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Tornado

2007 Schools Wikipedia Selection. Related subjects: Climate and the Weather

   A tornado in central Oklahoma. The tornado itself is the thin tube
   reaching from the cloud to the ground. The lower half of this tornado
   is surrounded by a dust cloud, kicked up by the tornado's strong winds
   at the surface.
   Enlarge
   A tornado in central Oklahoma. The tornado itself is the thin tube
   reaching from the cloud to the ground. The lower half of this tornado
   is surrounded by a dust cloud, kicked up by the tornado's strong winds
   at the surface.

   A tornado is a violently rotating column of air which is in contact
   with both a cumulonimbus (or, in rare cases, cumulus) cloud base and
   the surface of the earth. Tornadoes can come in many shapes, but are
   typically in the form of a visible condensation funnel, with the narrow
   end touching the earth. Often, a cloud of debris encircles the lower
   portion of the funnel.

   Most have winds of 110 mph (175 km/h) or less, are approximately
   250 feet (75 meters) across, and travel a few miles (several
   kilometers) before dissipating. However, some tornadoes can have winds
   of more than 300 mph (480 km/h), be more than a mile (1.6 km) across,
   and stay on the ground for dozens of miles (more than 100 kilometers).

   They have been observed on every continent except Antarctica; however,
   a significant percentage of the world's tornadoes occur in the United
   States. This is mostly due to the unique geography of the country,
   which allows the conditions which breed strong, long-lived storms to
   occur many times a year. Other areas which commonly experience
   tornadoes include New Zealand, western and southeastern Australia,
   south-central Canada, northwestern Europe, Italy, south-central and
   eastern Asia, east-central South America, and Southern Africa.

Etymology

   The word "tornado" is an altered form of the Spanish word tronada,
   which means "thunderstorm". This in turn was taken from the Latin
   tonare, meaning "to thunder". It most likely reached its present form
   through a combination of the Spanish tronada and tornar ("to turn");
   however, this may be a folk etymology.

   Some common, related slang terms include: twister, whirlwind, cyclone,
   funnel, wedge, tube, finger of God, Devil's tail, rope, or stovepipe.

Definitions

   A funnel cloud near Ardmore, Oklahoma.
   Enlarge
   A funnel cloud near Ardmore, Oklahoma.
   A shear funnel observed by Project VORTEX in North Texas in 1994.
   Enlarge
   A shear funnel observed by Project VORTEX in North Texas in 1994.
   A waterspout near the Florida Keys.
   Enlarge
   A waterspout near the Florida Keys.
   A landspout near North Platte, Nebraska on May 22, 2004.
   Enlarge
   A landspout near North Platte, Nebraska on May 22, 2004.

   A tornado is defined by the Glossary of Meteorology as "a violently
   rotating column of air, in contact with the ground, either pendant from
   a cumuliform cloud or underneath a cumuliform cloud, and often (but not
   always) visible as a funnel cloud.." A tornado does not necessarily
   have to be visible; however, the low pressures caused by the fast wind
   speeds (see Bernoulli's principle) usually cause water vapor in the air
   to condense into a visible condensation funnel. Strictly, the term
   tornado refers to the vortex of wind, connecting to both the surface
   and a convective cloud above, not the condensation cloud.

   A funnel cloud is a low-hanging, vertically rotating cloud, with no
   associated strong winds at the surface. Funnel clouds are not
   tornadoes, and not all funnel clouds develop into a tornado. However,
   many tornadoes are preceded by a funnel cloud aloft in which
   condensation descends from the parent storm as saturation occurs at
   progressively lower altitude. It is often difficult to tell the
   difference between a funnel cloud and a tornado from a distance. Most
   tornadoes produce strong winds at the surface while the visible funnel
   is still a good distance from the ground. This is usually marked by
   swirling dust and debris at the surface, confirming a tornadic
   circulation is on the ground. A shear funnel is a tiny, harmless funnel
   which occasionally forms underneath or on the sides of cumuliform
   clouds.

   Tornadoes commonly develop from a class of thunderstorms known as
   supercells. Supercells contain mesocyclones, in which rotation is
   organized. Most intense tornadoes (F3 to F5 on the Fujita Scale)
   develop from supercells. Very heavy rain, frequent lightning, strong
   wind gusts, and hail are also common in such storms. The largest hail
   generally comes from supercells, as a very strong and rotating updraft
   is usually required to suspend such large hailstones aloft.

