Quick access »
 

Multimedia

Winds

Wind, depression and anticyclone

The layer of (breathable) air around the Earth has a thickness that varies between 10,000 and 12,000 meters. The atmosphere (troposphere) reaches a height of 18km at the equator and about 7km at the poles. The average overall pressure at the Earth’s surface is 1015 hectopascals (Hpa), which is the average weight of the mass of air. At sea level, one cubic meter of air weighs about 1.2kg and at 16km altitude, only 160g. The weight of the air column defines the atmospheric pressure. However, this layer of air is not uniform.

There are very hot zones (equator) and very cold zones (poles). When they meet these layers of air form "lows", (depressions) and "highs", (anticyclones), hot air tending to rise, cold air tending to fall. Thus the atmosphere has a mountainous look - valleys being the depressions and mountains being the anticyclones. Similar to geography where the height is defined by contour lines, meteorologists determine lines of pressure, isobars (weather charts).

Wind speed is given as knots (1852 meters per hour) and readings are taken at a height of ten meters above sea level. _ _

(c) Christophe Launay/DPPI/Safran
© Christophe Launay/DPPI/Safran
Beaufort Scale
Force Description Wind speed Sea conditions Waves
0 Calm 0 - 1 Sea like a mirror 0 feet
1 Light Air 1 - 3 Ripples with the appearance of scales 1/4 - 1/2 feet
2 Light Breeze 4 - 6 Short, small-pronouced wavelets 1/4 - 1/2 feet
3 Gentle Breeze 7 -10 Large wavelets with some crests 2 feet
4 Moderate Breeze 11 - 16 Small waves with some whitecaps 4 feet
5 fresh Breeze 17 -21 Moderate waves with many whitecaps 6 feet
6 Strong Breeze 22 - 27 Large waves, extensive whitecaps, some spray 10 feet
7 Near Gale 28 - 33 Heaps of waves, blown down wing in streaks 14 feet
8 Gale 38 - 40 Moderately high waves 18 feet
9 Strong Gale 41 - 47 High waves with dense foam streaks 23 feet
10 Storm 48 - 55 Very high waves with long, overhanging crests 29 feet
11 Violent Storm 56 - 63 Exceptionally high waves 37 feet
12 Hurricane + de 64 The sea is completely white 45 and more