60 ON THE FORMATION OF RAINDROPS AND HAILSTONES. 



with the naked eye, and descend with a very appreciable 

 velocity so as to form a drizzle. When a coarse mist, such 

 as this, is superimposed over a fine mist, then rain must 

 ensue if the particles are water, and hail if they are ice. 



Although, as has been 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 the 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 pro- 

 bably 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 sub- 

 sequent descent, it may sometimes be seen to overtake an- 

 other flake and combine with it, the two descending 

 together. 



