Page:Popular Science Monthly Volume 60.djvu/506

498 Take a lofty cumulo-nimbus cloud such as rises in the summer afternoon 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 unsaturated vapor and marks the flat base of the cloud; the second is at the top of the saturated part and at the 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

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 reflected 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° Centigrade 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