<P> The main factor behind this shift is the high summer temperature difference between Central Asia and the Indian Ocean . This is accompanied by a seasonal excursion of the normally equatorial intertropical convergence zone (ITCZ), a low - pressure belt of highly unstable weather, northward towards India . This system intensified to its present strength as a result of the Tibetan Plateau's uplift, which accompanied the Eocene--Oligocene transition event, a major episode of global cooling and aridification which occurred 34--49 Ma . </P> <P> 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 . As India further cools during September, the southwest monsoon weakens . By the end of November, it has left the country . </P> <P> Monsoon rains impact the health of the Indian economy; as Indian agriculture employs 600 million people and comprises 20% of the national GDP, good monsoons correlate with a booming economy . Weak or failed monsoons (droughts) result in widespread agricultural losses and substantially hinder overall economic growth . Yet such rains reduce temperatures and can replenish groundwater tables, rivers . </P> <P> During the post-monsoon 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.It is called autumn . Winds spill across the Himalayas and flow to the southwest across the country, resulting in clear, sunny skies . Though the India Meteorological Department (IMD) and other sources refers to this period as a fourth ("post-monsoon") season, other sources designate only three seasons . Depending on location, this period lasts from October to November, after the southwest monsoon has peaked . Less and less precipitation falls, and vegetation begins to dry out . In most parts of India, this period marks the transition from wet to dry seasonal conditions . Average daily maximum temperatures range between 28 and 34 ° C (82 and 93 ° F). </P>

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