TRANSACTIONS OF SECTION A. 445 
The normal weather during the period is similar to that which obtains in anti- 
cyclonic 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 overthe 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 outside the field of this movement. 
~The direction of the movement 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 neighbouring 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 region 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 eleva- 
tion, 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 mainly determined India alone is subject to a double influx from two sea 
areas in opposite directions. The current from the Arabian Sea passes eastwards 
across the Malabar, Konkan, and North Bombay coasts, the peninsula and 
Central India. The Bengal current is deflected in the north of the Bay of 
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 1) is a debatable area of variable 
winds and low pressure. This trough of low pressure varies in position 
with the relative strengths of the two currents. The cyclonic storms of the 
period, which are of comparatively frequent occurrence, advance along the trough. 
Tt is hence a factor of considerable importance in determining the distribution of 
the rainfall of the period. The trough is purely a resultant of the peculiar con- 
ditions of the air movement, and is not in 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 
(1) 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 horizontal 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 described as catastrophic, due to 
impulsive action. 
It is preceded in India by a period of preparation (as it may be termed), when 
