276 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 51 



which the particles of water have a temperature more nearly that of 

 the freezing point, they cover themselves with a coating of clear 

 ice, upon which during a second ascent, such as frequently occurs in 

 the commotion within a thunder cloud, there is deposited a second 

 layer of subcooled particles, after which the hailstone, now become 

 heavier, again sinks and is again covered with clear ice. 



In this way is formed the cloudy milky nucleus with its surround- 

 ing concentric layers such as we find in hailstones. 



It seems quite natural that a regelation occurs when the hail- 

 stones already formed strike violently together and thus grow 

 together into the irregular forms that are frequent among hailstones. 



Thus it is that from the assumption that subcooled water particles 

 play an important role in thunder clouds, there follow easily and 

 naturally the series of phenomena that actually do accompany a 

 thunderstorm. 



Still there remains one great difficulty to be overcome in that it 

 is not easy to get any clear idea of the process of dissipation of the 

 subcooled condition. 



According to observations that have been frequently made, 

 among which I need only mention those of Assmann, Moedebeck, 

 and Gross, the freezing of the subcooled water at individual points 

 of the cloud does not spread throughout the whole cloud at once. 

 Whereas on the Brocken all subcooled microscopic droplets immedi- 

 ately freeze when they strike a solid body and gradually inclose 

 this body in a sheet of ice, and whereas in the oft-cited case of the 

 balloon voyage of 189 1, June 19, all the rigging of the balloon became 

 rapidly covered with ice, still the fog or cloud as such remained 

 unchanged. 



It is not easy to understand how this freezing spreads in a short 

 time throughout the larger part of a cloud, and yet this must be the 

 case if in fact the sudden rise in pressure, as already described, with 

 all its consequences is to occur. 



Whether this is brought about by the crystals of ice that fall from 

 the cirrus screen and by contact with the subcooled water particles 

 cause this sudden freezing, or whether some electrical process here 

 comes into play, are still open questions. 



On the other hand, I must consider the heavy showers characteris- 

 tic of thunderstorms as a proof that in these phenomena the above- 

 described dissipation does play a part. 



Moreover, it is not improbable that many thunderstorm rains 

 begin in the upper regions as sleet or hail and only become rain in 

 the lower strata of the atmosphere. 



