34 Southwest M<strong>on</strong>so<strong>on</strong>Ramakrishnan, K.P. andGopinatha Rao, B.1958 Some aspects of the n<strong>on</strong>–depressi<strong>on</strong> rain in peninsularIndia during the southwest m<strong>on</strong>so<strong>on</strong>, M<strong>on</strong>so<strong>on</strong>s of theWorld, pp. 195–208.Ramdas, L.A. 1958 The establishment, fluctuati<strong>on</strong>s and retreat of the southwestm<strong>on</strong>so<strong>on</strong> of India, M<strong>on</strong>so<strong>on</strong>s of the World, pp. 251–256.Rangarajan, S. and Mokashi,R.Y,Rao, K. N., George, C.J. andAbhyankar, V.P.1966 Some aspects of the statistical distributi<strong>on</strong> of upper windsover India, Indian J. <str<strong>on</strong>g>Met</str<strong>on</strong>g>. Geophys., 17, pp.. 25–28.1972 Nature of the frequency distributi<strong>on</strong> of Indian rainfall;M<strong>on</strong>so<strong>on</strong> and annual, Indian J. <str<strong>on</strong>g>Met</str<strong>on</strong>g>. Geophys. , 23, pp. 507–514.Rao, Y.P, 1962 Meridi<strong>on</strong>al circulati<strong>on</strong> associated with the m<strong>on</strong>so<strong>on</strong>s ofIndia, Indian J. <str<strong>on</strong>g>Met</str<strong>on</strong>g>. Geophys., 13, pp. 157–166.Tucker, G.B. 1959 Mean meridi<strong>on</strong>al circulati<strong>on</strong> in the atmosphere, Quart. J.R.<str<strong>on</strong>g>Met</str<strong>on</strong>g>. Soc, 85, pp. 209–224.
CHAPTER 3ONSET AND WITHDRAWAL OF THE MONSOON3.1 Perceptible changes <strong>on</strong> account of the m<strong>on</strong>so<strong>on</strong>, are (i) winds from about southwest,(ii) decrease in temperature from the heat of April and May and (iii) increase in rainfall. The first ismainly of interest to mariners, while other two have more general impact. Dates of <strong>on</strong>set can befixed by the changes in any of these features for any year or from climatological means of theseelements for a sufficiently l<strong>on</strong>g period. Development of these three features or more rec<strong>on</strong>diteaspects like air mass, meridi<strong>on</strong>al circulati<strong>on</strong> or easterly jet stream are not simultaneous. Westerliesset in the Arabian Sea in May but the rains <strong>on</strong>ly the next m<strong>on</strong>th. The India <str<strong>on</strong>g>Met</str<strong>on</strong>g>eorologicalDepartment has fixed the dates of <strong>on</strong>set and withdrawal with reference to the rather sharp increaseand decrease respectively shown by the five–day means of rainfall and the changes in thecirculati<strong>on</strong>. The appearance of m<strong>on</strong>so<strong>on</strong> synoptic features far ahead of the normal date is referredto as "temporary advance", if it is not maintained. The m<strong>on</strong>so<strong>on</strong> rains are some times not easy todistinguish from prem<strong>on</strong>so<strong>on</strong> thundershowers. C<strong>on</strong>sequently, the dates of <strong>on</strong>set and withdrawalof m<strong>on</strong>so<strong>on</strong> cannot be fixed uniquely.3.2 During the course of October there is <strong>on</strong>ce again a change from the circulati<strong>on</strong> patternof the southwest m<strong>on</strong>so<strong>on</strong> over the Indian area but the rains increase over south India. This changein circulati<strong>on</strong> should be rightly regarded as the end of the southwest m<strong>on</strong>so<strong>on</strong> and it is notappropriate to identify the much later decrease of rainfall in south India as the withdrawal of thesouthwest m<strong>on</strong>so<strong>on</strong>. The displacement of the m<strong>on</strong>so<strong>on</strong> air by c<strong>on</strong>tinental air mass anddevelopment of anti–cycl<strong>on</strong>ic flow would determine the dates of withdrawal over north and centralIndia.3.3 In the interior of the Peninsula, <strong>on</strong>set of the m<strong>on</strong>so<strong>on</strong> may not be seen immediately asa striking increase of rain. Nevertheless, the synoptic practice of fixing the date of <strong>on</strong>set ofm<strong>on</strong>so<strong>on</strong> in any year in different areas has a physical basis. The development of the circulati<strong>on</strong>pattern and invasi<strong>on</strong> of air mass are basic criteria and these create c<strong>on</strong>diti<strong>on</strong>s favourable foroccurrence of more frequent and increased amounts of rain. Whatever may be the uncertainty inforecasting m<strong>on</strong>so<strong>on</strong> advance, actual <strong>on</strong>set can be generally fixed within acceptable limits.3.4 Figure 3.1 shows the normal dates of the <strong>on</strong>set and withdrawal of the m<strong>on</strong>so<strong>on</strong> asdetermined from rainfall increase and decrease and other synoptic features. Figure 3.2 shows thefive–day rainfall figures at some stati<strong>on</strong>s to illustrate increase of rainfall at the <strong>on</strong>set. Both thedates of <strong>on</strong>set and withdrawal vary from year to year and may occur even outside the period ofJune to September, usually regarded as the southwest m<strong>on</strong>so<strong>on</strong> period. According to Ramdas,Jagannathan and Gopal Rao (1954), the standard deviati<strong>on</strong> of the dates of establishment ofm<strong>on</strong>so<strong>on</strong> al<strong>on</strong>g the west coast south of 20° N is 6 to 7 days. The earliest date was 17–22 daysbefore, while the most delayed arrival was 10–13 days after the normal date. Their criteria, beingslightly different from synoptic practice, resulted in earlier dates being fixed for the establishmentof m<strong>on</strong>so<strong>on</strong>. Bhullar (1952) gives the standard deviati<strong>on</strong> for the advance into Delhi as 7–8 daysfrom the mean date of 2nd July which he determined from the increase of rains at a number ofraingauges around. To illustrate the variability of the advance of m<strong>on</strong>so<strong>on</strong>, a histogram giving thedates of <strong>on</strong>set over Bombay from 1879 to 1975 is given in Fig. 3.3. The extreme dates of <strong>on</strong>set ofm<strong>on</strong>so<strong>on</strong> for some parts of the country are given in Table 3.1.
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CHAPTER 10OROGRAPHIC EFFECTS10.1 Or
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