RELATION TO THE DISTRIBUTION OE BAROMETRIC PRESSURE. 
367 
These results, for the three dry months, are consistent with the variations ot the 
length of the day, and the probable rates of decrease of temperature near the ground ; 
but the result for July is much too large, and implies either that the convective 
action does not extend to so great a height as in the other months, or that, owing to 
the frequency of small cyclonic disturbances in July, the mean of the gradients 
actually obtaining from day to day is considerably steeper than that deduced from 
the chart of monthly mean pressures. Both reasons, doubtless, concur in producing 
the result just found. 
The hypothesis of diurnal interchange between atmospheric strata lying at low and 
high levels, brought about by convection currents during the hours when the sun is 
shining, having thus been shown to account for the observed anomalies of the winds 
of Northern and Central India, both in direction and velocity, I shall now proceed 
to enquire whether the same hypothesis will suffice to explain the effect of unusual 
snowfall on the Himalaya in producing extraordinarily powerful and persistent N.W. 
winds over the Indian plains. 
Part III.—The Anomalous Effect of Heavy Snoiv in the Himalayan Region. 
In the year 1877, remarkable for the almost total failure of the summer rains, I 
pointed out, in a letter to the Government of the North-Western Provinces, the 
probability of a rule that when the winter rains of Northern India are light those of 
the summer are heavy, and vice versa. About the same time Mr. E. D. Archibald 
arrived independently at the same empirical law. The investigation which led up to 
this law was continued and extended, and two years afterwards I published a paper on 
the subject in the ‘ Indian Meteorological Memoirs/ vol. 1. At page 209 of that volume 
it is shown that out of a total number of 34 years of which the rainfall statistics are 
discussed 25 tell in favour of the rule that, in the North-Western Provinces, the 
winter rainfall and that of the succeeding summer vary in inverse directions, and 
only nine are against it. Mr. Blanford has since investigated the subject further, 
and shown that if, instead of the rainfall during the winter and spring months over 
the plains, we take the precipitation on the North-West Himalaya (which usually, 
but not always, varies, pari passu, with the rainfall on the plains), and compare it 
with the rainfall of the succeeding summer monsoon, the unfavourable instances, with 
one or two exceptions, due to special temporary causes, all disappear. 
The way, in which unusually heavy and late snowfall on the mountains exercises a 
retarding and weakening influence on the summer monsoon, is doubtless by keeping 
down the temperature of those regions, which constitute one of the goals towards 
which the monsoon winds blow. In such dry years, the westerly winds, which usually 
prevail over northern aTid western India during April and May, are strongly reinforced, 
and continue to blow- far into June or July; or even, as in 1868 and 1877, right on 
through the months of the summer monsoon until September or October. In his 
