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Tornado

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 causesfrom light to incredibleand its wind speedfrom 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 ]

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Tornadoes

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 .


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tornado

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.

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).

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tornado

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.

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tornado

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.

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tornado

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.

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tornado

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.

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tornado

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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tornado

tornadoforeshadow, shadow •Faldo •accelerando, bandeau, Brando, glissando, Orlando •eyeshadow •aficionado, amontillado, avocado, Bardo, Barnardo, bastinado, bravado, Colorado, desperado, Dorado, eldorado, incommunicado, Leonardo, Mikado, muscovado, Prado, renegado, Ricardo, stifado •commando •eddo, Edo, meadow •crescendo, diminuendo, innuendo, kendo •carbonado, dado, Feydeau, gambado, Oviedo, Toledo, tornado •aikido, bushido, credo, Guido, Ido, libido, lido, speedo, teredo, torpedo, tuxedo •widow • dildo • window •Dido, Fido, Hokkaidocondo, rondeau, rondo, secondo, tondo •Waldo •dodo, Komodo, Quasimodo •escudo, judo, ludo, pseudo, testudo, Trudeau •weirdo • sourdough • fricandeau •tournedos • Murdo

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Tornado

Tornado

A tornado is a rapidly spinning column of air formed in severe thunderstorms. The rotating column, or vortex, forms inside the storm cloud and grows downward until it touches the ground. Although a tornado is not as large as its parent thunderstorm, it 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. Due to a combination of geography and meteorology , the United States experiences most of the world's tornadoes. An average of 800 tornadoes strike the United States each year. Based on statistics kept since 1953, Texas, Oklahoma, and Kansas are the top three tornado states. Tornadoes are responsible for about 80 deaths, 1500 injuries, and many millions of dollars in property damage annually. While it is still impossible to predict exactly when and where tornadoes will strike, progress has been made in predicting tornado development and detecting tornadoes with Doppler radar.

Most tornadoes form in the Northern Hemisphere during the months of March through June. These are months when conditions are right for the development of severe thunderstorms. To understand why tornadoes form, consider the formation and growth of a thunderstorm. Thunderstorms are most likely to develop when the atmosphere is unstable, that is, when atmospheric temperature drops rapidly with height. Under unstable conditions, air near the surface that begins rising will expand and cool, but remains warmer (and less dense) than its surroundings. The rising air acts like a hot air balloon; because it is less dense than the surrounding air, it continues to rise. At some point, the rising air cools to the dew point where the water vapor in the air condenses to form liquid water droplets. The rising column of air is now a visible cloud. If the rising air, or updraft, is sustained long enough, water droplets will begin to fall out of the rising air column, making it a rain cloud.

This cloud will become a severe storm capable of producing tornadoes only under certain circumstances. Severe storms are often associated with a very unstable atmosphere and moving low-pressure systems that bring cold air into contact with warmer, more humid air masses. Such weather situations commonly occur in the eastern and Midwestern United States during the spring and summer months. Large-scale weather systems often sweep moist warm air from the Gulf of Mexico over these regions in a layer 1.21.9 mi (23 km) deep. At the same time, winds aloft (above about 2.5 mi [4 km] in altitude) from the southwest bring cool dry air over the region. Cool air overlying humid air creates very unstable atmospheric conditions and sets the stage for the growth of strong thunderstorms.

The warm surface air is separated from colder air lying farther north by a fairly sharp temperature boundary called a front. A low-pressure center near Earth's surface causes the cold air to advance into the warmer air. The edge of the advancing cold air, called a cold front, forces the warmer air ahead of the front to rise and cool. Because the atmosphere is unstable, the displaced air keeps rising and a cloud quickly forms. Rain that begins to fall from the 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. The water vapor in this air condenses to form more water droplets as it rises and cools. When water vapor condenses, it releases latent heat. This warms the air and forces it to rise more vigorously, strengthening the storm.

The exact mechanism of tornado formation inside severe thunderstorms is still a matter of dispute, but it appears that tornadoes grow in a similar fashion to the small vortices that form in draining bathtubs. Tornadoes appear to be upside down versions of this phenomenon. 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 usually contains some rotation , or vorticity. 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 spinning with arms outstretched. As the skater brings his or her arms inward, his or her rotational speed increases. In the same way, air moving into a severe storm begins to move in a tighter column and increases its rotational speed. A wide vortex is created, called the mesocyclone. The mesocyclone begins to build vertically, extending itself upward throughout the entire height of the cloud. The rapid air movement causes the surrounding air pressure to drop, pulling more air into the growing vortex. The lowered pressure causes the incoming air to cool quickly and form cloud droplets before they rise to the cloud base. This forms the wall cloud, a curtain-shaped cloud that is often seen before a tornado forms. The mesocyclone continues to contract while growing from the base of the storm cloud all the way up to 6.2 mi (10 km) above the surface. When the mesocyclone dips below the wall cloud, it is called a funnel cloud because of its distinctive funnel shape. This storm is on its way to producing a tornado.

A funnel cloud may form in a severe storm and never reach the ground. If and when it does, the funnel officially becomes a tornado. The central vortex of a tornado is typically about 328.1 ft (100 m) in diameter. Wind speeds in the vortex have been measured at greater than 220 mph (138 km/h). These high winds make incredible feats of destruction possible. They also cause the air pressure in the tornado to drop below normal atmospheric pressure by over 100 millibars (the normal day-to-day pressure variations we experience are about 15 millibars). The air around the vortex is pulled into this low-pressure zone where it expands and cools rapidly.

This causes water droplets to condense from the air, making the outlines of the vortex visible as the characteristic funnel-shaped cloud. The low pressure inside the vortex picks up debris such as soil particles, which may give the tornado an ominous dark color. The damage path of a tornado may range from 900 ft (275 m) to over 0.5 mi (1 km) wide.

Tornadoes move with the thunderstorm that they are attached to, traveling at average speeds of about 1030 mph (1545 kph), although some tornadoes have been seen to stand still, while other tornadoes have been clocked at 60 mph (90 kph). Because a typical tornado has a lifetime of about 510 minutes, it may stay on the ground for 510 miles. Occasionally, a severe tornado may cut a path of destruction over 200 mi (320 km) long. Witnesses to an approaching tornado often describe a loud roaring noise made by the storm similar to jet engines at takeoff.

The destructive path of tornadoes appears random. One house may be flattened while its neighbor remains untouched. This has been explained by the tornado skipping or lifting up off the surface briefly and then descending again to resume its destructive path. Studies made of these destructive paths after the storm suggest another possible explanation; some tornadoes may have two to three smaller tornado-like vortices circling around the main vortex. According to this theory, these suction vortices may be responsible for much of the actual damage associated with tornadoes. As they rotate around the main tornado core, they may hit or miss objects directly in the tornado's path depending on their position. The tornado's skipping behavior is still not completely understood.

When houses or other structures are destroyed by a tornado, they are not simply blown down by the high winds; they appear to explode. High wind passing over a house roof acts like the air moving over an airplane wing: it gives the roof an upward force or lift, which tends to raise the roof vertically off the house. Winds also enter the building through broken windows or doors pressurizing the house as one would blow up a balloon. The combination of these forces tends to blow the walls and roof off the structure from the inside out, giving the appearance of an explosion.

Tornado strength is classified by the Fujita scale, which uses a scale of one to six to denote tornado wind speed. Since direct measurements of the vortex are not possible, the observed destruction of the storm is used to estimate its "F scale" rating.

The single most violent tornado in United States history was the Tri-State tornado on March 18, 1925. Beginning in Missouri, the tornado stayed on the ground for over 220 mi (350 km), crossing Illinois, moving into Indiana, and leaving a trail of damage over 1 mi (1.6 km) wide in places. Tornado damage often is limited since they usually strike unpopulated areas, but the Tri-State tornado plowed through nine towns and destroyed thousands of homes. When the storm was over, 689 people had lost their lives and over 2,000 were injured, making the Tri-State the deadliest tornado on record.

On May 3, 1999, a storm started in southwestern Oklahoma, near the town of Lawton. By late in the day, it had grown into a violent storm system with 76 reported tornadoes. As the storm system tore across central Oklahoma and into Kansas, over 43 people were killed, over 500 injured and more than 1,500 buildings were destroyed. One of the tornadoes, classed as a F-5, was as much as a mile wide at times and stayed on the ground for over four hours.

The precise tracking and prediction of tornadoes is not yet a reality. Meteorologists can identify conditions that are likely to lead to severe storms. They 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. It is still not possible, however, to detect a funnel cloud by radar and predict its path, touchdown point, and other important details. Much progress has recently been made in the detection of tornadoes using Doppler radar.

Doppler radar can measure not just the distance to an object, but also its velocity by using the Doppler effect: if an object is moving toward an observer, radar waves bounced off the object will have a higher frequency than if the object were moving away. This effect can be demonstrated with sound waves. If a car is approaching with its horn sounding, the pitch of the horn (that is, the frequency of the sound waves) seems to rise. It reaches a peak just as the car passes, then falls as the car speeds away from the listener.

Doppler radar is used to detect the motion of raindrops and hail in a thunderstorm, which gives an indication of the motion of the winds. With present technology, it is possible to detect the overall storm circulation and even a developing mesocyclone. The relatively small size of most tornadoes makes direct detection difficult with the current generation of Doppler radar. In addition, any radar is limited by the curvature of Earth. Radar waves go in straight lines, which means distant storms that are below the horizon from the radar cannot be probed with this technique.

See also Atmospheric pressure; Clouds and cloud types; Weather forecasting; Weather forecasting methods; Weather radar; Weather satellite

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Tornado

Tornado ★ 1983

An army sergeant revolts against his superiors and the enemy when his captain leaves him stranded in Vietnam. 90m/C VHS . Giancarlo Prete, Tony Marsina, Alan Collins; D: Anthony M. Dawson.

