38 
Although; as has beeu shown, a raindrop cannot add 
considerably to its volume by condensing the vapour from 
the air through which it passes ; the reverse of this is not 
the case. The raindrop may be diminished by evaporation. 
Whenever a raindrop falls through dry air, that is, air of 
which the dew point is below tlie temperature, evaporation 
might, and would, go on to almost any extent, and the size 
of the drops be diminished until they entirely vanished. 
The heat for evaporation being supplied from the air, which 
would be warmer than the drop. 
The case of snow differs from that of hail. The snow 
crystals are clearly formed by the condensation of vapour, 
and not by the mere aggregation of particles of ice. In this 
case the latent heat developed in condensation is probably 
dissipated by radiation, the shape and smallness of the 
crystals causing them to descend very slowly, and so 
affording time for the radiation to produce an effect. 
But even in snow we see the effect of aggregation. The 
individual crystals never acquire a large size. But in their 
descent— the larger ones overtaking the smaller — they form 
into flakes. In this case the aggregation may be seen 
taking place. If when large flakes of snow are falling fast 
without wind the eye be fixed on a large flake as high 
as it can at first be perceived, and follow this flake in its 
subsequent descent, it may sometimes be seen to overtake 
another flake and combine with it, the two descending 
together. 
