1879.] H. F. Blanford—Rainfall Frequency at Calcutta. AT 
partly by the friction of the rain. But this is not invariably the case ; as I 
have more than once experienced a gusty though less violent wind, when no 
rain was falling. The only essential feature, which is apparently common 
to all storms of this class, including the dry and rainless dust-storms of the 
Punjab and hail-storms, is a more or less spasmodic and sudden subversion 
of vertical equilibrium, and rapid convection accompanied by eddying cur- 
rents, and generally, in Bengal, at least, by heavy rain. It is somewhat 
remarkable that their most frequent occurrence coincides with the most 
rapid cooling of the atmosphere, but it must be observed that as a general, 
if not invariable, rule the cloud canopy in which the storm originates has 
been formed during the day, and that this shields the subjacent air and earth 
surface from rapid cooling, while the higher strata are radiating freely into 
space. 
Lastly, in the cold season, falls of rain are distributed pretty evenly 
throughout the day, with a decided diminution during the two or three 
hours before and after midnight; as shown in the table, from 8 Pp. mM. 
to2a.m. This period corresponds with that of the strongly marked 
minimum of the rainy season; and may probably be referred to similar 
causes ; the atmosphere of the cold weather being normally of higher humi- 
dity than that of the hot season, and especially when a southerly wind sets 
in, which is always the precursor of rain. 
The general conclusion to be drawn from this discussion is, that the 
conditions which promote and determine precipitation, are different at 
different seasons. In the highly vapour-charged atmosphere of the rainy 
monsoon, and in a much less degree in the cold season, condensation is most 
promoted by increasing temperature, and the more active ascent of vapour 
determined thereby. In a minor degree, the opposite action, v¢z., nocturnal 
cooling, under certain conditions, produces the same effect, but this is com- 
plicated with those of the internal movements (the compression and expan- 
sion) of the atmosphere, which are another effect of the oscillation of 
temperature. In the comparatively dry atmosphere of the- hot weather, 
the precipitation is chiefly that of storms, which are spasmodic movements, 
arising from the subverted equilibrium of the superimposed strata; and 
these are most frequent when the atmosphere as a whole is cooling most 
rapidly. They are probably the effect of unequal cooling. 