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Tornado

Tornado

Tornado formation

Tornado characteristics

Tornado history

Prediction and tracking of tornadoes

Resources

A tornado is a rapidly spinning column of air formed in severe thunderstorms. The rotating column, or vortex, forms inside the storm cloud then grows downward until it touches the ground. Although a tornado is not as large as its parent thunderstorm, it can cause 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. Because of a combination of geography and meteorology, the United States experiences most of the worlds tornadoes. An average of 800 tornadoes strike the United States each year. Based on statistics kept since 1953, Texas, Oklahoma, and Kansas are the top three tornado states. Tornadoes are responsible for about 80 deaths, 1,500 injuries, and many millions of dollars in property damage annually. Although it is still impossible to predict exactly when and where tornadoes will strike, progress has been made in predicting tornado development and detecting tornadoes with Doppler radar.

Tornado formation

Most tornadoes form in the northern hemisphere during the months of March through June. These are months when conditions are right for the development of severe thunderstorms. To understand why tornadoes form, consider the formation and growth of a thunderstorm. Thunderstorms are most likely to develop when the atmosphere is unstable; that is when atmospheric temperature drops rapidly with height. Under unstable conditions, air near the surface that begins rising will expand and cool, but remains warmer (and less dense) than its surroundings. The rising air acts like a hot air balloon. Because it is less dense than the surrounding air, it continues to rise. At some point the rising air is cooled to the dew point, at which the water vapor in the air condenses to form liquid water droplets. The rising column of air is now a visible cloud. If the rising air, or updraft, is sustained long enough, water droplets will begin to fall out of the rising air column, transforming it into a rain cloud.

The cloud will become a severe storm capable of producing tornadoes only under certain circumstances. Severe storms are often associated with a very unstable atmosphere and moving low-pressure systems that bring cold air into contact with warmer, more humid air masses. Such weather situations commonly occur in the eastern and midwestern United States during the spring and summer months. Large

scale weather systems often sweep moist warm air from the Gulf of Mexico over these regions in a layer 1.2-1.9 mi (2-3 km) deep. At the same time winds aloft (above about 2.5 mi [4 km] in altitude) from the southwest bring cool dry air over the region. Cool air overlying humid air creates very unstable atmospheric conditions and sets the stage for the growth of strong thunderstorms.

The warm surface air is separated from colder air lying farther north by a sharp temperature boundary called a front. A low-pressure center near Earths surface causes the cold air to advance into the warmer air. The edge of the advancing cold air, called a cold front, forces the warmer air ahead of the front to rise and cool. Since the atmosphere is so unstable, the displaced air keeps rising and a cloud quickly forms. Rain that begins to fall from the cloud causes down-drafts (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. The water vapor in this air condenses to form more water droplets as it rises and cools. When water vapor condenses, it releases latent heat. This warms the air and forces it to rise more vigorously, strengthening the storm.

The exact mechanism of tornado formation inside severe thunderstorms is still a matter of debate, but it appears that tornadoes grow in a similar fashion to the small vortices that form in draining bathtubs.

Tornadoes appear to be upside down versions of the bathtub vortex phenomenon. 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 because the air around the base of a thunderstorm usually contains some rotation, or vorticity. 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 spinning with arms outstretched. As the skater brings his or her arms inward, his or her rotational speed increases. In the same way, air moving into a severe storm begins in a tighter column and increases its rotational speed. A wide vortex, called a mesocyclone, is created. The mesocyclone begins to build vertically, extending itself upward throughout the entire height of the cloud. The rapid air movement causes the surrounding air pressure to drop, pulling more air into growing vortex. The lowered pressure causes the incoming air to cool quickly and form cloud droplets before they rise to the cloud base. This forms the wall cloud, a curtain-shaped cloud that is often seen before a tornado forms. The mesocyclone continues to contract while growing from the base of the storm cloud all the way up to 6.2 mi (10 km) above the surface. When the mesocyclone dips below the wall cloud, it is called a funnel cloud because of its distinctive funnel shape. This storm is on its way to producing a tornado.

Tornado characteristics

A funnel cloud may form in a severe storm and never reach the ground. If and when it does, the funnel officially becomes a tornado. The central vortex of a tornado is typically about 300 ft (100 m) in diameter. Wind speeds in the vortex have been measured at greater than 300 mph (482 km/h). These high winds can be very destructive and also cause the air pressure in the tornado to drop below normal atmospheric pressure by over 100 millibars (the normal day-today pressure variations are about 15 millibars). The air around the vortex is pulled into this low-pressure zone where it expands and cools rapidly. This causes water droplets to condense from the air, making the outlines of the vortex visible as the characteristic funnel-shaped cloud. The low pressure inside the vortex picks up debris such as soil particles, which may give the tornado an ominous dark color. A tornado can act as a giant vacuum cleaner sweeping over anything unlucky enough to be in its path. The damage path of a tornado may range from 900 ft (375 m) to over 0.6 mi (1 km) wide.

Tornadoes move with the thunderstorm with which they are associated, traveling at average speeds of about 10-30 mph (15-48 km/h), although some tornadoes have been seen to stand still and others have been clocked at 60 mph (97 km/h). Because a typical tornado has a lifetime of about 5 to 10 minutes, it may stay on the ground for 5-10 mi (8-16 km). Occasionally, a severe tornado may cut a path of destruction over 200 mi (320 km) long. Witnesses to an approaching tornado often describe a loud roaring noise made by the storm similar to jet engines at take off. There is no generally accepted explanation for this phenomenon although it has been suggested that supersonic winds inside the vortex cause it.

The destructive path of tornadoes appears random. One house may be flattened while its neighbor remains untouched. This has been explained by the tornado skippinglifting up off the surface briefly then descending again to resume its destructive path. Studies made of these destructive paths after the storm suggest another possible explanation: tornadoes may have two to three smaller tornado like vortices circling around the main vortex like horses on a merry-go-round. According to this theory, these suction vortices may be responsible for much of the actual damage associated with tornadoes. As they rotate around the main tornado core, they may hit or miss objects directly in the tornados path depending on their position. Thus, if two houses were in the tornado path, one may be destroyed by a suction vortex. Yet, if the vortex had moved into a different position (and the next vortex had not yet taken its place), the neighboring house may escape damage. The tornados skipping behavior is still not completely understood.

When houses or other structures are destroyed by a tornado, they are not simply blown down by the high wind. Instead, they appear to explode. For many years it was believed that the low pressure of the tornado vortex caused such explosions. According to this theory, if the pressure outside a building drops very quickly, the air inside may not escape fast enough (through cracks, holes, and the like) to equalize the pressure difference. The higher pressure inside the building then pushes out windows or whole walls, and the structure looks like it has exploded. Studies of tornado damage have shown that buildings do not actually explode in this manner. Instead, high wind passing over a house roof acts like the air moving over an airplane wing: it gives the roof an upward force or lift that tends to raise the roof vertically off the house. Winds also enter the building through broken windows or doors, pressurizing the house as one would blow up a balloon. The combination of these forces tends to blow the walls and roof off the structure from the inside out, giving the appearance of an explosion.

Tornado strength is classified by the Fujita scale, which uses a scale of one to six to denote tornado wind speed. Because direct measurements of the vortex are not possible, the observed destruction of the storm is used to estimate its F scale rating.

Tornado history

Prior to 2003, the single-most violent tornado in U.S. history was the tri-state tornado on March 18, 1925. Beginning in Missouri, the tornado stayed on the ground for over 220 mi (354 km), crossing Illinois, moving into Indiana, and leaving a trail of damage over 1 mile (1.6 km) wide in places. Tornado damage often is limited because they usually strike unpopulated areas, but the tri-state tornado struck nine towns and destroyed thousands of homes. When the storm was over, 689 people had lost their lives and over 2,000 were injured, making the tri-state the deadliest tornado on record.

On May 3, 1999, a storm started in southwestern Oklahoma near the town of Lawton. By late in the day, it had grown into a violent storm system with 76 reported tornadoes. As the storm system tore across central Oklahoma and into Kansas, over 43 people were killed, over 500 people were injured, and more than 1,500 buildings were destroyed. One of the tornadoes, classed as an F5, was as much as 1 mile (1.6 km) wide at times and stayed on the ground for over 4 hours.

Another historic storm was the severe tornado outbreak of April 3-4, 1974. As a strong low-pressure system moved over the Midwest, an advancing cold front ran into warm Gulf air over the southern states. The resulting storm triggered 148 tornadoes across 13 states in the subsequent 24 hours, some reaching F4 and F5 in strength. As severe as this outbreak was, the death toll was less than half of that from the tri-state tornado because of advances in tornado forecasting and warnings.

Prediction and tracking of tornadoes

The precise tracking and prediction of tornadoes is not yet a reality. Meteorologists can identify conditions that are likely to lead to severe storms. They 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. It is still not possible, however, to detect a funnel cloud by radar and predict its path, touchdown point, and other important details. Much progress has recently been made in the detection of tornadoes using Doppler radar.

Doppler radar can measure both the distance to an object and its velocity by using the Doppler effect. If an object is moving toward an observer, radar waves bounced off the object will have a higher frequency than if the object were moving away. This effect can be demonstrated with sound waves. If a car is approaching with its horn sounding, the pitch of the horn (that is, the frequency of the sound waves) seems to rise. It reaches a peak just as the car passes, then falls as the car speeds away from the listener.

Doppler radar is used to detect the motion of raindrops and hail in a thunderstorm, which gives an indication of the motion of the winds. As of 2007, it is possible to detect the overall storm circulation and even a developing mesocyclone. The relatively small size of a tornado makes direct detection very difficult with the current generation of Doppler radar,

KEY TERMS

Dew point The temperature at which water vapor in the air condenses to form liquid water droplets.

