National Geographic : 1925 Aug
174 THE NATIONAL GEOGRAPHIC MAGAZINE TIIE TORNADO CLOUD, TERROR OF TIE SKIES When a writhing black serpent of cloud forms, it means grave danger for those below. It is really a great suction tube that picks up trees and stones and man and most of his works as easily as a vacuum cleaner sucks up grains of dust and wisps of lint. Tornadoes have occurred in nearly every section of the United States. The one pictured passed close to the State Capitol of Texas, at Austin, a point farther south than the usual range of these destructive storms. material, such as (lust motes-a sort of magic carpet-and crowd upon them. Dust usually is present abundantly, thanks to the winds and to volcanoes; and in the thickly peopled regions of the earth thousands of chimneys throw into the air in their smoke billions of potential cloud nuclei. Even over the center of the greatest ocean there are as many as 750 dust par ticles in a thimbleful of air, and over the streets and buildings of large cities per haps 200,000. In one cubic foot of air in any of our great cities there are twice as many dust motes as there are human inhabitants of the earth. Widely separated, the dust motes, with their vapor passengers, at first float about like asteroids in space, but gradually the cold of the upper regions causes more and more of the vapor molecules to jump out of their gaseous form and attach them selves to existing droplets until the latter are built into drops heavy enough to fall earthward. Sometimes the first drops of a thunder shower seem huge, as they flash past; but even the largest raindrops are relatively small. What is called popularly a "light rain," which just escapes being "drizzle," is made up of droplets only a trifle larger than the little black period that marks the end of this sentence. A "moderate rain" consists of drops with diameters only alout twice that of the period, and the distance through a drop from a "heavy rain" is alout that across four or five periods touching one another in a row. A rain classified as "excessive" has drops about the size of a capital "0" of T'Il (0eOGRAPHIIC'S type. \Vhen drops larger than the latter fall steadily, the downpour is called a cloudburst. And they can be only a little larger at that. Nature, through the laws of physics, has set strict limits both upon the size of raindrops and upon the speed at which they can fall: and the drop that attempts to pass either limit is promptly blown to pieces.