Page:Popular Science Monthly Volume 60.djvu/507

Rh found at the ground in the polar regions during the coldest days of winter. The obvious correction for the suffering of humanity due to the oppressive hot waves of summer is to bring down this upper air, or else arrange to visit it in floating houses.

The reader may have noticed that hail falls usually in the summer instead of in the winter, and have wondered what is the reason for it. The answer is simple. If we arrange a diagram so that freezing temperature is on the left and 100 degrees Fahrenheit on the right, and plot the limits of the rain, hail and snow regions in altitude, the result is shown in a set of curves which indicate the height at which it is possible for them to form respectively. In the summer when the

weather is warm there is more moisture in the air, it rises higher, till in extreme cases it may reach about seven or eight miles. The stages each become wider, though the hail is always confined to a narrow wedge shaped region whose highest place is about four miles. Now in thunderstorms when there is powerful congestion and stratification of air currents in a vertical direction, the conditions are favorable for the forming of snow balls first by congealing the flakes in the lower parts of the upper stage; these fall slowly through the freezing stage and are coated with a layer of ice; they drop through the rain stage and collect a thicker covering of ice till they arrive at the ground as hailstones. Such strata may be intermixed and arranged in alternate layers so that a nucleus of snow may fall through more than one pair of snow and freezing regions in succession, and thus cover itself with several layers of snow and ice, like an onion. Such stratified hailstones are often found when they are cut open. This theory seems to