Geostrophic Wind, winds balanced by the Coriolis and Pressure Gradient forces. An air parcel initially at rest will move from high pressure to low pressure because of the pressure gradient force (PGF). However, as that air parcel begins to move, it is deflected by the Coriolis force to the right in the northern hemisphere (to the left on the southern hemisphere). As the wind gains speed, the deflection increases until the Coriolis force equals the pressure gradient force. At this point, the wind will be blowing parallel to the isobars. When this happens, the wind is referred to as geostrophic.

Wind, air in motion. The term is usually applied to the natural horizontal motion of the atmosphere; motion in a vertical, or nearly vertical, direction is called a current. Winds are produced by differences in atmospheric pressure, which are primarily attributable to differences in temperature. Variations in the distribution of pressure and temperature are caused largely by unequal distribution of heat from the sun, together with differences in the thermal properties of land and ocean surfaces (see Meteorology). When the temperatures of adjacent regions become unequal, the warmer air tends to rise and flow over the colder, heavier air. Winds initiated in this way are usually greatly modified by the earth's rotation.

Winds may be classified into four major types: the prevailing winds, the seasonal winds, the local winds, and the cyclonic and anticyclonic winds (see Cyclone; Hurricane; Tornado).


Cited "Weather," Microsoft® Encarta® Online Encyclopedia 2004 © 1997-2004 Microsoft Corporation. All Rights Reserved.
"Wind," University of Illinois WW2010 Project © 1997 by the University of Illinois Board of Trustees. All Rights Reserved.