Doppler radar A type of radar that measures both the position and the velocity of an object.

Front A fairly sharp temperature boundary in the lower atmosphere.

Fujita scale A scale of one to six which 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.

Skipping The tendency of tornado damage to be random as if the tornado skips along in its path.

Suction vortices Secondary vortices that are theorized to be part of a tornado vortex. They may be responsible for the skipping behavior of tornadoes.

Unstable atmosphere The condition of the atmosphere when air temperature drops rapidly with height. Such conditions support rising air and contribute to strong thunderstorms.

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

Vorticity The tendency of an air mass to rotate.

Wall cloud The characteristic cloud that forms at the base of a thunderstorm before a funnel cloud appears.

however. In addition any radar is limited by the curvature of Earth. Radar waves go in straight lines, which means distant storms that are below the horizon from the radar cannot be probed with this technique.

Tornadoes, which have long fascinated people with their sudden appearance and awesome destructive power, are still subjects of intense scientific study. Research continues on the formation life history and detection of these most impressive storms.

See also Cyclone and anticyclone; Tropical cyclone.

Resources

BOOKS

Ahrens, Donald C. Meteorology Today. Pacific Grove, CA: Brooks Cole, 2006.

Bluestein, H. B. Tornado Alley: Monster Storms of the Great Plains. Oxford, UK: Oxford University Press, 2006.

Palmer, Tim, and Renate Hagedorn, eds. Predictability of Weather and Climate. New York: Cambridge University Press, 2006.

James Marti

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Tornado

Tornado

Tri-state tornado delivers death and destruction
Dangerous science: How tornadoes form
Life cycles of tornadoes
Consequences of tornadoes
The human factor
Technology connection
A matter of survival
For More Information

A tornado is a rapidly spinning column of air that extends from a thunderstorm cloud to the ground. The tornado rotates around a vertical axis of extremely low pressure called a vortex. Tornadoes are sometimes called "twisters" or "cyclones," although the term cyclone is also used to described hurricanes in the Indian Ocean.

A tornado destroys nearly everything in its path. A tornado's high winds (and to a lesser extent its low pressure), cause walls to buckle and roofs to be lifted off and carried away. The winds can uproot trees and pick up entire buildings, only to deposit them hundreds of yards away. People, animals, cars, and all types of household items have been tossed about by tornadoes.

In the early part of the twentieth century, tornadoes killed an average of two hundred people each year in the United States. Due to improvements in forecasting and emergency preparedness in recent years, this number has been reduced to a yearly average of fewer than fifty people killed. More people die each year in flash floods and from lightning than are killed by tornadoes. In terms of the total population, the death rate has plunged dramatically from 1.8 deaths per million people in the 1920s to fewer than 0.1 deaths per million people in the first decade of the twenty-first century.

While the United States has the greatest number of tornadoes per year, over one thousand, other parts of the world also experience tornadoes. Especially in poorer nations, tornadoes still take many lives each year. In north-central Bangladesh on May 13, 1996, more than seven hundred fatalities were caused by a tornado outbreak, the emergence of a group (also called a family) of tornadoes from a single thunderstorm. In August of 2006, at least seven tornadoes swept through mainland Europe causing thirty-eight injuries but no reported deaths.

Tri-state tornado delivers death and destruction

The deadliest tornado ever experienced in the United States swept through Missouri, Illinois, and Indiana on March 18, 1925. Known as the Tri-State Tornado, the monster had winds of more than 300 miles (483 kilometers) per hour, took 695 lives, and caused more than 2,000 injuries in 23 cities and towns. The tornado also set records for path length, at 219 miles (352 kilometers); and speed, at an average of 62 miles (100 kilometers) per hour. Its path width varied from 0.5 to 1 mile (0.8 to 1.6 kilometer). Over a three-and-a-half-hour period, it caused more than $16 million in damages—a huge sum by 1925 standards. Because of the massive amount of damage, it was classed as an F5 on the Fujita scale. The Fujita scale measures tornado intensity: F5 is used for the most violent storms.

The greatest number of fatalities in a single town occurred in Murphysboro, Illinois, where 234 people died. The towns of Gorham, Illinois, and Griffin, Indiana, were completely destroyed. Not a single building or home in any of those communities remained after the tornado went through.

The tornado killed farmers, schoolchildren, and factory workers. Miners, working far below ground while the tornado roared above, returned to the surface to find their families dead and their homes gone. Most people did not see the approaching funnel, the cone-shaped spinning column of air that hangs under the thunderstorm cloud, since it was obscured by blowing debris. The tornado came upon them so fast that they had little chance to find adequate shelter.

At least six other tornadoes from the same storm system also resulted in extensive damage and caused fifty-two deaths.

The tornado's beginnings

The day of the tornado was a relatively warm and humid one for the Midwest in March, at about 65°F (18°C). At the same time, a cold air mass was moving in from the north. Today's meteorologists would recognize the pattern as being ripe for tornado formation. The United States Weather Bureau (the precursor to the National Weather Service) had called for "rains and strong shifting winds" on that day, but not tornadoes.

The tornado touched ground briefly in Arkansas, but did no damage. It then rose back into the clouds and traveled to the northeast. At 1:00 pm, the funnel returned to Earth, just southwest of the small town of Annapolis (northwest of Ellington), in southeastern Missouri. There it killed its first victim: a farmer. The enormous twister turned from white to black as it sucked up everything it touched, including dirt, fences, trees, and barns. As the twister traveled, it ejected the items with deadly force. The tornado headed right for Annapolis's one-block-long business district and leveled every structure.

WORDS TO KNOW

conventional radar:
instrument that detects the location, movement, and intensity of precipitation, and gives indications about the type of precipitation. It operates by emitting microwaves, which are reflected by precipitation. Also called radar.
Doppler radar:
a sophisticated type of radar that relies on the Doppler effect (the change in frequency of waves emitted from a moving source) to determine wind speed and direction, as well as the direction in which precipitation is moving.
fair-weather waterspout:
relatively harmless waterspout that forms over water and arises either in conjunction with, or independently of, a severe thunderstorm. Also called nontornadic waterspout.
Fujita Intensity scale:
scale that measures tornado intensity, based on wind speed and the damage created.
funnel cloud:
cone-shaped spinning column of air that hangs well below the base of a thunderstorm cloud.
mesocyclone:
region of rotating updrafts created by wind shear within a supercell storm; it may be the beginnings of a tornado.
multi-vortex tornado:
tornado in which the vortex divides into several smaller vortices called suction vortices.
NEXRAD:
acronym for Next Generation Weather Radar, the network of 156 high-powered Doppler radar units that cover the continental United States, Alaska, Hawaii, Guam, and Korea.
suction vortices:
small vortices within a single tornado that continually form and dissipate as the tornado moves along, creating the tornado's strongest surface winds.
supercell storm:
the most destructive and long-lasting form of a severe thunderstorm, arising from a single, powerful convective cell. It is characterized by strong tornadoes, heavy rain, and hail the size of golf balls or larger.
tornadic waterspout:
tornado that forms over land and travels over water. Tornadic waterspouts are relatively rare and are the most intense form of waterspouts.
tornado:
rapidly spinning column of air that extends from a thunderstorm cloud to the ground. Also called a twister.
tornado cyclone:
spinning column of air that protrudes through the base of a thunderstorm cloud.
tornado family:
a group of tornadoes that develop from a single thunderstorm.
tornado outbreak:
emergence of a tornado family. Tornado outbreaks are responsible for the greatest amount of tornado-related damage.
vortex:
(plural: vortices) vertical axis of extremely low pressure around which winds rotate.
wall cloud:
a roughly circular, rotating cloud that protrudes from the base of a thunderstorm cloud; it is often the beginning of a tornado.
waterspout:
rapidly rotating column of air that forms over a large body of water, extending from the base of a cloud to the surface of the water.

The tornado's next target was another small town, Cape Girardeau. It reduced to ruins a school from which students had been dismissed just minutes earlier. Leaving a total of thirteen dead in Missouri, the tornado cut a path across the plains and into southwestern Illinois.

Mayhem in Murphysboro

The tornado next demolished 152 blocks of Murphysboro, Illinois. Murphysboro, population twelve thousand, was a former mining and agricultural community that had become a center for manufacturing and railroad transport. The townspeople were taken by surprise by the swirling black wall of winds that descended upon them. The winds sucked up houses and cars, snapped trees off at their bases and tossed them around as if they were matchsticks, and ripped water pipes right from the ground. Twelve hundred buildings were destroyed; even freight trains were carried away. Three brick school buildings collapsed on the children inside them, killing at least twenty-five students.

Many people were still trapped in piles of wreckage when fire erupted. Firefighting operations throughout the town were greatly hampered due to broken water mains. Even where water was available, the piles of collapsed houses, trees, and vehicles made it nearly impossible to get to the fires. Many

Eyewitness account of destruction in Murphysboro

May Williams, a religious lay worker from St. Louis, was in Murphysboro on March 18, 1925, to attend a revival held by the Reverend and Mrs. Parrott. This eye-witness account was first published in the St. Louis Post-Dispatch on March 22, 1925, and was reprinted in the May/June 2000 edition of American Heritage.

"Mrs. Parrott … had sung the first verse and chorus which we were repeating when it suddenly grew dark and there fell upon us what we thought was hail. Rocks began to break through. We were being showered with glass, stones, trash, bricks, and anything. I saw the concrete wall at the back of the hall collapse and come crumbling in. Then the roof started to give way. From outside as well as from within, we could hear terrible cries, yells, screams, and there was a great popping noise. The wind roared—I cannot describe it and it tore great handfuls of the roof above us…

"Then the storm passed. We went out into the street. We walked the city for an hour or more, terror-struck by what we saw. People went about almost without clothes, with no shoes on, wrapped in rugs or blankets. It was indescribable, the confusion. We picked our way among tangles of wires, trees, poles, brick and lumber to our rooms….

