Tornado

A tornado is a rapidly spinning column of air formed in severe thunderstorms. The rotating column, or vortex, forms inside the storm cloud (cumulonimbus), then grows downward until it touches the ground. When a tornado is visible but does not touch the ground, it is properly called a funnel cloud. A tornado in contact with a body of water is called a waterspout.

A tornado is capable of extreme damage because it packs very high wind speeds into a compact area. Tornadoes have been known to shatter buildings, drive straws through solid wood, lift locomotives from their tracks, and pull the water out of small streams. The United States experiences most of the world's tornadoes, averaging about 800 each year. Most of these tornadoes arise in the states of Texas, Oklahoma, and Kansas. On average, tornadoes are responsible for 80

deaths, 1,500 injuries, and millions of dollars of damage annually in the United States.

Tornado formation

Although tornadoes can occur at any time of the year, most form during the months of March through June, when conditions are right for the development of severe thunderstorms.

In a severe storm, rain that falls from a cloud causes downdrafts (sinking air) in the rear of the cloud. Meanwhile, the advancing edge of the storm has strong updrafts and humid air is pulled into the storm. As this humid air rises and cools, its water vapor condenses to form more water droplets, releasing heat in the process into the surrounding air. This latent heat, in turn, causes the air mass to rise ever more quickly, strengthening the storm.

As updrafts in a severe thunderstorm cloud get stronger, more air is pulled into the base of the cloud to replace the rising air. Some of this air may be rotating slightly since the air around the base of a thunderstorm always has a certain amount of vorticity or "spin."

Words to Know

Fujita Tornado Scale: A scale of six categories that rates tornado wind speed based upon the observed destruction of the storm.

Funnel cloud: A fully developed tornado vortex before it has touched the ground.

Latent heat: The heat released when water vapor condenses to form liquid water.

Vortex: A rotating column of a fluid such as air or water.

Vorticity: The tendency of an air mass to rotate.

Waterspout: Tornado in contact with a body of water.

As the air converges into a smaller area it begins to rotate faster due to a law of physics known as the conservation of angular momentum. This effect can be seen when an ice skater begins slowly spinning with arms outstretched. As the skater brings his or her arms inward, the skater's rate of rotation increases dramatically. In the same way, as air converges into the strong updraft of an intense thunderstorm, its rate of spin increases. Meteorologists still are unsure whether tornadoes form deep within clouds and extend downward or form underneath the cloud and extend upward. It is possible that both situations occur.

Tornado characteristics

Tornadoes move with the thunderstorm to which they are attached at an average speed of 35 miles (56 kilometers) per hour. They have an average path length of about 5 miles (8 kilometers). The diameter of a tornado can vary from 300 feet to 1 mile (90 meters to 1.6 kilometers). Tornadoes come in a variety of shapes and sizes, and often have an ominous dark color due to the soil and other debris they pick up as they move along.

Dust Devil

A dust devil is a relatively small, rapidly rotating wind that stirs up dust, sand, leaves, and other material as it moves across the ground. Dust devils also are known as whirlwinds or, especially in Australia, willy-willys. In most cases, dust devils are no more than 10 feet (3 meters) wide and less than 300 feet (100 meters) high.

Resembling mini-tornadoes, dust devils form most commonly on hot dry days in arid regions such as a desert. They originate when a layer of air lying just above the ground is heated and begins to rise in an updraft. Winds blowing in the area cause this rising air mass to rotate, either clockwise or counterclockwise. In some cases, wind speeds can easily exceed 50 miles (80 miles) per hour. Some large and powerful dust devils have been known to cause property damage. In the vast majority of cases, however, dust devils are too small to pose a threat to buildings or to human life.

Tornado strength is classified by the Fujita Tornado Scale, or F-scale. Developed by T. Theodore Fujita of the University of Chicago, the scale measures the power and destructiveness of tornadoes. The six categories of the scale (F0 through F5) classify a tornado by the amount of damage it causes—from light to incredible—and its wind speed—from 40 to more than 300 miles (64 to more than 482 kilometers) per hour. It is estimated that 90 percent of all tornadoes have wind speeds below 115 miles (185 kilometers) per hour.

Tornado history

The deadliest tornado in United States history was the Tri-State tornado on March 18, 1925. Beginning in Missouri, the tornado stayed on the ground for almost 220 miles (350 kilometers), moving into Illinois and Indiana. In places, it left a trail of damage almost 1 mile (1.6 kilometers) wide. The Tri-State tornado plowed through nine towns and destroyed thousands of homes. When the storm was over, 695 people had lost their lives and more than 2,000 were injured.

Another historic storm was the severe tornado outbreak of April 3-4, 1974. This so-called "Super Outbreak" triggered 148 tornadoes over 13 states, devastating an area from Alabama to Michigan. More than 300 people were killed and more than 5,000 were injured. Property damage was approximately $500 million.

