Describing the wind
Wind is air in horizontal motion. A wind velocity has two parts, a direction and a speed, and there are a handful of terms for how it varies that the exam expects you to have straight.
Direction and speed
- Wind direction is the direction the wind blows from, given in degrees true, except that ATC gives it in degrees magnetic.
- Speed is usually in knots. Some countries use metres per second, and the Met Office often works in km/h.
- Veering is a change of direction clockwise. Backing is a change anticlockwise. Both terms mean the same thing in both hemispheres.
Sudden changes
| Term | What it is |
|---|---|
| Gust | A brief increase, under a minute, local. Reported only if 10 kt or more above the mean |
| Lull | A brief decrease |
| Squall | An increase lasting a minute or more, over a wide area, often with cumulonimbus and cold fronts |
| Gale | Sustained wind over 33 kt, or gusts over 42 kt, and Joshi ties it to a depression |
| Hurricane force | Sustained wind over 63 kt |
Measurement
Surface wind is read from a wind vane for direction and a cup anemometer for speed. The ICAO standard position is 10 m, about 33 ft, above aerodrome level, clear of buildings. Upper winds come from tracking a radiosonde and from aircraft reports. Lines of equal wind speed on a chart are isotachs.
The geostrophic wind, pressure gradient and Coriolis
Above the friction layer the wind is close to a simple balance of just two forces. Get that balance and you can read both the direction and the speed of the wind straight off a pressure chart.
Pressure gradient force
The pressure gradient force acts from high pressure straight to low pressure, at right angles to the isobars. Its size is shown by the isobar spacing: close isobars, strong force, strong wind. Wind speed is directly proportional to the PGF.
Coriolis force
The Coriolis force is the apparent deflection caused by the earth's rotation. It acts at 90° to the wind, turning it to the right in the Northern Hemisphere and to the left in the Southern.
The geostrophic wind
The geostrophic wind is the balance of PGF against Coriolis only. It blows parallel to straight isobars, above the friction layer, at latitudes greater than about 15°, when the pressure pattern is not changing fast. Buys Ballot still holds: back to the wind in the Northern Hemisphere, low pressure on your left.
The gradient wind and the surface wind
Real isobars curve, and near the ground friction gets in the way. Both change the wind from the plain geostrophic value.
The gradient wind
When isobars are curved, a third force, the centrifugal force, joins the PGF and Coriolis. The gradient wind is the balance of all three, blowing parallel to the curved isobars.
| System | Centrifugal force | Gradient wind vs geostrophic |
|---|---|---|
| Depression, cyclonic | Opposes the PGF | Slower than geostrophic. The geostrophic wind scale over reads |
| Anticyclone | Adds to the PGF | Faster than geostrophic. The geostrophic wind scale under reads |
The surface wind
Below about 2000 to 3000 ft, surface friction slows the wind. A slower wind means a weaker Coriolis force, so the PGF wins and the wind is pulled across the isobars, towards the low.
Diurnal variation
By day, thermal mixing couples the surface to the stronger air above, so the surface wind veers and increases, peaking about 1500. By night the mixing stops, the surface wind backs and decreases, and the 1500 ft wind does the opposite, so a windshear develops between 1500 ft and the surface at night, which matters on an approach. Over the sea the daily change is small.
Local winds
Small scale winds set up by local heating and terrain. They are too small for the Coriolis force to matter, so they do not obey Buys Ballot's law.
Land and sea breezes
| Sea breeze | Land breeze | |
|---|---|---|
| When | Day, land warmer than sea | Night, land cooler than sea |
| Direction | From sea to land | From land to sea |
| Strength, reach | About 10 kt, 8 to 14 NM inland, more in the tropics | About 5 kt, about 5 NM offshore |
The sea breeze veers with time under the Coriolis force. Practical effects at a coastal aerodrome: the landing direction can reverse between the two breezes, sea fog can be carried inland by day, and small clouds mark the sea breeze front over the coast.
Slope winds
- Katabatic. Cold, dense air draining down a slope, mainly at night. About 10 kt, strongest with snow cover, a clear sky and a slack pressure gradient. It feeds valley frost and fog and leaves an inversion. Example: the Bora of the northern Adriatic.
- Anabatic. Air warmed on a sunlit slope rising up the hill by day. A light wind, about 5 kt, weaker than the katabatic.
The Fohn wind
A warm, dry wind on the lee side of a mountain range. Moist air is forced up the windward side, cooling at the DALR then the SALR once saturated, and rains out, so its dew point falls. On the lee side it descends and warms at the DALR over a greater height than it cooled at the SALR going up. The result is a lee side base temperature much higher than the windward side, gains over 10°C are common.