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The word monsoon is derived from the Arabic word ‘mausim’, meaning season. It was first used by Arabic navigators to describe the seasonal winds of the Arabian Sea. These generally blow from the north-east for one half of the year, and from the south-west for the other half. Although the term monsoon actually means a seasonal wind, it is often used to refer to the torrential rainfall associated with these winds.

Monsoons occur mainly in tropical regions – northern Australia, Africa, South America and the USA. However, the best known area affected by monsoons is south-east Asia, particularly India. During the winter, air over the Siberian plateau becomes colder than air over the surrounding seas, producing a large anticyclone with winds circulating clockwise, thus causing cool north-easterly winds to blow across India and its neighbouring countries. This brings dry, pleasant weather, and has a marked drying effect on the land. During April and May the winds abate, causing temperatures to rise rapidly to over 35 °C.

In the summer the process reverses. The Siberian plateau is now warmer than the seas, and low pressure develops over these seas. The winds circulate anticlockwise and approach India from the south-west, bringing very moist air. These south-westerly winds bring a drop in temperature and heavy downpours of rain. In fact, during this monsoon, which generally lasts from June to September, India receives virtually all its rainfall for each year.

The mountains of southern India split the summer winds. The western arm of the monsoon is deflected northwards, by the western Ghats, to Bombay and then on to Pakistan. The eastern arm travels up through the Bay of Bengal to Calcutta and Assam, and is deflected north-westwards by the Himalayas. On average, the winds arrive in southern India about six weeks before they arrive in north-west India.

The heaviest monsoon rainfalls occur where the winds blow side-on to the hills. The higher the hills and more moist the air, then the greater the amount of rainfall. These factors give Cherrapunji, inWeb page reproduced with the kind permission of the Met Office Assam, one of the highest rainfalls in the world; the western monsoon winds having travelled an extra distance over the warm seas of the Bay of Bengal, then meeting the Himalayas. On average, Cherrapunji has an annual rainfall total of nearly 11 metres, the maximum monthly amount occurring in June. Bombay, in the eastern monsoon, receives about 1.8 metres with the maximum monthly total in July. In comparison, Delhi registers only 64 cm of rainfall each year (about the same as London), with the maximum monthly total occurring in both July and August. At Madras the pattern of rainfall is different because the monsoon winds blow along the coast. Here, the rainfall increases gradually through the summer months with larger amounts falling in October and November, owing to tropical cyclones travelling westwards across the Bay of Bengal.

Monsoon hazards

Monsoon rainfalls are unreliable in that the amount varies considerably from year to year. Low rainfalls cause great problems for agriculture and water supplies in general. On the other hand, even moderate rainfalls can cause flood hazards. The eastern monsoon releases most of its rainfall in the Ganges plain, causing flooding to low-lying areas where the river flows into the Bay of Bengal. In the Indus river the flood problem is often made worse because the monsoon rainfalls can coincide with high river levels in its tributaries, caused by water from the melting mountain snows of the Himalayas. Web page reproduced with the kind permission of the Met Office

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