JUST south of the Himalayas on the Shillong Plateau in the far northeast of India lies a meteorologically famous little town called Cherrapungee. It has the second highest average annual rainfall in the world its normal yearly total of 11,430mm is about 10 times that of Ireland, and is exceeded only by the 11,680mm average of Mount Waialeale in Hawaii.
Cherrapungee also holds another two long standing records between August 1860 and July 1861 it experienced 26,000mm of rain, the most ever anywhere in a 12 month period, and the 9,300mm measured in July 1861 is a global record for a calendar month.
The heavy rain at Cherrapungee comes from the summer monsoon which sets in around this time every year on the Indian subcontinent. During the winter, a large anticyclone over Siberia dominates the climate of India, and air moving clockwise around this area of high pressure is responsible for the cool, dry northeasterly winds of the so called winter monsoon.
When spring arrives, however, the sun moves north of the equator and shines straight down on southern parts of Asia the continent responds more quickly than the sea to the heat of the sun, and as the air over land expands and becomes lighter, a low pressure area develops over the region.
It is this low pressure which brings about the dramatic change. Moving anti clockwise around the low, warm moist air from the ocean streams in over India from the southwest, reversing the existing winds. This warm, humid air is forced to rise as it passes, over terrain that slopes gently upwards towards the Himalayan mountains, bringing heavy showers, thunderstorms and torrential rain to Cherrapungee and its ilk.
The rains continue intermittently until late September when, with the coming of autumn, the continental anti cyclone over Siberia reasserts itself and brings back the dry, northeasterly winds of the winter season.
This Indian summer monsoon begins in late May or early June, and delivers in the few months of its duration about 90 per cent of the sub continent's annual rainfall.
The rains are a vital life giving force, and if they are delayed or weaker than usual the rice fields are inadequately watered, agricultural production suffers and great hardships are visited on large sections of the local population.
The word "monsoon" itself comes from the Arabic mausim, meaning "season", and although the term is applied mainly to the phenomenon as it affects India and other parts of southeast Asia, similar systems exist elsewhere - in Australia, South Africa, and South America, for example - where large temperature differences develop between the oceans and the continents.