Prevailing Winds and General Circulation Patterns
Over some zones around the earth, winds blow predominantly in one direction throughout the year and are usually associated with the rotation of the earth; over other areas, the prevailing direction changes with the seasons; winds over most areas also are variable from day to day so that no prevailing direction is evident, such as, for example, the day-to-day changes in local winds associated with storms or clearing skies. Around the equator there is a belt of relatively low pressure known as the doldrums, where the heated air is expanding and rising; at about lat. 30°N and S there are belts of high pressure known as the horse latitudes, regions of descending air; farther poleward, near lat. 60°N and S, are belts of low pressure, where the polar front is located and cyclonic activity is at a maximum; finally there are the polar caps of high pressure.
The prevailing wind systems of the earth blow from the several belts of high pressure toward adjacent low-pressure belts. Because of the earth's rotation (see Coriolis effect), the winds do not blow directly northward or southward to the area of lower pressure, but are deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The wind systems comprise the trade winds; the prevailing westerlies, moving outward from the poleward sides of the horse-latitude belts toward the 60° latitude belts of low pressure (from the southwest in the Northern Hemisphere and from the northwest in the Southern Hemisphere); and the polar easterlies, blowing outward from the polar caps of high pressure and toward the 60° latitude belts of low pressure.;g687;none;1;g687;;;block;;;;no;1;71808n;45371n;;;;;wind-glob687;;;left;stack;;;;;;;;left;stack;2745n;;;Global wind patternsCE5
This zonal pattern of winds is displaced northward and southward seasonally because of the inclination of the earth on its axis and the consequent migration of the belts of temperature and pressure. In addition, the pattern is considerably modified by the distribution of land and water, especially in the temperate regions, where temperature differences between land and water are greatest. In winter, areas of high pressure tend to build up over cold continental land masses, while low-pressure development takes place over the adjacent, relatively warm oceans. Exactly the opposite conditions occur during summer, although to a lesser degree. These contrasting pressures over land and water areas are the cause of monsoon winds.
Superimposed upon the general circulation of winds are many lesser disturbances, such as the extratropical cyclone (the common storm of the temperate latitudes), the tropical cyclone, or hurricane, the tornado, and the derecho; each of these storms moves generally along a path that follows the direction of the prevailing winds.
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