Nov., 1921 THUNDERSTORMS 25 



because of the processes just explained. The negative electrons 

 in the meantime are carried up into the higher portions of the 

 cumulus, where they unite with the cloud particles and thereby 

 facilitate their coalescence into negatively charged drops. 

 Hence, the heavy rain of a thunderstorm should be positively 

 charged, as it almost always is, and the gentler portions neg- 

 atively charged which very frequently is the case. 



"Such in brief, is Dr. Simpson's theory of the origin of the 

 electricity in thunderstorms, a theory that fully accounts for 

 the facts of observation and in turn is itself abundantly sup- 

 ported by laboratory tests and simulative experiments. 



" If this theory is correct, and it seems well founded, it must 

 follow that the 07ie essential to the formation of the giant 

 cumukis cloud, namely, the rapid uprush of moist air, is also 

 the one essential to the generation of the electricity of thunder- 

 storms. Hence the reason why lightning seldom if ever occurs 

 except in connection with a cumulus cloud is understandable 

 and obvious. It is simply because the only process that can 

 produce the one is also the process that is necessary and suf- 

 ficient for the production of the other." 



4. Turbulence of the Cumulus Cloud. 



That the large cumulus clouds, especially those that pro- 

 duce thunderstorms, are exceedingly turbulent within with 

 violent vertical motion, as demanded by the theory just out- 

 lined, is evident to even the casual observer. Furthermore the 

 testimony of those balloonists who have had the trying ordeal 

 of passing through the heart of a thunderstorm confirms the 

 facts of observation. Since these are the only clouds, appar- 

 ently, characterized by this high degree of turbulence, it may 

 be well to pause a moment and ask why these motions — niotions 

 which, in the magnitude of their vertical components and 

 degree of turmoil, are never exhibited by clouds of any other 

 kind nor are they met with elsewhere by either manned, sound- 

 ing or pilot balloons. Without going into very great detail, it 

 may be pointed out, as has been done by von Bezold'*, that 

 the heat liberated by the sudden condensation from a state of 

 supersaturation, and also from the sudden congelation of 

 undercooled cloud particles, would cause an equally sudden 

 expansion of the atmosphere, resulting in turbulent motions 

 analogous to those observed in the large cumulus clouds. 



