Global and Local Wind Patterns

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 Wind formation along a) the coast lines, and b) mountain ranges
Figure 1 from Wind Rose: Wind formation along a) the coast lines, and b) mountain ranges

Winds are generated as a result of two factors, the non-uniform heating of the earth by the sun and the rotation of the earth. Equatorial regions receive the most radiation, whereas polar regions receive the least. The difference in ground temperatures between the equator and the poles induces a global circulation pattern where hotter (and lighter) air rises near the equator, and colder (and heavier) air sinks at the poles. As a result, overall wind flow direction is from the poles toward the equator close to the surface and from the equator toward the poles in the upper atmosphere. The detailed flow pattern, however, is much more complicated than this. The upper atmosphere wind (called geostrophic wind) is largely driven by the earth’s rotation and temperature (and thus pressure) differences. Close to the earth’s surface, other factors such as mountains, valleys, and shorelines are important in establishing the local wind patterns.

Sea breezes occur during the daytime when landmasses are heated more quickly than the sea. Sand has a lower heat capacity than water and cannot hold solar heat as effectively, resulting in its temperature rising above that of the water nearby. As the air rises above the hotter land, air from the cooler sea moves to replace it, resulting in a sea breeze. At night, the land gives off heat more quickly and its temperature drops faster than the surrounding sea, resulting in land breezes. At dusk, there is often a period of tranquility when the temperatures of land and sea are equal (Figure 1a from Wind Rose).

Valley and mountain breezes are due to a combination of both differential heating and local topography. As the sun rises, it hits the mountain tops first and, as the day progresses, the mountain slopes, causing differential heating between the two. As warmer air rises off the slopes, cool valley air moves up to fill the vacuum (valley breeze). In the afternoon, as the sun sets, the opposite occurs and we have mountain breezes (Figure 1b from Wind Rose).


(1) Toossi Reza, "Energy and the Environment:Sources, technologies, and impacts", Verve Publishers, 2005

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