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People at a flooded market in Varanasi, India, following the arrival of monsoon rains in August Billions of people depend on the arrival of the monsoon every summer to the parched lands of southern Asia. Monsoons are winds that reverse direction each year. This happens when there are great differences in temperature between the oceans and the land. Cool, dry winds are replaced in the summer by warm, moist winds from the Indian Ocean. Heavy rains then arrive.

How monsoons occur

The yearly changing pattern of the winds blowing across India

During the winter, cool, dry air flows southwards from northern Asia. For six months, India has a drought as there are no moist winds blowing in from across the Indian Ocean (1). Eventually, the land heats up in the summer sun and the air above it becomes warmer than the air over the ocean. Moist air now flows in, bringing heavy rains (2).

Almost half of the world's population lives in areas affected by the Asian monsoons.

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