ATMOSPHERIC CIRCULATION

large-scale wind systems arranged in several eastwest belts that encircle the Earth. In the subtropical high-pressure belts near latitudes 30 N and 30 S (the horse latitudes), air descends and causes the trade winds to blow westward and equatorward at the Earth's surface. These merge and rise in the intertropical convergence zone near the equator and, as the antitrades, blow eastward and poleward at altitudes of 2 to 12 km (1 to 7 miles). Part of the antitrade flow descends in the subtropical high-pressure belts, and the remainder merges at high altitudes with the midlatitude westerly winds farther north. Between latitudes 15 N and 15 S, a layer of high-speed winds, the Krakatoa winds, blow 20 to 40 km (12 to 25 miles) above the surface. The descending air in the subtropical high-pressure belts diverges near the surface; the air that does not flow equatorward flows eastward and poleward as the midlatitude westerlies. These extend to great altitudes and contain jet streams, high-velocity air currents around the 10-kilometre (6-mile) level in both hemispheres. Poleward of 60 N and 60 S, the winds generally blow westward and equatorward as the polar easterlies. In the northern polar regions, where water and land are interspersed, the polar easterlies give way in summer to variable winds. The zonal wind belts are much more uniform and constant in the Southern Hemisphere because there is little land to disturb the circulation. In the Northern Hemisphere, a number of large, semipermanent high- and low-pressure centres exist over the continents and oceans during various parts of the year; their winds prevent the zonal wind belts from extending continuously around the world.