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The summer monsoon is an Indian wind bringing welcome rain

Posted in Geography, Nature, Science on Wednesday, 27 June 2012

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This edited article about the monsoon originally appeared in Look and Learn issue number 742 published on 3 April 1976.

Indian Monsoon, picture, image, illustration

Indian monsoon by Gerry Wood

History books honour the famous Portuguese explorer Vasco da Gama as the first European to have sailed to India. But for the final stage of his voyage, across the Indian Ocean, he owed much to his pilot.

The pilot, lent to him by an East African ruler, was an Arab, familiar with those waters. They sailed from Malindi, in what is now Kenya, in the year 1498. By setting the correct course, the Arab pilot was able to pick up the strong south-westerly wind that blows towards India in the summer months. Landfall was safely made at the port of Calicut, on the south-west coast.

The wind that helped the explorer on his way is known to us as the summer “monsoon”. It is a word derived from the Arabic mausin, or possibly the Malayan monsin, both meaning “season”. For it is a seasonal wind.

Winds are movements of air caused by differences of density. Heavier, or denser, air tends to push into areas of less density. One of the ways in which air is made less dense, or lighter, is by raising its temperature. As a result, winds often blow from colder to warmer regions.

Across the Atlantic and Pacific Oceans there are “trade winds”, which regularly blow towards the warm region of the Equator. Because of the Earth’s rotation, they do not blow straight in from north and south, but slant in from north-east and south-east.

The Indian Ocean, too, has trade winds. But from May to September something strange happens. Instead of winds from both north-east and south-east, the south-easterlies seem to overrun the Equator. Swinging round, and coming now from the south and south-west, they flow across the Indian sub-continent.

The usual explanation of the upsetting of the trade wind pattern is that, under the heat of the summer sun, the air over the continental land-mass of Asia is much hotter and less dense than that over the oceans. As a result air is drawn in from the south-west and south, and, in China and south-eastern Asia, from the south-east.

From October or November onwards, the reverse happens, the monsoon blowing from land to sea. Across India the winter monsoon blows from the north-east.

The movement of air between land and sea can be observed on a much smaller scale at any seaside resort during otherwise calm weather. As the air over the land warms up by day, denser air moves in from the sea to create a “sea breeze”. During the night, as the air ashore cools and grows heavier, the breeze will blow seawards.

Some experts today consider this explanation of the Asian monsoon winds to be incomplete, and over-simplified. They claim that the behaviour of the monsoons is at least partly due to the effects of global air currents high in the atmosphere. These have a much greater influence on climate than was once realised.

The term “monsoon” is now used to describe any wind system like those that exist in Asia. Such winds are also found, for example, in West Africa and the Caribbean region. But for the people of India the monsoon means especially the wind that blows from the Indian Ocean in summer – and above all it means the rains which that wind normally brings. For many millions of Indians these rains mean the difference between famine and survival.

Any air current that blows across a large expanse of water picks up moisture. The summer monsoon of India is heavily laden with it. But, unlike our Atlantic westerlies, it does not shed its rain in moderate quantities over a long period. Instead it builds up a stormy “front” which moves across India. From June onwards, each region can expect to experience what is called the monsoon “burst” – a sudden onset of torrential rainfall. The rain descends not in drops but in sheets, and normally continues for several weeks.

Now the all-pervading dust is laid, and, within days, the parched land begins to grow green. Rice begins to flourish in the flooded paddy fields, water-tanks and irrigation ditches are filled, and hope revives in the hearts of the peasant farmers.

Yet, though the Indian farmers depend so much on the monsoon, it has often proved a treacherous friend to them. In some years it has failed altogether. In others it has caused such flooding that crops have been destroyed and top soil washed away.

Great efforts have been made to improve conditions – as for example, with the huge irrigation dam and reservoir at Hirakud, in eastern India. But over large areas there are still only primitive means of controlling the water when the rains come, so that the peasant folk are largely at the mercy of Nature.

When the summer monsoon begins to die away in October, and before the north-east monsoon sets in, there is an unsettled period. At this time another danger may threaten – that of hurricanes, called “cyclones” in this part of the world, or “typhoons” in the China seas. When these occur, even in years when the monsoon has been kind, the farmer’s work may be in vain.

It is not only human living conditions that are affected by the monsoon climate. In parts of India and south-eastern Asia there exists a type of vegetation that is peculiar to monsoon lands. This is the monsoon forest. In the wet season, it is not unlike the evergreen tropical rain forest, though less dense, and with trees that do not grow quite so tall. But in the dry months, the trees shed their leaves, and the forest assumes a bleak appearance.

The lands where the climate is ruled by the Asian monsoons are estimated to hold about half the world’s population. So it is fair to say that these great seasonal winds are among the most important factors in human life.

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