Thunderstorm Electricity. 



637 



coalescence, the maximum strength of field could not exceed 

 one volt per centimetre; for in stronger fields the positive 

 ions would travel downwards relatively to the air faster than 

 the drops. 



Small uncharged drops do not generally coalesce on coming- 

 in contact, but, as Lord Rayleigh has shown, electrification of 

 the drops causes coalescence, and indeed he has suggested 

 that the large drops of thunderstorms may be due to this 

 cause. In his experiments, it is true, the conditions deter- 

 mining coalescence were such that the coalescing drops 

 carried a charge of electricity while the drops condensed on 

 the ions have a charge which is probably too small to have 

 any effect of this kind. But there is little doubt that coales- 

 cence would also take place between neutral drops in an 

 electric field, i. e. between drops of which the impinging 

 surfaces were oppositely charged by induction. Experiments 

 could alone decide what strength of field would be required 

 to cause coalescence. 



We should not necessarily get an upper cloud of more than 

 very small vertical thickness, even if the upward air-current 

 were sufficient to support the drops when first condensed on 

 the negative ions from the supersaturated vapour. 



For the critical supersaturation (i. e. the supersaturation 

 necessary to cause condensation on the negative ions) will be 

 reached at a level which will be continually rising. The 

 critical supersaturation will be attained by a given portion 

 of the air when it has risen to a definite height, not above the 

 present position of the upper surface of the lower cloud, but 

 above the level which that upper surface had when the 

 portion of air under consideration escaped from it. The level 

 of critical supersaturation would then in the absence of con- 

 densation travel upwards with a velocity depending upon that 

 of the upper surface of the lower cloud. 



The size of the drops which separate out when the critical 

 supersaturation is first reached depends of course on the 

 number of negative ions present, and can be calculated for 

 given conditions. If these initially formed drops are suffi- 

 ciently small to be carried upwards faster than the upward 

 velocity of the level of critical supersaturation (which we may 

 call the critical velocity), then a layer of cloud will be formed 

 continually increasing in thickness by the condensation of 

 drops at this level of critical supersaturation. Ice-particles 

 would be specially likely to be carried upwards with more 

 than the critical velocity. If, however, the drops condensed 

 on the negative ions are large enough to be carried up with 

 less than the critical velocity, the critical supersaturation 