"[After dark] everything was on fire, it seemed. There was no light except the flare of flames. There was no water. We were black from head to foot…. Every place that stood was turned into a hospital. We visited the high school where the doctors were sewing up wounds, giving emergency treatment, and where other helpers were hauling out the dead. We saw numberless torn and bleeding bodies…. They were dynamiting the city now in their effort to stop the flames, and the roar of the explosions added to the horror of the fires' glare. Everything was ghastly."

trapped people were burned alive. There were 234 deaths and 623 injuries in Murphysboro that day. Damages amounted to $10 million.

Devastation in De Soto

The tornado's next target was De Soto, Illinois, population six hundred. People who witnessed the tornado described a boiling black mass of clouds, continually flashing lightning, with a huge funnel swinging like a pendulum. As the tornado blew through De Soto's residential area, it lifted houses, and the people inside them, straight up into the air. Every building in De Soto more than 10 feet (3 meters) tall was demolished. Roof beams were later found 15 miles (24 kilometers) from where they began. The tornado struck a school, ripping off the roof and causing the walls to collapse. One-hundred-and-twenty-five students and teachers were buried in the rubble. Thirty-three of them died, and the rest sustained serious injuries. Sixty-nine people in De Soto and the surrounding area were killed.

Path of destruction continues

In the town of West Frankfort (the largest on the twister's path, with a population of 18,500), 127 people were killed

and 410 were seriously injured. Within a span of a few minutes the tornado cut a swath through one-third of Frankfort's residential district, leveling 250 buildings. Property damage was an estimated $2 million.

Before leaving Illinois, the tornado tore through Akin, Gorham, McLeansboro, Logan, Benton, Enfield, Bush, Thomsonville, Carmi, Crossville, and Parrish. In each town there were fatalities and significant property damage. The destruction of factories and businesses left many of the survivors jobless. Parrish was 90 percent destroyed; there were twenty-two deaths and sixty injuries in the town of 270. Gorham was entirely destroyed and saw thirty-four deaths (more than half the town's residents were injured or killed). In all, the tornado took about 610 lives in Illinois.

The Oklahoma City twister of 1999

On May 3, 1999, more than seventy tornadoes, spawned by a dozen supercell thunderstorms, blew through central Oklahoma and southern Kansas. The twisters killed forty-three people in all: thirty-eight in Oklahoma and five in Kansas. Nearly 750 people in Oklahoma and 150 in Kansas were injured during the outbreak. The number of deaths and injuries likely would have been much greater if not for the lengthy advance warning given by weather agencies and the up-to-the-minute reporting by local television stations.

The tornado outbreak was the costliest in the nation's history. More than 10,500 homes and 47 businesses in Oklahoma, and some 1,500 buildings in Kansas, were destroyed. Property damages totaled nearly $1.5 billion. Eleven counties in Oklahoma were declared disaster areas.

Throughout the evening of May 3, the National Weather Service placed forty-four of Oklahoma's seventy-seven counties under tornado watch and issued almost two hundred separate tornado warnings. When meteorologists determined that an F5 tornado was headed straight for metropolitan Oklahoma City, police drove through neighborhoods warning residents to take cover.

The tornado had maximum winds of 318 miles (512 kilometers) per hour, the highest wind speed ever recorded. The measurement was taken by a truck-mounted Doppler radar placed in the tornado's path. The tornado was at the upper end of the F5 tornado category. If its wind speed had been just 1 mile per hour greater, it would have been classified as F6, a classification no tornado has ever attained. The tornado was powerful enough to rip pavement from roads and grass from the ground, and hurl freight trains from their tracks. In some places the tornado was accompanied by softball-sized hail.

The monstrous tornado remained on the ground for several hours, twisting out a path about 90 miles (145 kilometers) long. It struck densely populated areas of Oklahoma City, killing thirty-six people in the city and surrounding suburbs and demolishing entire neighborhoods. The Oklahoma City area alone suffered more than $1 billion in damage.

At around 4:00 pm, the tornado entered southwestern Indiana. Traveling at a speed of 73 miles (117 kilometers) per hour, it pounded

the towns of Griffin, Poseyville, Elizabeth, Owensville, and Princeton, killing at least 71 people in the state. Griffin, population 400, was wiped off the map; every one of its 200 buildings succumbed to the storm. Any wall left standing after the tornado was reduced to ashes in the fire that consumed the wreckage. In Princeton, with 9,850 residents, the tornado killed 45 people. The Tri-State Tornado finally dissipated at 4:30 pm, 10 miles (16 kilometers) northeast of Princeton.

Relief efforts

Word of the disaster was carried to nearby cities by train. The response was rapid and extensive. Doctors and nurses came to tend the wounded, and entire fire departments arrived to battle the blazes. Medical supplies were brought in on trains, and outbound trains carried the wounded to hospitals. Coffins and flowers were sent from St. Louis and Chicago to ease the pain of burying the dead.

Dangerous science: How tornadoes form

The process of tornado formation remains somewhat mysterious to meteorologists (scientists who study weather and climate). Tornadoes are tricky research subjects because the time and place of their emergence and the paths they travel are nearly impossible to predict. Adding to the difficulty, tornadoes are usually short-lived. In recent years, with the advent of Doppler radar (a sophisticated instrument that determines wind speed and direction, as well as the direction in which precipitation is moving) and computer modeling systems, researchers have developed a likely explanation for how tornadoes develop.

Tornadoes originate within severe thunderstorms—usually in the most destructive and long-lasting breed of severe thunderstorms, called supercell thunderstorms. The first stage of a developing tornado is a mesocyclone, a region of rotating, upward-blowing columns of wind within a mature thunderstorm. The mesocyclone starts in the center region of the thunderstorm cloud and grows downward. Air from below is sucked upward into the low-pressure center (or vortex) of the mesocyclone, making it longer and skinnier. At the same time, the spinning of the mesocyclone becomes more rapid.

When the mesocyclone protrudes below the base of the cloud, it is considered a tornado cyclone. The tornado cyclone grows as updrafts of air rush into the zone of extremely low pressure in its vortex. When the tornado cyclone hangs well below the base of the thunderstorm cloud and has a pronounced cone shape, it is considered a funnel cloud.

The funnel cloud continues to grow downward by attracting air from below. When the incoming air rises, it cools. If the air is moist enough, the water within it condenses and forms a cloud, making the funnel cloud visible. When the funnel cloud reaches the ground, it is classified as a tornado.

Tornadoes travel at a range of speeds, averaging 34 miles (55 kilometers) per hour. While some tornadoes barely inch along, others sprint at 150 miles (241 kilometers) per hour.

The Fujita scale of tornado intensity

The Fujita scale, created by the late tornado expert T. Theodore Fujita (1920–1998), places tornadoes into six categories. The categories, F0 through F5, are defined by the damage created by the tornado. F0 and F1 tornadoes are "weak"; F2 and F3 tornadoes are "strong"; and F4 and F5 tornadoes are "violent." A Fujita label can only be applied to a tornado once it has passed through an area and the damage has been assessed. Based on the extent of the damage, one can estimate the tornado's wind speeds. (It is very difficult to measure tornado wind speeds directly because regular wind instruments cannot withstand a tornado's strong winds. Recently, however, truck-mounted Doppler instruments, correctly placed in the path of a tornado, have made direct measurements in a handful of tornadoes.)

It is important to remember that the Fujita scale rates tornadoes according to intensity, not size. It is a common misconception that the largest tornadoes are the strongest and that the smallest tornadoes are the weakest. This, however, is not always the case.

F0: Light damage; broken branches, signs and billboards damaged. Winds less than 72 mph (116 kph).

F1: Moderate damage; mobile homes pushed from foundations/overturned, surfaces peeled off roofs. Winds 73 to 112 mph (117 to 180 kph).

F2: Considerable damage; mobile homes demolished, large trees snapped/uprooted, light-object missiles generated. Winds 113 to 157 mph (181 to 251 kph).

F3: Severe damage; roofs and some walls torn from well-constructed houses, heavy cars lifted off the ground and thrown. Winds 158 to 206 mph (252 to 330 kph).

F4: Devastating damage; well-constructed houses leveled, cars thrown and large missiles generated. Winds 207 to 260 mph (331 to 417 kph).

F5: Incredible damage; houses lifted from foundations or disintegrated; automobile-sized missiles carried through the air. Winds 261 to 318 mph (418 to 512 kph).

Appearance of tornadoes

Tornadoes come in a variety of shapes, sizes, and colors. Tornadoes may look like funnels, upside-down bells, elephants' trunks, or long ropelike pendants. Some tornadoes are white, while others appear gray, brown, or black. If the air sucked into the vortex cools to the temperature at which the water vapor within it condenses, then a cloud forms within the tornado. The cloud gives the tornado a white or gray appearance. A tornado will be dark brown or black if debris and dust gets picked up from the ground and spun up into the vortex. Occasionally a tornado picks up red dirt or clay, giving it a reddish color.

"Weak" and "strong" tornadoes

Just less than 70 percent of all tornadoes are classified as "weak tornadoes," with spinning winds of 75 to 110 miles (118 to 177 kilometers) per hour. The damage path created by those tornadoes is, on average, 30 to 190 feet (10 to 60 meters) wide by 5 miles (8 kilometers) long. Contact with the ground lasts less than ten minutes. Weak tornadoes are responsible for less than 5 percent of all tornado deaths.

"Strong tornadoes," with spinning winds of 110 to 200 miles (180 to 320 kilometers) per hour, account for about 29 percent of all tornadoes. Strong tornadoes create a path of destruction that is up to 1 mile (1.6 kilometers) wide and several miles (kilometers) long. They stay on the ground for about twenty minutes and account for almost 30 percent of tornado deaths.

