The Advancing Southwest Monsoon Season, Distribution of Annual Rainfall

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The Advancing Southwest Monsoon Season

It is the rainy season for most parts of India. It starts with the inflow of southwest monsoons which strike the coast of Kerala normally in the first week of June and cover most of India by mid-July. This weather condition continues till September. The arrival of these warm moisture laden winds brings a total change in the weather conditions. Their arrival causes sudden rains which bring down the temperatures considerably. The decline in temperature is between 5°C to 10°C. The sudden onset of rain is called break of monsoons or the burst of monsoons. The arrival of these winds may be delayed by a week or two depending upon the pressure conditions over northern plains and over the Indian ocean. The peninsular shape of India divides these southwest monsoons into two branches:

  • The Arabian Sea Branch: It strikes the western coast of India and causes heavy rains on the western slopes of the Western Ghats. After crossing the Western Ghats, these winds cause less rainfall on the eastern slopes as they gain temperature while descending. This area is, therefore, known as the rain shadow zone. Southwest monsoons striking along the coast of Saurastra and Kachchh and pass over Rajasthan and beyond to meet the Bay of Bengal branch. These winds cause widespread rain in these states and western Himalayan region.

  • The Bay of Bengal Branch: It is divided into two subbranches after striking the eastern Himalayas. One branch moves towards the east-northeast direction and causes heavy rains in the Brahmaputra valley and northeast hills of India. The other branch moves towards northwest along the Ganga valley and the Himalayan ranges causing heavy and widespread rains over vast areas. In this region, the amount of rainfall decreases from east to west owing to the progressive decrease in humidity of these winds.

The amount and timing of rainfall and intervening duration of wet and dry spells varies from year to year. This is known as the vagaries of the monsoons.

Mean Temperature and Pressure (July)

Mean Temperature and Pressure (July)

The Retreating Southwest Monsoon Season

The southwest monsoons start retreating in the first week of September from Pakistan border in northwest India. Thus, these winds withdraw earlier from the regions where they reach the last. The retreat of these winds takes place due to weakening of the low-pressure area over the north-western parts. This happens due to low temperatures caused by apparent shift of the sun towards the equator and also owing to the widespread rains bringing down temperatures perceptibly. Consequently, the air pressure starts decreasing. Such changes in the patterns of atmospheric pressure cause southwest monsoons to withdraw. Hence, this period is known as the season of retreating southwest monsoons. By the end of October, these winds retreat from most of northern India. As a result, fair weather conditions prevail over this region.

Image of Maps

Image of Maps

The low-pressure area lying over northwest India is transferred to the middle of the Bay of Bengal by the end of October. As a result of these unstable conditions, severe cyclonic storms originate in the Bay of Bengal. These cyclonic storms strike along the eastern coast of India causing widespread rain in the coastal regions. Sometimes very severe storms cause damage to the standing crops, cattle, property, the lines of transports, communication and even electricity. Tamil Nadu coast receives maximum of its rainfall during October and November.

Distribution of Annual Rainfall

The north-eastern parts of Jammu and Kashmir and extreme western Rajasthan receive a rainfall of less than 20 cm. On the other hand, the west coastal plains, sub-Himalayan areas of northeast India including the Shillong plateau receive more than 200 cm of annual rainfall. Southern slopes of Khasi and Jayantia Hills, particularly the Cherrapunji valley receive the highest rainfall exceeding 1000 cm. Starting from the southern coast of Gujarat, the isohyet of 200 cm runs somewhat parallel to the coast of Western Ghats up to Kanyakumari. To the east of Western Ghats, the rainfall drops abruptly below 60 cm over interior Maharashtra and Karnataka. Most parts of Punjab, Haryana, central and eastern Rajasthan, and western Gujarat also receive rainfall below 60 cm. Starting from the south-western parts of Jammu and Kashmir, the isohyet of 100 cm moves eastwards up to east of Allahabad from where it bends to the west and southwest, running over western Madhya Pradesh, eastern Maharashtra, and northern Andhra Pradesh, it joins eastern coast near Visakhapatnam. To the west and southwest of this isohyet, the areas receive less rainfall. Some parts of Coromandel coast receive a rainfall of more than 100 cm. The areas receiving less than 100 cm of rainfall depend on the means of irrigation for agricultural activities.

Average Annual Rainfall In India

Image of Average Annual Rainfall in India

In India, distribution of rainfall particularly of the southwest monsoon has a close relationship with the relief. Hence it is even described as relief or orographic rainfall. The large places with higher altitude have greater chance to catch more rainfall than the places with less altitude.

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