The Indian summer monsoon typically lasts from June-September, with large areas of western and central India receiving more than 90% of their total annual precipitation during the period, and southern and northwestern India receiving 50%-75% of their total annual rainfall. Overall, monthly totals average 200-300 mm over the country as a whole, with the largest values observed during the heart of the monsoon season in July and August.
At the Equator the area near India is unique in that dominant or frequent westerly winds occur at the surface almost constantly throughout the year; the surface easterlies reach only to latitudes near 20° N in February, and even then they have a very strong northerly component. They soon retreat northward, and drastic changes take place in the upper-air circulation. This is a time of transition between the end of one monsoon and the beginning of the next. Late in March the high-sun reaches the Equator and moves farther north. With it go atmospheric instability, convectional (that is, rising and turbulent) clouds, and rain. The westerly subtropical jet stream still controls the flow of air across northern India, and the surface winds are north easterlies.
five special conditions- 1) The Sea Surface Temperature (SST) gradient between the north Atlantic and the north Pacific (conditions during December of the previous year and January of present year)
2) SST of the equatorial south Indian Ocean (conditions during February and March of the present year)
3) Mean Sea Level pressure in east Asia (conditions during February and March of the present year)
4) The surface air temperature over northwest Europe (conditions during January of the present year)
5) The warm water volume of the equatorial Pacific Ocean (conditions during February and March of the present year).
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