hardly warranted in considering the tornado as an exaggerated thunderstorm.
Desert Whirlwinds.—Dust spouts are common in desert regions. A little after sunrise during warm weather, the still air next to the ground becomes very much warmer than the air at the distance of a few hundred feet above the ground. In time the unstable equilibrium is upset and chimneys of updraught are formed here and there, carrying columns of fine dust to a height of several hundred feet. At a distance the dust columns are strongly outlined. When the cold air has settled to the ground the whirl and its dust column ceases. Later in the day, the setting in of a steady wind puts an end to the unequal warming of the air.
Apache Indians have made use of the desert whirls as signals, creating them by setting fire to the spines of a columnar cactus that is common in the southwestern states. The burning of the spines at the right moment made enough heat to start the up- draught. When the warm air at the surface has been pressed upward the descending air is perceptibly colder at times.
Waterspouts.—If the whirl of the updraught over water increases to a velocity whereby the skin friction of the wind overcomes the cohesion of the water, a waterspout is formed. The whirl of the updraught is strong enough to whisk the water into the air, at the same time whirling it into a mist. Undoubtedly some of the water drawn into the air is vaporized. When the spout breaks, a considerable part of the water in the air drops in a torrential deluge. Popular tradition has it that sea water drawn into a spout falls as fresh water—a tradition that is contrary to the facts of the case.
White squalls are fair-weather whirlwinds over the water. In many instances there is not enough condensation in the air to form a cloud; occasionally, however, a bit of misty cloud, the “bull’s eye,” is visible. At the surface, the wind is strong enough to whisk the water into white spray, but the whirl is not strong enough to draw it into the updraught.
