monsoon, a majorwind system that seasonally reverses its direction—such as one that blows for approximately six months from the northeast and six months from the southwest. The most prominent monsoons occur inSouth Asia, Africa,Australia, and the Pacific coast ofCentral America. Monsoonal tendencies also are apparent along theGulf Coast of theUnited States and in central Europe; however, true monsoons do not occur in those regions.

For further information on prominent monsoon regions,seeIndian monsoon,West African monsoon,Malaysian-Australian monsoon, andNorth American monsoon.

The primary cause of monsoons lies in different warmingtrends over land and sea, though other factors may be involved. Seasonal changes intemperature are large over land but small over ocean waters, and monsoons blow from atmospheric heat sinks (that is, cold regions with high atmospheric pressure) toward heat sources (warm regions characterized by low atmospheric pressure). Consequently, monsoon winds typically travel from sea to land insummer and from land to sea inwinter. For example, the heat source involved in theIndian summer monsoon resides over thePlateau of Tibet and the eastern foothills of theHimalayas, while the heat sink occurs over the southernIndian Ocean and Madagascar, a region where relatively cloud-freeair cools by emittinginfrared, or “long-wave,” radiation into space. Likewise, the heat source for the Australian summer monsoon resides over the area in which many meteorologists call the “Maritime Continent,” a region made up of parts ofSoutheast Asia and the islands of Indonesia and thePhilippines, while the heat sink resides over Siberia.

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Most summer monsoons have a dominant westerly component and a strong tendency to ascend and producecopious rainfall, which occurs as a result of thecondensation of water vapour in the rapidly rising air. The intensity and duration of theserains, however, are not uniform fromyear to year. Conversely, the winds of winter monsoons have a prevailing easterly component and a strong tendency todiverge, subside, and causedrought.

The poleward limits of monsoon systems are often sites of sharp changes in wind direction. InIndia, for example, the monsoon blows from the southwest in July and August, and north of India the winds are from the east. In northern Australia the monsoon arrives from the northwest during January-February. At the southern limit of the Australian monsoon, the winds turn easterly.

Climatic patternsreminiscent of monsoons also occur in areas outside of the prominent monsoon regions. In central Europe, where the average wind direction in summer differs some 30° to 40° from that of the Atlantic, there are monsoonal tendencies that occur not as a continuous flow but rather intermittently within frontal depressions, bringing cool, cloudyweather,rain, andthunderstorms. Some see in this climatic pattern a true monsoon, but it is obvious that it is only an “embryo monsoon” that results in weather singularities. The latitude is too high for a true monsoon to arise. In addition, the Gulf Coast of the United States is prone to climatic patterns with monsoonal tendencies; however, the seasonally consistent windscharacteristic of true monsoons do not emerge.