   Stronger tornadoes are also those most observed to have multiple
   vortices (or, subvortices) which are many columns of violently spinning
   air rotating around a common centre. Multivortex structure occurs in
   smaller tornadoes as well as other circulations. A satellite tornado is
   a term for a weaker tornado which forms very near a large, strong
   tornado, contained within the same mesocyclone. The satellite tornado
   may appear to " orbit" the larger tornado (hence the name), giving the
   appearance of one, large multi-vortex tornado. However, a satellite
   tornado is a distinct funnel, and is much smaller than the main
   funnel.. Occasionally a single storm may produce multiple different
   tornadoes and mesocyclones, this process is known as cyclic
   tornadoegenesis. Tornadoes produced from the same storm constitute a
   tornado family . An anticyclonic tornado rotates clockwise in the
   Northern Hemisphere and counterclockwise in the Southern Hemisphere.

   Occasionally, many tornadoes are spawned from the same general storm
   system. While there is no single agreed upon definition, multiple
   tornadoes spawned by the same general storm system with no break in
   activity for 6 to 24 hours (depending on definition in use) is
   considered a tornado outbreak. The qualifying numbers vary, currently
   for the United States, about ten tornadoes constitutes an outbreak.
   When there is a break of activity for more than 6 to 24 hours, it
   usually is considered a separate outbreak. If spawned from the same
   general system, it may be referred to as an extended tornado outbreak.
   A period, of at least several successive days, of continuous or near
   continuous very high tornado activity consisting of a series of tornado
   outbreaks (spawned by multiple weather systems) is a tornado outbreak
   sequence, or sometimes, an extended tornado outbreak.

   A waterspout is a tornado over water. Although most tornadoes over land
   are associated with severe thunderstorms, most scientists consider all
   waterspouts—including "fair weather" waterspouts—to be tornadoes.
   Although the National Weather Service considers waterspouts as a
   tornadic meteorological phenomenon, waterspouts are not counted in
   official records unless they strike land. "Fair weather" waterspouts
   are less-severe relatives of classic tornadoes and are almost always
   weak (F0 or F1 on the Fujita Scale), and spawn from non-rotating
   thunderstorms, or even regular summer showers. Typically, such
   waterspouts moving onto land cause little or no damage, and dissipate
   within minutes. However, strong waterspouts from supercells can cause
   significant damage if they impact land areas. In addition, strong
   tornadoes can move over lakes or over the ocean, becoming waterspouts,
   without losing intensity.

   A landspout is an unofficial term for a tornado not associated with a
   mesocyclone. Landspouts most often are weak, featuring a small
   condensation funnel which often does not appear to reach the ground,
   and are often marked by a tall tube of dust and/or debris reaching as
   far up as the parent cloud. Though usually weaker than classic
   tornadoes, they are tornadoes, and can cause serious damage.

   A gustnado is a small, vertical swirl associated with a gust front or
   downburst. Because they are technically not associated with the cloud
   base, there is some debate as to whether or not gustnadoes are actually
   tornadoes. These usually cause localized areas of heavier damage among
   areas of straight-line wind damage caused by the gust front.

   A dust devil is also a vertical swirling column of air. These phenomena
   resemble tornadoes, but are rarely as strong as even the weakest
   tornadoes, and form under clear skies. Dust devils are not considered
   tornadoes because they form during fair weather, and are not associated
   with convective clouds. However, they can, on occasion, result in major
   damage and fatalities, especially in arid areas.

   Tornado-like circulations occasionally occur near large, intense
   wildfires and are called fire whirls. They are not generally tornadoes,
   though are if they connect the surface to a pyrocumulus or other
   cumuliform cloud above. Fire whirls usually are not as strong as
   tornadoes associated with thunderstorms, however, they can become quite
   intense as there are cases of up to F3 damage.
   A sequence of images showing the birth of a tornado. First, the
   rotating cloud base lowers. This lowering becomes a funnel, which
   continues descending while winds build near the surface, kicking up
   dust and other debris. Finally, the visible funnel extends to the
   ground, and the tornado begins causing major damage. This tornado, near
   Dimmitt, Texas, was one of the most well-observed violent tornadoes in
   history.
   Enlarge
   A sequence of images showing the birth of a tornado. First, the
   rotating cloud base lowers. This lowering becomes a funnel, which
   continues descending while winds build near the surface, kicking up
   dust and other debris. Finally, the visible funnel extends to the
   ground, and the tornado begins causing major damage. This tornado, near
   Dimmitt, Texas, was one of the most well-observed violent tornadoes in
   history.

Life cycle

   Most tornadoes follow a recognizable life cycle. The cycle begins when
   a strong thunderstorm develops a rotating mesocyclone a few miles up in
   the atmosphere, becoming a supercell. As rainfall in the storm
   increases, it drags with it an area of quickly descending air known as
   the rear flank downdraft (RFD). This downdraft accelerates as it
   approaches the ground, and drags the rotating mesocyclone towards the
   ground with it.

