While it might seem like they appear out of nowhere to wreak havoc on houses and land, tornadoes don't simply come into being without provocation. Typically, the most important factor in the creation of these devastating phenomena is the existence of a thunderstorm. Thunderstorms produce an upward flow of warm, low-pressure air that merges with and augments already growing clouds. As these clouds move, they can sometimes form a vortex known as a mesocyclone, which is the precursor to a tornado and is usually about 2 to 6 miles (3 to 10 kilometers) wide. Once a mesocyclone forms, it has a 50 percent chance of becoming a tornado, usually within about 30 minutes [source: NOAA]. As rainfall in the storm increases, a powerful downward suction may form, dragging the mesocyclone down toward the ground. When the mesocyclone makes contact with the ground, the cyclone becomes a full-fledged tornado.
Thunderstorms generate enormous amounts of energy. In fact, the average thunderstorm releases around 10 million kilowatt-hours of energy, which is roughly equivalent to a 20-kiloton nuclear warhead [source: Britannica]
While tornadoes usually originate from storms, there are other events that can bring about a tornado. In certain circumstances, tornadoes can be formed without a thunderstorm present. Whenever air -- or a fluid -- achieves a high enough level of rotation around a downward force, there's a chance of a vortex being created. In deserts, cool wind flowing over hot ground can sometimes result in a swirling pillar of sand and air called a dust devil. Even more unusual is the phenomenon known as a fire whirl, which is a tornado of wild, uncontrollable flames that manifests during a wildfire. Perhaps the most common of all, though, is the vortex that forms in shower drains; the cyclical force of water as it runs down the drain results in a miniature whirlpool.
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