376 MR. S. A. HILL ON THE WINDS OF NORTHERN INDIA, AND THEIR 
Combining the differences of these figures with those for the same pair of stations 
in Table XIX., we get the following results for the whole summer half-year, that is 
to say, for the six months May to October inclusive, during which almost the whole 
rain of the year falls on the Gangetic plain. 
Table XXI.—Pressure Differences between Sutna and Panikhet for the Summer 
Half-Year. 
Year. 
May. 
June. 
July. 
August. 
September. 
October. 
Mean. 
1877 . • 
+ 142 
+ 75 
+ 52 
+ 26 
+ 76 
+ 79 
+ 75 
1879 . . 
+ 59 
+ 85 
-25 
+ 12 
- 5 
+ 31 
+ 26 
1880 . . 
+ 1G0 
+ 116 
-18 
+ 30 
+ 69 
+ 101 
+ 76 
1883 . . 
+ 166 
+ 93 
-40 
+ 16 
+ 18 
+ 58 
+ 52 
1884 . . 
+ 144 
+ 110 
-59 
-27 
-23 
+ 62 
+ 34 
In Table XXI. the relation between the pressure differences and the rainfall 
variation becomes quite clear. The driest years, 1877 and 1880, had the largest 
gradients for westerly winds, that is to say, in those years, easterly winds at high levels 
over the plains of Northern India were probably infrequent, or restricted to an un¬ 
usually narrow belt along the Himalayas ; on the other hand, the wet years, 1884 and 
1879, had the smallest gradients, or in those years the easterly upper currents were 
steadiest, and reached their widest extension ; while in the moderately dry year, 
1883, there was a considerable gradient for westerly winds, but nothing like so great 
as in 1877 and 1880. If the month of May be left out in striking the mean for the 
season, the relation indicated becomes still more distinct; thus— 
Year. 
1877. 
1879. 
1880. 
1883. 
1884. 
Mean gradient for 5 months . . 
+ 62 
+ 20 
+ 60 
+ 29 
+ 13 
Rainfall variation per cent. . 
-35 
+ 5 
-20 
-17 
+ 16 
It appears, therefore, from this analysis of the observations, that the theoretical 
deductions from Koppen’s convection hypothesis are fairly substantiated b} r experience; 
that in years of unusually heavy snowfall, and consequent cold, over the North-West 
Himalaya the pressure gradients producing westerly winds in the upper atmosphere 
are intensified, and that in consequence of this the easterly winds, which prevail as 
far south as the Ganges up to the level of the lower cirrus clouds in the rainy season 
of average years, are greatly restricted in horizontal extent, and doubtless also in 
height as well as in time, the result being a diminution of the rainfall, sometimes to 
a disastrous degree. 
Though these conclusions are substantiated by the average of the observations of 
five or six months, they are not completely borne out by those of any single month, 
