Figure 8.27 A tornado rips through Dimmitt, TX
Tornadoes are the most powerful weather phenomenon known. A tornado is an intense system of low pressure with violent updrafts and converging winds. Though tornadoes have been intensely studied for years, the mechanism that actually creates them still eludes us. Tornadoes have been documented in most all the regions of the Earth, though they are most prevalent in the United States.
Figure 8.28 Supercell thunderstorm are especially capable
of spawning tornadoes.
Tornadoes are spawned from severe thunderstorms. Wind shear, where winds are traveling at different speeds and from different directions aloft cause rotation of air about a horizontal axis within the thunderstorm. The rotating circulation is tilted into the vertical by the updrafts of air in a severe thunderstorm. As the rotating air increases in height and shrinks in size a mesocyclone is formed. For whatever reason, a tornado funnel is spawned within the mesocyclone. [ Simulate tornadic conditions at National Geographic's "Force of Nature: Tornadoes" site. ]
The funnel can remain aloft, twisting and turning without wreaking much havoc below, but is most destructive when it touches the ground. A tornado can vary in diameter from a few hundred feet to greater than a mile. Tornadoes typically move across the surface at speeds ranging from 22 - 33 mph (10 - 15 meters per second). [ Watch "Hunt for the Supertwister from PBS].
Figure 8.29 Tornado Formation
Tornadoes that develop over water are called waterspouts. Waterspouts my orignate over water or a tornado passes from land to water. View the video "Watersputs" for more.
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