182 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1937 



that of the earth. FaUing drops will thus attract to their negative 

 undersurfaces the upward moving positive ions. Drops which are 

 negatively charged when they leave the cloud therefore may become 

 positively charged during their passage to earth. 



COMPARISON OF THUNDERSTORM THEORIES 



To facilitate comparison of the three theories outlined, they are 

 summarized in table I. Of these theories the breaking-drop theory 

 appears to be the most attractive. Profuse breaking of raindrops 

 must occur in the violent convection sj'stems kno^vn to be present in 

 thunderclouds. Laboratory experiments have proved that when 

 drops are broken up in an air stream they become electrified. It is 

 natural to infer that a similar action on a veiy large scale occurs in 

 thunderclouds. Yet there is one serious discrepancy. This is that 

 laboratory experiments indicate that the water drops become positively 

 charged in breaking, and that therefore the lower most active region 

 of the thundercloud should be a region of positive charge concentra- 

 tion. This is in direct disagreement with measurements of the polarity 

 of lightning-discharge currents through transmission-line towers, 

 which mdicate that a very high percentage of strokes striking trans- 

 mission lines are from negatively charged clouds. ^Vhile, as Lewis 

 and Foust '^ have suggested, there is a possibility that the trans- 

 mission towers and lines may exert a directive action on strokes when 

 the clouds are negatively charged and thus cause the liigh percentage 

 of negative strokes to towers, still the discrepancy is so great that for 

 the present the breaking-drop theory, in spite of its attractiveness, 

 cannot be completely accepted. 



Table I. — Comparison of theories of the electrification of thunder stor 7ns 



It is probable that all three of the electrifjnng processes embodied 

 in the theories described account in part for the electrification of 

 thunderclouds. Which of them is the most important, however, 

 cannot be decided from our present knowledge. As R. A. Watson 



" Lewis, W. W., and Foust, C. M., Lightning investigation on transmission lines: HI, Amer. Inst. Elec. 

 Eng. Trans., vol. 52, pp. 475^80, 1933. 



