RAIN AND ITS ORIGIN IN THUNDERSTORMS. 409 



100 times to give the charges measured. There is no reason for considering that this 

 would be impossible with violent and widespread ascending currents. 



The air which passes through the accumulation of water at the head of the 

 ascending current carries with it the negative electricity separated during the 

 splashing. This electricity is rapidly absorbed by the cloud particles through which 

 the air streams in its upward course, and it is very probable that large negative 

 charges could in consequence be accumulated in the cloud. Thus the cloud over the 

 ascending current will consist of negatively charged water particles, and these will 

 coalesce to form rain having a negative charge. There is no means of estimating 

 what negative charge might be expected, but there is no reason for considering that 

 it should be smaller than the positive charge brought down by the water which 

 has been broken up several times at the head of the ascending current. Thus it 

 would appear that the process could provide both the positively and negatively 

 charged rain actually observed. 



The quantitative estimate which lias just been made has been based on values 

 which cannot be considered as being anything more than the roughest approximations. 

 It shows, however, that it is not necessary to assume ascending currents of more than 

 8 metres a second to supply enough electricity for a considerable amount of lightning 

 discharge, and that given reasonably rapid ascending currents, sufficient separation of 

 electricity could take place to account for the most violent thunderstorms. 



The preceding discussion will now be summed up in an account of the probable 

 mechanism of a thunderstorm. 



When extensive ascending currents occur in air which is not exceedingly dry, the 

 formation of cumulus clouds with possible precipitation will naturally follow. As 

 the ascending currents become more and more rapid large amounts of water will be 

 held in suspension, until finally, when they attain a greater velocity than 8 metres 

 a second, all water will be retained. As a consequence there will be considerable 

 breaking up of drops in the air accompanied by a separation of electricity, by which 

 the water becomes positively charged and the air negatively charged. 



The electrical effects react on the rate of splashing. Uncharged drops combine 

 only with difficulty, and rebound from one another as though they were solid. 

 Charged drops, on the contrary, combine with facility to form single large drops.* 

 Thus as the water becomes more and more highly charged the drops will the more 

 rapidly grow to the size necessary for them to be broken again, and as a 

 consequence the greater will be the splashing and the greater the rate of electrical 

 separation. 



An ascending current must at some place in its ascent spread out horizontally and 

 so have its vertical velocity reduced. At the part of the current where the velocity 

 falls below 8 metres a second a large accumulation of water will in all probability 

 take place, and this will be the seat of the greatest amount of separation of 



* RAYLEIGH, 'Roy. Soc. Proc.,' vol. 28, p. 406, 1879. 



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