<P> The Arabian Sea Branch of the Southwest Monsoon first hits the Western Ghats of the coastal state of Kerala, India, thus making the area the first state in India to receive rain from the Southwest Monsoon . This branch of the monsoon moves northwards along the Western Ghats with precipitation on coastal areas, west of the Western Ghats . The eastern areas of the Western Ghats do not receive much rain from this monsoon as the wind does not cross the Western Ghats . The Bay of Bengal Branch of Southwest Monsoon flows over the Bay of Bengal heading towards northeast India and Bengal, picking up more moisture from the Bay of Bengal . The winds arrive at the Eastern Himalayas with large amounts of rain . Mawsynram, situated on the southern slopes of the Eastern Himalayas in Shillong, India, is one of the wettest places on Earth . After the arrival at the Eastern Himalayas, the winds turns towards the west, travelling over the Indo - Gangetic Plain at a rate of roughly 1--2 weeks per state, pouring rain all along its way . June 1 is regarded as the date of onset of the monsoon in India, as indicated by the arrival of the monsoon in the southernmost state of Kerala . </P> <P> The monsoon accounts for 80% of the rainfall in India . Indian agriculture (which accounts for 25% of the GDP and employs 70% of the population) is heavily dependent on the rains, for growing crops especially like cotton, rice, oilseeds and coarse grains . A delay of a few days in the arrival of the monsoon can badly affect the economy, as evidenced in the numerous droughts in India in the 1990s . The monsoon is widely welcomed and appreciated by city - dwellers as well, for it provides relief from the climax of summer heat in June . However, the condition of the roads take a battering each year . Often houses and streets are waterlogged and the slums are flooded in spite of having a drainage system . This lack of city infrastructure coupled with changing climate patterns causes severe economical loss including damage to property and loss of lives, as evidenced in the Bombay floods of 2005 . Bangladesh and certain regions of India like Assam and West Bengal, also frequently experience heavy floods during this season . And in the recent past, areas in India that used to receive scanty rainfall throughout the year, like the Thar Desert, have surprisingly ended up receiving floods due to the prolonged monsoon season . </P> <P> The influence of the Southwest Monsoon is felt as far north as in China's Xinjiang . It is estimated that about 70% of all precipitation in the central part of the Tian Shan Mountains falls during the three summer months, when the region is under the monsoon influence; about 70% of that is directly of "cyclonic" (i.e., monsoon - driven) origin (as opposed to "local convection"). </P> <P> Around September, with the sun fast retreating south, the northern land mass of the Indian subcontinent begins to cool off rapidly . With this air pressure begins to build over northern India, the Indian Ocean and its surrounding atmosphere still holds its heat . This causes the cold wind to sweep down from the Himalayas and Indo - Gangetic Plain towards the vast spans of the Indian Ocean south of the Deccan peninsula . This is known as the Northeast Monsoon or Retreating Monsoon . </P>

Which type of air masses have the greatest effect on weather in north america