India has tropical monsoon climate. Indian climate is greatly influenced by the presence of Himalayas in the north and the Indian Ocean in the south. The Himalayas obstruct the monsoon winds and provide rain throughout the country as well as protect from the cold winds which blow from the North
Latitude and the monsoon winds are the other major factors affecting the Indian climate. The Tropic of Cancer divides India into two most equal climatic zones namely the northern zone and the southern zone. The cause of the Indian climate is the ‘Monsoon winds’.
The south - west monsoons from the Arabian Sea and Bay of Bengal bring rainfall to the entire country. The changes in the upper air circulation over Indian land mass influence the climate of India to a great extent. Thunder storms cause upto 25 cm of rainfall along the Kerala and Karnataka coasts and about 10 cm in the interior of South India. These rains are known as 'Cherry Blossoms' in Karnataka.
El-Nino is a weather system that comes once in each 3 to 7 years that fetches drought, overflow of flood and further weather limits to different parts of the world. El-Nino is utilized in the country for forecasting lengthy range monsoon rainfall.
India receives 90% of the total rainfall from monsoons. Monsoons are the seasonal winds which blow during six months of summer from ocean to land and for the six months of winter from land to sea. They are the winds blowing out from the land-mass of north-western India towards the Indian Ocean. Throughout the summer season, the winds drive from marine to the continents so that the wet winds results in rainfall in this season. Throughout the winter, the direction of the winds is upturned so that they drive from continents towards the sea.
The South West Monsoon would forms the main monsoon season in India (June to August). The north - east monsoons are comparatively minor monsoons confined to a smaller area of the country. It brings rain mainly to Tamil Nadu. On the basis of monsoonal variations there are four seasons in India namely the Cold Weather Season (December to February), the Hot Weather Season (March to May), the Advancing Monsoon (June to September) and the Retreating Monsoon (October to November).
During the cold dry season, the mean temperature is about 24°C to 25°C in Southern India and about 10°C in the plains of the North. January is the coldest month. The weather is very fine with clear skies, enjoyable sun and light winds. However these fine weather conditions at intervals get disturbed by shallow cyclonic depressions known as western disturbances. They originate over the east Mediterranean Sea and travel eastwards across Iran and Pakistan before entering north western part of India. The rainfall caused by these winds decreases towards the east and the south. These winds cause light rains in the northern plains especially in Punjab. This is ideal for the cultivation of rabi crops. These winds cause heavy snowfall in the Himalayas.
With the northward apparent movement of the sun the intense heat shifts from the south to the north. During May the atmospheric temperature rises to about 48°C in north western India. Towards the end of May a low pressure comes into being, extending from the Thar Desert to the Chotanagpur plateau.
This low pressure brings hot, dry winds known as Loo in Punjab, Haryana, East Rajasthan and Uttar Pradesh. In Kerala and Coastal Karnataka pre - monsoon showers set in towards the end of summer. These are known as mango showers. Violent thunderstorms bring rain in Assam and Bengal. These disastrous storm are known as Kalbaisakhi or Norwesters.
India gets around 60% of its entire rainfall during the south west monsoon season. The South west moonsoon appears to enter the Indian subcontinent in two branches, the Arabian Sea branch and the Bay of Bengal branch. The Arabian Sea causes wide rainfall in the western as well as central states. The Bay of Bengal branch gives rainfall to the north eastern states and eastern coastal plains. The west coast of Peninsular India receives heavy rainfall because the on-shore winds are obstructed by the Western Ghats. The Bay of Bengal branch enters West Bengal and Bangladesh from the south and the south - east. Beyond Bengal this branch sub-divides into two, one branch enters the Brahmaputra valley and causes heavy rainfall in the north and north - eastern regions.
The Khasi - Jaintia hills of Meghalaya obstruct these winds and cause heavy rainfall in these regions. Cherrapunji and Mawsynram are in this region. The other branch moves towards the North West and through the Ganga plains it reaches Punjab Haryana plains and joins the Arabian Sea branch.
The retreat of the monsoons is marked by clear skies and rise in temperature. The reason of this retreat is that the monsoon trough of low pressure becomes weaker and is gradually replaced by high pressure. The land is still moist. The combination of high temperature and humidity gives rise to an oppressive weather known as October heat. The low pressure environment is moved to the Bay of Bengal by early November. This shift of the low pressure areas marked by Cyclonic depressions originates over the Andaman Sea and cross the eastern coast of India Peninsula resulting in heavy and wide spread rain on the coast of Tamil Nadu and Sri Lanka.