plain. Along with this, the Tibetan plateau gets intensely heated and causes strong vertical air currents and the formation of low pressure over the plateau at above 9 kms altitude.

They then flow as the southwest monsoon. The Indian peninsula divides them into two branches - the Arabian Sea branch and the Bay of Bengal branch. The Bay of Bengal branch strikes the Bengal coast and the southern face of the Shillong plateau. Then, it gets deflected and flows westward along the Gangetic valley. The Arabian Sea branch arrives at the west coast of India and moves north ward. Both the branches reach India by the beginning of June, which is known as the ‘onset of monsoon’. They gradually spread over the entire country in four to five weeks. The bulk of annual rainfall in India is received from south-west monsoon. The amount of rainfall is very high along the west coast due to the Western ghats, and in north- east India due to the high peaked hills. Tamil Nadu coast (Coramandel), however, remains mostly dry during this season as it is in the rain shadow area of the Arabian Sea branch and is parallel to the Bay of Bengal branch.

October - November is a period of transition from hot wet conditions to dry winter conditions. The retreat of the monsoons is marked by clear skies and rise in temperature. The land is still moist. Owing to the conditions of high temperature and humidity, the weather becomes rather oppressive. This is commonly known as “October heat”.

The low pressure conditions which once prevailed over north-western India move far South to the centre of the Bay of Bengal by early November. During this period, cyclonic depressions are common which originate over the Andaman area. These tropical cyclones are often very destructive. The thickly populated deltas of the Godavari, Krishna and Cauveri are their targets. No year ever goes disaster free. Occasionally, these tropical cyclones visit Sundarbans and Bangladesh too. Bulk of the rainfall of the Coromandel Coast is derived from depressions and cyclones.