44 H. F. Blanford—Rainfall Frequency at Calcutta. [No. 1, 
Hours p. M. 
Noon to 13 
13 to 14 
15 to 16 
| 21 to 22 
| 22 to 23 
| 23 to Midn. 
Rains : June to October, | 543) 537 
Hot season: March to 
477| 464| 413] 397| 343] 321| 261| 289! 963 
Vie Vavelalatsietelelotelenavelaters 86| 388) 47) 66] 69} 102) 99) 125) 109} 81) 61) 34 
Cold season: November 
to February, ..... ces 24! 25) 27) 281° 83) QO Dai D7" Se tye 2 
The variation in the rainy months is, then, almost identical with that 
above described, the chief difference being that after the afternoon maxi- 
mum, the decline is more rapid. The heavy rains of the monsoon months 
are, then, more particularly rains of the day time, favoured and accelerated 
by the diurnal rise of temperature, and declining with the decline of the 
sun’s heat. . In a nearly saturated atmosphere, the rapidity with which 
vapour ascends from lower to higher levels, and eventually becomes dynami- 
cally cooled and condensed, depends on the temperature, increasing indeed 
as the square of the absolute temperature. The relative humidity of the 
lower atmosphere (as tested in our observations), does not follow the same 
course of variation. Indeed, as may be seen in fig. 7, this course is exactly 
the inverse of that of temperature, but as far as can be judged from casual 
observation, the formation and dispersion of cumulus cloud, indicating the 
state of saturation at heights of from 2000 to 7000 or 8000 feet, is equally 
determined by the rise and fall of the temperature, and in its mode of 
formation the rain-cloud of the summer monsoon is essentially cumulus. 
The hour of least frequent rainfall, which in the summer monsoon would 
seem to be between 10 and 11 P. m,, is probably also that of least cloudiness. 
The horary variation of cloud is not known for Calcutta, but I found some 
time since on examining the registers of a number of Bengal stations, at 
which the cloud proportion had been recorded for some years at 4 and 10 
A. M. and Pp. M. that the average at 10 p. mM. was very considerably below 
that observed at other hours. Kreil has noticed a similar fact at Vienna, 
and Neumayer in his discussion of the Observations of the Flag-Staff Obser- 
vatory at Melbourne, also finds that, on the average of the year, there is a 
strongly marked minimum about this hour. Kreil explains this tendency 
to the dispersion of cloud, after sunset, by the compression which the lower 
atmospheric strata undergo, in consequence of the general contraction and 
subsidence of the mass ; to which action he also refers the coincident baro- 
