NCERT Class 9 Solutions: Climate (Contemporary India-I) Chapter 4– Part 5
Q 3. Why does the rainfall decrease from the east to the west in Northern India?
The Bay of Bengal is the main branch of the monsoon winds which moves from northeast and then hits Himalayas to return westwards by covering the northern plains.
As the winds move westward, their moisture contains tends to reduce as they cause rains along Indian subcontinent. So that the rainfall decreases from east to west in northern India.
The Arabian Sea branch does not contribute much and exhaust most of its moisture when it hit Western Ghats.
Following Image show the annual rainfall in India.
Q 4. Give reasons as to why.
(i) Seasonal reversal of wind direction takes place over the Indian subcontinent?
Wind Directions: Monsoon and Easterlies
The prevailing wind direction in India is north east due to north-easterly winds which blow from sub-tropical high pressure belt of northern hemisphere. The monsoon, causes the seasonal reversal in wind direction so that the windows now flow from the seas and oceans in the south towards the land mass in north. Such a reversal due to monsoons causes most of the rainfall received in India.
The Intertropical Convergence Zone or ITCZ
The key for understanding the monsoon and wind reversal is to look closely at the difference between annual temperature trends over land and sea. The position of maximum solar insolence oscillates from the Tropic of Cancer to the Tropic of Capricorn.
The northeast and southeast trade winds (easterlies) converge in this low pressure zone, which is also known as the Intertropical Convergence Zone or ITCZ. However the exact latitude of maximum low pressure region created by solar insolence changes its position depending on time of year. When the ITCZ shifts towards East Asia, sub-Saharan Africa, Australia and the southern parts of North America, there is rainfall in these regions.
During Indian summers, the Thar Desert and adjoining areas of the northern and central parts of the Indian subcontinent heat up. Due to high solar insolence and the heating of Earth’s surface the air along the surface gets warm and rises up creating a low pressure over Indian Subcontinent. This low pressure causes continuous movement of moist wind from seas and oceans in the south to the land masses. This wind moves from seas in the south to the landmass in the north thus reversing the direction
The bulk of rainfall in India is concentrated over a few months.
The monsoon begins from the first week of June and it covers almost whole country by mid-July so the months of June - August get the bulk of rainfall in India.