NCERT Class 9 Solutions: Climate (Contemporary India-I) Chapter 4– Part 3
(ii) Why does India have a monsoon type of climate?
Wind Directions: Monsoon and Easterlies
The prevailing wind direction in India is 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 cause continuous movement of moist wind from seas and oceans in the south to the land masses.
Branches of Monsoons and Timing
Like we saw above, in April and May (onset of summer), a low-pressure zone develops over the Indian subcontinent. First the moisture-laden winds from the Indian Ocean rush in causing monsoon rains, which start in end of May or the first week of June. The met-department declares the onset of monsoon over Kerala if 60% of the 14 enlisted stations falling in the southern states report a rainfall of 2.5mm or more for any two straight days after May 10. Beyond Kerala, the monsoon gets divided into two parts: the Arabian Sea branch and the Bay of Bengal branch. By the first week of July, monsoon winds reach almost all over India.
Monsoons and the Orography (Mountains)
Once the moisture laden winds start travelling over India, they rise and cool down causing precipitation over most of India.
(iii) Which part of India does experience the highest diurnal range of temperature and why?
Northwestern part of India experiences the highest diurnal range of temperature because of the Thar Desert and also because this region does not have the controlling influence of the ocean.
(iv) Which winds account for rainfall along the Malabar Coast?
Monsoon winds from the southwest (the Arabian Sea branch) is mainly responsible for the rainfall along the Malabar Coast.