| Wind, in the most general sense, is the movement of air.
It occurs at all scales, from local breezes generated by heating of land
surfaces and lasting tens of minutes to global winds resulting from solar
heating of the planet, lasting eons. The two major influences on the atmospheric
circulation are the differential heating between the equator and the poles,
and the rotation of the planet (Coriolis effect). Because of the first,
and the fact that, warm air rises and cool air falls, there arise circulations
that (on a non-rotating planet) would lead to a equator-to-pole flow in
the upper atmosphere and an pole-to-equator flow at lower levels. Because
of the second, this simple situation is vastly modified in the real atmosphere.
In almost all circumstances the horizontal component of the wind is much
larger than the vertical - the exception being violent convection.
Given a difference in barometric pressure between two air masses, a wind will arise between the two which tends to flow from the area of high pressure to the area of low pressure until the two air masses are at the same pressure, although this will be strongly modified by the Coriolis effect. While all winds are the movement of air more or less parallel to the Earth's surface, they come in a variety of forms. There are global winds, such as the wind belts which exist between the atmospheric circulation cells. There are upper-level winds, such as the jet streams. There are synoptic winds that result from pressure differences in surface airmasses at the middle latitudes, and there are winds that come about as a consequence of geographic features such as oceans, lakes, mountains, and deserts. Mesoscale winds are those which act on a local scale, such as gust fronts. At the smallest scale are the winds which blow on a scale of only tens to hundreds of metres and are essentially unpredictable, such as dust devils and microbursts. Finally, there are special-case winds that come about as a consequence of local geography. Wind can also shape landforms, via a variety of eolian processes. |
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