This season is called Rainy Season.
This season starts in the first week of June in the eastern parts of the country. The south-west winds start blowing in South India, sometimes in the end of May or in the beginning of June. Characteristics of S.W. Winds
(1) The Time of the Blowing of Winds.
The main characteristic of these winds is that these winds start blowing sometimes early and sometimes late.
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(2) The Decreasing Order of Rainfall.
The rainfall received from the monsoons when they move into the land from sea is of a decreasing order. The decreasing order of rainfall is shown in the following table:
(3) The Distance from the Mountains and the Amount of Rainfall.
The monsoon winds blow along the Himalayas. The places which are away from the mountains get lower amount of rainfall.
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There is a direct connection between the distance from the mountains and the decrease of rainfall. For example, the rainfall (in cm) at Mussoorie is 222, at Bareilly 110, at Meerut 83 and at Gwalior 50.
Along with the S.W wind depressions, cyclonic storms and tropical storms of the north of Bay of Bengal also move up the Ganga Valley.
These wind structures are related to waves (also called troughs) in the upper air easterly jet streams. Their frequency is about 10 per month in the summer season. It is due to the shift of the easterlies.
(4) Dry Season Gaps.
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Many times the rainfall stops for days together in the area of monsoon winds. It is known as Monsoon Breaks or Lulls. It is a gap of dry season. The cause of this gap is Iropieal Depressions whose pattern goes on changing.
The intensity of the Monsoon rain depends upon the frequency of these depressions but the real mechanism is still not properly known. Nowadays meteorologists relate it to the shift of Westerly Jet Streams to an area north of the Tien Shan in China.
A jet, easterly in lower stratosphere, is usually stationed at about 15°N. It is connected with global heat exchange mechanism and is responsible for monsoon breaks and bursts. The thermal depression in north-west India capped by a dynamic depression aloft is also a phenomenon which pulls monsoon and causes monsoon bursts.
The eastern coast of India remains dry in this season because it is situated in the rain shadow area of these monsoons and is also parallel to the wind in the Bay of Bengal.
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(5) The Dry Area around Rajasthan and Neighbourhood.
The monsoons enter India in two branches. One branch is South-West Monsoon which enters India via Bay of Bengal. Another branch comes from the Arabian Sea and reaches the Himalayas after crossing Gujarat and Rajasthan.
This branch provides scanty rainfall while crossing these areas because this is an area of high pressure and subsidence of upper air takes place with adiabatic tendencies. Upper air temperature inversion also prevents condensation in sandy terrain of Rajasthan.
Temperature.
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On account of rains due to S.W Monsoons, the temperature falls by 5° to 8°C. This rain gives a great relief from the intense heat. However, as soon as the clouds disappear and the rain stops, the temperature starts rising very fast. The condition of high temperature with corresponding high humidity in the air makes life very uncomfortable.