# Motion in the atmosphere

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Motion in the atmosphere
ATC Chapter 6

Aim To understand large and small scale wind changes in the atmosphere

Objectives Review background relevant to wind
Describe geostrophic wind and state its effect on the atmosphere Define wind gradient Explain changes in surface winds with pressure Discuss different locals winds State and explain different types of turbulence

1. Background Wind The 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 differences Air flows with these temperature differences from a region of higher pressure to a region of lower pressure

1. Background Measurement of wind
Wind speed is measured using instruments such as an anemometer and the direction with a wind vane These 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 etc To 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

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 south The 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 widescale However, 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.

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 wind When 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

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.

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.

The change in pressure across a given distance is the pressure gradient The pressure gradient is a force that is directed from a high pressure to low pressure and is called the pressure gradient force The 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.

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 rotation As air moves from high to low pressure in the northern hemisphere, it is deflected to the right by the coriolis force As 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 winds Winds blowing at the poles will be deflected the greatest. The coriolis force is zero at the equator

2. Geostrophic wind Geostrophic wind
There are 2 forces acting on a moving airstream, they are: The coriolis force The pressure gradient The geostrophic wind is the theoretical wind that would result when the coriolis force balances the pressure gradient The geostrophic wind is directed parallel to the isobars The 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.

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.

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 spaced This changes the geostrophic winds so they are no longer geostrophic winds but instead are in gradient wind balance Around 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 inwards Around a low pressure in the southern hemisphere, the wind will tend to blow clockwise with the pressure gradient force greater than the coriolis force

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

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 winds The local winds which can affect us are: Sea breeze Land Breeze Katabatic wind Anabatic wind Fohn wind Low level Jetstream

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 temperature Incominig heat affects only a shallow layer of earth as the land is a poor conductor, whereas the water spreads the heat through a considerable depth The warm air above the land expands and rises This warm air moves towards the cooler column over the sea

5. Local Winds Land Breeze
In coastal regions at night, land breezes may develop Land breezes flow from the land to the sea The earth loses heat at night due to radiation cooling The land loses heat a lot more quickly than the water Eventually, 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 sea This causes a pressure difference between the land and the sea so therefore the wind will flow from land to sea Land breezes are not as strong as sea breezes due to the smaller temperature difference therefore has a smaller pressure gradient.

5. Local Winds Katabatic winds
At night, the earth is cooled by terrestrial radiation When the air over a sloped terrain is cooled it becomes denser therefore wants to drain to the lower levels These winds are known as katabatic winds They 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 flow The 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)

5. Local Winds Anabatic winds
An anabatic wind is the opposite of a katabatic wind On 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 slope This 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

5. Local Winds Fohn wind When 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.

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 anticyclone The 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.

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 turbulence The different types of turbulence are: Mechanical turbulence Thermal convection Inversion turbulence Frontal turbulence Clear Air Turbulence Mountain Waves

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 turbulence The severity of the turbulence depends on: Stability of the air Strength of the wind Nature of the obstructions Mechanical turbulence is greatest below 500’AGL

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

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 street Drifting thermals

6. Turbulence Thermal convection – thermal street
Thermal street is as it sounds, a street of thermals When thermals are in a moving air mass, they can be carried downwind causing a line of thermals Thermal streets are often characterized or identified by a line of cumulus clouds that lie parallel to the wind

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 strength The 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 air Heating source Light winds

6. Turbulence Inversion turbulence
As we have previously discussed, an inversion is caused due to a temperature change in the lower levels of the atmosphere With this sharp temperature change there will be varying degrees of turbulence The degree of turbulence that can be encountered when flying through an inversion are: The depth of the inversion The amount of temperature rise throughout the inversion The value of any vertical windshear The IAS of the aircraft

6. Turbulence Frontal turbulence
A front has the ability to create strong turbulence This 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 zone The relative movement of the air masses Temperature differential between the air masses Speed of the front Degree of instability The presence of any associated thunderstorms

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 borders In Australia, often airlines like to fly their eastward legs in Jetstream's Flying 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

6. Turbulence Mountain waves or standing waves
Airflow over a ridge or mountain may disturb flow up to a great height When the air flows up a mountain and reaches the ridge it may be smooth When 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 zones Conditions necessary for formation of mountain waves are: Wind strength 25-30kts Wind speed increase with height Stable layer. If the airstream is sufficiently moist, cloud may form in the ascending sections producing lens-shaped clouds called lenticular clouds

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 seatbelts Hold the flight attitude, minimise large elevator inputs and accept changes in altitude The following can be adopted to avoid turbulence Don’t fly underneath or near thunderstorms Avoid flying underneath large cumulus clouds Avoid flying in the lee of hills and mountains Avoid flying low level over rough ground with strong winds.

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.

Questions?