

Electricity of Atmospheric Precipitation. 7 



B. The " Breaking Drop-" Theory. 



This theory is founded on the observed fact that when a 

 drop of water is broken up into smaller drops, the water 

 becomes positively charged and the air negatively charged. 

 If there is any large amount of breaking of drops there is 

 bound to be a large separation of electricity. In thunder- 

 storms, which are known to have associated with them 

 violent ascending currents of air, there is no doubt that the 

 water-drops are much broken and, as shown in the paper in 

 which the theory was formulated *, there is no doubt that 

 all the chief electrical phenomena of thunderstorms can be 

 accounted for by the theory. When the theory was 

 published in 1909 there were not sufficient observations on 

 the electricity of non-thunderstorm rain and snow to test the 

 theory in respect to these. It is proposed now to extend the 

 theory to them. 



Non-thunderstorm rain. — For the theory to explain the 

 electrification of rain there must be a certain amount of 

 breaking of drops. The difficulty, however, is that accord- 

 ing to experiments made by Leonard f, drops of water of 

 less diameter than 4 mm. are not broken by falling through 

 air. As in non-thunderstorm rain the majority of the drops 

 are much smaller than this, it would appear that there would 

 be little or no breaking of drops. That this conclusion is 

 not strictly true can be observed in a simple manner. If 

 one stands during a rain shower in such a position that the 

 falling drops can be seen against a dark background, then 

 after a little practice the following will be noticed. The 

 drops will be seen to be falling in parallel lines inclined from 

 the vertical, the direction being determined by the wind. 

 Suddenly, as a gust of wind comes, the angle of fall will 

 change and the drops will become confused. At this moment 

 a large number of very fine drops will appear. Whether 

 these small drops are formed by the breaking of the drops 

 due to the wind itself, or as a result of collisions, it is im- 

 possible to say. but there can be no doubt of the breaking of 

 the drops by one cause or another. 



Then again there is the question of one drop overtaking 

 another and colliding with it. Leonard has shown that such 

 collisions are probably rare with real raindrops. At the 

 same time there must be a certain amount of collision 

 through one cause or another. When two drops collide it is 

 very unlikely that they unite and continue as one large drop. 

 On the contrary, they will " splash " against one another and 



* Loc. cit. 



t Leonard. Met. Zbit. xxxix. p. 2o7 (1904). 



