RELATION TO THE DISTRIBUTION OF BAROMETRIC PRESSURE. 
3G9 
variation curve of vapour-tension in a dry atmosphere near the earth’s surface, which 
is the same in all parts of India. This shows a rapid fall of the absolute humidity of 
the air after 8 or 9 o’clock in the forenoon, reaching its minimum about the time of 
greatest heat, and a more or less sudden rise before sunset, which it is difficult to 
account for on any other supposition than that it coincides with the cessation of the 
convective movement.” Having given thus clearly and concisely the evidence that the 
explanation of the diurnal hot winds is probably to be found in Koppen’s hypothesis, 
Mr. Blanford does not go on to explain how the abundance or paucity of the 
snowfall on the mountains will affect their intensity. From what has gone before, 
however, it will be evident that their intensity is likely to be affected only by tw T o 
causes : (1) the intensity of the convective action which brings about an interchange 
between the higher and lower atmospheric strata; and (2) the pressure gradients and 
consequent intensity of the winds prevailing at high levels. The former, which 
depends on the rate of variation of temperature with elevation, is doubtless affected 
by rainfall over the plains in what is ordinarily the dry season ; for such rainfall 
invariably lowers the temperature at the ground surface, though, from theoretical 
reasons, it would appear to raise it by the liberation of latent heat at the level where 
precipitation occurs, the double effect being a decrease in the rate of vertical decre¬ 
ment. The cooling effect of occasional showers over the plains is, however, a transient 
phenomenon, compared with that of the snows on the mountain slopes; hence it is 
probably by an increase of the baric gradient at high levels, owing to the sinking of 
the planes of equal pressure over the cold mountain zone, that the westerly winds are 
intensified to the extent observed in dry years. 
A thorough investigation of this part of the subject would demand the preparation 
of charts of the distribution of pressure at a high level for each of a considerable 
number of years, a comparison of the gradients on which, month by month, with the 
rainfall and the prevailing wind directions over the plains, would enable us to com¬ 
pletely verify or disprove the hypothesis. It is to be hoped that, in future, such high- 
level'^ charts may be prepared for the monthly mean pressures, as regularly as the 
charts at sea-level now published in the Annual Reports on the Meteorology of India. 
Such charts could not fail to be instructive. The labour of preparing such a series 
would, however, be very great, and all that can be attempted now is to ascertain as 
nearly as possible what were the high-level gradients prevailing over the Gangetic 
valley in the typical months of a few years of marked characteristics as regards 
rainfall. 
During the last eight years for which the Meteorological Reports have been pub¬ 
lished, the rainfall of the Punjab, Rajputana, Central India, the North-West Provinces, 
* The elevation, 10,000 feet, adopted in the charts attached to this paper, has been chosen only 
because it is expressed by a convenient round number, and because it lies above all the mountains in 
India except the snowy ranges. It may be found in practice more convenient to adopt some other 
level. 
3 B 
MDCCCLXXXVII.— A. 