Only 2 percent of all tornadoes are considered "violent tornadoes." The spinning winds of those tornadoes surpass 250 miles (400 kilometers) per hour, gusting to 320 mph (500 kph). A violent tornado

creates a path of destruction more than 1 mile (1.6 kilometer) wide and some 100 miles (160 kilometers) long. A tornado of this intensity can remain on the ground more than two hours. Violent tornadoes, despite their relatively small numbers, account for nearly 70 percent of all tornado deaths.

Life cycles of tornadoes

Similar to the process by which thunderstorms form, tornadoes evolve through a number of stages. The first stage is called the dust-whirl stage. This stage is marked by the formation of a short funnel cloud protruding downward from the base of the thunderstorm cloud. The funnel cloud causes the swirling around of debris on the ground beneath it. Surface winds in this stage are rarely strong enough to cause any damage.

The second stage is called the organizing stage. During this stage, the funnel cloud extends further downward and increases in strength.

A tornado is at its most destructive in the mature stage. The funnel reaches all the way to the ground in this stage and remains in contact with the ground until it dissipates. During this stage, the tornado attains its greatest width and is nearly vertical. This stage lasts only fifteen minutes, on average.

During the mature stage, some tornadoes become multi-vortex tornadoes. In these tornadoes, the vortex divides into several smaller vortices called suction vortices. Suction vortices are responsible for the strongest surface winds. They continually form and dissipate as the powerful tornado moves along.

As a tornado enters the shrinking stage, its funnel narrows. Friction with the ground causes the funnel to tilt. In this stage, which lasts an average of seven to ten minutes, the tornado creates a narrower path of destruction than it did during the mature stage.

The last stage is called the decay stage. In this stage, the funnel narrows further until it is shaped like a rope. It usually twists and turns several times before fragmenting and dissipating.

Not all tornadoes proceed through these five stages. Minor tornadoes may dissipate after the organizing stage, or proceed directly from the organizing stage to the decay stage.

Where tornadoes occur

Tornadoes occur wherever severe thunderstorms occur, mainly in the temperate middle latitudes (the regions of the world that lie between of 30° and 60°, north and south). About 75 percent of the world's tornadoes occur in the continental United States. Tornadoes in the United States are, on average, the most severe in the world.

Tornadoes are also common in Australia, New Zealand, South Africa, Argentina, Bangladesh, and much of central Europe as far south as Italy and as far north as Great Britain. Occasional tornadoes have occurred as far north as Stockholm, Sweden, and Saint Petersburg, Russia. Tornadoes sporadically occur in Japan, eastern China, northern India, and Pakistan. Very weak tornadoes sometimes form in tropical regions.

An eyewitness account of a tornado

In the following paragraphs, Roy S. Hall of Texas recounts his experience inside a tornado on May 3, 1948. This story was published in full in Weatherwise, January-February 1998.

"Since noon, thunderstorms had been developing to the west and the southwest, muttering and grumbling, miles away…. The squall, now about two miles away, was coming directly at us…. Lightning was striking all around the house now, adding its horror to the fast-rising din…. The deafening noise outside was growing in intensity by the second, and I realized a tornado was right on us.

"And then very suddenly, when I was in the middle of the room, there was no noise of any kind…. But I could still feel the house tremble and shake under the impact of the wind…. I saw it was growing lighter in the room.

"The light, though, was so unnatural I held the thought for a moment that the house was on fire. The illumination had a peculiar bluish tinge….

"There was a tremendous jar, the floor slid viciously under my feet…. All around objects flashed upward. I sensed the roof of the house was gone.

"I know the house had been lifted from its foundation, and feared it was being carried through the air…. I sensed a vast relief when I saw that we were still on the ground. The house had been jammed against trees on the east and south and had stopped, partly off its foundation.

"The relief I experienced, however, was very brief. Sixty feet south of our house something had billowed down from above and stood fairly motionless, save for a slow up-and-down pulsation…. For a second I was too dumbfounded even to try to fathom its nature, then it burst on my befuddled brain with a paralyzing shock: I was looking at the inside wall of the lower end of the tornado funnel; we were within the tornado itself!

"The bottom of the rim was about twenty feet off the ground, and its leading edge had doubtless destroyed our house as it passed over. The interior of the funnel was hollow … and, owing possibly to the light within the funnel, appeared perfectly opaque. Its inside was so slick and even that it resembled the interior of a glazed sandpipe.

"The bottom of the funnel was about 150 yards across. Higher up it was larger, and seemed to be partly filled with a bright cloud, which shimmered like a fluorescent light.

"Where I could observe both the front and back of the funnel's insides, the terrific whirling could be plainly seen…. It looked as if the whole column were composed of rings or layers, and when a higher ring moved on toward the southeast, the ring immediately below slipped over to get back under it. This rippling motion continued on down toward the lower tip.

"The peculiar bluish light was now fading, and abruptly was gone. Instantly it was again dark as night. With the darkness, my hearing began to come back…. The tornado had passed."

Tornadoes in the United States

Tornadoes in the United States are formed by the interaction of cold, dry air migrating south from Canada and the warm, moist air traveling north from the Gulf of Mexico. An average of eight hundred to nine hundred tornadoes occur yearly in the United States, accompanying about 1 percent of all thunderstorms. Most of the tornadoes are short-lived and strike sparsely populated areas. The state that experiences the greatest number of tornadoes is Texas.

Who's who: Howard Bluestein and the Storm Chasers

The 1995 film Twister featured fictional "storm chasers." These adventurers sped along prairie roads in an instrument-loaded vehicle in the hopes of sighting an elusive tornado or violent storm.

University of Oklahoma meteorology professor Howard Bluestein is also the star of a movie, albeit a movie made for public television, but he is a real storm chaser. He knows that things are quite different from what was portrayed in the movie. The most important difference between the real world and the movie is that storm chasers are very careful to not place themselves in the path of the oncoming storm. They are there to gather information, not to risk their lives.

When he is out in the field, Bluestein plays it safe. "I think that we pretty much know where a tornado might form, and we're very careful not to get in its path," said Bluestein in an interview published in the April/May 1996 issue of Weatherwise magazine. "The greatest dangers that we have are dangers of driving under bad conditions. Roads that are wet. Narrow country roads…. Lightning is also a big scare. We try to stay inside the van if we see a lot of lightning."

Bluestein attended the Massachusetts Institute of Technology, where he majored in electrical engineering until his senior year. Then he switched to meteorology. He went on to complete a Ph.D. in tropical meteorology. Bluestein joined the faculty of the University of Oklahoma in 1976, where he has been ever since.

Since 1977, Bluestein has been leading teams of storm chasers, armed with cameras and portable Doppler radars, all over the Oklahoma countryside. They are seeking answers to questions such as: Why do some thunderstorms spawn tornadoes while others do not, and, how exactly do tornadoes form?

"You need a source of rotation and an updraft to spin it up. Everyone knows that," explained Bluestein in an interview published in the July 12, 1996, Chronicle of Higher Education, when asked about tornado formation. "What people don't agree on are the sources of rotation and the sequence of events that leads to the rotation, which produces the seeds of the tornado. We need to understand where the original vortex comes from."

Bluestein and his tornado-chasing colleagues have greatly advanced our understanding of tornadoes. Before tornado chasers began their quest two decades ago, all that was known about tornado formation was

that tornadoes originated within supercell thunderstorms. The data collected by storm chasers has enabled forecasters to develop the computer models that more accurately, and farther in advance, predict when and where tornadoes will strike.

"Tornado Alley" is the part of the United States that has the highest per-area concentration of tornadoes. This region encompasses north-

central Texas, Oklahoma, Kansas, Nebraska, and South Dakota. Central Oklahoma has the greatest number of tornadoes per acre. Tornadoes also occur frequently throughout the Mississippi Valley, and in the Midwest, and all the way east to Massachusetts. Tornadoes occur very rarely west of the Rocky Mountains.

Tornadoes occur year-round in the United States, but with the greatest frequency in the spring and early summer (April through June). Their lowest rate of occurrence is in December and January. Tornadoes may develop at all times of day and night, with the peak time between

2:00 pm and 6:00 pm Forty percent of all tornadoes occur during that period. The least likely time for tornado formation is just before sunrise.

Consequences of tornadoes

A tornado leaves a path of near-total destruction in its wake. It picks up and carries away cars, trees, and roofs and walls of buildings. Objects the size of washers and dryers have been picked up by a tornado and transported several miles. They can carry entire buildings for hundreds of yards (meters).

Waterspouts

Waterspouts are tornado-like phenomena that occur over large bodies of water. They are rapidly rotating columns of air that extend from the base of a cloud to the surface of the water. Where the waterspout makes contact with the surface, water sprays upward. Contrary to what many people believe about waterspouts, water is not drawn upward through the funnel, into the cloud. Rather, the moisture in a waterspout comes from water vapor that has condensed within rising air and formed a cloud.

There are two types of waterspouts: tornadic waterspouts and fair-weather (or nontornadic) waterspouts. A tornadic waterspout is a tornado that has formed over land and traveled over the water; it is the strongest type of waterspout and relatively rare. Fair-weather waterspouts constitute the vast majority of waterspouts. Unlike the tornadic variety, fair-weather waterspouts form over the water and can arise independently of severe thunderstorms.

Fair-weather waterspouts are formed when winds converge (travel toward a common point) and cause the surface air to rise. As the air rises, it cools, and the moisture within it condenses into cloud droplets. The condensation releases latent heat (energy released or absorbed by a substance as it undergoes a phase change), which fuels the continued upward motion of moist air from the surface.

