Monsoon is a term usually used to refer the seasonally changing rain-bearing winds, which affects the Indian subcontinent. The monsoon winds are caused by the extreme heating or cooling of the land masses, in relation to the surrounding seas.
During the northern summer, the Indian land mass is relatively warmer than the Indian Ocean. This results in the onset of the southwesterly monsoon winds, which brings moisture from the Indian Ocean and pours it over the land. Once the rain lashes over the land, it cools but the latent heat release into the atmosphere keeps the meridional land-sea thermal contrast functional. The summer monsoon during June-September provides 80-90% of the total rainfall over the Indian subcontinent.