Worlds of David Darling
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A body of air moving relative to the Earth's surface. The world's major wind systems, or general winds, are set up to counter the unequal heating of the Earth's surface and modified by the Earth's rotation. Surface heating, at its greatest near the equator, creates an equatorial belt of low pressure (see doldrums) and a system of convection currents transporting heat toward the poles.

The Earth's rotation deflects the currents of the northern hemisphere to the right and those of the southern hemisphere to the left of the directions in which they would otherwise blow, producing the northeast and south east trade winds, the prevailing westerlies, and the polar easterlies. Other factors influencing general wind patterns are the different rates of heating and cooling of land and sea and the seasonal variations in surface heating. Mixing of air along the boundary between the westerlies and the polar easterlies – the polar front – causes depressions in which winds follow circular paths, counterclockwise in the northern hemisphere and clockwise in the southern hemisphere (see cyclone).

Superimposed on the general wind systems are local winds – winds, such as the chinooks, caused by temperature differentials associated with local topographical features such as mountains and coastal belts, or winds associated with certain cloud systems.

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