Meteorology Wind © Crown copyright No part of this presentation may be reproduced without the permission of the issuing authority. The views expressed in this presentation do not necessarily reflect the views or policy of the MOD. CGS Ground School

Air is a fluid and like all fluids will flow from high to low pressure. A good example would be a canal lock: Gates closed. A has more water and therefore more pressure than B. Gates open. Water flows from A to B (high to low pressure) until the water levels (pressures) are equal. Fluid pressure AB

Air is a fluid and like all fluids will flow from high to low pressure. A good example would be a canal lock: The greater the pressure difference, the faster the flow from high to low. Fluid pressure AB Large pressure difference

Air is a fluid and like all fluids will flow from high to low pressure. A good example would be a canal lock: The greater the pressure difference, the faster the flow from high to low. Fluid pressure AB Small pressure difference

The wind speed can be calculated from a synoptic chart by measuring the pressure gradient (change of pressure per unit distance). The tighter the isobars, the stronger the wind. Air pressure In the same way air will flow from high pressure to low pressure. Again, the greater the pressure difference the greater the flow. HighLow

This is caused by the Coriolis effect. Geostrophic force HighLow As the air begins to move it is acted on by another force, the geostrophic force. The geostrophic force is due to the difference in velocity of points near the equator compared with points near the poles.

Coriolis effect produces a force that prevents the wind flow directly from high to low pressure. Consider two identical objects placed on the surface of the Earth; one at the North Pole, the other at the equator. Both objects (ball bearings in this case), are stationary relative to the Earth beneath them, but when we consider how far each one travels in a day, we would see that they were travelling at very different speeds. 0 mph Coriolis effect 0 mph

The object at the equator will also rotate 360° but will travel approximately 24,000 miles in the same 24 hour period, therefore it has a forward speed of 1,000 mph. Any object (including air) will initially carry the momentum from the point on the Earth from which it originated. Therefore, if we now push our objects towards each other, there is a strange effect. 0 mph 360° per 24 hrs 0 mph Coriolis effect In 24 hours the object at the North Pole will rotate through 360° but will not have any forward speed. 0 mph 360° per 24 hrs 1000 mph

The ball bearing heading towards the equator is going slower than the Earth it travels over, and is deflected to the right. The ball bearing heading towards the pole is travelling faster than the Earth it travels over and is also deflected to the right. This is known as the Coriolis effect. In the Northern hemisphere, objects are deflected to the right, in the Southern hemisphere, they’re deflected to the left. Both objects are travelling at their original speeds over parts of the Earth’s surface that are travelling at different speeds. 0 mph 360° per 24 hrs 0 mph Coriolis effect 0 mph 360° per 24 hrs 1000 mph

Coriolis effect If we replace the ball bearings with parcels of air, the same effect can be seen. The overall effect in the northern hemisphere is that air flows clockwise around an anticyclone, and anti-clockwise around a depression. HighLow

Coriolis effect Buys Ballot's Law is a practical application of the previous information. It states – “In the Northern hemisphere, with your back to the wind, the low pressure system is on your left” HighLow

Coriolis effect Another application of this information comes during cross country flying. If the aircraft drifts right it is flying from high to low pressure. If it drifts left it is flying from low to high pressure. HighLow

We have seen that the Coriolis effect deflects the wind to the right. Since winds near the ground are slowed by friction, obstructions etc., they travel more slowly and are less affected by the geostrophic force. They are therefore deflected less, travel more directly from high pressure to low pressure and are said to have "backed". It is for these reasons that the wind appears to increase and veer with increases in height. H L Coriolis effect

H Sea breeze A sea breeze, as the name implies, occurs close to the coast. It is usually light (10 kts) and rarely extends more than 20nm inland. The day often starts with a light off-shore breeze. As the sun rises the land begins to heat up faster than the sea. The air in contact with the land begins to warm and therefore rises. L As the air rises the pressure near the surface, over the land, reduces. Air starts to flow in from the sea to equalise the pressure difference. This is the sea breeze. The rising air over the land causes the pressure at altitude over the land to increase. The air flowing in from the sea causes the airmass over the sea to subside and the pressure at altitude to reduce. L At altitude the airflow is from the land (high pressure) to the sea (low pressure). However the airflow is spread over a much greater height band and its effect is negligible. In the UK a sea breeze is most likely to occur in the summer, when the weather is settled with warm days and cloudless nights.

Sea breeze weather The onset of a sea breeze is often sudden and may be marked by a squall accompanying the change in wind direction. If an area of sea fog or low cloud exists just off shore then the sea breeze will carry this inland, causing a rapid deterioration in cloud base and/or visibility.

Coastal airfields Consider the effect of a sea breeze at a coastal airfield. Initially the sea breeze blows perpendicularly to the coast. But towards the end of the afternoon the Coriolis effect causes the wind to veer. As night falls the land cools quicker than the sea. This often leads to the process being reversed and a light off-shore breeze developing during the night. This whole cycle is then repeated the following day.

Winds and terrain When a wind meets a range of mountains its progress is impeded much like a river being blocked by large boulders. Valley winds Where a valley breaks this barrier the wind will flow at a much increased velocity. The Mistral is a typical example, which flows along the Rhone valley at speeds of up to 75 kts.

Winds and terrain During a sunny day the side of a hill facing the sun will warm up and conduct this heat to the air above it. The warm air then rises and flows up the hill. The resulting wind is termed an Anabatic wind.

Winds and terrain At night the side of a hill will cool down and conduct this heat loss to the air above it. The cool air then descends and flows down the hill. The resulting wind is termed a Katabatic wind. Katabatic winds are usually more significant than anabatic winds. They are particularly significant in fjords and steep valleys.

Describing winds The wind is usually described by giving its direction of origin and its speed. In aviation it is normally given in degrees and knots, separated by a "/" and with the units omitted. 200/20 350/10

Describing winds When the wind direction changes clockwise it is said to have veered. 200/20 270/20 Veered

Describing winds When it changes direction anti-clockwise it is said to have backed. 200/20 270/20 Backed

When the wind direction changes clockwise it is said to have veered. When it changes direction anti-clockwise it is said to have backed. Sudden, brief increases in wind velocity are called gusts. They are extremely important to pilots of light aircraft. Temporary increases in wind velocity which are of a longer duration are called squalls. They are usually associated with rapidly moving cold fronts, troughs or thunderstorms. Describing winds

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