Fair-weather waterspouts are arc-shaped and may appear alone or in clusters. They are relatively small, ranging in diameter from 10 to 325 feet (3 to 100 meters). The speed of their rotating winds only approaches 50 miles (80 kilometers) per hour, and they move along the water at a slow pace. Fair-weather waterspouts usually last ten to fifteen minutes, dissipating when cool air gets drawn into the funnel. They are rarely strong enough to cause damage, except to small boats directly in their path.

Fair-weather waterspouts are common in coastal areas of all tropical oceans. They occur with the greatest frequency near the Florida Keys. The Keys get nearly one hundred fair-weather waterspouts per month during the summer, for a total of four hundred to five hundred waterspouts annually. Waterspouts are also a common sight over the Mediterranean Sea. They occur occasionally over large inland bodies of water, such as the Great Lakes and Utah's Great Salt Lake.

There are reports of tornadoes sucking up the contents of a pond, including frogs and toads, and raining them down further along the storm's path. A violent tornado in 1931 picked up a train car weighing 83 tons (75 metric tons), with 117 passengers on board, and dumped it in a ditch 82 feet (25 meters) away. One tornado snatched a motel sign from Broken Bow, Oklahoma, and dropped it 30 miles (48 km) away in Arkansas. In another tornado a canceled check was blown 305 miles

(491 kilometers), from Great Bend, Kansas, to just outside of Palmyra, Nebraska.

The human factor

Tornadoes occur throughout such a large geographic area that, especially in the United States, they are virtually impossible to avoid. At present, there is an average of fewer than fifty deaths per year in the United States because of tornadoes. Most of those fatalities occur when people are crushed in collapsing buildings. The best way to reduce the number of deaths caused by tornadoes is for people to have to access to sturdy, tornado-proof structures.

The danger of manufactured housing

Manufactured and mobile homes are particularly vulnerable to tornadoes: not only do they lack basements (the safest place during a tornado), but they are relatively flimsy and lightweight. Even those mobile homes tethered to the ground can usually be picked up and tossed by a tornado's extreme winds. Paul Hebert, director of Miami's National Weather Service forecast office, described mobile homes in a news report as "very vulnerable to any tornado." The National Oceanic and Atmospheric Administration (NOAA) stated that it is safer to be in a ditch with one's hands over one's head than it is to be in a mobile home, if in the path of a tornado.

Ideally, people living in mobile home parks would have access to a concrete structure in which they could take shelter. Unfortunately, only a handful of states require that mobile home park operators provide tornado shelters for their residents.

Homeowners urged to build "safe rooms"

In 1999 the Federal Emergency Management Agency (FEMA) began a push to get owners of homes in tornado-prone areas to construct their own storm shelters or "safe rooms." FEMA provided designs for shelters, which were made of metal-reinforced panels, in basements, crawl spaces, closets, bathrooms, storage areas, and other interior parts of the house. One requirement of a "safe room" is that it have its own walls, separate from the rest of the house, so that it could remain standing even if other parts of the house were knocked down.

"When constructed according to the plans," stated James Lee Witt, director of FEMA, "the safe room can provide protection against winds of up to 250 miles per hour and projectiles [flying objects] traveling at 100 miles per hour."

Weather report: Exploding chicken feathers

A tornado-related oddity is the de-feathering of chickens. Reports of naked chickens abound after tornadoes strike rural areas. Professional and amateur meteorologists alike have been perplexed by this phenomenon for years. The popular theory that the low pressure of the tornado causes the feathers to "explode" off a chicken has been shown to be false. The fact is that a pressure drop great enough to cause feathers to explode off a chicken would also cause the whole chicken to explode.

A more likely explanation for de-feathering is that the frightened chicken induces what is called the "flight molt" response. This self-protective response, evoked by a chicken that is being threatened by a predator, causes the chicken's feathers to loosen. That way, when the predator chomps, it gets a mouthful of feathers instead of a mouthful of chicken. During a tornado, the feathers loosened by the terrified chicken's flight molt response are merely blown away.

Technology connection

Forecasters presently have the capability to issue tornado warnings, which are severe weather advisories that mean a tornado has been sighted and may strike a specific area, fifteen to thirty minutes ahead of time (eighteen minutes on average). Tornado watches, which are severe weather advisories

Tornado destroys Jarrell, Texas

Violent thunderstorms developed across central Texas in the afternoon of May 27, 1997, spawning three tornadoes. The first of those tornadoes brought death and destruction to the town of Jarrell, Texas. Jarrell, with a population of fourteen hundred people, experienced twenty-seven deaths and $40 million in property damage.

The tornado that struck Jarrell was rated as F5 on the Fujita scale, meaning it created "incredible" damage. The Austin/San Antonio National Weather Service Office called the twister "a tragic tornado of unimaginable proportions." When the tornado first touched down northwest of Jarrell, it was a thin, rope-shaped funnel. It quickly grew until its base was 0.5 miles (0.8 kilometers) wide and its winds surpassed 261 miles (420 kilometers) per hour. As the tornado headed toward homes in the Double Creek subdivision, just west of downtown Jarrell, it ripped asphalt off the road and demolished one business.

Residents of the Double Creek subdivision, given just twelve minutes warning about the approaching tornado, had few places to seek shelter. Their houses were built on cement slabs and had no basements. Some people jumped in cars and attempted to escape the tornado, while others huddled inside their homes. When the tornado blasted through the area it destroyed fifty homes, making them look as if they had been bulldozed from their foundations. Twenty-seven people inside those homes, including some whole families, were killed.

Debris from the destroyed homes was later found 2 miles (3.2 kilometers) away. The tornado also carried away dozens of cars, ripped bark from trees, pulled grass from the ground, and killed three hundred head of livestock.

Lax mobile home regulations in Florida

The deadliest tornado outbreak in Florida's history occurred in the late evening of February 22 and early morning of February 23, 1998. The tornadoes, produced by three supercell thunderstorms, took 42 lives and caused 260 injuries. More than 800 residences were destroyed and 3,500 others were damaged. Most of the people killed during the tornadoes were in mobile homes or trailers. (There are 800,000 mobile homes or trailers in Florida, accounting for 16 percent of the state's residences.)

Following Hurricane Andrew in 1992, Florida's most costly natural disaster in recent history, the Florida legislature enacted new building standards for mobile homes. The new standards required that mobile homes be constructed with 2-inch by 6-inch (5-centimeter by 15-centimeter) lumber and be anchored to the ground by cables. Mobile homes constructed in this way would be able to withstand winds of 100 to 110 miles (161 to 177 kilometers) per hour. However, safety advocates argued that this standard was insufficient to protect occupants from all hurricanes, not to mention tornadoes. While hurricane winds range in strength from 74 to 180 miles (119 to 290 kilometers) per hour, tornado winds can reach 318 miles (512 kilometers) per hour.

Moreover, 90 percent of Florida's mobile homes are exempt from the 1992 law's requirements, since they were built prior to that time. Homes constructed between 1976 and 1992 had to be able to withstand winds of 90 miles (145 kilometers) per hour; before 1976 there were no construction requirements at all. To make matters worse, Florida law does not require mobile home parks to provide tornado-proof shelters for their residents.

stating that conditions are ripe for tornadoes to develop, may be issued as far as three hours in advance. The advance warning given for tornadoes is far shorter than the twelve to twenty-four hours given for hurricanes, however. The reason for this difference is that tornadoes are much smaller, develop more rapidly, and are far more difficult to detect than hurricanes. Nonetheless, great strides have been made in recent years in tornado detection. The present warning time is a tremendous improvement over the two-minute notice given in the mid-1970s.

Today's tornado forecasting arsenal includes surface weather stations, satellite images, radar images, and computer models. The cornerstone of the forecasting effort is NEXRAD (Next Generation Weather Radar), a network of 156 high-powered Doppler radars that were installed across the United States in 1996. Doppler radar is a sophisticated type of radar that relies on the Doppler effect, the change in frequency of waves emitted from a moving source, to determine wind speed and direction, as well as the direction in which precipitation is moving. (Radar, which is an acronym for "radio detection and ranging," works by emitting short radio waves called microwaves that can reflect off of precipitation.)

Doppler radar can look within a storm system and map out patterns of air circulation, including wind rotation. This information allows forecasters to identify thunderstorms or tornadoes in their earliest stages. Data from NEXRAD is sent, via high-speed computers, to National Weather Service (NWS) centers and field offices around the country.

Experiment: Tornado in a bottle

You can't make a real tornado in a bottle, but you can simulate its movement with water. Here is what you need:

  • two 2-liter clear, clean, empty plastic bottles
  • water
  • 1-inch metal washer
  • duct tape

First, fill one of the bottles two-thirds full of water and put the washer over the opening. Then, turn the second bottle upside down and place it on the washer. Use the duct tape to fasten the two containers and the metal washer together very securely.

Turn the tornado maker, so that the bottle with the water is on top, then swirl the bottle in a circular motion. Watch as a "tornado" forms in the top bottle and rushes into the bottom bottle.

At the NWS's Storm Prediction Center in Norman, Oklahoma, meteorologists analyze weather data from NEXRAD and other systems twenty-four hours a day. When they detect conditions that could give rise to tornadoes, they notify the weather center in that area. The local weather center may then issue a tornado watch or warning. Weather forecasters also rely on information delivered by SKYWARN, a network of trained weather spotters who watch for tornadoes and report their findings.

Extending the warning time

One of the greatest challenges before meteorologists today is learning more about how tornadoes are formed. This task involves determining why some thunderstorms create tornadoes, while others do not. When tornadoes are better understood, forecasters will be able to lengthen the advance warning time they give; with longer warning times, people have a greater opportunity to get out of harm's way.

Killer tornadoes in Bangladesh

Some of the world's deadliest tornadoes have occurred in Bangladesh, a small country on India's eastern border. Tornadoes there cause more deaths than in more developed countries, such as the United States, in part because Bangladesh does not have a sophisticated tornado prediction and advance warning system. The relatively high frequency of tornadoes, the lack of sturdy shelters, and the high population density all contribute to tornadoes' high death toll in Bangladesh.

