632 Dr. G. 0. Simpson on the Wilson- Gerdien 



a surface in which the air is both dusty and saturated, and 

 in this layer condensation will take place. As soon as this 

 occurs the drops will lag" behind the ascending current, and 

 will soon be overtaken by the region of supersaturated air. 

 Drops thus fed by supersaturated air will grow more rapidly 

 than the drops in the upper layer of the cloud below, and in 

 consequence will lag behind the ascending current at a greater 

 rate than the latter. Thus the supersaturated stratum which 

 had begun to form will rapidly decrease in size and finally 

 disappear, the top of the cloud becoming once more the 

 boundary between the ascending air and dusty air which it 

 is displacing. 



In other words, an extensive supersaturated stratum could 

 never form above an ascending current. 



(v.) We will, however, for the sake of the discussion, 

 assume that a supersaturated stratum may, as proposed by 

 the theory, come into existence, and then consider whether 

 such a stratum could exist during a thunderstorm. 



It has been pointed out several times that in order to have 

 a supersaturated stratum we must have a total absence of 

 nuclei on which the water could condense. Now every drop 

 of water which leaves the condensation layer has to pass 

 through the supersaturated stratum on its way to the earth, 

 and if the electrical discharges are to be as violent and 

 frequent as we often observe during thunderstorms there 

 must be a very considerable rainfall from the condensation 

 layer. It must be admitted that each rain-drop as it passes 

 through the supersaturated stratum will act as a very 

 efficient nucleus for the condensation of the water vapour, 

 and that no mass of air through which heavy rain is falling- 

 could long remain supersaturated. 



Thus we see that a supersaturated stratum would be an 

 impossibility underneath a condensation layer, condensing 

 water at the rate necessary to bring into play the violent 

 electrical discharges observed during thunderstorms. Hence, 

 we are forced to conclude that the probability of the forma- 

 tion of an extensive supersaturated stratum in the atmosphere 

 is extremely small, and that if even once formed it could 

 not long exist after precipitation commenced from the 

 atmosphere above it. 



(vi.) We will now turn to actual phenomena observed 

 during thunderstorms, and see in how far they support the 

 theory. 



A peculiar formation for thunder-clouds is demanded by 

 the theory : there must be a lower cloud to separate out the 

 dust; then a dust- and cloud-free supersaturated layer of 



