RELATION TO THE DISTRIBUTION OF BAROMETRIC PRESS ERE. 
341 
the drier months of the year there is a decided diminution of vapour pressure at the 
hottest hours of the day, when west winds, similar to, or identical with, the “hot 
winds,” are blowing. The following hourly values of the pressure of vapour at 
Allahabad will illustrate this. They are taken, with some corrections derived from 
subsequent observations, from a paper on the “ Results of Meteorological Observations 
at Allahabad,” published in 1881 in the first volume of the ‘Indian Meteorological 
Memoirs/ 
Table ITI.— Diurnal Variation of the Pressure of Vapour at Allahabad. 
Month. 
Mi 1- 
nigkt. 
2 h. 
4 k. 
6 h. 
8 h. 
10 h. 
12 h. 
14 1). 
16 h. 
18 k. 
20 k. 
22 k. 
January 
// 
■342 
// 
•334 
// 
•325 
// 
•313 
// 
•326 
// 
"355 
•369 
•366 
// 
•372 
•384 
•369 
// 
•352 
May .... 
580 
•585 
•588 
•617 
•621 
•590 
•557 
•527 
•515 
•534 
•552 
•575 
July. . . . 
■9^8 
925 
•923 
•919 
•943 
•955 
•958 
•964 
•956 
•949 
•940 
•935 
October 
•656 
•650 
- 648 
•655 
•683 
•672 
•652 
•646 
•654 
•671 
■666 
■662 
It will be seen that, whereas, in the month of July, the quantity of vapour in 
the air increases, with slight irregularities, as the temperature rises from 6 A.m. up to 
2 P.M., and diminishes again steadily as the temperature falls after 2 P.M., this 
regular march is interrupted in the other months by a fall in the middle of the day. 
The fall is very slight in January, but considerable in October, making the afternoon 
minimum in that month slightly lower than that of the early morning. In May, 
however, and in the other dry hot months, the afternoon fall so completely masks the 
variation due to the diurnal changes of temperature, as seen in July, that the morning 
minimum is altogether obliterated. 
Whence comes this excessive dryness in the afternoons, which is not merely a 
decrease of relative humidity, due to rise of temperature, but a diminution of the 
absolute quantity of vapour in the air near the ground, from an average pressure of 
G21 inch in May, at 8 o’clock in the morning, to '515 inch at 4 in the afternoon? It 
may be caused by the wind coming from drier regions to the north-westward ; but 
this can hardly be the true cause, for in May there is very little decrease in the mean 
pressure of vapour as we go north-westwards over the Gangetic plain, and there 
is still less in April. For example, we have for three selected stations on the plain 
of the Ganges, arranged in order from SE. to NW., the following values for the 
mean pressure of vapour. 
Allahabad. 
Lucknow. 
Bareilly. 
// 
April ...... 
•414 
•431 
•421 
May. 
•553 
'565 
•549 
