The monsoons in India form two branches: the first, the southwest monsoon, sweeps from the Arabian Sea and drenches the Malabar coast of western India and then sweeps down towards Sri Lanka. The second, the southeast, moves northward around the same time from the Bay of Bengal and drenches Bangladesh and eastern Indian and then curves off towards the northwestern part of the country.
The southwest monsoon blows in from sea to land and usually breaks on the west coast early in June and reaches most of South Asia by the first week in July. Because of the critical importance of monsoon rainfall to agricultural production, predictions of the monsoon’s arrival date are eagerly watched by government planners and agronomists who need to determine the optimal dates for plantings.
With onset of summer in India in April, the land heats up more rapidly than the ocean and the monsoon winds begin to blow from west to east from the Arabian Sea inland across the Indian subcontinent. At the beginning of the summer monsoon “a low pressure area forms as heated air above the land expands, and rises and warm ocean air moves in to take its place. Passing over the hills and highland, the ocean winds then drop their moisture as torrential summer rains.” The rain falls from June to September. The wind blow from west to east from April or May to September.
During the southeast monsoon (the second or autumn monsoon) the situation is reversed. As the land cools down more rapidly than the sea, a low-pressure area develops over the ocean from October to December, and the dry monsoon winds blow steadily seaward from east to west.
In ancient times and even today dhows steered by Arab, Persian and Indian mariners utilized the monsoon winds to travel across the Arabian Sea between the Middle East, the Persian Gulf and India. Sinbad the sailor is said to have reached China from Arabia by riding the monsoons and Islam reached India as well as Malaysia and Indian on the same winds.