400 



NA TURE 



August 25, 1904 



is also the goal of the lower air movement of the south-east 

 trades circulation at this time. The equatorial belt of 

 calms is hence the termination of the lower air movement 

 of the south-east trades and north-east monsoon. It is 

 chiefly an area of uptake, and of outflow northwards and 

 southwards, to replace the lower air inflow from the distant 

 south and north. The influ.x to the Indian land area occurs 

 chiefly or entirely in the upper and (perhaps) middle atmo- 

 sphere. There is also, as indicated by the wind directions 

 in the lower Assam and Burma hills, an influx from the 

 adjacent seas in the upper portion of the lower atmosphere.' 

 The diurnal land and sea breezes alternate with great 

 regularity on the west coast south of Gujarat during this 

 period, but probably do not contribute to the general upper 

 influx compensating in part or whole the lower outflow. 



The circulation over the Indo-oceanic region hence consists 

 at this time of two semi-independent circulations, with a 

 common sink or goal for the lower air movement, which 

 shifts with the season and with the relative strengths of 

 the two movements. It is hence probable that they react 

 on each other to some e.xtent, and possible that general 

 abnormal actions may affect the two similarly. 



The normal weather during the period is similar to that 

 which obtains in anticyclonic periods during the summer 

 in Central Europe — viz., the prevalence of light winds, with 

 clear or lightly clouded skies, low humidity, moderate 

 temperature, and large diurnal range of temperature, with 

 a bracing, exhilarating atmosphere. 



It is interesting to note that the air movement in India 

 itself is from opposite directions in Northern India and the 

 peninsula, with a belt of unsteady movement over the area 

 of the Vindhya and Satpura hill ranges. The variations of 

 weather conditions from the normal are as a rule inverse 

 in these two regions — viz., Extra-tropical and Tropical 

 India. 



The season of the opposite air movement is present in its 

 most complete form in July and August, and lasts from 

 the beginning or middle of June to the middle or end of 

 September. It commences as a lower air movement in an 

 anticyclonic region over the South Indian Ocean, and is 

 thence continued northwards to Abyssinia, South Arabia, 

 India, and Burma. Persia, Afghanistan, and Baluchistan 

 (where dry hot north-west winds chiefly prevail) are out- 

 side the field of this movement. The direction of the move- 

 ment is from south, with more or less easting to the south of 

 the equator, and with more or less westing to the north 

 of the equator, dependent in part upon the earth's rotation 

 and in part upon local conditions and the influence of neigh- 

 bouring land areas, and hence more effective in the Bay of 

 Bengal than in the Arabian Sea. This lower air current 

 advances over an extensive tropical oceanic region before 

 it reaches Southern Asia, and hence arrives charged with 

 vast stores of aqueous vapour, which it discharges chiefly 

 over the peninsulas of Southern Asia and the mountain 

 \egion of Abyssinia. 



The regions of rainfall indicate the areas of upward 

 •movement terminating the lower advance of the current. 

 The circulation is undoubtedly maintained in large part 

 by the release or addition of energy due to the condensation 

 of its enormous stores of aqueous vapour. The lower air 

 movement is of very considerable elevation, estimated at 

 15,000 to 20,000 feet in India. Above it is the outward 

 upper return movement, in part only compensatory, and in 

 part probably slowly filling up the Central and Southern 

 Asian low-pressure region. The movement exhibits some 

 interesting features in India, due to the fact that of the 

 three areas to which it is mainlv determined India alone is 

 subject to a double influx from two sea areas in opposite 

 directions. The current from the Arabian Sea passes east- 

 wards across the Malabar, Konkan, and North Bombay 

 coasts, the peninsula and Centr.al India. The Bengal 

 current is deflected in the north of the Bay and Bengal, and 

 advances in a westerly direction up the Gangetic plain. 

 Between the areas or fields of the two currents (roughly 

 proportional to their relative strength and importance — 

 viz., about 2 to i) is a debatable area of variable winds and 

 low pressure. This trough of low pressure varies in posi- 

 tion with the relative strengths of the two currents. The 

 1 In India the lower atmosphere maybe defined as from o to 5000 feet, 

 the middle atmosphere from 5000 to 15,000 or 20,000 feet, and the upper 

 atmosphere above 20,000 feet. 