On April 1-2, 1977, a tornado outbreak struck the Madaripur district, 80 miles (128 kilometers) outside of Bangladesh's capital city, Dhaka, killing 505 people. The twisters also left more than six thousand people injured and hundreds of thousands homeless. "The tornado … lasted two to three minutes and left behind a trail of devastation," read a report about the most destructive twister in the April 3 Bangladesh Observer. "Hundreds of houses were razed to the ground and a large number of cattle head killed and injured. Trees were uprooted and the damage to standing crop is colossal." In the next day's paper, a reporter noted: "Not a single dwelling nor a tree I found standing erect and there was hardly any family which did not suffer losses."

On April 26, 1989, Dhaka, Bangladesh, was hit with another violent twister, the deadliest ever recorded. The tornado killed at least 1,109 people and injured about fifteen thousand. Some forty-eight thousand huts were demolished, leaving one hundred thousand people homeless. The twister also destroyed much of the rice crop (the primary food in the region) and many fruit trees. The tornado came on the heels of a massive flood, which just six months earlier had already damaged the rice crop.

Another massive tornado struck central Bangladesh on May 12, 1996. The tornado killed an estimated 760 people and caused 34,000 injuries in a cluster of villages 95 miles (152 kilometers) north of Dhaka. Many of the casualties were believed to have been caused by corrugated tin roofs that flew through the air like missiles. The winds were so strong that they ripped clothes right off of people, leaving some survivors reluctant to emerge from their hiding places until clothing was distributed by relief agencies.

In the late 1990s, the National Weather Service sponsored a tornado research program called VORTEX, the Verification of the Origins of Rotation in Tornadoes Experiment. VORTEX staffers placed very high frequency, truck-mounted Doppler radar units in the paths of oncoming severe thunderstorms (storms that were likely to produce tornadoes). When a tornado formed along the path, the equipment took readings

of wind gusts, rainfall, and hail, producing a three-dimensional view of the tornado throughout its life cycle.

A more recent tornado project, called STEPS (the Severe Thunderstorm Electrification and Precipitation Study), took place from May 22 through July 16, 2000. The purpose of STEPS, which was coordinated by the National Center for Atmospheric Research, was to identify the processes within towering thunderstorm clouds that give rise to tornadoes and lightning. STEPS researchers, stationed across the plains of Kansas and Colorado, used Doppler radars and a lightning mapping system to produce three-dimensional pictures of storms. They also launched weather balloons directly into thunderheads (towering cumulonimbus clouds likely to produce thunderstorms) to take readings of atmospheric conditions.

NOAA Weather Radio

NOAA Weather Radio, also called the "Voice of the National Weather Service," presents continuous, extensive, local weather information all across the United States. The service is operated by the National Oceanic and Atmospheric Administration, and reports are prepared by local offices of the National Weather Service. Weather reports are usually four to six minutes long and are updated every one to three hours (more frequently during rapidly changing or severe weather). Weather Radio also broadcasts special reports relevant to specific regions. For instance, one region may present marine reports, while another presents agriculture reports and climatological forecasts.

Weather radio broadcasts are transmitted on seven different high-band FM frequencies, ranging from 162.400 to 162.550 megahertz. Picking up the broadcasts requires a special receiver called a "weather radio." In some locations weather radio is carried on certain radio bands like the weather band; citizens band; and some automobile, aircraft, and marine bands. In a few places it can be picked up on standard AM/FM radios.

When severe weather is forecast, NOAA Weather Radio sounds an alarm. That signal alerts the listener to turn up the radio and stay tuned. Some receivers can be programmed to automatically switch on whenever a hazardous-weather alarm is activated.

NOAA Weather Radio broadcasts are sent out from four hundred transmitters throughout the United States, Puerto Rico, Guam, and Saipan (a tiny island north of Guam in the North Pacific Ocean). Each transmitter sends information to receivers within a 40-mile (64-kilometer) radius—the area for which the report is relevant. Presently weather radio broadcasts have the potential to reach about 90 percent of the American population, provided they have the appropriate receivers.

A matter of survival

Tornado watches and warnings are issued in advance of tornadoes and are intended to alert people in a given area that dangerous weather is brewing. During a tornado watch, stay tuned to your radio or television for updates. Prepare to move quickly to a safe place in the event that a tornado warning is issued. Watch the sky, as well. You may spot the warning signs of a tornado before forecasters announce a tornado warning.

In the event of a tornado warning, television and radio programs are interrupted and, in many communities, sirens blare. Immediately move to a safe place when a tornado warning is issued. You may have very little time before a tornado strikes.

If you live in a tornado-prone region, it is wise to have areas designated as "tornado shelters" in your home, at school, and at work. The best place for a tornado shelter is in a basement. If there is no basement, select an interior room (bathrooms and closets are best) or hallway on the first floor, away from windows. Keep a first aid kit and a flashlight with extra batteries in your tornado shelter.

When a tornado warning is issued

  • Go to your designated tornado shelter; or crouch beneath the stairs, a heavy workbench, a mattress, or a sturdy piece of furniture.
  • Do not open windows! It was once believed that opening windows would keep buildings from "blowing up" by allowing the indoor and outdoor pressure to equalize. It is now known that opening windows only increases pressure on the opposite wall, making it more likely that the building will collapse.
  • If you're outside, seek shelter in a strong building. Stay away from windows and doors. Flying glass and other debris are major tornado hazards.
  • If you're in a car, leave it and go into a nearby building. If that's not possible, leave your car and crouch in a ditch or depression or next to a strong building. Do not stay in the car. Tornadoes can pick up cars and hurl them through the air. When the car is dropped to the ground, it may crash with the force of a 100-mile-per-hour (161-kilometer-per-hour) head-on collision.
  • If you're in a mobile home, leave. Even properly secured mobile homes can be lifted up by a tornado. Go to a designated tornado shelter or crouch in a ditch.
  • Lend assistance to very young children, elderly people, those who are mentally or physically disabled, and people who don't understand the tornado warning due to a language barrier.
  • Protect yourself by lying face down with your knees drawn up under you. Put your head down by your knees and cover the back of your head with your hands.

How to tell when a tornado is coming

One warning sign of a tornado is a wall cloud, a roughly circular cloud that is 1 to 4 miles (1.5 to 6.5 kilometers) in diameter and that hangs beneath the thunderstorm cloud. A wall cloud may appear up to one hour before a tornado strikes. Another warning sign is a funnel cloud. This looks like a dimple bulging from the underside of the cloud, which quickly lengthens. A funnel cloud will only be visible if the air rising within it condenses, creating a gray or white funnel. Another indication that a funnel cloud is present is the swirling of debris on the surface. Sometimes, however, falling rain or darkness obscures a funnel cloud.

Two other warning signs of a tornado are the sky turning dark green or hail falling from a large thunderstorm. Some tornadoes make a roaring sound, like several freight trains, loud enough to be heard miles away. Other tornadoes, however, travel quietly.

[See AlsoClimate; Hurricane; Thunderstorm; Weather: An Introduction ]

For More Information

BOOKS

Bradford, Marlene. Scanning the Skies: A History of Tornado Forecasting. Norman, OK: University of Oklahoma Press, 2001.

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

Mogil, H. Michael. Tornadoes. Saint Paul, MN: Voyageur Press, 2003.

Robinson, Andrew. Earth Shock: Hurricanes, Volcanoes, Earthquakes, Tornadoes and Other Forces of Nature. New York: W. W. Norton, 2002.

WEB SITES

"Tornado" Federal Emergency Management Administration. 〈http://www.fema.gov/hazard/tornado/index.shtm〉 (accessed March 23, 2007).

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Tornado

Tornado

A tornado is a rapidly spinning column of air formed in severe thunderstorms. The rotating column, or vortex, forms inside the storm cloud then grows downward until it touches the ground. Although a tornado is not as large as its parent thunderstorm , it 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. Due to a combination of geography and meteorology , the United States experiences most of the world's tornadoes. An average of 800 tornadoes strike the United States each year. Based on statistics kept since 1953, Texas, Oklahoma, and Kansas are the top three tornado states. Tornadoes are responsible for about 80 deaths, 1500 injuries, and many millions of dollars in property damage annually. While it is still impossible to predict exactly when and where tornadoes will strike, progress has been made in predicting tornado development and detecting tornadoes with Doppler radar .

Tornado formation

Most tornadoes form in the northern hemisphere during the months of March through June. These are months when conditions are right for the development of severe thunderstorms. To understand why tornadoes form, consider the formation and growth of a thunderstorm. Thunderstorms are most likely to develop when the atmosphere is unstable; that is when atmospheric temperature drops rapidly with height. Under unstable conditions, air near the surface that begins rising will expand and cool, but remains warmer (and less dense) than its surroundings. The rising air acts like a hot air balloon ; since it is less dense than the surrounding air it continues to rise. At some point the rising air cools to the dew point where the water vapor in the air condenses to form liquid water droplets. The rising column of air is now a visible cloud. If the rising air, or updraft, is sustained long enough water droplets will begin to fall out of the rising air column, making it a rain cloud.

This cloud will become a severe storm capable of producing tornadoes only under certain circumstances. Severe storms are often associated with a very unstable atmosphere and moving low pressure systems that bring cold air into contact with warmer, more humid air masses. Such weather situations commonly occurs in the eastern and Midwestern United States during the spring and summer months. Large scale weather systems often sweep moist warm air from the Gulf of Mexico over these regions in a layer 1.2-1.9 mi (2-3 km) deep. At the same time winds aloft (above about 2.5 mi [4 km] in altitude) from the southwest bring cool dry air over the region. Cool air overlying humid air creates very unstable atmospheric conditions and sets the stage for the growth of strong thunderstorms.