On May 3, 1999, a storm started near the town of Lawton in southwestern Oklahoma. By the end of the day, it had grown into a violent storm system with a reported 76 tornadoes. As the storm system tore across central Oklahoma and into Kansas, more than 40 people were killed, over 500 were injured, and more than 1,500 buildings were destroyed. One of the tornadoes in the system, classified as an F5, had a diameter of 1 mile (1.6 kilometers) at times and stayed on the ground for more than 4 hours.

Tornado prediction and tracking

The precise tracking and prediction of tornadoes is not yet a reality. Meteorologists can identify conditions that are likely to lead to severe storms and can issue warnings when atmospheric conditions are right for the development of tornadoes. They can use radar to track the path of thunderstorms that might produce tornadoes. Yet it is still not possible to detect a funnel cloud by radar and predict its path, touchdown point, and other important details. Scientific research in this area continues.

[See also Atmospheric pressure; Cyclone and anticyclone; Thunderstorm ]

tornadoes Tornadoes are small and short-lived but highly destructive phenomena. Because of their severe nature and small size, comparatively little is known about their detailed structure: measurement and observation within them is difficult. They comprise elongated funnels of cloud which descend in a snake-like fashion from the base of a well-developed cumulonimbus cloud, eventually making contact with the ground beneath. They represent a zone of marked convergence into the cloud base generated by the intense updraughts that feed the cloud system, and are manifested as a narrow columnar zone of upwardly rotating violent winds, perhaps exceeding 100 metres per second. The vortex core is at very low atmospheric pressure, so that cloud develops within the funnel, and pressure gradients can approach an estimated 25 mbar (hPa) per 100 m. This compares to the most extreme pressure gradients of about 20 mbar per 100km around a larger-scale cyclone.

A tornado's impact as a hazard is extreme. There are three damaging factors at work. First, the winds are often so strong that objects in the tornado's path are simply removed or very severely damaged. The pronounced rotational element also tends to twist objects from their fixing, and strong uplift can carry some debris upwards into the cloud. More typically, the heavier material is thrown tangentially outwards by the system's rotational winds: the second source of hazard. The third source of damage is the very low atmospheric pressure near the vortex centre. When a tornado approaches a building, external pressure is rapidly reduced, and unless there is a nearly simultaneous and equivalent decrease in internal pressure, by opening windows or doors, the walls and roof may explode outwards in the process of equalizing the pressure differences.

Many tornadoes have a short life. The convectional activity which creates the source cloud is itself highly variable, and a single cloud can spawn a number of different tornado vortices, either simultaneously or in sequence, beneath different areas of the cloud, as parts of it develop and decay. Movement is generally with the parent cloud, perhaps with the funnel twisting sinuously across the ground beneath. Once contact with the ground is made, the track of a tornado at ground level may frequently extend for only a few kilometres, though there are examples of sustained tracks extending over hundreds of kilometres. The diameter of the funnel is rarely more than 200 m; track length and width are consequently modest, and the probability of being affected is comparatively remote—though this is no consolation to those afflicted.

Tornadoes, being associated with extreme atmospheric instability, show both seasonal and locational preference in their incidence. ‘Favoured’ areas are temperate continental interiors in spring and early summer, when insolation is strong and the air may be unstable, although many parts of the world can be affected by tornado outbreaks at some time or another. The Great Plains of the USA, including Oklahoma, Texas and, of course, Kansas, have by far the highest global frequency, and are particularly prone at times when cool, dry air from the Rockies overlies warm, moist ‘Gulf’ air.

Graham Sumner

Bibliography

Eagleman, J. R.,, Muirhead, V. U.,, and and Willems, N. (1975) Thunderstorms, tornadoes and building damage. Lexington Books, London.

TORNADOES

TORNADOES. A product of an unusually powerful thunderstorm, a tornado is a naturally occurring atmospheric vortex of air spiraling at a very high speed, usually about 250 miles per hour or more, forming a funnel, and extending from the ground to the base of a convective cloud. The shape of the funnel depends on air pressure, temperature, moisture, dust, rate of airflow in the vortex, and whether the air in the tornado's core is moving upward or downward. A tornado can also have multiple vortices. Double vortices are often produced when the upper vortex turns in the direction opposite to the circular motion of the lower vortex. Because of all these factors, very few tornadoes look like true funnels. Tornadoes cause one-fifth of natural-disaster losses each year in the United States. The most intense tornadoes can toss a car a half-mile or shatter a house. However, about 80 percent of tornadoes are weak and cause no more damage than severe winds. A tornado can last fewer than 10 seconds or more than two hours. Tornadoes can occur singly or in swarms. There is no agreement among experts on any single theory of tornado formation.

The typical tornado has ground contact for about six miles, marking a path up to 500 feet wide. Tornadoes travel as fast as 35 to 60 miles per hour. The average number of tornadoes in the United States ranges between 700 and 800 per year, exceeding 1,000 in some years, most notably 1973,1982,1990, and 1992. Tornadoes occur most frequently in Texas, followed by Oklahoma and Kansas. Most tornado fatalities happen in the deep South and generally total fewer than 100 per year, although 350 people died in the 1974 tornado that swept through Alabama, Georgia, Tennessee, Kentucky, and Oklahoma on 3 and 4 April.

