Monsoons – circulating winds

Springtime in India brings more than May flowers — it’s also when the monsoon winds might begin to blow. We’ll answer a listener’s question about monsoons — and talk about the rains they bring. I’d like to know how and why monsoons start — and what they are.

Some definitions of “monsoon” focus on the rain that usually comes with them — but, in fact, a monsoon is a circulating wind. As summer approaches, land heats up faster than ocean. Warm air rises over the land. Moist air flows inland from a nearby ocean — thus the monsoon winds. Meanwhile, air is sinking over the ocean, completing a circulation.

Monsoon winds carry moisture from the ocean. When the air rises, it cools and the moisture rains out. The most torrential rains on Earth result where monsoon winds run into mountains — for example, in Asia, into the Himalayan-Tibetan plateau complex. Northern India has been known to be soaked by some five meters — or 15 feet — of rain in 15 days.

Next October, in the northern hemisphere, after six months of showers, the monsoon winds will change direction. In autumn, the land cools faster than the ocean, so air rises over the ocean. Air flowing over the land then comes from the continental interior. This air is cool and dry.

Meanwhile, also in October, in the tropics and subtropics of the southern hemisphere, the summer monsoon season will begin.

The Asian monsoon has some unique features. It is a large enough monsoon that it is affected by the rotation of the Earth, which complicates the circulation. In addition, the presence of mountains helps to block and guide the winds and also causes the rain to fall in an even more concentrated way if the winds encounter the mountains. Moreover the mountains themselves constitute a source of heat, though absorption of solar radiation, which is elevated. The East African highlands block the winds from going into Africa, and steer them up into southern Asia. Once over the land, they encounter the Himalayas that force the air to rise and cause heavy rains to occur.

The monsoon rains are not steady. Sometimes monsoon winds don’t bring rain, and crops fail. Partly this seems to be because of weather systems that interfere. Partly it is because the rains and cloud block the sun out, and the rains cool the earth’s surface. The result is that the air does not get so buoyant, and the monsoon slows down for a while, until the land heats up again.

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