Contents preface (VII) introduction 1—37

2.3.1. Characteristics of Precipitation in India
India receives more than 75% of its annual precipitation during the monsoon season (June to September). The monsoon (i.e., south-west monsoon) originates in the Indian ocean and appears in the southern part of Kerala by the end of May or the beginning of June. The monsoon winds, then, advance and cover the entire country by mid-July.The monsoon season is, however, not a period of continuous rainfall. The temporal and spatial variability of the magnitude of rainfall results into regions of droughts and floods. Relatively speaking, Assam and the north-eastern reigon are the heavy rainfall regions (with average annual rainfall ranging from 2000-4000 mm) and U.P., Haryana, Punjab, Rajasthan, and Gujarat constitute low rainfall regions (with average annual rainfall less than about 1000 mm). Western Ghats receive about 2000-3000 mm of annual rainfall. Around mid-December, the western disturbances cause moderate to heavy rain and snowfall (about 250 mm) in the Himalayas and Jammu and Kashmir and other northern regions of the country. Low pressure areas formed in the Bay of Bengal during this period cause some rainfall in the south-eastern parts of the country.
The temporal variation of annual rainfall at a given place is expressed in terms of the coefficient of variation, C_v defined as

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C =	100	× standard deviation	=	100σ m − 1	(2.1)
v		mean		P

The coefficient of variation of the annual rainfall for different places may vary between 15 (for regions of high rainfall) and 70 (for regions of scanty rainfall) with an average value of about 30.