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A seasonal wind, especially in south Asia. In summer, which is the monsoon season, the winds normally blow from the sea to the land and bring rain, but in winter there is a complete change of direction and the winds blow out from the land, giving dry weather. Some monsoon regions are very wet. Cherrapunji in India, for example, receives over 11,000 mm (433 in) per year, but others can be dry, such as the Thar Desert between India and Pakistan where the annual rainfall is less than 250 mm (10 in). The major monsoon areas are in Asia, where the seasonal reversal of wind is greatest. This is because the largest continent, Asia, is adjacent to the largest ocean, the Pacific. In the small continents of South America, Africa, Australia, and North America, the monsoonal effects are less marked. These small continents do not have such wet summers or such dry winters, and are sometimes referred to as "eastern marginal" rather than true monsoon.

Caused by low pressure areas over land masses in the summer (see diagram), monsoons bring wet winds from the sea, and high pressure areas in the winter causing dry winds to blow from the land. During the summer (top) large areas of mainland Asia are heated by the Sun. The air over these regions expands and rises forming regions of low pressure. Wet winds from the sea then blow into these areas giving the summer monsoons. In the winter (bottom), the situation is reversed and regions of high pressure are formed over the land. Dry winter monsoon winds then blow out to sea. The paths of these winds are deflected due to the Coriolis effect.

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