no 



NATURE 



[January 26, 1922 



culminating in unstable conditions productive of 

 violent thunderstorms ; (3) a, wet season, June to 

 September, dominated by the south-west mon- 

 soon, a powerful current bringing heavy rainfall 

 everywhere in India except the north-west corner 

 and those parts situated on the lee side of the 

 mountain ranges; (4) a short transition period 

 embracing the month of October, during which 

 the south-west monsoon is retreating with belated 

 rains on the Madras coast. 



Now, in order to strike at the root of the 



Fig. 2. — Average wind and pressure distribution around India in July. Pressure in inche 



prevalent misconception that the south-west mon- 

 soon current is due essentially to the heated 

 surface of India itself. Dr. Simpson points to the 

 outstanding seasonal anomaly in the climate of 

 India. The anomaly in question is the fact that 

 the hottest month of the year in India as a whole 

 is not July, but May, coming, that is to say, just 

 before the high solstice, instead of just after, as 

 in England and most countries. The mean day 

 and night temperature for the whole of India in 

 NO. 2726, VOL. 109] 



May is 88-7° F., with a large part of the northern 

 central region more than 90°, whereas in July the 

 mean is only 835°, with the area more than 90^ 

 relegated to the Thar Desert in the north-west. 



Clearly, in all the more northern portions of 

 India which lie away from the nearly non-seasonal 

 equatorial regimen controlling the climate of 

 Ceylon and the extreme south of the peninsula, 

 the temperature ought to continue rising until 

 July, and the fact that after May it appreciably 

 declines is evidently the result of the cutting off 

 of sunshine by the dense canopy 

 of cloud and rain rolled in by the 

 south - west monsoon. Why, 

 then, does not the south-west 

 monsoon burst in May? Be- 

 cause in that month the summer 

 low-pressure system to the north- 

 west of India is not in a suffi- 

 ciently advanced stage of develop- 

 ment. It is not iintil June 

 that this low-pressure area and, 

 contemporaneously, the high- 

 pressure area in the South Indian 

 Ocean become pronounced 

 enough to induce the south-east 

 trade wind to cross the equator,, 

 thereby to become deflected by 

 the rotation of the earth into the 

 current which feeds the south- 

 west monsoon. The difference is 

 illustrated in Figs, i and 2, which 

 show the average distribution of 

 wind and pressure over a large 

 area surrounding India in May 

 and July. The difference between 

 the two maps will be brought out 

 more fully in relation to the mon- 

 soon rainfall. Meanwhile, let 

 there be noted what is exhibited 

 with much greater distinctness in 

 maps ^ of wind and pressure for 

 India alone, that in both months, 

 but more conspicuously in May, 

 the isobars, with corresponding 

 deflection of the wind arrows, 

 bend southward in crossing the 

 Indian land-mass — away, that is 

 to say, from the centre of low 

 pressure in the north — signifying 

 that there actually is some in- 

 draught due to India itself,, 

 though it is only a superposed 

 secondary feature, giving the iso- 

 bars their precise trend — a local modification of 

 the general Asiatic circulation. 



Now to explain the great meteorological char- 

 acteristic of the south-west monsoon, viz. the 

 heavy rainfall. The diagram Fig, 3 was devised 

 by Dr. Simpson to represent the chief alignments 

 of mountains in and around India (thick-lettered 

 lines), and the chief air-stream lines of the south- 



'-i See Dr. Simpson's original paper, and Sir John Eliot's " Climatologicai 

 Atlas of India." 



