India is a large country, nearly half the size of the United States. It is situated in southern Asia, and is bordered by China, Nepal, and Bhutan on the north; Myanmar and Bangladesh on the east; Pakistan and Afghanistan on the northwest; and by the Indian Ocean on the southeast, south, and southwest.
The country extends from 8° to 33°N, and includes vast plains like the Ganges valley and high mountains like the Himalayas - the highest in the world.
The wide variety in the terrain makes for a wide variety of climatic conditions. These range from permanent snowfields to tropical coastlands; from areas of virtual desert in the northwest plains to fertile, intensively cultivated rice fields in the northeast.
The climate of India is dominated by the great wind system known as the Asiatic monsoon. This is completely unlike the prevailing wind system that operates in many countries, i.e., a wind that prevails from the same direction throughout the year.
The monsoon reverses direction at certain times of the year. For some months it will blow steadily from the southwest; for other months, from the northeast.
From June to October the country is influenced by the moist, rain-bearing monsoon from the southwest. On some mountain ranges, facing the sea, rainfall can be very heavy indeed.
The coolest, driest time over most of the country is from December to February, when light northerly winds bring clear skies and little rain. From March to May the climate becomes hotter and hotter and the drought continues. The rains only come when the wind turns again to the southwest.
On average, the arrival of the rains - the 'burst of the monsoon', as it is called - comes to the south of India during late May or early June.
It will reach the north about six weeks later. In some years, the rains will be torrential; in other years they may be light or locally variable, in which case the monsoon will be said to have 'failed'. Results for food crops can, of course, be disastrous.
India can be conveniently divided into seven climatic regions: the northern mountains or Himalayas, the northern plains from the Ganges delta to just northwest of Delhi, the Rajasthan Desert, the Deccan plateau, the west coast, the southeast coastlands, and Assam in the extreme northeast of the country.
The Northern Mountains
This region includes the Himalayas and their foothills. Here some rain can occur all the year round. In winter light rain or snow is brought by disturbances travelling from the west. The main rainy season, however, is from July to October during the southwest monsoon.
Winters are pleasant and cool at lower levels but it can get quite hot before the 'burst' of the monsoon. At intermediate levels, from 1,800 m/6,000 ft to 2,450 m/8,000 ft, the summer climate is very pleasant and cool.
Sikkim has a range of climate which varies with altitude as described for Nepal. Kashmir (see table for Srinagar) and hill stations such as Shimla and Darjeeling are popular refuges from the heat of the plains.
The Northern Plains
Extending from the Punjab to the Ganges delta, this low-lying region is everywhere hot and generally dry from March until June. Some occasional thunderstorms occur at this season, more particularly in the east.
With the arrival of the main monsoon rains in July, temperatures drop a little in the more cloudy weather but the high humidity causes this season to be almost as unpleasant as the preceding hot season and the nights are particularly sticky.
Rainfall decreases from east to west and to the west and northwest of Delhi conditions verge on desert. During the winter season from December to February the weather is generally sunny and dry. The nights and early mornings can feel quite chilly but the days are warm and pleasant.
Some light rain may occur in the west and no part of the region is completely dry at this time. The contrast between the wetter east and drier west is well shown by comparing the climatic tables for Kolkata and Delhi.
The Rajasthan Desert
This is the eastern part of what before the partition of India used to be called the Thar or Great Indian Desert. Here the climate is similar to that found in the Sind province of Pakistan to the west, and the conditions round the year are well illustrated by the climatic table for Jacobabad in Pakistan.
Annual rainfall is almost everywhere below 500 mm/20 in per year and in many places only half this. This area is one of the hottest parts of the world from May to July and the arrival of the monsoon with some light rain and more cloud makes very little difference to temperatures, so that July, August, and September are unpleasantly hot and humid. The cool season from November to March is warm, sunny and dry.
The Deccan Plateau
The interior of the centre and south is a low plateau with a different climate from that of the coastlands. The three main seasonal divisions of the year apply equally well here, but rainfall is generally moderate compared with the coastlands and, in the northwest, rather low.
On occasions, during the hot season, temperatures can approach those of the northern plains. Altitude is the main control on temperature, but towards the south even the cool season is typically tropical in temperature when warm sunny days are moderated by dry heat and pleasant cool evenings (see the table for Hyderabad).
The West Coast
This consists of a narrow coastal plain backed by a steep mountain barrier, the Western Ghats. Rainfall is abundant and heavy during the southwest monsoon season. The heat can be very oppressive because of the humidity throughout the year, particularly in the hot season.
Some hill stations in the Western Ghats have a pleasant climate during the hot season, but are very cloudy and wet during the monsoon. Towards the south some rain can occur at any time of the year and the monsoon arrives earlier.
The table for Mumbai is representative of conditions at sea level.
The Southeastern Coastlands
Here the main rains do not occur until the period October to December, and are often associated with tropical storms or cyclones developing in the Bay of Bengal. Because of the lack of cloud, the period of southwest monsoon from June to September can be very unpleasant, since temperature and humidity are high.
The east coast north of Banda has its main rainy season at the time of the southwest monsoon, but this area is occasionally affected by very heavy rain and strong winds caused by tropical cyclones between July and November (see the table for Chennai).
This area is almost detached from the rest of India by Bangladesh. It is a region of plains and mountainous tracts. Its climate is similar to that of the northern plains and Himalayas, depending on altitude.
Here some significant rainfall can occur in the period March to May but the main rainy season from June to October is, in places, very wet indeed and Cherrapunji at an altitude of 1,300 m/4,300 ft has the distinction of being one of the three wettest places in the world with an annual rainfall averaging 10,800 mm/425in!
The table for Dhubri is representative of the lowlands.
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