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The phenomenon of the monsoons is certainly very old. According to meteorologists, it is a complete replacement of the dry hot air by the equatorial maritime air up to an altitude of three to five kilometers over the land and water surface. By and large this phenomenon is confined to tropical lands lying between 20 degree North and 20 degree South. But in the Indian subcontinent it is greatly influenced by the Himalayan ranges bringing the whole subcontinent under the sway of these moist equatorial winds for a season ranging between two to five months. It accounts for 75 to 90 per cent of the annual rainfall just from June to September.

By early June, very low pressure conditions prevailing over the north-western plains get further intensified. At this stage they are powerful enough to attract the trade winds of Southern Hemisphere.

These south-east trade winds are of oceanic origin. Coming from the Indian Ocean they cross the equator and enter the Bay of Bengal and the Arabian Sea, only to be caught up in the air circulation over India. Passing over the equatorial warm currents they bring with them moisture in abundance. After crossing the equator they follow a south-westerly direction. This is why they are known as the south- west monsoons.

The rain bearing winds are strong. They blow at an average speed of 30 km per hour. Barring the extreme north-west they overrun the country in a month's time. Though they follow a south-westerly direction, as they approach the land, their direction is modified by the relief and thermal low pressure over north-west India. To begin with, the Indian peninsula divides the monsoons into two branches. They are the Arabian sea branch and the Bay of Bengal sea branch.

Distribution of rainfall received from south-west monsoons is very largely governed by the relief or topography. For instance, the windward side of the Western Ghats registers a rainfall of over 250 centimetres.

On the other hand, the leeward side of these Ghats is hardly able to receive 50 centimetres. Again the heavy rainfall in the north-eastern states can be attributed to their hilly ranges and the eastern Himalayas. The rainfall in the Northern plains goes on decreasing from east to west. During this particular season, Calcutta receives about 120 centimetres, Patna 102 cm, Allahabad 91 cm and Delhi 56 cm.

The south-west monsoons retreat in the months of October and November.

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