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This is also called the monsoon season, the southwest monsoon season, the wet season and the hot-wet season. The weather conditions all over the country change with the onset of the monsoon winds. High heat, high humidity, extensive clouding and several spells of moderate to heavy rain with strong surface winds are the chief characteristics of this season.
(a) Temperature:
There is a significant fall in temperature with the beginning of rainy season. The June temperature in south India is 3° to 6°C lower than the May temperature.
Similarly July temperature in northwest India is 2° to 3°C lower than the June temperature. But once the temperature falls from its dry summer level, it remains more or less uniform throughout the rainy season. However, the temperature rises again in September with the cessation of rains and secondary maximum temperature period is experienced all over the country.
Also there is rise in temperature whenever there is break in the monsoons and rainfall does not occur for a number of days. Night temperatures are more uniform than the day temperatures. The diurnal range of temperature is small due to clouds and rains. It ranges between 4°C and 8°C when the monsoon is fully established.
The highest temperatures of over 32°C are experienced in the Thar Desert of Rajasthan. At places especially west of the Aravali the temperature may be as high as 38° to 40°C. This is due to lack of clouds and the predominance of continental airmass. The remaining parts of northwest India also have temperatures above 30°C.
The temperatures are quite low over the Western Ghats due to high elevation and also due to heavy rainfall, but the rain-shadow area is comparatively wanner on account of low elevation and scanty rainfall. The coastal areas of Tamil Nadu and adjoining parts of Andhra Pradesh have temperatures above 30°C because they receive little rainfall during this season.
(b) Pressure and Winds:
The temperatures in northwest India are still very high as a result of which low pressure conditions prevail there. The most conspicuous feature of the surface pressure distribution during this season is an elongated trough across the Ganga basin right upto the head of Bay of Bengal. This is called the monsoon trough.
There are frequent changes in its location and intensity depending upon the weather conditions. The atmospheric pressure in most parts of north India is less than 1,000 mb. It increases steadily southwards where it ranges between 1,008 mb. and 1,010 mb. The isobar of 1,009 mb. crosses parts of Kerala and Tamil Nadu besides Arabian Sea and the Bay of Bengal.
Under the influence of the above mentioned pressure distribution, winds blow in a southwest to northeast direction from Arabian Sea and Bay of Bengal. They maintain this direction throughout peninsular India. But their direction undergoes a change in Indo-Gangetic plain where they move from east to west.
(c) Rainfall:
India’s three fourths of the total annual rainfall is received during this season. In some areas it is much more than this average. For example, the average rainfall over the plains of India in this season is about 92 cm, or about 87 per cent whereas during the remaining part of the year only 14 cm of rainfall occurs. This season has the maximum number of rainy days as a result of which it is called the ‘wet season’.
Rainfall during this season all over the country is caused by southwest monsoons coming from the Indian Ocean. Figure 5.18 shows isolines of normal dates of arrival of the monsoons in different parts of the country. It is clear from this map that the normal date of the arrival of the monsoon is 20th May in Andaman and Nicobar Islands.
It is worth mentioning that the advance of the monsoon is much more in the Bay of Bengal than in the Arabian Sea. This is evident from the pronounced curve shown by isolines of the monsoon onset in figure 5.18. The monsoon current advances to nearly 20°N latitude in Bay of Bengal by the third week of May, when it is still south of Kerala at about 7°N latitude in the Arabian Sea.
The normal date of onset of the southwest monsoon over Kerala i.e. the first place of entry in the mainland of India is 1st June. The monsoons advance with startling suddenness accompanied with a lot of thunder, lightning and heavy downpour. This sudden onset of rain is termed as monsoon burst. Although the normal date of onset of the southwest monsoon on the southern tip of the peninsula is 1st June, the actual onset may be earlier or later than this date. On 60% occasions, the onset occurs between 29th May and 7th June. The earliest onset was on 11th May in 1918 and 1955, while the most delayed onset was on 18th June in 1972. Satellite imagery is used to identify the advance of the monsoon on a day to day basis.
The progress of the monsoon winds beyond south Kerala is in the form of two branches viz. the Arabian Sea branch and the Bay of Bengal branch. The Arabian Sea branch gradually advances northwards. It reaches Mumbai by 10th June and spreads over Saurashtra-Kuchchh and central parts of the country by 15th June.
The progress of the Bay of Bengal branch is no less spectacular. It spreads rather rapidly over most of Assam. The normal date of its arrival at Kolkata is 7th June. On reaching the foothills of the Himalayas the Bay branch is deflected westward by the Himalayan barrier and it advances up the Gangetic plain.
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