Page:Handbook of Meteorology.djvu/171

 Tornadoes are less frequent than thunder-storms, but they are the most violent and destructive storms that come into the experience of humanity. The term is loosely applied to almost every violent wind; it is incorrectly applied to any secondary storm that is not a true whirlwind, or “twister.” There are no definite conditions known by which tornadoes may be forecast; but when the path of a northerly storm dips southward and increases in intensity, tornadoes are likely to occur.

The tornado is a whirling storm, and the whirl becomes so rapid that the vortex develops into a black funnel-cloud. The funnel is usually observed first in the air. As the whirl increases, the funnel gradually extends downward to the ground. No measured velocity of the whirl is known to have been made; but calculations based on the weight and the surface of bodies moved by the wind show that the velocity, in various instances, has exceeded 500 miles per hour.

The first visible warning of the tornado is the gathering of a bank of very dense cloud, usually in a westerly quadrant—southwest, west or northwest. The color of the cloud bank varies. Not infrequently it appears much like the smoke from a burning hay barn, or a strawstack; quite frequently it is a dark greenish gray. The color depends on the position of the observer with reference to the sun. The cloud bank is always in tumultuous commotion within itself.

It is in this cloud bank that the funnel of the tornado forms. In some instances, as the tip of the funnel approaches the ground, an inverted funnel is formed at the ground, quickly joining the funnel hanging from the cloud. The funnel is the destructive part of the tornado. It uproots trees, or twists their trunks to the breaking point. Wherever the tornado passes through woodlands its path is marked by uprooted, shattered and twisted trunks of trees. When the funnel strikes a building the latter bursts outwardly. In various instances a roof has been carried in fragments a distance several miles away. Wooden railway bridges have been dismembered and splintered beyond repair, and steel bridges have been torn from their abutments and crumpled into shapeless heaps. Chickens have been almost completely plucked; straw and twigs have been driven