128 



And what I have said of thunder clouds applies also 

 to hail clouds. In the upper strata, clouds are not always 

 of vaporous formation; their vapour in the chilly upper 

 regions having changed to snow, when they roll down 

 upon the heated minimum they are discharged in icy 

 globules ol varying size according to the bulk of the- 

 snowflakes from which they form. The nature of the 

 hailstone with its coating of ice and snowy centre points 

 to its history in the clouds. The hail which we see in 

 early spring and autumn unattended by storms is formed 

 obviously in the same way by the passage of snow 

 through warmer air. 



Hail and thunder-clouds are easily distinguishable 

 from ordinary rain-clouds. The latter are a dark smoke- 

 gray whilst the hail-cloud has a whitish appearance with 

 borders of lurid hue. 



The so-called claps of thunder always move in one 

 direction and are remarkable for their graduated course, 

 the graduation corresponding to the passage of the va- 

 pour cloud over the heated area of barometrical mini- 

 mum. 



If I were asked why thunder and lightning occur not 

 on the atmospheric surface of the minimum but at some 

 distance beneath, I should argue that the descending 

 hydrogen mass presents a certain weight which must 

 have above it a stratum of air more solid than that which 

 is moving, and secondly, that not every stratum of warm 

 air can convert vapour cloud into dry vapour: for this 

 purpose the heat must be great, and such heat is found 

 only in lower strata. 



The heat of the atmosphere before a storm in summer 

 is clearly visible to the eye. The observer has only to 

 turn his glance in the direction of the approaching 

 stormcloud and he will plainly see the surface of the 

 air tinted with the colour of steel heated to a straw- 

 coloured yellow. 



What awful force is exhibited in explosions of dry 

 vapour is well-known by bitter experience. A single 



