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monsoon(mŏnso͞on) [Arab., mausium=season], wind that changes direction with change of season, notably in India and SE Asia. To a lesser degree, monsoonal winds also develop in portions of all other continents except Antarctica. The change of wind direction is caused by the differences in temperatures of landmasses in contrast to that of oceans. For example, the dry, or winter, monsoon of Asia is largely the result of an area of high pressure that develops over S Siberia. From this area dry winds blow outward, crossing India from northeast to southwest and SE Asia from north to south. The wet, or summer, monsoon is caused by low pressure that develops over S Asia as the landmass warms. Moisture-laden air over the oceans is drawn toward this center of low pressure. The air cools as it ascends the slopes of mountain barriers; it can no longer retain moisture, resulting in heavy rainfall.
a steady, seasonal movement of air along the earth’s surface and in the lower portion of the troposphere. Monsoons are characterized by abrupt changes in direction from winter to summer and from summer to winter; these changes are apparent over vast regions of the earth. In each of the seasons, one wind direction prevails over the others; with a change of season, the direction is altered by 120°-180°. Monsoons cause an abrupt change in the weather (dry and relatively clear to humid and rainy, and vice versa). For example, over India there is a southwestern summer (wet) monsoon and a northeastern winter (dry) monsoon. Between monsoons there are comparatively brief transitional periods with variable winds.
Monsoons with the greatest stability and wind velocity occur in certain regions of the tropics (particularly in Equatorial Africa, in the countries of southern and southeastern Asia, and in the southern hemisphere down to the northern parts of Madagascar and Australia). Weaker monsoons are encountered over limited territories in subtropical latitudes (in particular, in the southern Mediterranean Sea, in northern Africa, near the Gulf of Mexico, along East Asia, in South America, in southern Africa, and in Australia). Monsoons are also noted in certain regions of the middle and higher latitudes (for example, in the Far East, in southern Alaska, and along the northern edge of Eurasia). In a number of places there is merely a tendency for monsoon formation; these regions experience a seasonal change in the prevailing wind directions, but the directions are characterized by less intraseason stability.
Monsoon air currents, like all the manifestations of general atmospheric circulation, are caused by the location and interaction of land masses with low and high atmospheric pressure (cyclones and anticyclones). The specific features are that with a monsoon, the interdependent positioning of these air masses is maintained for a long time (for an entire season), and the disruption of this relationship coincides with the breaking up of the monsoon. In certain regions of the earth, where the cyclones and anticyclones are characterized by rapid movement and frequent change, monsoons do not occur. The vertical depth of the monsoon currents in the tropics is 5–7 km in the summer and 2–4 km in the winter. Above the currents the air moves in a fashion normal for the corresponding latitudes (easterly in the tropics and westerly in the higher latitudes).
The basic cause of monsoons are the seasonal movements of atmospheric pressure and wind, which are related to changes in the incidence of solar radiation and, as a consequence, the varying thermal conditions on the earth’s surface. From January to July the areas of reduced atmospheric pressure near the equator and the poles, as well as the two zones of subtropical anticyclones in each hemisphere, move toward the north; from July to January they move southward. The related wind zones also move along with these planetary zones of atmospheric pressure. These wind zones also have global dimensions—the equatorial zone of westerlies, the easterlies in the tropics (trade winds), and the westerlies of the temperate latitudes. Monsoons occur in regions of the earth that are located within one such zone during one season and within the neighboring zone during the opposite season, provided that the wind conditions of the seasons are relatively stable. Thus, generally speaking, the distribution of monsoons is subordinate to the laws of geographic zoning.
Another reason for the formation of monsoons is the uneven warming and cooling of the sea and of large land masses. For example, over Asia there is a tendency for a greater incidence of anticyclones in the winter and of cyclones in the summer. This is in contrast to conditions over the adjacent ocean waters. Owing to the presence of the enormous continent to the north, the equatorial westerlies in the basin of the Indian Ocean reach far into southern Asia during the summer, forming the summer southwesterly monsoon. In the winter these winds give way to the northeasterly trade wind (the winter monsoon). In latitudes outside of the tropics, owing to the steady winter anticyclones and the summer cyclones over Asia, monsoons also occur in the Far East, in the USSR (the southerly and southeasterly summer monsoon and the northerly and northwesterly winter monsoon), and along the northern edge of Eurasia (in the summer, northeasterly winds prevail; in the winter there are southerly and southwesterly winds).
For the particular features of the climate and weather in individual monsoon regions see.
REFERENCESPedelaborde, P. Mussony. Moscow, 1963. (Translated from French.)
Khromov, S. P. “Musson kak geograficheskaia real’nost’.”/zv. Vsesoiuznogo geograficheskogo ob-va, 1950, vol. 82, fasc. 3.
Khromov, S. P. “Mussony v obshchei tsirkuliatsii atmosfery.” A. I. Voeikov i sovremennye problemy klimatologii Leningrad, 1956.
Drozdov, O. A. , and O. G. Sorochan “Kratkii obzor rabot, vypolnennykh v Rossi i SSSR po kharakteristike mussonov.” Tr. Glavnoi geofizicheskoi observatorii, 1961, fasc. 111.
S. P. KHROMOV