NO. 18 I 7, VOL. 70] 



cyclonic storms of the period, which are of comparatively 

 frequent occurrence, advance along the trough. It is hence 

 a. factor of considerable importance in determining the dis- 

 tribution of the rainfall of the period. The trough is purely 

 a resultant of the peculiar conditions of the air movement, 

 and is not the cause of that movement ; in other words, it 

 is determined by it, and does not determine it. 



The transformation of the double circulation of the north- 

 east monsoon period into the single circulation of the south- 

 west monsoon over the Indo-oceanic region next requires 

 consideration. It is evident that the chief stages in this 

 change are (i) the discontinuance of the vertical movement 

 over the equatorial belt ; (2) the extension of the trade 

 winds of the south-east trades across the equatorial belt, 

 with an accompanying increase of pressure and of hori- 

 zontal air movement ; (3) the continuance of that northerly 

 movement over the Indian seas into the peninsulas of 

 Southern Asia. 



The marine data of the Indian seas collected during the 

 past fifteen years establish fully that this transformation 

 is primarily due to actions in the Indian Ocean, producing 

 a movement resembling in many respects that of a bore 

 or storm wave. The actual transition may hence be de- 

 scribed as catastrophic, due to impulsive action. 



It is preceded in India by a period of preparation (as it 

 may be termed), when pressure and other conditions are 

 slowly established in Southern Asia, which directly con- 

 tribute to the advance of the monsoon winds over the Indian 

 seas, but which in no way assist the preliminary burst 

 across the equator, the first stage towards the establish- 

 ment of the south-west monsoon circulation. 



This preliminary period is the hot-weather season, last- 

 ing from about the middle of March to the middle of June 

 (on the average in Northern India). During this period 

 temperature increases rapidly until the last week in May 

 or first week of June, when maximum day temperatures 

 ranging between 120° and 125° are usually recorded in the 

 driest and hottest interior districts of Northern and Central 

 India. Pressure decreases pari passu in the heated land 

 areas of Southern Asia, which become areas of low pressure 

 and indraught relative to the neighbouring seas. The in- 

 draught only extends to a comparatively short distance land- 

 wards and seawards from the coasts, more especially in 

 the larger sea area, the Arabian Sea, over the centre of 

 which light variable or northerly winds obtain even imme- 

 diately before the advance of the monsoon currents. In 

 the interior of Northern and in Central India exceedingly 

 dry and hot westerly winds prevail with great steadiness. 



The weather in India during this period depends almost 

 entirely upon local thermal actions and contrasts of tempera- 

 ture and humidity conditions. Skies are generally free 

 from cloud, but the air is more or less charged with dust 

 and is excessively dry (humidities of i to 5 being of 

 occasional occurrence in North-Western India). 



The characteristic features of the dry season are hence 

 most strikingly exhibited immediately before the advent of 

 the wet monsoon. There is no gradual change over the 

 greater part of India from one to the other such as would 

 occur if the furnace, or Central Asia hot area, theory were 

 correct. Over small isolated portions of India, including 

 Tenasserim, Arakan, Lower Burma, Assam, Bengal, and 

 Malabar, thunderstorms giving more or less heavy down- 

 pours occur in increasing frequencv during the period. The 

 rainfall is considerable to large in amount in these areas, 

 and is of much agricultural value in some districts — e.g., 

 in Assam for the tea crop. In those areas the transition to 

 the rainv season is much less abrupt and spasmodic, the 

 chief differences being that the rainfall in the wet season 

 is more general and frequent, larger in amount, and rarely 

 accompanies thundersfonns. 



The transformation from the hot weather to the rains 

 is usually effected between June i and 15. It commences 

 in the equatorial belt with a considerable increase of pressure 

 and air movement accompanying a strong rush of southerly 

 winds, the continuation of south-east trade winds, across 

 the equator. If the burst be sufficiently strong the rush 

 is continued northwards over the Indian seas as a wave 

 of disturbance, squally weather, heavy rain, and much 

 violent electric discharge or action, invading areas 

 characterised previously by light and variable winds and 

 fine weather. The distin-bance usually increases with its 