Although tornadoes have been reported in every state, most occur in the Gulf States and in the Midwest. The west-to-east airflow across the United States is interrupted by the Rocky Mountains, which push the air currents upward; they fall suddenly as they reach the Great Plains. If moisture-laden air is pulled in from the Gulf of Mexico and meets the high dry air over the plains, that confluence creates the conditions for a tornado. Tornado season begins in early spring in the deep South and progresses northward, with two-thirds of tornadoes occurring from March to June. Tornadoes are most likely to form in late afternoon, but they can occur at any time of day on any day of the year.

The National Severe Storms Forecast Center in Kansas City, Missouri, is responsible for issuing warnings of approaching tornadoes. Tornado predictions are based on meteorological conditions in combination with unusual patterns on the weather radar. Although the approach of a tornado can be forecast only 50 percent of the time, warnings have become important in reducing the death toll.

BIBLIOGRAPHY

Eagleman, Joe R. Severe and Unusual Weather. New York: Van Nostrand Reinhold, 1983.

Grazulis, Thomas P. The Tornado: Nature's Ultimate Windstorm. Norman: University of Oklahoma Press, 2001.

Mary AnneHansen

See alsoDisasters ; Great Plains ; Meteorology ; Midwest .

tornado dark, funnel-shaped cloud containing violently rotating air that develops below a heavy cumulonimbus cloud mass and extends toward the earth. The funnel twists about, rises and falls, and where it reaches the earth causes great destruction. The diameter of a tornado varies from a few feet to a mile; the rotating winds may attain velocities of 200 to 300 mi (320–480 km) per hr, and the updraft at the center may reach 200 mi per hr. The Enhanced Fujita scale is the standard scale for rating the severity of a tornado as measured by the damage it causes. A tornado is usually accompanied by thunder, lightning, heavy rain, and a loud "freight train" noise.

In comparison with a cyclone or hurricane , a tornado covers a much smaller area but can be very violent and destructive. Under the right conditions, however, a large storm system can produce multiple (more than a hundred in rare cases) and longer-lasting tornadoes over a wide area, leading to widespread damage. The atmospheric conditions typically required for the formation of a tornado include great thermal instability, high humidity, and the convergence of warm, moist air at low levels with cooler, drier air aloft. Wind shear at the back of large thunderstorm can create horizontally spinning vortices that are pulled into the stormcloud by updrafts to form a mesocyclone, a rotating, upward-flowing columnar air mass; a tornado may form from the base of an intense mesocyclone.

Although tornadoes have occurred on every continent except Antarctica, they are most common in the continental United States, where tornadoes typically form over the central and southern plains, the Ohio valley, and the Gulf states. The area where the most violent storms commonly occur in the United States is known as Tornado Alley, which is usually understood to encompass the plains from N central Texas north to the Dakotas, with the peak frequency located in Oklahoma. A tornado typically travels in a northeasterly direction with a speed of 20 to 40 mi (32–64 km) per hr, but tornadoes have be reported to move in a variety of directions and as fast as 73 mi (117 km) per hr—or to hover in one place. The length of a tornado's path along the ground varies from less than one mile to several hundred. Tornadoes occurring over water are called waterspouts .

Bibliography: See J. Verkaik and A. Verkaik, Under the Whirlwind: Everything You Need to Know about Tornadoes but Didn't Know Who to Ask (1998); H. B. Bluestein, Tornado Alley: Monster Storms of the Great Plains (1999).

tor·na·do / tôrˈnādō/ • n. (pl. -does or -dos) a mobile, destructive vortex of violently rotating winds having the appearance of a funnel-shaped cloud and advancing beneath a large storm system. ∎ fig. a person or thing characterized by violent or devastating action or emotion: a tornado of sexual confusion. DERIVATIVES: tor·nad·ic / -ˈnādik; -ˈnadik/ adj.

tornado Funnel-shaped, violently rotating storm extending downwards from the cumulonimbus cloud in which it forms. At the ground its diameter may be only c.100m (310ft). Rotational wind speeds range from 160 to 480km/h (100 to 300mph). Tornadoes occur in deep low pressure areas, associated with fronts or other instabilities. They are most frequent in the Midwest and s USA.

tornado A relatively small-scale (about 100 m diameter) ‘twisting’ or rotating column of air, like a funnel, with high wind speeds and great destructive force over the narrow path of its movement. Such systems are especially frequent in unstable air conditions in the central parts of the USA.

tornado A relatively small-scale (about 100 m diameter) ‘twisting’ or rotating column of air, like a funnel, with high wind speeds and great destructive force over the narrow path of its movement. Such systems are especially frequent in unstable air conditions in the central parts of the USA.

tornado †violent thunderstorm of the tropical Atlantic XVI; rotatory storm of Africa, etc. XVII (ternado). perh. orig. alt. — Sp. tronada thunderstorm (f. tronar thunder), later assim. to tornar TURN; see -ADO.

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