Winds 

So far we have mentioned winds but not discussed them in detail. This is now the time to do so. Wind is defined as the flow or air over a surface. The wind direction is the direction it comes from. So a North Easterly Wind is coming from the North East and not going to the North East. In aviation we give this a compass direction in numerals. In this case 315 degrees. Strength or Speed is measured in knots. it is normally written like this 315/10. Indicating 315 heading at 10 Knots. If the wind changes compass direction clockwise it is said to Veer. If it changes anti clockwise it is said to back. 

In the atmosphere the pressure gradient force starts a parcel of air moving. This is caused by varying air pressures as indicated on weather charts by isobars. Air will normally flow from a High Pressure area to a Low Pressure area. Just like a hill the steeper the hill is the faster the car will go down it. Therefore the steeper the pressure gradient the stronger the wind. On weather charts the wind is stronger the closer the isobars are together. 

Because the earth is rotating this deflects the air to the right in the Northern Hemisphere and to the Left in the Southern Hemisphere. This force is known as  the Coriolis Force. The wind therefore instead of flowing at right angles to the isobars towards high and low pressure is now is turned and flows along the isobars. This is called the Geostrophic wind. 

Wind flows around a High pressure system (Anticyclone) in a clockwise direction in the Northern Hemisphere and Anti Clockwise (Cyclone) around a Low Pressure system. That said there are exceptions. For the pilot this is most important. 

The surface wind affects pilots for the take off and landing phase of the flight. On average the surface wind can vary by 25 degrees to the isobars and the wind is normally two and a half times less than the wind at 2000 ft. The wind is less because of it hitting the earth and friction slows it down. Also other effects can cause it to act differently. Wind shear is when a sudden change in direction or speed is caused. Large hangers or woods or other obstacles can cause windshear. As the wind hits them it is diverted off at a different angle. It is important to pilots due to the landing and take off effects and distances and climb gradients of aircraft. Temperature can also affect wind at the surface level. This leads us on to the next part of the wind study.

The Anabatic and Katabatic Wind

In areas of high ground the surface wind acts different to the wind at upper levels. This is caused by temperature. During the day heat from the sun warms the surface in valleys and the sides of hills. This causes the air to rise and flows up the sides of the hills. This is great news for glider and paragliding pilots as they use this effect to gain lift and height. This is why the new sport of paragliding has taken off especially in the ALPS. Paragliding pilots can now stay up for hours using these currents by flying up and down ridge lines gaining lift. My son is a keen paragliding pilot and often he spends hours doing just this. The wind flowing up the slope is called the Anabatic Wind . Gravity stops the wind from being too strong. This wind is known as the Anabatic Wind.  Late afternoon to early morning as the sun sinks the opposite effect starts Air flows down the valleys and hills from the top. This is known as the Katabatic wind. (Remembered by the cats come out at night). Mist often forms in the valleys as a result of this cool air making the dew point temperature rise. This can be accelerated by snow and ice on the tops of mountains making the air colder before it starts its descent down the hillside. These conditions can affect the direction.

Coastal winds

Now lets look at Coastal winds. Air over the land in summer rises as it is warmed. Air from the sea is cooler and then rushes in to replace the rising warm air. This rising air starts to circulate  by turning out to sea. The result of this is windshear. Imagine an aircraft coming into land here. At 2000 ft the wind direction could be Southerly yet at the Surface would be Northerly. Recently I landed at the coastal airport of Corfu around midday. The tower gave the surface wind as 350 at 6 knots  as I came down to 1800 feet six miles out the EFIS showed a 10 knot tailwind.  As I came over the threshold the windsock showed a headwind.

Windsocks & Crosswinds

If the runway windsock is out to almost 90 degrees, it will indicate a wind of 25-30 knots. The windsock also shows the direction the surface wind is blowing. Pilots on landing or taking off in in a crosswind must take extra care. In a crosswind the wing on the wind side will generate more lift and the pilot will have to correct this by turning the ailerons slightly into the wind side  (to a left turn position if the wind was coming from the left) to stop the wing lifting as you accelerate. This could cause the leeward wing to strike the ground if it was a low wing aircraft, and was not corrected. The pilot would have to apply opposite rudder, to keep the aircraft straight because the aircraft would weathercock into wind by swiveling on the undercarriage. Instead of taking off you could run the aircraft onto the grass and an accident could occur.

 Mountain Wind Effects

Valley Winds

On most day the wind will blow up and over mountain ranges but if the lapse rate for the day is small the wind may blow down the valleys instead in any direction. Around the world these winds down different valleys have names. In the Rockies it is known as the Chinook. In France the Mistral blows down the Rhone Valley. This is caused by High Pressure to the North of the mountains and Low pressure in the Mediterranean  to the South. Likewise the Bora in the Adriatic produces strong offshore winds.

The Fohn Wind

Strong Wind normally flows up a mountain slope. As it does the moving air cools. If it contained a lot of water vapour it would reach its dew point with thick cloud on the windward side at the top of the mountain. If rain falls from the cloud it releases latent heat. The wind descending on the leeward side ( The site sheltered from the wind) of the mountain will be warmer and drier. Visibility around the top of the mountain will be poor. Downdraughts could endanger a Paraglider or Light aircraft forcing it down into high terrain. Therefore this is a dangerous condition and pilots should be aware of this.

Sometimes strong winds blowing up the mountains cause air to be deflected upwards high into the air above the mountains this can cause severe turbulence to aircraft passing through the smooth air that was above it. The aircraft is suddenly hit by a massive updraught which may be rolling around itself.. Sometimes you can see this by the presence of a rolling Cumulus type cloud normally of small size above the mountains. This can also be dangerous. These updraughts are called standing waves.

Clear Air Turbulence (CAT)

This is caused by windshear.  A change of wind speed of 4 knots per thousand feet can cause a nasty bump. At 6 knots per thousand feet it can be quite severe. The problem with this type of turbulence is that it cannot be detected. Sometimes pilots  predict it in areas and then fly through it safely. They then get it in places where they didn't expect it.

(As a rule of thumb its when the stewardess has just put your flight meal on your lap and you are just about to get stuck into it CAT will find you.)

Jet Streams

Air over the Equator is normally warmer. We also know that air flows from High to Low pressure areas. Therefore most air flows North or South from the Equator. We also know the spinning earth deflects air to the left or right depending which hemisphere we are in. The effect of this at height causes wind to to flow at great speeds. Sometimes this air is concentrated into an area a few hundred miles wide and a thousand miles long with a depth of a few miles. This is a bit like a river but of course instead of water flowing it is air. Winds in the centre of one of these areas can blow at 200 knots normally just below the tropopause line. This river of air is known as a jetstream.  This is shown on weather charts and is one of the things pilots look for.

You have heard mentioned in the Crossing the Atlantic lecture in which I stated that Westbound Aircraft avoid the jet streams ands Eastbound aircraft use them. This can add an hour to a journey flying Westbound over the Atlantic.

On average the Jetstreams in the North Atlantic  blow Westwards at 60 knots.

For those of you that like to fly around the world with your flight simulator below are some averages to give you an idea

Europe to Australia Westerly at 25 Knots until the Middle East is reached.

Middle East to India wind Westerly at 60 knots

From India to Singapore the wind slackens but is still westerly.

After Singapore the winds go Easterly and are Light. and then from Darwin to Sydney they go Westerly again at 60 knots.

I hope this gives you some ideas and explains some of the common terms used.

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