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TornadoesWhether it's a slender, graceful tube or a massive, roaring wall of dirt and debris, the tornado is among the most fascinating and frightening of atmospheric phenomena. Each year more than 1,000 twisters touch down in the United States, more than in any other nation by far. However, U.S. death and injury tolls have dropped considerably in recent decades due to better warnings. As part of its research on severe storms, NCAR joins its colleagues to delve into the processes that drive tornado formation and evolution. Virtually all tornadoes develop out of thunderstorms. Dust devils, the rotating columns of dust you might see on a hot, sunny day, are distant cousins of tornadoes. Hurricanes are much larger systems that develop over the tropical ocean. (However, tornadoes can develop within a landfalling hurricane's spiral bands of thunderstorms.) What these weather systems have in common are warm, rising air and rotation. Strong thunderstorms tend to form on a boundary between air masses, similar to a small-scale cold front, that pushes surface air upward. Moisture in the warm air adds to the air's potential buoyancy. As springtime unfolds, the reappearance of unstable, sun-heated air near the surface, combined with still-strong winds aloft, results in frequent tornado activity in the U.S. Gulf Coast states in March. The zone moves north and west toward the upper Midwest by June and July. Tornadoes are rare west of the Continental Divide, where moisture for strong storms is usually lacking. However, every U.S. state, including Alaska and Hawaii, has reported tornadoes. The Ganges Delta of eastern India and Bangladesh is another favored area for strong tornadoes. Parts of Europe, South Africa, and Australia also report a number of tornadoes each year, most of them relatively weak. Tornado types and researchOn the U.S. High Plains from Colorado to Montana, the most frequent type of tornado is a landspout—a relatively weak vortex much like a waterspout, the tornado-like vortex observed over oceans or lakes. Both landspouts and waterspouts form when a weak spin-up near ground level (or sea level) is stretched and narrowed by rising air currents within a building cumulus or cumulonimbus cloud. Like a figure skater pulling in her arms to speed up a spin, the rotation intensifies as it gets stretched upward and focused inward.
The most intense tornadoes form out of supercells: discrete, long-lived severe thunderstorms. Most supercells form in an environment where wind direction or speed changes sharply with height. This wind shear produces bundles of air that rotate along a horizontal axis, like a giant rolling pin. If these bundles flow into a supercell, they can enhance the storm's rotation. Using computer models in the early 1980s, NCAR scientists were among the first to discover and analyze this process. What finally produces a tornado from a supercell? The answer is still not clear, but field work in the 1990s yielded some important clues. NCAR joined a number of agencies and universities in 1994–95 for a project called VORTEX. Instrument-studded cars and aircraft, along with portable Doppler radar, captured the life cycle of several tornadoes in detail. The data, which are still being analyzed, hint that a strong downdraft at the rear of a supercell may combine with the storm's intense updraft to produce small-scale wind shear that may be key to a tornado's development. Tornadoes vary tremendously in appearance. Even the same funnel cloud can look different when viewed at different times or from different angles. Many tornadoes emerge from a wall cloud, a rotating cloud hanging from a rain-free part of a supercell. However, most wall clouds do not produce tornadoes. In arid or semiarid climates, a tornado funnel may look clear or translucent even when the circulation extends fully to the ground. Large tornadoes may resemble a cloud on the ground rather than a narrow funnel. Especially in the eastern United States, rain may wrap around a tornado's circulation, making it hard to spot the vortex.
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