8 Southwest M<strong>on</strong>so<strong>on</strong>Fig. 2.5
<str<strong>on</strong>g>Met</str<strong>on</strong>g>eorological <str<strong>on</strong>g>M<strong>on</strong>ograph</str<strong>on</strong>g> : Synoptic <str<strong>on</strong>g>Met</str<strong>on</strong>g>eorology No. 1/1976 9In the north Indian plains, a minimum rainfall belt runs from northwest Rajasthan to thecentral parts of West Bengal, practically al<strong>on</strong>g the axis of the m<strong>on</strong>so<strong>on</strong> trough. Rainfall decreasesgenerally from the hills of the western and eastern ghats towards the coast.2.5.2 Rainfall decreases very rapidly southwards al<strong>on</strong>g west coast from 9.5 o N toKanyakumari. The rainfall at Kanyakumari in this seas<strong>on</strong> is about the same as in the Great IndianDesert. To the east of the western ghats between 8 o N and 10 o N, rainfall decreases c<strong>on</strong>siderablywith a very steep gradient across the eastern slopes. Rainfall is <strong>on</strong>ly 2 cm in some places in thecoastal strip in extreme south Tamil Nadu. With all the significant amounts of rainfall occurringover the ghats, a saving feature of ec<strong>on</strong>omic interest is that all the important rivers of south Indiaemerge out of the western ghats to flow east through the plains having rainfall of the order of thatin west Rajasthan .2.5.3 Hills and mountain ranges cause striking variati<strong>on</strong>s in rainfall distributi<strong>on</strong>. On thesouthern slopes of the Khasi–Jaintia hills rainfall is over 800 cm while to the north, in theBrahmaputra valley, it drops to about 120 cm. Cherrapunji ’ s annual rainfall of 1142 cm (atelevati<strong>on</strong> of 1313 m) is obviously due to orographic lifting but its magnitude requires to bequantitatively explained. From the west coast, rainfall increases al<strong>on</strong>g the slopes of the westernghats and rapidly decreases <strong>on</strong> the eastern lee side. No definite informati<strong>on</strong> is available about theincrease of rainfall with elevati<strong>on</strong> and the height at which the rainfall attains the highest value. Inthe higher reaches of the western ghats, there are places with seas<strong>on</strong>al rainfall of 500 cm. Within80 km <strong>on</strong> the lee side, rainfall is <strong>on</strong>ly 40 cm.2.5.4 From the coast of West Bengal and the hills of Orissa, rainfall decreases inland.Further westwards, the Chota Nagpur hills, the Maikala Range and the Mahadeo hills cause anincrease of rainfall, with lesser amounts in the valleys in between. The Gir hills in Kathiawar havemore rainfall than the neighbourhood. Mount Abu in Aravallis has a rainfall of 169 cm while thesurrounding plains have <strong>on</strong>ly 60 to 80 cm .2.5.5 Across northern India, a line of rainfall minimum runs from 28.5 o N, 75 o E to 25 o N, 88 oE which is paradoxically close to the m<strong>on</strong>so<strong>on</strong> trough. Area to south of this rainfall minimumfalls in the track of m<strong>on</strong>so<strong>on</strong> depressi<strong>on</strong>s which are resp<strong>on</strong>sible for much of the rainfall. In tractsfurther to north, there is probably the influence of the Himalayas in increasing the rainfall. Apartfrom this, there is also a decrease of rainfall from east to west, from about 120 cm in West Bengalto less than 20 cm in the Great Indian Desert in west Rajasthan.2.5.6 In the Himalayas, observati<strong>on</strong>s are extremely scanty, particularly from higherelevati<strong>on</strong>s where there is added difficulty of measuring snowfall. Rainfall measured in rivervalleys may not be representative of the hill slopes. Between the Great Himalayan Range and theplains, there are the Pir Panjal, the Siwalik and the Mahabharat Ranges. Most of the availableobservati<strong>on</strong>s are from these ranges. Rainfall increases up the slopes of these foot hills, presumablydecreases <strong>on</strong> their northern slopes and increases again <strong>on</strong> the Himalayan slopes. Annual rainfall atChaunrikharka (2,700 metres) is 228 cm and at Namche Bazar (3,300 metres) <strong>on</strong>ly 94 cm (Dharand Narayanan, 1965). Both are in Nepal and the distance between the two is hardly 16 km.Therefore, we can tentatively c<strong>on</strong>clude that above some elevati<strong>on</strong> near 3 km, rainfall maydecrease with height <strong>on</strong> the Himalayan Range. In the eastern Himalayas, rainfall is more than inthe western porti<strong>on</strong>s. In the east, annual rainfall of 400 cm has been recorded but less than 200 cmin the west.2.5.7 Rainfall in the Andaman and Nicobar Islands in the southwest m<strong>on</strong>so<strong>on</strong> seas<strong>on</strong> isabout 140 to 190 cm, while in Laccadives (Minicoy and Aminidivi Islands) and Maldives in theArabian Sea, it is <strong>on</strong>ly about 100 cm though both the groups are in the same latitude belt. Calicut<strong>on</strong> the mainland in the west coast, however, gets 235 cm, more than the Bay Islands.
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