Buys Ballot’s Law states that: If an observer stands with his back to the wind, the lower pressure is on his left in the northern hemisphere, and on his right in the southern hemisphere.

A corollary of this law is that if an aircraft is experiencing starboard drift in the northern hemisphere the aircraft is heading towards low pressure

Wind speed is usually given in knots, but some countries give the speed in metres per second and the Met.

Wind direction is always given as the direction from which the wind is blowing. It is normally given in degrees true, but wind direction given to a pilot by ATC or in an ATIS will be given in degrees magnetic.

Although Föhn winds blow in the Alps, the name is used generically to describe winds which blow with similar effect in other parts of the world. One such wind is the Chinook which blows on the Eastern side of the Rocky mountains of North America.

Surface wind is measured by a wind vane which aligns itself with the wind direction, and an anemometer which measures the speed. An anemometer is a set of 3 hemispherical cups which rotate on a shaft with the effect of the wind. The speed of rotation of the shaft is directly proportional to the windspeed. The rotation is used to drive a small generator, the output of which is then displayed on a gauge which is calibrated in knots.

Both vane and anemometer are positioned 33 ft (10 m) AGL and they are located clear of buildings and obstructions which could affect the airflow and hence accuracy. An anemograph records windspeed and sometimes direction.

Buys Ballot’s Law states that: If an observer stands with his back to the wind, the lower pressure is on his left in the northern hemisphere, and on his right in the southern hemisphere.

A corollary of this law is that if an aircraft is experiencing starboard drift in the northern hemisphere the aircraft is heading towards low pressure. This is illustrated below.

As a result of the effect of thermal mixing, there can be a regular change in the surface wind in each 24 hr period. It veers and increases by day reaching maximum strength about 1500 hrs.

in the below diagram to the right is high pressure 

Easterly and westerly jets are found in northern hemisphere

whereas Southern Hemisphere has westerly jet streams only

The surface wind over land is backed by 30 degrees from the geostrophic wind and its speed is reduced by 50%.

the Geostrophic Wind Speed will increase as latitude reduces.

wind is the movement of air from high to low pressure horizontally

Since surface friction has reduced the wind velocity, resulting in a reduction in the Coriolis Force, the PGF is now more dominating. This causes the wind to blow across the isobars towards the low.

Due to friction the surface wind is slower than the geostrophic (freestream) wind.

In NH the surface wind Backs.

In SH the surface wind Veers.

Coriolis Force, (CF), is the force caused by the rotation of the earth.

It acts 90° to the wind direction causing air to turn to the right in the northern hemi-sphere and to the left in the Southern hemisphere. CF is maximum at the poles and minimum at the equator.

The gradient wind occurs when the isobars are curved.

The Geostrophic Wind blows parallel to straight isobars.

A wind blowing against a mountain is impeded. If the barrier is broken by a gap or valley, the wind will blow along the valley at an increased speed due to the restriction.

The Föhn Wind is a warm dry wind which blows on the downwind side of a mountain range. It is a local wind in the Alps

if air is moving steadily around a high, then the centrifugal force acts with the Pressure Gradient Force (PGF), increasing the velocity of the wind.

true altitude and indicated altitude remains same hence no cross wind

90 km =  48.6 nautical mile

The Geostrophic Wind only blows above the friction layer. Within the friction layer the wind speed is reduced because of surface friction. Therefore the Coriolis Force will reduce, causing the two forces to be out of balance. Remember that the friction layer varies depending upon the nature of the surface and the time of the day. There-fore, the height of the Geostrophic Wind will vary. Generally though it is considered to be between 2,000 – 3,000ft.

The Föhn Wind is a warm dry wind which blows on the downwind side of a mountain range. It is a local wind in the Alps.

If moist air is forced to rise up a mountain side, it will quickly become saturated and will cool adiabatically as it rises. After reaching the condensation level, cloud will form and the air will cool at the SALR.

Wind is caused by air flowing from high pressure to low pressure. The Earth’s rotation prevents that flow from being direct, but deflects it side to side(right in the Northern Hemisphere and left in the Southern), so wind flows around the high and low pressure areas

in day we get sea breeze

night land breeze

on downwind we get winds from left

on finals we get winds from right

Air tries to move from high to low pressure and this will generate a pressure gradient force which develops into the wind velocity that we feel

The pressure gradient is the difference in pressure between consecutive isobars divided by the distance between them, this is illustrated in the figure below. Closely spaced isobars imply a steep pressure gradient (common in low pressure systems) and widely spaced isobars imply a shallow pressure gradient (common in high pressure systems). The pressure gradient produces an attractive force that will tend to pull molecules of air towards lower pressure areas. The greater the pressure change and the greater the pressure gradient force for a given distance the faster the wind velocity.

As we go higher in the troposphere, the density decreases and the temperature effect overwhelms the pressure effect, hence winds become progressively westerly with height. The strongest winds are to be found

Gradient Wind In A Depression If air is moving steadily around a depression, then the centrifugal force opposes the PGF and therefore reduces the wind speed. You can see this on the left hand side of the image shown below.

The gradient wind speed around a depression is less than the geostrophic wind for the same isobar interval. Hence if the Geostrophic Wind Scale (GWS) is used, it will overread.

Wind flow is clockwise around a low in the S. hemisphere.

Flying away from the L means the wind must be from the left for it to also be flowing clockwise around the low.

Wind flow at Low altitude vs high altitude changes as a result of surface friction preventing flow being truly parallel to the isobars.

The pressure system mentioned is a Low so air will tend flow into the Low, in addition to clockwise around this S. hemisphere Low.

You’re flying away from the low at low altitude (therefore friction effects), the wind has a tendency to flow into the low so there will also be a slight headwind component.

The Bora. This wind is part valley and part katabatic. It blows down the north Adriatic with high pressure over Central Europe and the Balkans and a low over the Adriatic. The wind speed is around 70 kts with great gusts exceeding 100 kts in places. The Bora is strongest and most frequent in winter.

Gradient Wind In A High

Looking to the right of the diagram, if air is moving steadily around a high, then the centrifugal force acts with the Pressure Gradient Force (PGF), increasing the velocity of the wind.

The gradient wind speed around an anticyclone is greater than the geostrophic wind for the same isobar interval. Hence if the Geostrophic Wind Scale (GWS) is used, it will underread.

Buys Ballot’s Law states that

if an aircraft is experiencing starboard drift in the northern hemisphere the aircraft is heading towards low pressure

if an aircraft is experiencing starboard drift in the southern hemisphere the aircraft is heading towards high pressure and away from low

After sunset. The land will cool rapidly whilst the sea will retain its heat. There will be an increase in pressure at the surface over the land whilst the pressure over the sea will fall – there will be a land breeze. The speed will be about 5 kts and the breeze will extend about 5 nms out to sea.

mean speed taken over 10 minutes , gust reported 10 knots

Polar frontal depressions are large scale low pressure areas

A back is a change in wind direction in an anti-clockwise direction it happens in low pressure areas in northern hemisphere.

Jet streams ,The direction of jet streams is generally westerly, maximum speeds occur near the tropopause, 200 kts have been recorded in Europe