The Primary Wind Belts
The air moves not only as local winds, but also in a general circulatory system over the entire earth. This planetary circulation operates at two levels—at the earth's surface and in the upper portion of the troposphere, the layer of the atmosphere nearest to the earth.
Planetary circulation, like local wind, is caused by pressure differences. Convectional circulation is, in part, a cause of these differences, but there are other causes as well. Among these are the effects of one part of the circulatory system on the other parts.
The circulatory system can be divided into five general zones, called primary wind belts. The zones are not well defined, but are broken and constantly shift and migrate with the sun.
The Doldrums lie approximately between 5 north and south of the equator, the area receiving the largest amount of heat from the sun. Like other wind belts, the doldrums shift with the sun, reaching as far as 17 or 18 north latitude in July and far into the Southern Hemisphere in January. Air pressure is generally relatively low, and the air moves chiefly upward from the warm earth. At a high elevation, the air stops rising and moves north and south toward the polar areas. Weather in the doldrums is usually hot, humid, and rainy. Surface winds are light and variable.
The Trade Winds extend from the doldrums to as much as 30 north and south. These winds are created by surface air moving from high-pressure areas in the subtropics to replace air rising in the doldrums. Turned westward by the rotation of the earth, the trades blow with great steadiness from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere. They are steadiest over oceans. (Trade, in this case, has the old meaning of "steady track or course.")
The Horse Latitudes, at about 30 north and south latitudes, form a belt between the trade winds and the prevailing westerlies. Here part of the high-altitude air flowing poleward accumulates, descends to earth, and becomes warmer because of compression. The barometer shows the air pressure to be high. Some of the world's largest deserts are located here.
Horse-latitude winds are weak and shifting. Calms are common. The weather is usually clear and bright. Part of the descending air feeds the trade winds; part flows to the prevailing westerlies.
The Prevailing Westerlies extend from the horse latitudes to about 60 north and south. Although the wind is highly variable in direction because of traveling high-pressure areas (anticyclones) and low-pressure areas (cyclones), the wind comes from the west more often than from other directions. Air flowing toward the poles from the horse latitudes is here deflected eastward to become a west wind. In the middle latitudes of the Southern Hemisphere, where there are few land areas to interfere with the westerlies, wind speeds are greatest. The region from 40 to 50 south is notorious among seamen as "the roaring forties."
The Polar Easterlies extend from about 60 north and south latitude to the poles. Near the poles the shrinkage of air due to cooling favors the inflow of air aloft from warmer latitudes. This causes a polar high-pressure area from which winds move toward the equator to the subpolar low-pressure areas, located at about 60 north and south. These winds are deflected by the earth's rotation to become east winds, or the polar easterlies.