THE THUNDERSTORM— EVANS AND McEACHRON JgX 



There are normally present in each cubic centimeter of the atmos- 

 phere about 1,000 positive and 800 negative "small" ions ^' having 

 mobiUties of about a centimeter per second under the action of a 

 field of a volt per centimeter, as well as 1,000 to 60,000 "large" ions 

 of much smaller mobilities. These ions are, according to Wilson, 

 greatly increased in number in thunderclouds by ionization attending 

 the strong electrical fields in such clouds. The positive ions travel 

 toward the negatively charged earth with a velocity dependent upon 

 the field strength in the region through which they are passing. 

 Similarly, the negatively charged ions travel away from the earth. 



Wilson points out that the rain drops falling or rising in the air 

 currents of the thunderstorm must meet many such ions. However, 

 for the electrification to start it is necessary to consider, as in the 

 Elster and Geitel theory, the separation of charge in drops due to the 

 effect of the field of the earth. Since the earth is negatively charged 

 these drops must be polarized with their lower surface positively 

 charged and their upper surface negatively charged. Drops falling 

 toward the earth faster than the velocity of positive ions in that 

 direction will not be overtaken by them. They therefore cannot 

 acquire a positive charge through attraction of such positive ions to 

 the upper negatively charged surface of the drops. On the other 

 hand, the positive ions which the drop catches up with in its fall will 

 be repelled by the positive charge on the lower surface of the drop and 

 so cannot contact it. Negative ions, however, which the drop meets 

 will be attracted to the lower surface of the drop. The larger faster- 

 falling drops by repeated contacts with negative ions thus become 

 negatively charged. As they accumulate in the lower part of the 

 cloud, their field adds greatly to that of the earth in polarizing the 

 drops above them and thus aids in the electrification process. Drops 

 falling more slowly than the positive ions will be overtaken by them. 

 The positive ions will be attracted to the upper negatively charged 

 surface of the drops. By repeated contacts these drops will thus 

 become charged positively. Smaller droplets carried upward by the 

 air currents will likewise become charged positively. Thus Wilson 

 conceives that the upper region of a cloud becomes positively charged 

 and the lower region negatively charged. 



Wilson explains the discrepancy between the apparent preponder- 

 ance of clouds with negatively charged bases and Simpson's measure- 

 ments which show a preponderance of positively charged rain from 

 thunderclouds by reasoning similar to that just given. He points out 

 that water drops below the negatively charged base of the cloud will 

 be polarized with their tops positive and their bottoms negative. 

 This follows since the negative cloud field will be much stronger than 



" Hess, V. F,, The electrical conductivity of the atmosphere and its causes, D. Van Nostrand Co., New 

 York, 1928. 



