A tornado is a strong atmospheric vortex that originates in a cumulonimbus cloud and then reaches the surface of land or sea, assuming the shape of a dark hose or spout. Its upper part exhibits a funnel-shaped column of rotating winds that blends with the clouds. This atmospheric phenomenon has two movement: rotatory, turning around a vertical axis of a spiraling updraft and a straight downdraft; and sideways, moving horizontally in the direction the storm is heading, creating a path of destruction. Like a hurricane, the rotatory movement is counter-clockwise in the northern hemisphere, while it is clockwise in the southern hemisphere. A tornado is a violent storm characterized by powerful winds that originate around a center of low atmospheric pressure. In the United States and Canada, it usually takes place in the Spring and Summer, especially, between March and July.
The height of a tornado may attain 800–1,500 m. Air within a tornado usually rotates at high speeds, simultaneously rising upward in a spiral and sucking in dust and water. The velocity of rotation is several tens of meters per second. In connection with the lowering of air pressure inside the funnel, there is condensation of water vapor. The water vapor, as well as the funnel cloud, dust, and water, makes the tornado visible. The diameter of a tornado above sea is measured in tens of m, and above dry land in hundreds of meters. The Fujita and the Torro scales are used to rate the strength of a tornado, with the former going from F1 to F5, while the latter from T0 to T11. To be considered a tornado, the vortex of winds must be in contact with both the ground and the clouds above.
The formation of tornadoes is associated with extremely unstable atmospheric stratification and it arises in the warm sector of a center of low pressure, most frequently in advance of a cold front. It travels in the same direction as the cyclone, which moves at a rate of 10–20 m/sec. While in existence, a tornado travels along a path, whose length usually ranges between 10 and 100 miles, although the 1925 Tri-State tornado tore a 219-mile-long path (a 352 km-long track), wreaking havoc as it went.
Tornadoes are accompanied by thunderstorms, rain, and hail. They almost always cause large-scale destruction when sweeping the ground because they draw in objects in their path. The objects the pick up are lifted to considerable heights and moved over great distances. Tornadoes formed at sea, which are called waterspouts, pose a grave danger to ships. When they occur over dry land, they are called twisters in the United States.
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| Above, the 1925 Tri-State tornado, one of the most powerful in history. |
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| The 1979 Red River Valley tornado tearing a path of destruction in Wichita Falls, Texas. |