178 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1937 



field increases as the generation process goes on within tlio cloud. 

 When the field reaches a certain critical value a discharge between 

 cloud and earth takes place. 



HOW THUNDERCLOUDS BECOME ELECTRIFIED 



It is certain that the electrification of thunderclouds is closely 

 connected with the turbulent convection systems which form them. 

 This was veiy evident during an intensive 2-year field study ^ of 

 thunderstorms made from a mountain top in Colorado by one of the 

 writers of this article. In the course of some 300 storms there were 

 numerous occasions when the shape and nature of the storm cloud 

 and its electrical activity were plainly visible at the same time. 

 Clouds with dense well-developed heads, showing evidence of violent 

 internal convection systems through many and continually changing 

 surface protuberances, were practically always active electrically. 

 On the other hand, clouds which did not indicate the presence of such 

 convection systems were not electrically active. 



The exact manner in which these convection currents take part 

 in the electrification of thunderclouds is not completely known. 

 That they separate the small raindrops and cloud particles from the 

 large raindrops, and that by doing so they separate positive and 

 negative electricity within the cloud, is generally agreed. But 

 whether tlicy cause the electrification of the raindrops by breaking 

 them up, or by bringing them into contact with each other while they 

 are polarized and then separating them again, or by helping to bring 

 positive and negative air ions in contact with polarized raindrops, is 

 a controversial subject. The three best known theories of thunder- 

 cloud electrification, each involving one of the foregoing methods of 

 electrification, will now be described. 



Simpson's breaking-drop theory 



Simpson's ^ * breaking-drop theory of thundercloud electrification 

 is probably the best known and most generally accepted. In this 

 theory, a major role is played by violent upward air currents, which, 

 for reasons to be explained later in the section on "Causes of Thunder- 

 stoi-m Formation", exist in active thunderstorms. These upward 

 air currents cany up moisture which condenses as it rises. The con- 

 densing water vapor combines into drops. When these attain a size 

 and weight such that the force of gravity can cause them to move 

 against the rising air currents, they fall. Joining with other drops 

 as they fall, they grow larger and an increasingly greater number of 

 them are broken up by the action of the upward air currents. The 



* Evans, E. A., A study of the mechanism of the liphtning discharge, Stanford Univ. thesis, October 1932. 

 » Simpson, O. C, Electricity of rain and its origin in thunderstorms, Phil. Trans. Roy. Soc, ser. A, vol. 

 209, pp. 379-413, 1909. 

 < Simpson, G. C., The mechanism of a thunderstorm, Proc. Roy. Soc, ser. A, vol. U4, pp. 37&-401, 1927. 



