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Monsoon

Understanding the monsoon

The monsoons, which help balance global temperatures and sustain life on earth, affect a vast area of the globe - from Africa across Asia to the Pacific; northern China and the Himalayas to north Australia; and even Mexico and parts of Central America - directly influencing the lives of over half the world's population. In India, 50% of the arable land is irrigated solely by monsoon rains.

Two monsoons

There are two Indian monsoon seasons. The summer or south-west monsoon comes in from the direction of Africa, and brings heavy rain to the west coast and large areas of northern India between June and August. This is when Asia breathes in. The winter, or north-east monsoon, sweeps down from the plateaus of Asia and the Himalayas, and brings rain and cooler weather to south-east India between October and December. This is when Asia breathes out. For a thousand years or more Arab dhows have relied on this phenomenon - and the `trade winds' - to sail across the Arabian Sea to India ahead of the main force of the south-west monsoon, returning with fresh cargo on the winds of the north-east monsoon. It's not surprising that the word `monsoon' derives from the Arabic word `mausim', meaning `season'.

How it works

The key to understanding the basic monsoon mechanism lies in the fact that land heats up and cools more quickly than the sea; the latter holding its temperature more or less steady. As the sun moves north bringing our summer heat, the land steadily gets hotter and hotter, while the temperature of the ocean lags far behind. The effect on a huge land mass like Asia as the hot air rises over the land, leaving below a vast area of low pressure, is to draw in massive amounts of air from over the ocean, where higher pressures are maintained. This is the south-west monsoon pattern (it is the wind, and not the resulting rain, which is defined as the monsoon).

Around September, with the sun fast retreating south, the northern land mass begins to cool rapidly. As it cools, air pressure builds up over the land. Meanwhile the oceans are still holding their warmer summer temperatures. The cooler high pressure air over inland Asia then starts to move down towards the lower pressure areas over the ocean, and our north-east monsoon is born. Asia breathes out, and we enjoy our October-December period of rains. Meanwhile the air moving out from inland Asia is replaced and balanced by warmer air moving in northwards from the oceans at around 40,000 feet.

October-December in Auroville

For most of India, the main monsoon is the south-west monsoon. Only in Tamil Nadu does one see a major difference, because this area fails to pick up anything but the remnants of the summer monsoon. It's the north-east monsoon which saves us here in Auroville, picking up moisture over the Bay of Bengal and bringing us approximately two thirds of our annual rainfall.

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