A monsoon is a seasonal wind system that shifts its direction from summer to winter as the temperature differential changes between land and sea. Monsoons often bring torrential summer rains, such as on the Indian subcontinent where the summer monsoon delivers three-quarters of the country's annual precipitation. But sometimes the shift in wind directions can bring a "dry monsoon," which can mean drought in regions where the prevailing winds blow moisture offshore.
In India, Australia and other equatorial regions, monsoons move from cold, high pressure systems to warmer areas of less resistance with low atmospheric pressure. Thus in hot summer months as the land heats up, monsoons blow inland from the sea. Come winter, the ocean retains its warmth while the land cools rapidly, and the monsoons head back out to sea. This change in direction establishes a seasonal pattern of rainfall that many farmers depend on in monsoon regions.
The summer monsoons in India typically blow from the southwest, bringing huge amounts of rain from the Indian Ocean to the warmer land. Some high-elevation areas of India receive up to 500 centimeters (200 inches) of rain from June to September alone. Similar to summer monsoons across the globe, the rains of the Indian summer monsoon are produced as winds push moisture-laden air high into the atmosphere, where it condenses and falls in heavy precipitation.
In the cooler winter months, the direction of monsoon winds changes as it shifts to follow the warm air back out to sea. As moisture leaves the cooling land, the "dry monsoon" season can be responsible for extensive drought in some regions. But the winter season is not as uniform as the summer monsoons and, according to the Indian state of Maharashtra's Department of the Environment, northeastern India near the Himalayas receives half of its annual precipitation during this time, and even the southwestern states of Kerala and Karnataka receive rain until December.
Although true monsoons are associated with the equatorial tropics, some weather patterns at higher latitudes are similar to monsoon rains, earning the moniker of "embryo monsoons." Central Europe, for example, experiences severe summer thunderstorms, and winds can change direction from summer to winter, but the weather events are sporadic and lacking in prevailing monsoonal patterns. Similarly, central Mexico and parts of the American Southwest receive heavy seasonal rains, but consistent winds are not present as the harbinger of a true monsoon.
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