The warm surface air is separated from colder air lying farther north by a fairly sharp temperature boundary called a front. A low pressure center near the earth's surface causes the cold air to advance into the warmer air. The edge of the advancing cold air, called a cold front, forces the warmer air ahead of the front to rise and cool. Since the atmosphere is so unstable the displaced air keeps rising and a cloud quickly forms. Rain that begins to fall from the 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. The water vapor in this air condenses to form more water droplets as it rises and cools. When water vapor condenses it releases latent heat . This warms the air and forces it to rise more vigorously, strengthening the storm.

The exact mechanism of tornado formation inside severe thunderstorms is still a matter of dispute but it appears that tornadoes grow in a similar fashion to the small vortices that form in draining bathtubs. When the plug is pulled in a bathtub, water from other parts of the tub rushes in to replace that going down the drain. If the water has any swirl in it, the drain soon has a little vortex.

Tornadoes appear to be upside down versions of this phenomenon. 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 usually contains some rotation , or vorticity. 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 spinning with arms outstretched. As the skater brings his or her arms inward, his or her rotational speed increases. In the same way air moving into a severe storm begins in a tighter column and increases its rotational speed. A wide vortex is created, called the mesocyclone. The mesocyclone begins to build vertically, extending itself upward throughout the entire height of the cloud. The rapid air movement causes the surrounding air pressure to drop, pulling more air into growing vortex. The lowered pressure causes the incoming air to cool quickly and form cloud droplets before they rise to the cloud base. This forms the wall cloud, a curtain-shaped cloud that is often seen before a tornado forms. The mesocyclone continues to contract while growing from the base of the storm cloud all the way up to 6.2 mi (10 km) above the surface. When the mesocyclone dips below the wall cloud it is called a funnel cloud because of its distinctive funnel shape. This storm is on its way to producing a tornado.


Tornado characteristics

A funnel cloud may form in a severe storm and never reach the ground. If and when it does, the funnel officially becomes a tornado. The central vortex of a tornado is typically about 328.1 ft (100 m) in diameter. Wind speeds in the vortex have been measured at greater than 220 mph (138 km/h). These high winds make incredible feats of destruction possible. They also cause the air pressure in the tornado to drop below normal atmospheric pressure by over 100 millibars (the normal day-to-day pressure variations we experience are about 15 millibars). The air around the vortex is pulled into this low pressure zone where it expands and cools rapidly. This causes water droplets to condense from the air, making the outlines of the vortex visible as the characteristic funnel shaped cloud. The low pressure inside the vortex picks up debris such as soil particles, which may give the tornado an ominous dark color . A tornado can act as a giant vacuum cleaner sweeping over anything unlucky enough to be in its path. The damage path of a tornado may range from 900 ft (300 m) to over 0.5 mi (1 km) wide.

Tornadoes move with the thunderstorm that they are attached to, traveling at average speeds of about 10-30 mph (15-45 kph), although some tornadoes have been seen to stand still, while other tornadoes have been clocked at 60 mph (90 kph). Since a typical tornado has a lifetime of about five to 10 minutes, it may stay on the ground for 5-10 mi (8-16km). Occasionally, a severe tornado may cut a path of destruction over 200 mi (320 km) long. Witnesses to an approaching tornado often describe a loud roaring noise made by the storm similar to jet engines at takeoff. There is no generally accepted explanation for this phenomenon although it has been suggested that supersonic winds inside the vortex cause it.

The destructive path of tornadoes appears random . One house may be flattened while its neighbor remains untouched. This has been explained by the tornado "skipping"—lifting up off the surface briefly then descending again to resume its destructive path. Studies made of these destructive paths after the storm suggest another possible explanation: tornadoes may have two to three smaller tornado-like vortices circling around the main vortex like horses on a merry-go-round. According to this theory, these "suction vortices" may be responsible for much of the actual damage associated with tornadoes. As they rotate around the main tornado core they may hit or miss objects directly in the tornado's path depending on their position. Thus if two houses were in the tornado path one may be destroyed by a suction vortex but the vortex had moved into a different position (and the next vortex had not yet taken its place) by the time it reached the next house. The tornado's skipping behavior is still not completely understood.

When houses or other structures are destroyed by a tornado, they are not simply blown down by the high winds: they appear to explode. For many years it was believed that the low pressure of the tornado vortex caused such explosions. According to this theory, if the pressure outside a building drops very quickly the air inside may not escape fast enough (through cracks, holes, and the like) to equalize the pressure difference. The higher pressure inside the building then pushes out windows or whole walls, and the structure looks like it had exploded. Studies of tornado damage have shown that buildings do not actually explode in this manner. Instead, high wind passing over a house roof acts like the air moving over an airplane wing: it gives the roof an upward force or lift which tends to raise the roof vertically off the house. Winds also enter the building through broken windows or doors pressurizing the house as one would blow up a balloon. The combination of these forces tends to blow

the walls and roof off the structure from the inside out giving the appearance of an explosion.

Tornado strength is classified by the Fujita scale, which uses a scale of one to six to denote tornado wind speed. Since direct measurements of the vortex are not possible the observed destruction of the storm is used to estimate its "F scale" rating.


Tornado history

Prior to 2003, the single most violent tornado in United States history was the Tri-State tornado on March 18, 1925. Beginning in Missouri, the tornado stayed on the ground for over 220 mi (350 km), crossing Illinois, moving into Indiana, and leaving a trail of damage over one mile (1.6 km) wide in places. Tornado damage often is limited since they usually strike unpopulated areas, but the Tri-State tornado plowed through nine towns and destroyed thousands of homes. When the storm was over, 689 people had lost their lives and over 2,000 were injured making the Tri-State the deadliest tornado on record.

On May 3, 1999, a storm started in southwestern Oklahoma, near the town of Lawton. By late in the day, it had grown into a violent storm system with 76 reported tornadoes. As the storm system tore across central Oklahoma and into Kansas, over 43 people were killed, over 500 injured and more than 1,500 buildings were destroyed. One of the tornadoes, classed as a F-5, was as much as a mile wide at times and stayed on the ground for over four hours.

Another historic storm was the severe tornado outbreak of April 3-4, 1974. As a strong low pressure system moved over the Midwest, an advancing cold front ran into warm Gulf air over the southern states. The resulting storm triggered 148 tornadoes over 13 states in the next 24 hours, some reaching F4 and F5 in strength. As severe as this outbreak was, the death toll was less than half of that from the Tri-State tornado because of advances in tornado forecasting and warnings.


Prediction and tracking of tornadoes

The precise tracking and prediction of tornadoes is not yet a reality. Meteorologists can identify conditions that are likely to lead to severe storms. They 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. It is still not possible, however, to detect a funnel cloud by radar and predict its path, touchdown point, and other important details. Much progress has recently been made in the detection of tornadoes using Doppler radar.

Doppler radar can measure not just the distance to an object, but also its velocity by using the Doppler effect : if an object is moving toward an observer, radar waves bounced off the object will have a higher frequency than if the object were moving away. This effect can be demonstrated with sound waves . If a car is approaching with its horn sounding, the pitch of the horn (that is, the frequency of the sound waves) seems to rise. It reaches a peak just as the car passes, then falls as the car speeds away from the listener.

Doppler radar is used to detect the motion of raindrops and hail in a thunderstorm, which gives an indication of the motion of the winds. With present technology it is possible to detect the overall storm circulation and even a developing mesocyclone. The relatively small size of a tornado makes direct detection very difficult with the current generation of Doppler radar. In addition any radar is limited by the curvature of Earth . Radar waves go in straight lines, which means distant storms that are "below the horizon" from the radar cannot be probed with this technique.

Tornadoes, which have long fascinated people with their sudden appearance and awesome destructive power, are still subjects of intense scientific study. Research continues on the formation life history and detection of these most impressive storms.

See also Cyclone and anticyclone; Tropical cyclone.

Resources

books

Battan, Louis J. Weather. Engelwood Cliffs: Prentice-Hall Inc., 1985.

Battan, Louis J. Weather in Your Life. New York: W.H. Freeman & Co., 1983.

Danielson, Eric W., James Levin, and Elliot Abrams. Meteorology. 2nd ed. with CD-ROM. Columbus: McGraw-Hill Science/Engineering/Math, 2002.

Hardy, Ralph, Peter Wright, John Kington, and John Gribben. The Weather Book. Boston: Little, Brown and Co., 1982.

Lewellen, W.S. "Tornado Vortex Theory." In The Tornado: ItsStructure, Dynamics and Hazards. Washington, DC: American Geophysical Union, 1993.

McNeill, Robert. Understanding the Weather. Las Vegas: Arbor Publishers, 1991.

Mogil, H. Michael, and Barbara G. Levine. The Amateur Meteorologist. New York: Franklin Watts, 1993.

periodicals

Schmidlin, Thomas. "Unsafe At Any (Wind) Speed." Bulletin of the American Meteorological Society 83, no. 12 (2002): 1821-1830.


James Marti

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Dew point

—The temperature at which water vapor in the air condenses to form liquid water droplets.

Doppler radar

—A type of radar that measures both the position and the velocity of an object.

Front

—A fairly sharp temperature boundary in the lower atmosphere.

Fujita scale

—A scale of one to six which 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.

Skipping

—The tendency of tornado damage to be random as if the tornado skips along in its path.

Suction vortices

—Secondary vortices that are theorized to be part of a tornado vortex. They may be responsible for the "skipping" behavior of tornadoes.

Unstable atmosphere

—The condition of the atmosphere when air temperature drops rapidly with height. Such conditions support rising air and contribute to strong thunderstorms.

Vortex

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

Vorticity

—The tendency of an air mass to rotate.

Wall cloud

—The characteristic cloud that forms at the base of a thunderstorm before a funnel cloud appears.

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