Tornado

:For other uses of Tornado, see Tornado (disambiguation).

Tornado formation

[[Image:Tornado_radar_hook_echo.gif|thumb|300px|left|A classical "hook echo" as seen in the strongest tornado in the 1999 Oklahoma Tornado Outbreak. At the time of this image, the tornado was crossing Interstate 44 near the Canadian River, after producing F5 damage in Bridge Creek, Oklahoma, and before more F5 damage in Moore, Oklahoma|Moore. The bright red colors at the tornado location represent not rain or hail—but the aggregate signature of car parts, pieces of houses, shredded tree branches, dirt and other debris, hoisted thousands of feet skyward by the tornado vortex. Some survivors of this storm were interviewed and had their story broadcast on The Weather Channel's drama "Storm Stories" by meteorologist Jim Cantore.

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Source: U.S. NOAA National Weather Service]]

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Tornadoes develop from thunderstorms, most frequently supercell thunderstorms, though they also occur within squall lines and hurricanes. They are believed to be produced when cool air overrides a layer of warm air, forcing the warm air to rise rapidly, though tornadoes over water (waterspouts) frequently are observed forming in the absence of convection or apparent strong surface temperature differences. Tornadoes, lightning, and sometimes hail are associated with thunderstorms. Many tornadoes appear at the tail end of mesocyclones. On weather radar screens, a characteristic "hook echo" marks the area where tornadoes are likely to exist.

Related Topics:
Supercell - Thunderstorm - Squall line - Hurricane - Waterspout - Lightning - Hail - Mesocyclone - Weather radar - Hook echo

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Exactly how tornadoes form is complex and not fully understood. When thunderstorms develop, an increase in wind speed and/or a large change in direction with height ("wind shear") produces a horizontal, spinning area of air. The strong updrafts within the thunderstorm can draw this area of rotation up from horizontal to vertical. Towards the end of this area of rotation (the mesocyclone) is often a lower area of rain-free cloud and can be seen as a rotating "wall cloud". If the rotation intensifies, funnel clouds can develop within this area and form tornadoes when they reach the ground. Why the rotation can intensify and form tornadoes is not understood.

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Tornadoes normally rotate in a cyclonic direction as the warm air thunderstorms usually form in sweeps north and jet streams come from the west, creating a situation in which the storms rotate. In the northern hemisphere, this rotation is counterclockwise, and in the southern hemisphere, clockwise. The tornadoes usually rotate the same way. Sometimes opposite direction swirls develop under a thunderstorm. About 1 in 100 tornadoes rotate in an anticyclonic direction.

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Tornado damage to man-made structures is the result of high wind velocity and the associated windblown debris. Tornadic winds have been measured well in excess of 300 mph (480 km/h). Tornado season in North America is generally March through October, although tornadoes can occur at any time of year. They tend to occur in the afternoons and evenings: over 80 percent of all tornadoes strike between noon and midnight.

Related Topics:
Mph - North America

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Tornadoes can be nearly invisible, marked only by swirling debris at the base of the funnel. While tornadoes are invisible at night, some nocturnal tornadoes have been observed glowing diffusely due to lightning activity. Verified observations by Hall and others suggest a cellular structure inside tornadoes. Some tornadoes are composed of several mini-funnels. A tornado must by definition have both ground and cloud contact.

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Waterspouts

A waterspout is a tornado that occurs over water, usually not in association with a supercell thunderstorm. Although waterspouts are always tornadoes by definition, they are not counted in official tornado records unless they hit land. They are smaller and weaker than the most intense Great Plains tornadoes, but still can be quite dangerous. Waterspouts can overturn small boats, damage ships, and cause fatalities and significant damage when hitting land. http://www.spc.noaa.gov/faq/tornado/ USA Today article on waterspouts

Related Topics:
Supercell - Great Plains

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