354 MR. S. A. HILL OH THE WINDS OF NORTHERN INDIA, AND THEIR 
however, arises that the pressure differences of any two months may have the same 
small mean value, while in one month the barometer stood at a uniform heiefit all over 
the country, and in the other there may have been a series of more or less violent 
disturbances causing strong winds ; that, in short, it is useless to compare mean values 
instead of actual observations. In North India, however, no feature of the meteorology 
is more remarkable than the simultaneity of barometric movements over a large extent 
of country; all the usual fluctuations, which are very numerous, though of small 
range, even in the most settled weather, occurring with the most absolute uniformity 
over an extent of country often larger than France, Germany, and Austria. Small 
disturbances, of the nature of cyclonic storms, do occur occasionally, more especially 
during the rainy season and in the cold weather, during the month of January or about 
the end of December ; but, while they, doubtless, make the mean wind velocity, for the 
months mentioned, somewhat greater than it would otherwise be, they do not per¬ 
ceptibly interfere with the regular march of its annual variation. 
Table X. also shows, contrary to what might be expected from the preceding part 
of this Paper, that the velocity is much more distinctly related to the actual tempera¬ 
ture of each month than to the daily range of the thermometer. The explanation of 
this probably is that in the interior of Northern India the daily range at all seasons 
is sufficient to set up convection currents, while the velocity, of the winds which 
descend from a considerable elevation over the plains is dependent upon the pressure 
gradients prevailing at high levels. Now these gradients are doubtless subject to a 
distinct annual range, depending on the temperature ; for, if the plains be heated 
more than usual relatively to the Himalayan slope, the result will be to raise the 
planes of equal pressure and make them incline towards the mountains, thus giving 
rise to westerly upper currents, the velocity of which will, ccBleris paribus, be 
proportional to the temperature difference between plains and mountains. 
Now, if we compare the mean temperature for stations on the plains of the North- 
Western Provinces and Oudh, given in Table X., with that of the mountain slope at 
a height of about 5700 feet, as computed from the observations of Chakrata, Panikhet, 
Pithoragarh, and Kathmandu, we find the following differences, which are subject to 
a very distinct annual valuation, nearly coincident in phase with that of wind velocity 
on the plains : — 
Month :— 
January. 
February. 
March. 
April. 
May. 
June. 
c 
o 
O 
O 
o 
o 
Temperature difference . 
13-4 
17-1 
1ST 
22-1 
24-2 
21-3 
Month :— 
July. 
August. 
September. 
October. 
November. 
December. 
O 
O 
O 
° 
o 
O 
Temperature difference . 
16T 
15-3 
15-4 
15-3 
12-0 
10-0 
