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30 INDIA : PHYSICAL ENVIRONMENT Inter Tropical Convergence Zone (ITCZ) Inter Tropical Convergence Zone (ITCZ) The Inter Tropical Convergence Zone (ITCZ) is a low pressure zone located at the equator where trade winds converge, and s , it is a zone where air tends to as end. In July, ther ITCZ is located around 20°°N-25°°N lati udes ( ver the Gangetic plain), sometimes called the monsoon rough. This monsoon trough encourages the developmentain), sof thermal lowd over north and northwest India. Due to the shift of ITCZ, the trade winds of the southern hemisphere cross the equator between 40 °° and 60°°E longitudes and start blowing fromhern hemisphere cross the equator between 40° and 60°E longitudes and start blowing from ssouthwest to northeast due to the Coriolis force. It becomes southwest monsoon. In winter, tthe ITCZ moves southward, and so the reversal of winds from northeast to south and ssouthwest, takes place. They are called northeast monsoons. of Western Ghats and Assam receive high gets intensely heated. This causes the rainfall during June-September whereas the formation of an intense low pressure in the southern plateau remains dry due to its northwestern part of the subcontinent. Since leeward situation along the Western Ghats. the pressure in the Indian Ocean in the south of the landmass is high as water gets heated T HE N ATURE OF INDIAN M ONSOON slowly, the low pressure cell attracts the Monsoon is a familiar though a little known southeast trades across the Equator. These conditions help in the northward shift in the climaticphenomenon.Despitetheobservations position of the ITCZ. The southwest monsoon spread over centuries, the monsoon continues to puzzle the scientists. Many attempts have may thus, be seen as a continuation of the been made to discover the exact nature and southeast trades deflected towards the Indian subcontinentaftercrossing the Equator. These causation of monsoon, but so far, no single winds cross the Equator between 40°°E and theory has been able to explain the monsoon fully. A real breakthrough has come recently 60°°E longitudes. when it was studied at the global rather than at regional level. Systematic studies of the causes of rainfall in the South Asian region help to understand thecausesandsalientfeaturesofthemonsoon, particularly some of its important aspects, such as: (i) The onset of the monsoon. (ii) Break in the monsoon. Onset of the Monsoon Towards the end of the nineteenth century, it was believed that the differential heating of land and sea during the summer months is the mechanism which sets the stage for the monsoon winds to drift towards the subcontinent. During April and May when the sun shines vertically over the Tropic of Cancer, thelargelandmassinthenorth ofIndianocean Figure 4.1 : Onset of Monsoon 2026-27