498 



POPULAR SCIENCE MONTHLY. 



Take a lofty cumulo-nimbus cloud such as rises in the summer after- 

 noon before a thunderstorm, from whose base rain may perhaps be falling, 

 while the top is even higher than Mount Blanc piled upon the summit of 

 the Himalayas. See Chart 2. There are sections which should be passed 

 through the cloud, so as to divide it into three parts, which in fact 

 differ from one another physically though they look alike to the 

 observer. The lowest plane separates the saturated from the un- 

 saturated vapor and marks the flat base of the cloud; the second is at 

 the top of the saturated part and at thfe beginning of the freezing 

 stage; the third is at the top of the freezing and at the bottom of the 

 frozen stage. The freezing stratum is thin and is the place where the 



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Chart 2. The distribution of pressure, temperature, and vapor tension in a lofty cumulo- 

 nimbus cloud, observed by the Weather Bureau, July 29, 1896. 



saturated vapor is passing into water at freezing temperature before 

 it can crystallize into ice. A cloud has, therefore, these three portions, 

 the lower consisting of vapor, the middle of water and the top of ice 

 or snow. They appear to be alike because the light from the sun is re- 

 flected from drop to drop and from flake to flake in its passage through 

 the cloud. The diagram gives the pressures, temperatures and vapor 

 tensions at the ground, and at the several stages, while the height is 

 indicated in miles, meters and feet. This illustration is taken from 

 one of the loftiest clouds ever observed, and it was computed that the 

 temperature fell from 26.4° Centigrade at the ground to — 59° Centi- 

 grade or — 74° Fahrenheit at the height of 8.8 miles. In summer time 

 at the top of a high cloud the same temperature prevails that may be 



