Distribution of winds and rainfall over India in summer monsoon
- The word ‘Monsoon’ has been derived from the Arabic word ‘Mausim’ meaning season.
- These monsoon winds have such a far reaching influence on India’s climate that it is termed as ‘Monsoon type of Climate’.
- The Southwestern summer monsoons occur from July through September.
- Monsoon winds are the most important periodic winds (the direction of these winds changes with the change in seasons)
- The winds that reverse their direction with the change of seasons are called monsoon winds.
- This complete reversal of winds in India brings about a sudden change in seasons- the harsh summer season suddenly giving way to the eagerly awaited monsoon or rainy season.
- During summer, the monsoon winds blow from sea towards land and during winter from land towards seas.
- The Asiatic monsoon is the result of interaction of both planetary wind system and regional factors, both at the surface and in the upper troposphere.
- In the summer monsoon season, the atmospheric pressure is low all over the Indian peninsula as the sun is apparently situated at the right angle from the tropic of cancer.
- The isobar runs more or less parallel to the coast indicating differences in pressure conditions over land and sea.
- There is marked change in the direction of and speed of the winds from the winter conditions.
- The winds are by and large light and variable. But, in May and June, high temperature in Northwest region causes steep pressure gradient which is often of the order of 1.0 to 1.5 mb. per degree latitude. It gives rise to hot dust laden wind called Loo.
- Loo are hot and dry winds, which blow very strongly over the northern plains of India and Pakistan in the months of May and June.
- Their direction is from west to east and they are usually experienced in the afternoons.
- Their temperature varies between 450 C to 50 C.
- It has a severe drying effect on vegetation leading to widespread browning in the areas affected by it
- Dust storms cause drizzle in northern plains. Light showers are also experienced in Kerala, West Bengal and Assam. In Kerala, these pre-monsoon showers are popularly known as ‘Mango Showers’. In West Bengal and Assam, they are called Norwesters or ‘Kalbaisakhi’. But they are only regional variation of the wind system.
Distribution of rainfall
- In summer, because of India’s inverted triangular shape, the land is heated progressively as the sun moves northwards.
- There are three distinct areas of relative upper troposphericwarmth—namely,
(1) above the southern Bay of Bengal,
(2) above the Plateau of Tibet, and
(3) across the trunks of the various peninsulas that are relatively dry during this time.
These three areas combine to form a vast heat-source region
- This causes a low pressure area over the northern and central Indian subcontinent.
- To fill this void, the moisture-laden winds from the Indian Ocean rush in to the subcontinent. These winds, rich in moisture, are drawn towards the Himalayas.
- The Himalayas act like a high wall, blocking the winds from passing into Central Asia, and forcing them to rise. As the clouds rise their temperature drops and precipitation occurs.
- The southwest monsoon is generally expected to begin around the beginning of June and fade away by the end of September.
- The moisture-laden winds on reaching the southernmost point of the Indian Peninsula, due to its topography, become divided into two parts: the Arabian Sea Branch and the Bay of Bengal Branch.
- The Arabian Sea Branch of the Southwest Monsoon first strikes the western coast of India and causes heavy rains (orographic rainfall) on the western slopes of western ghats, between 200-400 cms for the season.
- After crossing the western ghats, these winds cause less rainfall on the eastern slopes as they gain in temperature while descending. This area is therefore known as rain shadow area. This explains why interior parts of Maharastra, Karnataka and Telangana get meagre rains from these winds.
- Southwest monsoons striking along the coast of Saurastra and Kutchch 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.
- Rajasthan does not have much rainfall due to temperature inversion.
- Tamil Nadu coast does not have rainfall as the rain bearing winds run parallel to the coast.
- The Bay of Bengal Branch of Southwest Monsoon is divided into two sub-branches after striking eastern Himalayas.
- One branch moves towards east northeast direction and causes heavy rains in Brahmaputra valley and Northeast hills of India.
- The other branch moves towards northwest along the Ganga valley and Himalayan ranges causing heavy and widespread rains over vast areas. In this region, the amount of rainfall decreases from east to west owing to progressive decrease in humidity of these winds.
- The winds arrive at the Eastern Himalayas with large amounts of rain. Mawsynram, situated on the southern slopes of the Khasi Hills in Meghalaya, is one of the wettest places on Earth.
- The North East and East Plateau have rainfall in the range of 150-200 cms.
- Snowfall occurs in Himalayas, while the plains have cyclonic rainfall between 100-200 cm which decreases from east to west with maximum in the Tarai region.
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