NCERT Class 9 Solutions: Climate (Contemporary India-I) Chapter 4– Part 6
(iii) The Tamil Nadu coast receives winter rainfall.
Around September, with the sun moving towards the southern hemisphere, the northern land mass of the Indian subcontinent begins to cool. With this air pressure begins to build over northern India, however a little south the Indian Ocean and its surrounding is still warm. Therefore, the cold wind from high pressure region in Himalayas and Indo-Gangetic Plain is forced towards the Indian Ocean. This is known as the Northeast Monsoon or Retreating Monsoon.
When these winds move south, the dry cold wind picks up moisture from the Bay of Bengal. This moisture when it reaches over peninsular India is forced up due to Eastern Ghats and causes rainfall. Cities like Chennai, which get less rain from the Southwest Monsoon, receive rain from this Monsoon. About 50% to 60% of the rain received by the state of Tamil Nadu is from the Northeast Monsoon.
(iv) The delta region of the eastern coast is frequently struck by cyclones.
Formation of tropical cyclones
Tropical cyclones form over warm ocean waters near the equator deriving their energy from warm, moist air as fuel. The warm, moist air over the ocean rises upward from near the surface. As the air moves up there is a low pressure area on the surface, the surrounding areas with higher air pressure pushes in to the low pressure area. Meanwhile the air which rises cools down and the moisture releases its latent heat of condensation providing more fuel to the rising air. As the warmed, moist air rises and cools off, the water in the air forms clouds. The whole system of clouds and wind spins and grows, fed by the ocean's heat and water evaporating from the surface.
Storms that form north of the equator spin counterclockwise. Storms south of the equator spin clockwise. This difference is because of (Coriolis Effect) Earth's rotation on its axis.
As the storm system rotates faster and faster, an eye forms in the center. It is very calm and clear in the eye, with very low air pressure. Higher pressure air from above flows down into the eye.
Tropical cyclones usually weaken when they hit land, because they are no longer being "fed" by the energy from the warm ocean waters. However, they often move far inland, dumping many inches of rain and causing lots of wind damage before they die out completely.
In the Bay of Bengal, conditions of warm waters exist throughout the year. In the North Indian basin, storms are most common from April to December, with peaks in May and November.
Arabian Sea gets Fewer Tropical Cyclones than Bay of Bengal
The same solar insolence is received by Arabian Sea as Bay of Bengal, however Bay of Bengal has more river drainage and hence has less surface salinity. Since fresh water evaporates faster than saline water, Bay of Bengal has more moisture content to feed tropical cyclone.
Also any cyclones which form in the Arabian Sea and strike the west coast are weaker and lose intensity on striking mighty Western Ghats. The cyclones which form in the Bay of Bengal are not only stronger but are able to move further inland causing severe damage to property and life.
(v) Parts of Rajasthan, Gujarat and the leeward side of the Western Ghats are drought-prone.
Parts of Rajasthan and Gujarat Are Drought Prone
These western parts of India cannot get rainfall from monsoon winds directly arriving from Bay of Bengal. The retreating monsoon winds have already lost much of their moisture by the time they get to these areas. Additionally the height of the Aravalli Range is not high enough to block the clouds and cause rain.
Leeward Side of Western Ghats is Drought Prone
The Arabian Sea branch of the southwest monsoons is divided into three distinct streams on arriving in the mainland of India.
The first stream strikes the west coast of India perpendicular to Western Ghats and gives extremely heavy rainfall on the windward side [400 to 500 cm]. The leeward side however remains dry [30-50 cm]. There is a narrow belt of marked aridity on the immediate leeward side of the Western Ghats. But once it is passed, the air starts rising again and the amount of rainfall increases further east.
The second stream enters Narmada—Tapi troughs (narrow rift valley) and reaches central India.
The third stream moves parallel to the Aravalli Range without causing much rainfall. Consequently the whole of Rajasthan is a desert area.