THE THUNDERSTORM 



By E. A. Evans and K. B. McEachron 

 Pittsfield Works, General Electric Co. 



[With two plates] 



In this article the writers have attempted to gather together in 

 one place sufficient information on tiie electrification of thunder- 

 clouds, the causes of their formation, and their characteristics, to 

 give engineers a general understanding of the thunderstorm. To 

 this end the writers have drawn on their o^vn personal experiences in 

 field studies of lightning and have selected and interpreted the ideas 

 of a number of investigators who have presented their findings in 

 numerous publications, many of which are not easily accessible to 

 the average engineer. In a number of instances, additional field 

 data have been added to round out or corroborate the findings and 

 deductions of previous investigators. The characteristics of actual 

 thunderstorms are described to amplify the conceptions obtained 

 from descriptions of "ideal" models which, for ease in understanding 

 their operation, necessarily have to be simple. Finally, an explana- 

 tion is given as to how a laiowledge of the characteristics of thunder- 

 storms and of the factors affecting their formation and travel can be 

 applied in lightning protection problems. 



ELECTRICAL PICTURE OF THE THUNDERSTORM 



It is desirable first to review briefly the operation of the thunder- 

 cloud generator before explaining its formation and characteristics. 

 As a helpful picture of this generator, the lower part of a thunder- 

 cloud can be visualized as one plate of a physically huge condenser, 

 the air as the dielectric, and the ground, or another part of the cloud, 

 as the other plate. It must not be forgotten that the cloud is not a 

 conductor but consists of a multitude of poorly conducting water 

 droplets suspended in an insulating medium, the air. The charge 

 of the cloud is not distributed on its surface as on the plates of a 

 metallic condenser, but is a volume charge distributed on water drop- 

 lets and air ions throughout considerable regions in the cloud. 



The cloud charge attracts to the ground beneath and near it an 

 equal amoimt of charge of opposite sign. Between the charge on 

 the cloud and the charge on the ground an electrical field exists just 

 as between the charges on the plates of any other condenser. This 



' Reprinted by permission from the General Electric Review, September 1936. 



31508—38 13 177 



