February 21, 1919] 



SCIENCE 



187 



phere — normal atmospheric electricity. Ob- 

 servations, however, seem definitely to exclude 

 this assimiption. Thus, while the difference 

 in electrical potential between the surface of 

 the earth and a point at constant elevation is, 

 roughly, the same at all parts of the world, 

 the number and intensity of tliundcrstorms 

 vary greatly from place to place. Further, 

 while the potential gradient at any given place 

 is greatest in winter, the number of thunder- 

 storms is most frequent in summer, and while 

 the gradient in the lower layer of the atmos- 

 phere, at many places, usually is greatest from 

 8 to 10 o'clock, both morning and evening, 

 and least at 2 to 3 o'clock p.m. and 3 to 4 

 o'clock A.M., no closely analogous relations 

 hold for the thunderstorm. 



But how, then, is the great amount of elec- 

 tricity incident to a tliunderstorm generated? 

 Fortunately an answer to this question based 

 on careful experiments and numerous obser- 

 vations, and that greatly aids our understand- 

 ing of the interrelations between the various 

 thunderstorm phenomena, has been given by 

 Dr. G. C. Simpson, of the Indian Meteorolog- 

 ical Department. 



The chief conclusions drawn bj' Simpson 

 from his obsei-vational data, and supported by 

 numerous subsequent observations by other 

 persons at widely separated places, are : 



(a) That the charge on thunderstorm rain, 

 amounting often to 5 to 10 electrostatic units 

 per cubic centimeter, usually is positive. 



(b) That, on the whole, the quantity of 

 positive electricity brought down is more than 

 three times greater than the negative. 



While these observations were being secured 

 a number of well-devised experiments were 

 made to determine the electrical effects of each 

 obvious process that takes place in the thun- 

 derstorm. 



Freezing and thawing, air friction, and 

 other things were tried, but none produced 

 any electrification. Finally, on allowing drops 

 of distilled water to fall tlirough a vertical 

 blast of air of sufficient strength to produce 

 some spray, positive and important results 

 were found, showing: (1) That breaking of 



drops of water is accompanied by the pro- 

 duction of both positive and negative ions. 

 (2) That three times as many negative ions as 

 positive ions are released. 



Xow, a strong upward current of air is one 

 of the most conspicuous features of the thun- 

 derstorm. It is always evident in the turbu- 

 lent cauliflower heads of the cumulus cloud — 

 the parent, presumably, of all thunderstorms. 

 Besides, its inference is compelled by the oc- 

 curence of hail, a frequent thunderstorm phe- 

 nomenon, whose formation requires the carry- 

 ing of raindrops and the growing hailstones 

 repeatedly to cold and therefore high, alti- 

 tudes. And from the existence of hail it is 

 further inferred that an updraft of at least 

 8 meters per second must often occur within 

 tlie body of the storm, since, as experiment 

 shows, air of normal density must have ap- 

 proximately this upward velocity to support 

 the larger drops, those of 4 nmi. or more in 

 diameter, and, because of its greater weight, 

 even a stronger updraft to support the average 

 hailstone. 



Exi>eriment also shows that raindrops of 

 whatever size can not fall through air of 

 normal density whose upward velocity is 

 greater than about 8 meters per second, nor 

 themselves fall with greater velocity through 

 still air; that drops large enough, 4.5 mm. in 

 diameter and up, if kept intact, to attain 

 through the action of gravity a greater veloc- 

 ity than 8 meters per second with reference 

 to the air, whether still or in motion, are so 

 blo\vn to pieces that the increased ratio of 

 supporting area to total mass causes the re- 

 sulting spray to be carried aloft or, at least, 

 left behind, together with, of course, all 

 original smaller drops. Clearly, then, the up- 

 drafts within a cumulus cloud frequently must 

 be strong and therefore breiik up at about the 

 same level, that of maximum rain accumula- 

 tion, innimierable drops which, through coales- 

 cence, have grown beyond the critical size; 

 and thereby, according to Simpson's experi- 

 ments produce electrical separation within the 

 cloud itself. Obviously, under the turmoil 

 of a thunderstorm, such drops may be forced 



