1879.] H. F. Blanford—Rainfall Frequency at Calcutta. 45 
metric rise and maximum. In any case, this coincidence of minimum rain- 
iness, minimum cloudiness and the semi-diurnal maximum of pressure, is an 
important fact of observation. 
The rapid rise of rain-frequency after midnight corresponds, though 
less exactly, to the nocturnal fall of pressure ; but, as on the average of the 
year, the secondary maximum is not reached till some time after sunrise, viz., 
about 6 a.m. Inthe Melbourne curve of cloud variation, this is also 
about the epoch of the diurnal maximum, and as already remarked itis that 
of minimum temperature and maximum humidity at the ground surface. 
The slight fall that ensues continues till between 8 and 9, which is about 
an hour in advance of the epoch of maximum pressure. It would seem 
therefore that the tendency to the precipitation of rain is a somewhat com- 
plex function of the temperature and pressure variations; or inasmuch as 
the latter is an effect of the former, of the temperature variation producing 
two conditions which are in part mutually antagonistic in their effect on 
the rainfall, To sum up the results of this discussion, I would suggest the 
following as a possible explanation of the rainfall variation. The cooling 
of the atmosphere after 3 Pp. M. in the first place checks the production and 
ascent of vapour, as well as of convective atmospheric currents, and (adopt- 
ing Kreil’s explanation of the barometric tides) causes a rise of pressure in 
the lower atmosphere as a consequence of the sinking and compression of 
the atmospheric mass. These effects bring about a dispersion of cloud and 
‘a fall of rainfall frequency from the absolute maximum to the absolute 
minimum of the 24 hours. About 10 p. um. the compression having reached 
its maximum, re-expansion sets in, and, in conjunction with continued cool- 
ing, raises the relative humidity of the cloud-forming strata, and conse- 
quently the tendency to the formation of cloud and rain. When the 
re-expansion ceases about 3 or 4 A. M., the loss of heat is still operative 
in the same direction, though less powerfully ; but, after sunrise, the direct 
effect of the solar heat is to diminish cloud and rainfall, while raising the 
pressure of the lower atmosphere ; and it is not until this increasing pressure 
‘has nearly attained its maximum, and the ascent of vapour has become 
sufficiently active to prevail over these first effects, that the formation of 
cloud* and rainfall proceed actively, and attain their afternoon maximum ; 
this condition coinciding with the highest temperature and the greatest 
activity of diffusing vapour and convective currents. 
This explanation, I must remark, is suggested solely by a consideration 
of the several coincident phenomena, and presupposes an atmosphere highly 
charged with vapour, such as is that of the summer monsoon. It would be 
‘impossible to predict the course of the changes a priori, because the several 
-* This is of course an assumption as regards the cloud maximum. 
