds.
34 6. Turbulence Classification of turbulence Classification of turbulence can be found in AIP GEN 3.4 appendix 2, but below is an outline on the different classifications.
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