Theory of Thunderstorm Electricity. 631 



under all circumstances be an impossibility, but it is not 

 difficult to show that we have here one of those cases in 

 which the probability is so small that for practical purposes 

 the word " improbable " may be replaced by " impossible." 



The steps o£ the process leading up to the formation of 

 dust-free air and then to a fourfold supersaturation have been 

 so carefully described by Wilson and Gerdien that it is 

 surprising to find that they both have left out of account a 

 step which appears to be of vital importance to the theory ; 

 that is, the step by which the first few metres of the super- 

 saturated stratum come into existence. 



Wilson and Gerdien both commence their reasoning with 

 a cumulus cloud ready formed, and having a supersaturated 

 layer above it, into which more dust-free air is continually 

 being passed from the cloud itself. We will start still further 

 back in the history of the process, and consider the formation 

 of the cloud and then of the supersaturated stratum. 



Thunderstorms generally occur on warm calm days, and 

 usually follow hot cloudless mornings. This is not always 

 the case, but as we can only go into typical instances, it will 

 be as well to consider a storm forming under some such 

 conditions. We have, then, to start with a cloudless sky, 

 and the air near the ground rapidly increasing in temperature 

 as the dav advances. First weak ascending currents form ; 

 then as the ascending currents become stronger and penetrate 

 higher condensation takes place at the upper surface of one 

 of the more extensive of them. Thus a cumulus cloud is 

 formed in dusty air with dusty air above it as well as below 

 it. Now the ascending current will go on rising, and so 

 will be continually pushing its way into the dusty air above. 

 We will grant that the top of the ascending current is com- 

 posed of air which has passed through the cloud, and is in 

 consequence dust free, still it must be remembered that this 

 air cannot push back the air around it without mixing with 

 it to some extent, and so will have little chance of becoming 

 supersaturated. Thus the dust-free air which rises from the 

 cloud becomes contaminated with the surrounding dusty air, 

 condensation takes place on it, and the cloud grows as fast 

 as the ascending current rises. It may be urged that the 

 first dust-free air which gets through the cloud may mix with 

 the dusty air around, but that the resulting mixture may not 

 be saturated; in this way a partition might be formed 

 between the supersaturated air which subsequently passes 

 out of the cloud and the dusty air above. This, however, 

 neglects the fact that on account of the continuous rises 

 there will always be ultimately, above the supersaturated air, 



