Wind
Wind is simply air in motion, moving from high pressure to low pressure. Winds are described by the direction they blow from - an easterly wind lows from the east, a westerly from the west. Prevailing winds (winds which occur frequently in a particular direction) are arranged in a series of belts around the globe. This pattern is the result of differences in the speed the Earth rotates and the different amounts of solar heating it gets, depending on where a place is between the poles and the equator.
There are many smaller scale winds and wind patterns produced by a number of processes. Mountains often produce their own local winds, and wind currents can be shaped as they are forced to rise or funnelled through valleys, greatly increasing their strength. As an air mass descends a mountain slope it is compressed and becomes a warm dry wind. The effect is most noticeable in spring when these winds can melt snow very quickly. They are called föhn winds in the Alps and chinook ("snow-eaters") in western North America and they can sometimes cause avalanches.
Local Winds have names
Other winds are truly local and are caused by a daily pattern of air-flow up and down valleys.
Katabatic winds occur at night when cool air sinks down mountain slopes and valleys. Anabatic winds occur when sun-warmed air rises and gently flows up mountain slopes and valleys.
Local winds in different countries have special names and often have particular characteristics associated with them, such as the Mistral - a dry and cold wind common to the Rhone Valley in France.
In deserts under hot sunshine, thermals can rise very strongly and produce short-lived whirlwinds. These dust-devils or sand-devils are a fairly common sight on sunny days in dry, open country. Intense heating of a patch of ground by the Sun may create a spiralling column of hot air that can reach more than 100 m (330 ft) in height. Lightweight material, such as sand or dust, is lifted up and briefly whirled around. Whirlwinds rarely last more than a few minutes after which the air column gently collapses.
