Atpl Met Notes Wind

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  • Topic: Wind, Wind gradient, Equator
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  • Published : October 19, 2010
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Measurement of wind, with an ANEMOGRAGH, placed 10m above ground away from obstructions.


Gust and lull. Gusts of wind are increases in the prevailing wind speed or rel short duration, meausred in secs. A lull is whe the wind speed decreases for a few secs.

Gust factor used to warn pilots about turbulence assoc with gusting surface winds.

Gust factor = range of fluctuations in gust and lulls/mean wind speed x 100

Eg max gust 35kts, min lull 15kts, means speed 25kt, ans 80%.

Squal, is a surface wind increasing by 16kts or more to become 22kts or more for at least 1min.

Gale force wind is when surface wind has a mean value of 33kts or more, averaged over 10min, or gusting upto 42kts or more.

Tropical Storm.
A tropical depression has winds below 34kts.
A tropical storm has winds from 34 kts to 64kts
A topical revolving storm has winds of 64kts or more.


VEERING when wind changes direction clockwise
BACKING when wind changes direction anti-clockwise.

Definitions are applicable to both N and S hemispheres.

Backing (anti-clock)Veering (clockwise)

Wind, or the movement of a parcel of air is influenced by forces acting upon it. These are PFG and GF.

Pressure Gradient Force (PGF) – (due to pressure gradient)

The primary cause of wind is horizontal pressure differences. Wind always flow from high to low pressure. The force to produce this movement is the PGF. The direction of this force will be towards the low, perpendicular to the isobars. Magnitude depends on pressure change over unit distance.

The closer the isobars the higher the pressure gradient, the greater the wind strength. The PGF varies inversely with isobar spacing.

Pressure gradient increase = wind speed increase.

Geostropic Force (GF) – (due to rotation of earth)

Once the air starts moving over the surface of the rotating earth, it will tend to follow a straight path in space. However due to the different rotational speeds of the earth at different latitudes (ie slower at the poles, fast at the equator), the air will follow a curved path relative to the earth below it.

Thus it can be said that air moving over a distance and for a period of time will:

Turn to the right in the N hemisphere and to the left in the S hemisphere.

This deflection/change in direction is caused by the Geostropic force(GF), aka Coriolis Effect. The GF will act at right angles to the movement of the air, causing it to turn.

GF = 2 ( ( V sin (

Where: ( = angular velocity of earth rotation,
( = density,
V = wind speed (geostrophic wind speed)
( = latitude

hence, GF will be min (zero) at equator and max at the poles.

GF is directly proportional to wind speed (V).

Note: the GF effects only wind direction, not wind speed however GF is effected proportionally by wind speed (the stronger the wind, the greater the deflecting force). Ie wind speed change effects the GF, but GF cannot alter the wind speed, The relationship is one way!

Geostropic wind (GW) or ‘free wind’ is due to the interaction of the PGF and GF only, and no other forces. This wind blows above friction layer.

The direction of the GW is determined through Buys Ballot’s Law, which states:

If you stand with your back to the wind in the Northern hemisphere, the low pressure is on your left, in the Southern hemisphere the low is on your right.

Magnitude of the GW is determined by the Geostrophic Wind Scale. The scale gives the wind speed expected for a given distance between isobars.

It shows that for a given isobar spacing, the wind speed increases with a lowering in latitude, (max at equator, min at poles).

The geostropic wind speed is inversely proportional to the latitude, ie:

For the same pressure gradient, the lower the latitude, the greater the wind speed.

To summarise, for a wind to be geostrophic it has to:

1) blow above the friction layer
2) be at a latitude...
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