A persistent south-westerly wind is driven by a warm air mass and this results in low pressure forming over Southern Asia as it is warmed - centred above the Ganges Valley in Northern India. Air from the high pressure zone over the Indian and Western Tropical Pacific Ocean travels northwards toward the low pressure over the land, bringing torrential rain
Why?
The overhead sun shifts towards the tropic of cancer, pulling the ITCZ north with it (up to 30dN). The intense heating of the region of Northern India, Pakistan and Central Asia produces a large area of low pressure. The monsoon climate tends to have its highest temperature just before the rainy period (as this is when the intense heating occurs so is where moisture is picked up - when it condenses into clouds, radiation is absorbed/reflected). Once the season starts, clouds block incoming solar radiation to reduce monthly temepratures. As this warm air rises, it draws warm moist equatorial and tropical maritime air from the Indian Ocean. As the air crosses the geographical equator, it is diverted to the North East due to the Coriolis effect; they become south westerly winds.
As the winds pass the western Ghats, bordering the Arabian sea, and the Himalayas, it delivers a substantial level of precipitation - Cherrapunji recieves 13,000mm of rain in these four months. This orographic and conventional uplift is exacerbated by latent heat released by condensation which creates more instability and uplift
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