The southwesterly monsoon is a prevailing wind that blows during the wet seasons of April to October. Very heavy rains are brought to India and to countries surrounding the Indian Ocean. These include Pakistan, Bangladesh, Burma, Thailand, the Arabian Peninsula (Oman, Saudi Arabia, the United Arab Republic, and Yemen) and neighboring African countries that border the Indian Ocean and the Arabian Sea. The rains are very heavy and bring the majority of the rainfall to the region each year. There are places in India that receive over 200 in. of rain per year (over 500 cm.). The warm moisture in the southwesterly monsoon falls as rain over the foothills of the Himalaya Mountains. The southwest Indian monsoon is the source of water for most agriculture in the region. If the monsoon fails, or is not strong, there will be drought with the starvation of millions.
The Tibetan Plateau affects the Indian monsoon. The vast Himalayan Mountain system has reached great heights because the India plate is elevating it. Theorists argue that the land's formation and the development of the Indian monsoon began about 8 million years ago. However, other scientists have hypothesized that an older monsoon existed that also was tied to the Tibetan plateau.
From June through September, the southwestern monsoon wind prevails in India. In the Great Indian
Thar Desert, and the areas to the north and south of it, the Indian subcontinent develops very hot temperatures during the long days of the summer months. The hot air causes lower pressure to prevail in the area. Moisture laden air flows into the Indian subcontinent from the Indian Ocean. When the warm moist air reaches the Himalayas, storm clouds form, because the great height of the Tibetan plateau blocks the moist air. The warm air rises to heights where it cools enough that rain falls in varying amounts. When the oceanic southwest monsoon reaches the Indian subcontinent, it splits into two parts. The eastern part becomes the Bay of Bengal branch; the western part becomes the Arabian Sea Branch of the Southwest Monsoon.
The Bay of Bengal branch of the Southwest Monsoon flows over the Bay of Bengal and northward toward Calcutta and then meets the eastern area of the Himalayan Mountains, where it drops huge quantities of rain over Northeast India, northern Burma, and Bangladesh. Cherrapunji, located on the slopes of the Himalayas in Shillong, is one of the wettest spots on Earth. The remaining portion of the Bengal Branch of the Southwest Monsoon then flows westward over the Grand Trunk Road in the Indo-Gangetic Plain. Great quantities of rain pour down during the height of the seasonal monsoon. The Arabian Sea Branch of the Southwest Monsoon bounces off of the Western Ghats of the Indian subcontinent. The monsoon then moves northward, producing rain along the west coast of the Indian subcontinent. The Ghats are steps, or hills that are shorter steps to the Decca plateau. The eastern Ghats are located on the Bay of Bengal side of the Indian subcontinent. They receive considerably less rain than do the western Ghats, which are on the Arabian Sea side of the Indian subcontinent.
Without the monsoon, India would be much more arid. About 80 percent of the rainfall in India is associated with the monsoon. The huge agricultural population in India is totally dependent upon the monsoon rains for growing rice and other crops. In the 1990s, a few days delay affected the future crops. On the other hand, the Bengal Bay Branch of the
Indian monsoon has regularly brought floods. The monsoon is also a great benefit to Indian city-dwellers because it brings relief from the heat and rains that wash away debris from the cities. The first day of June is considered the normal date for the beginning of the Southwest Monsoon. However, in September, the northern part of the Indian subcontinent begins to cool rapidly. The denser air now blocks the flow of the Southwest Monsoon and, instead, reverses in the direction of the retreating monsoon (Northeast Monsoon) that flows from the Indian subcontinent into the Indian Ocean.
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