India is a large country - about a third of the size of the USA, and includes vast plains like the Ganges valley and also the world's highest mountains - the Himalayas.
The wide variety of terrain leads to a wide variety of climatic conditions. These range from permanent snowfields to tropical coastlands; from areas of virtual desert in the north-west to fertile, intensively cultivated rice fields in the north-east.
The Indian climate is dominated by the great wind system called the Asiatic monsoon which is totally unlike any other country's prevailing wind system. The monsoon reverses direction at certain times of the year. From June to October India is influenced by the moist rain-bearing monsoon from the south-west and on some mountain ranges facing the sea, rainfall can be very heavy indeed. The coolest, driest period over most of India is from December to February when light northerlies bring clear skies and virtually dry weather. From March to May the climate becomes very hot and the drought continues. Usually, the monsoon reaches the south during late May or early June, reaching the north about six weeks later. In some years the rains are torrential, but in other years they will only be light. The monsoon is then reckoned to have failed, leading to disastrous crop-growth problems.
India can be divided into seven climatic regions: the northern mountains, the northern plains, the Rajastan Desert, the Deccan plateau, the west coast, the south-east coastlands and Assam in the extreme north-east.
The northern mountains region includes the Himalayas and their foothills. Some rain can occur all year round and in winter light snow is brought by disturbances from the west. The main rainy season is from July to October (during the south-west monsoon). Winters are pleasant and cool at lower levels, but it can get hot before the monsoon burst. At intermediate levels (1,800-2,500 metres) the summer climate is very pleasant and cool, and hill stations such as Darjeeling are popular refuges from the heat of the plains.
Extending from the Punjab to the Ganges delta, the Northern Plains are a low-lying region, and are hot and generally dry from March to June. Occasional thunderstorms do occur at this season, especially in the east. When the monsoon arrives in July, temperatures drop slightly in the cloudier weather but the high humidity makes this season almost as unpleasant as the preceding hot season - particularly at night. Average rainfall decreases from east to west, and north-west of Delhi conditions are virtually desert. From December to February the weather is generally sunny and dry. 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 east and west is typified by looking at rainfall data for July: 325 mm (12.8") at Calcutta, but 180 mm (7.1") at Delhi.
The Rajastan Desert has annual average rainfall figures as low as 250 mm (10") in places. This area is one of the world's hottest places from May to July, and the arrival of the monsoon makes little difference to the temperatures. Indeed July to September are unpleasantly hot and humid. Average maximum temperatures typically reach 46 °C (114 °F) in June, and a sweltering 53 °C (127 °F) has been recorded. The 'cool' season is from November to March and is warm, sunny and dry.
The Deccan peninsula is a low plateau with a different climate from the coastlands. The three main seasonal divisions apply equally well here, but rainfall is generally moderate or low. During the hot season, temperatures can approach those of the northern plains (Average maximum temperature of 40 °C/112 °F in May at Hyderabad).
The west coast consists of a narrow coastal plain backed by a steep mountain barrier (the Western Ghats). Rainfall is abundant and heavy during the monsoon season, and the heat can be very oppressive because of the humidity. Some hill stations in the Western Ghats have a pleasant climate during the hot season, but are very wet and cloudy.
In the southern coastlands, the main rains occur in October to December, and are often associated with cyclones or tropical storms developing in the Bay of Bengal. Because of the lack of cloud, the period of the south-west monsoon from June to September can be very unpleasant since temperature and humidity are very high.
Assam (in the extreme north-east) is almost detached from the rest of India by Bangladesh. It is a region of plains and mountains and has a similar climate to that of the northern plains and Himalayas, depending on altitude. Significant rainfall can occur from March to May, but the main rainy season (June to October) is very wet indeed. Cherrapunji (altitude 4,300 feet/1,300 m) has the distinction of being one of the three wettest places in the world, with an annual average rainfall of 10,800 mm (425 ").