What experience have you had of tornadoes?

Tornado

Tornado Photo courtesy of NSSL

A tornado is a violently rotating column of air. At its top is in a storm cloud, the most intense of which are called supercells, and its funnel touches the ground. The rotating winds are so strong that they can do major damage. If there is enough water vapour or dust they will be clearly visible during the day. They generally occur where a front of cold air meets a body of warm humid air. The warm air rises and the cool air falls, creating horizontal spin. Vertical wind shear (variation of wind speed with height) plus the nearby downdraft of cooler drier air causes the spin to shift to the vertical. The middle of the vortex is at low pressure. Strong tornadoes can pick up objects like cars with ease and destroy buildings. Tornadoes may be accompanied by lightning and hail. Tornadoes which are over water are called waterspouts. Waterspouts are not usually associated with supercell thunderstorms, and they are not counted in tornado statistics unless they hit land.

In the USA tornadoes are measured according to the Fujita scale with a range of 0 to 5. Winds of up to 130 meters per second or 300 miles per hour have been observed. Usually the wind speed is estimated from damage after the event rather than from direct readings. Tornadoes can be up to 5 or more kilometers wide at ground level, and can track along the ground for well over a hundred kilometres. About two percent of tornadoes have wind speeds exceeding 200 mph. Usually tornadoes and the storms that form them rotate counterclockwise in the northern hemisphere and clockwise in the southern hemisphere, but about 1 percent of tornadoes spin in the opposite (anticyclonic) direction.

The area from Nebraska to Texas and Oklahoma in the USA is known as ‘tornado alley’, as there are more strong tornadoes there than elsewhere in the country. The cold dry air comes from the Rockies and the arm moist air from the Gulf of Mexico. Tornadoes there are more common in late spring and summer. While the USA tends to have the most violent tornadoes, England (roughly the size of Oklahoma) followed by the Netherlands tend to have the most per unit area, though these rarely case much damage. For example, on 3-4 April 1974 the USA experienced 148 tornadoes in 16 hours in 13 states. On 21 November 1981 just one part of Britain experienced 105 tornados in five and a half hours. Central Oklahoma has the most tornadoes per unit area in the USA. It varies a lot, but the UK has on average about 33 tornadoes reported a year in an area 38 times smaller than the USA, so there is twice the chance of seeing a tornado compared to the U.S. Tornadoes have been seen in all 50 states of the USA, and around the world. In North America most tornadoes occur between March and October, and between noon and midnight. Florida has the highest number of tornadoes per unit area in the States. Contiguous USA has about 800 to 1,000 tornadoes per year.

Vulnerable areas in the USA prepare for the tornado season with education and precautions. Many people buy NOAA Weather Radios which activate when the National Weather Service issues severe weather warnings. There are also sirens and warnings on TV and radio. People are advised to go to a central, windowless room or corridor of a solid building, below ground if possible, and to shelter against falling debris under a strong table, doorway or beam. Some regions have storm cellars. Mobile homes, vehicles and underneath bridges are unsafe. If you cannot get out of the path of a storm or into a study building, get into a ditch. Families should have plans for how they will contact each other after a tornado, as they will not necessarily have been together. People should also keep emergency supplies in their homes.

Forecasting tornadoes is a far from exact science, and America relies on storm watchers as well as technology to spot them. Some people actually chase storms, whether for the thrill or for the science.

Two Dead After Tornado Hits Wyoming Town

A tornado destroyed 40 to 50 mobile homes in Wright, Wyoming, killing two people and injuring about a dozen. One person was missing. There were only a few minutes warning.

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What experience have you had of tornadoes?

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