India Meteorological Department (IMD) follows the international standard of four seasons with some local adjustments: winter (December to February), summer (March to May), monsoon or rainy season (June to September), and a post-monsoon period (October and November).
Traditionally, North Indians note six seasons or Ritu, each about two months long. These are the spring season (Sanskrit: vasanta), summer (grīṣma), monsoon season (varṣā), autumn (śarada), winter (hemanta), and prevernal season (śiśira). These are based on the astronomical division of the twelve months into six parts. The ancient Hindu calendar also reflects these seasons in its arrangement of months.
The word monsoon was used by Arab traders to describe a system of seasonal reversal of winds along the shores of the Indian Ocean.
Monsoons always blow from cold to warm regions.
The southwest monsoon arrives in two branches: the Bay of Bengal branch and the Arabian Sea branch. The latter extends towards a low-pressure area over the Thar Desert and is roughly three times stronger than the Bay of Bengal branch. The monsoon typically breaks over Indian territory by around 25 May, when it lashes the Andaman and Nicobar Islands in the Bay of Bengal. It strikes the Indian mainland around 1 June near the Malabar Coast of Kerala. By 9 June, it reaches Mumbai; it appears over Delhi by 29 June. The Bay of Bengal branch, which initially tracks the Coromandal Coast northeast from Cape Comorin to Orissa, swerves to the northwest towards the Indo-Gangetic Plain. The Arabian Sea branch moves northeast towards the Himalayas. By the first week of July, the entire country experiences monsoon rain; on average, South India receives more rainfall than North India. However, Northeast India receives the most precipitation. Monsoon clouds begin retreating from North India by the end of August; it withdraws from Mumbai by 5 October and completely by Oct 15. As India further cools during September, the southwest monsoon weakens.
India's highest recorded one-day rainfall total occurred on 26 July 2005, when Mumbai received 944 mm (37 in). In recent years the Cherrapunji-Mawsynram region in Meghalaya has averaged between 9,296 and 10,820 millimetres (366 and 426 in).
Onset dates and prevailing wind currents of the southwest summer and northeast winter monsoons.
During the post-monsoon or autumn months of October to December, a different monsoon cycle, the northeast (or "retreating") monsoon, brings dry, cool, and dense air masses to large parts of India. The northeast monsoon, which begins in September, lasts through the post-monsoon seasons, and only ends in March.
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