General Information

In contrast to thunderstorms, cyclones often cover thousands of square miles, and the winds blow toward the center of an area of low air pressure. They blow in spirals because they are deflected by the rotation of the Earth. The direction of turn is usually counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Tornadoes, sometimes miscalled cyclones, arise when the conditions that cause thunderstorms are unusually violent. Winds blowing in opposite directions around a strong updraft start a narrow, violent whirl. Centrifugal force throws the air away from the center, leaving a core of very low pressure. This low-pressure core acts as a powerful vacuum, tearing the roofs from houses. Around the edges of the whirl, wind speeds may reach 300 miles (480 kilometers) per hour or more. At first, the tornado's funnel is whitish because it is composed of minute water droplets. After touching down, the funnel becomes dark because of all of the debris it has sucked up. This debris can include rocks, tree limbs, and parts of buildings; tornadoes have been known to pick up automobiles, horses, and whole trees. When the edge of the funnel slams into a building, the debris acts as a circular saw, ripping through everything it touches. A tornado usually moves toward the east at 25 to 40 miles (40 to 64 kilometers) per hour, cutting a narrow swath through everything in its path. Fortunately, tornadoes are only a few hundred feet wide; the edge of one may destroy all of the houses on one side of a street while leaving those on the other side completely unscathed. A tornado that passes over water is called a waterspout.