The period June to September is referred to as
the 'Southwest Monsoon' period. Southwest Monsoon period is the
principal rainy season for the Indian subcontinent. This is the
summer monsoon period where the southwest monsoon holds away
over the country. The whole country receives nearly 75% of its
rainfall during this period.
Southwest monsoon sets in over the extreme
southwestern tip of the peninsula by the end of May. The onset
of Monsoon is characterized by a sudden spurt of rainfall
activity. It progresses inland in stages and covers the
entire country by the middle of July.
It starts retrieving from the extreme northwest by the beginning
of September, progressively receding southwards. Tamil Nadu is
considered as a rain shadow region as it lies on the eastern (leaward
side) side of western ghats. The total annual rainfall is
maximum over the southernmost part of the state.