I. By early June, the low pressure condition over Northern Plains intensifies.ii. It is powerful enough to attract the trade winds of the Southern Hemisphere.a. These south-east trade winds originate over the warm subtropical areas of the southern oceans.b. They cross the equator and blow in a south-westerly direction entering the Indian Peninsula as the South-west Monsoon.c. As these winds blow over warm oceans, they bring abundant moisture to the subcontinent.d. These winds are strong and blow at an average speed of 30 km per hour.e. With the exception of the extreme north-west, the monsoon winds cover the country in about a month.iii. The inflow of the South-west Monsoon into India brings about a total change in the weather.a. Early in the season, the windward side of the Western Ghats receives very heavy rainfall (more than 250 cm).b. The Deccan Plateau and parts of Madhya Pradesh also receive some amount of rain in spite of lying in the rain shadow area of the Western Ghats.c. Maximum rainfall of this season is received in the north-eastern part of the country.1) Cherrapunji and Mawsynram in the southern ranges of the Khasi Hills receive the highest average rainfall in the world (maximum-recorded rainfall 1080 mm).2) Rainfall in the Ganga valley decreases from the east to the west.3) Rajasthan and £arts of Gujarat get scanty rainfall.iv. Another characteristic feature associated with the monsoon is its tendency to have 'breaks' in rainfall. In other words it has wet and dry spells.a. Monsoon rains take place only for a few days at a time.b. They are interspersed with rainless intervals.v. These breaks in monsoon are related to the movement of the monsoon trough.For various reasons, the trough and its axis keep on moving northward or southward, which determines the spatial distribution of rainfall.a. When the axis of the monsoon trough lies over the plains, rainfall is good in these parts.b. On the other hand, when the axis shifts closer to the Himalayas, there are longer dry spells in the plains,(a) Widespread rains occur in the mountainous catchment areas of the Himalayan Rivers.(b) These heavy rains bring devastating floods.(c) They cause damage to life and property in the plains.vi. The frequency and intensity of tropical depressions too, determine the amount and duration of monsoon rains.a. These depressions form at the head of the Bay of Bengal and cross over to the mainland.b. The depressions follow the axis of the "monsoon trough of low pressure".vii. Monsoon is known for its uncertainties and vagaries.a. The alternation of dry and wet spells varies in intensity frequency and in duration.b. It causes heavy floods in one part, while it is responsible for droughts in the other.c. It is often irregular in its arrival and its retreat.d. These uncertainties disturb the entire farming schedule of crores of farmers all over India.