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The climate of India is difficult to lay due to the country's large geographic size and varied topography. Many regions have their own microclimates (e.g. in mountain tops), and the mean climatic conditions in Kashmir (extreme north) are very different from those in the extreme south.''
India's climate is strongly influenced by The Himalaya and the Thar Desert. The Himalaya ensure, by acting as a barrier to the cold north winds from Central Asia, that northern India is warm or mildly cool during winter and hot during summer. So, although the Tropic of Cancer (the dividing line between the tropical and sub-tropical regions) passes almost through the middle of India, India as a whole is considered to be a tropical country.
India has three distinct seasons:
Summer
Summer in northwestern India lasts from April to July, and in the rest of the country from March to June. The temperatures in the north rise as the vertical rays of the Sun reach the Tropic of Cancer. The hottest month for the western and southern regions of the country is April, while for the northern regions it is May. By May, most of interior India experiences mean temperatures over 32°C and maximum temperatures exceeding 40°C. Temperatures of 49°C and higher have been recorded in parts of India during this season. Near the coast the temperature hovers around 36°C, and the proximity of the sea increases the level of humidity. In southern India, the temperatures are higher on the east coast by a few degrees compared to the west coast.
Altitude affects the temperature to a large extent, with the higher parts of the Deccan plateau and hills being relatively cooler. The Himalayan and Nilgiri hill stations offer some respite from the heat with a high temperate of 25°C.
Monsoons
The monsoons come as a relief from the heat and parched landscape. The rains bring down the temperature, and make the surroundings lush and green. It is the best season to go hiking and trekking. The monsoons are intricately linked to the economy as a good monsoon results in a booming economy. The rains fill the ground water tables and reinvigorate rivers and lakes.
Southwest monsoons
The southwest monsoons supply over 80% of India's annual rainfall. There are two branches to the monsoon, the Bay of Bengal branch, and the Arabian Sea branch, extending to the low pressure area over the Thar Desert in Rajasthan. The Arabian Sea branch is roughly three times stronger than the Bay of Bengal branch.The monsoon makes its presence felt by the end of May. It starts around the 29 May, hitting the Andaman and Nicobar Islands in the Bay of Bengal. It strikes the mainland in Kerala by 1 June. By 9 June, it reaches Mumbai, and Delhi by 29 June. The Bay of Bengal monsoon moves in a northwest direction whereas the Arabian Sea monsoon moves northeast. By the first week of July, the entire country experiences rain. But usually southern India receives more rainfall than northern India.
Cherrapunji, Meghalaya, one of the world's wettest places, receives 10,000 mm (400 inches) of rainfall. Currently this record is held by Mausamram, a village about 5 km west of Cherrapunji. The monsoons start its withdrawal by the last week of August from northern India. It withdraws from Mumbai by 5th October and eventually the entire country by the end of October.
Northeast monsoons
After the withdrawal of the south-west monsoon, the northeast monsoon sets in by November. Though much of India remains largely unaffected by this monsoon, it is very crucial for Tamil Nadu as it receives over 60% of rainfall due to northeast monsoon. This weather system produces rain primarily in the states of Tamil Nadu, Orissa, Andhra Pradesh, Kerala and southeast corner of Karnataka. In Tamil Nadu, November is the wettest month of the year. But for the rest of the nation, this system supplies only 20% annual rainfall.Due to very active weather systems in the Bay of Bengal, many cyclones take place during this season. They generally occur between September and December, and mainly affect the eastern coast of India. Many cyclones have led to widespread devastation along the east coast of India. Many deaths and widespread destruction of property is reported every year in Andhra Pradesh, Orissa, Tamil Nadu and West Bengal. These same cyclones often hit the neighbouring Bangladesh. A super cyclone that hit Orissa on 29-30 October, 1999 caused worst damage in known history and unofficial estimates put the loss of life around 30,000 to 35,000. Cyclones are rare on the west coast and mainly affect Gujarat and to even lesser extent Kerala.
Winter
The temperature gradually falls in the country after September. As the vertical rays of the sun move south of the equator, the country experiences cool weather with temperatures decreasing by about 0.5°C for every 1° latitude moved north. December and January are typically the coldest months with mean temperatures of 10 to 15°C in the northwest and the Himalayan region. The mean temperatures increase towards east and south, where it is between 20 to 25°C.
In northwest India, October and November are cloudless. This leads to a high diurnal range of temperatures during these months. It ranges between 16 to 20°C in northwest India as well as across much of Deccan Plateau, and 12 to 14°C in the coastal strip. Entire Himalayan range, from Kasmhir to Arunachal Pradesh in northeast India, receives significant snowfall. However, the rest of north India, i.e., plains, does not receive snow. But the minimum temperature in the plains falls below freezing occasionally, not for more than a couple of days in December and January. Highs in Delhi range between 16° to 21°C. Night-time temperatures range between 2 to 8°C. Further north in the Punjabregion the low does fall below freezing in the plains: to around -6°C in Amritsar. Frost sometimes occurs, but the hallmark of the season is the notorious fog which disrupts daily life.
Northern India does receive some rainfall due to the western disturbances originating in the Mediterranean Sea. As these disturbances travel eastwards, unable to "climb" the Himalaya they drop their rain over north India.
Eastern India has a much milder climate. It has mild days and cool nights. Highs range from 23°C in Patna to 26°C in Calcutta and lows average 8°C in Patna to 14°C in Calcutta. The cold winds over the Brahmaputra River lower the temperatures.
In Southern India, inner Maharashtra, Madhya Pradesh, and some parts of Karnataka and Andhra Pradesh experience cooler weather. Minimum Temperatures in western Maharshtra, Madhya Pradesh and Chhattisgarh hover around 10°C. It is to about 16°C in the southern Deccan Plateau. Coastal areas and low-level interior tracts are warm with highs of 30°C and lows of 21°C. The Nilgiri range is the exception, where the lows can fall below freezing.
Withdrawal of monsoons
This is not a true season as such. Many text books however, refer to this as a separate season. This season lasts between September and December depending on its location. The weather turns more dry and the grass starts to dry up. This season marks the transition from wet to dry climate in most parts of India. Highs range between 34°C and 28°C.
Autumn and spring
Autumn and spring seasons only occur in the Himalayan states. These regions have a temperate season and experience 5 seasons annually.
Records
The highest temperature recorded in India was 50.6 °C (123.08 °F) in Alwar in 1955. The lowest was −45 °C (−49 °F) in Kashmir. Recent claims of temperatures touching 55 °C (131 °F) in Orissa have been met with some scepticism by the meteorological department, based largely on the method of recording of such data. These claims are not true.
"Climate of India"