The regularity of the onset of the rainy seasons is a marked feature of the monsoon. The precipitation is caused by the confrontation of differing air masses, convection, and orographic effects. The summer period is characterised by moist southwesterly winds, causing the transport of warm, moist air from the oceans to the subcontinent. Quantities of rain vary but intensities are high. The geographical distribution of annual average precipitation is unequally distributed over the earth.
Heavy precipitation is observed over large areas of the tropical continents.
Predominantly convectional rain occurs in this area around the equator.