   As the mesocyclone approaches the ground, a visible condensation funnel
   appears to descend from the base of the storm, often from a rotating
   wall cloud. As the funnel descends, the RFD also reaches the ground,
   creating a gust front that can cause damage a good distance from the
   tornado. Usually, the funnel cloud begins causing damage on the ground
   (becoming a tornado) within minutes of the RFD reaching the ground.

   Initially, the tornado has a good source of warm, moist inflow to power
   it, so it grows until it reaches the mature stage. During its mature
   stage, which can last anywhere from a few minutes to more than an hour,
   a tornado often causes the most damage, and can in rare instances be
   more than one mile across. Meanwhile, the RFD, now an area of cool
   surface winds, begins to wrap around the tornado, cutting off the
   inflow of warm air which feeds the tornado.

   As the RFD completely wraps around and chokes off the tornado's air
   supply, the tornado begins to weaken, becoming thin and rope-like. This
   is the dissipating stage, and the tornado often fizzles within minutes.
   During the dissipating stage, the shape of the tornado becomes highly
   influenced by the direction of surface winds, and can be blown into
   fantastic patterns.

   As the tornado enters the dissipating stage, its associated mesocyclone
   often weakens as well, as the rear flank downdraft cuts off the inflow
   powering it. In particularly intense supercells, tornadoes can develop
   cyclically. As the first mesocyclone and associated tornado dissipate,
   the storm's inflow is concentrated into a new area closer to the centre
   of the storm. If a new mesocyclone develops, the cycle may start again,
   producing a new tornado. Occasionally, the old, or occluded
   mesocyclone, and the new mesocyclone produce a tornado at the same
   time.

   Though this is a widely-accepted theory for how most tornadoes form,
   live, and die, it does not explain the formation of smaller tornadoes,
   such as landspouts, long-lived tornadoes, or tornadoes with multiple
   vortices. These each have different mechanisms which influence their
   development—however, most tornadoes follow a pattern similar to this
   one.

Characteristics

   A wedge tornado, nearly a mile wide.
   Enlarge
   A wedge tornado, nearly a mile wide.
   A rope tornado in its dissipating stage. The horizontal lines in the
   foreground are power cables.
   Enlarge
   A rope tornado in its dissipating stage. The horizontal lines in the
   foreground are power cables.
   A multiple-vortex tornado outside of Dallas, Texas on April 2, 1957.
   Enlarge
   A multiple-vortex tornado outside of Dallas, Texas on April 2, 1957.

Shape

   Most tornadoes take on the traditional appearance of a narrow funnel, a
   few hundred yards across, with a small cloud of debris near the ground.
   However, tornadoes can appear in all manner of shapes and sizes.

   Small, relatively weak landspouts might only be visible as a small
   swirl of dust on the ground. While the condensation funnel may not
   extend all the way to the ground, if associated surface winds are
   greater than 40 mph (64 km/h), it is considered a tornado.

   Large single-vortex twisters, often violent, can look like a large
   wedge stuck into the ground, and are known as wedge tornadoes or
   wedges. Wedges can be so wide that they appear to be a block of dark
   clouds. Even experienced storm observers may not be able to tell the
   difference between a low-hanging cloud and a wedge tornado from a
   distance.

   Tornadoes in the dissipating stage can appear like narrow tubes, or
   ropes, twisting into all manner of curls, twists, and s-shapes. These
   tornadoes, such as the one pictured at right, are roping out, or
   becoming a rope tornado. Multiple-vortex tornadoes can appear as a
   family of swirls circling a common centre, or may be completely
   obscured by condensation, dust, and debris, appearing to be a single
   funnel.

   In addition to these appearances, tornadoes may be obscured completely
   by rain or dust. These tornadoes are especially dangerous, as even
   experienced meteorologists might not spot them.

Size

   In the United States, an average tornado is around 500 feet (150 m)
   across, and stays on the ground for 5 miles (8 km). While this is the
   average, there is an extremely wide range of tornado sizes, even for
   typical tornadoes.

   Weak tornadoes, or strong but dissipating tornadoes, can be exceedingly
   narrow, sometimes only a few feet across. In fact, a tornado was once
   reported to have a damage path only 7 feet (2 m) long.

   On the other end of the spectrum, wedge tornadoes can have a damage
   path a mile (1.6 km) wide or more. A tornado which affected Hallam,
   Nebraska on May 22, 2004 was at one point 2.5 miles (4 km) wide.

   In terms of path length, some meteorologists believe that the Tri-State
   Tornado, which affected parts of Missouri, Illinois, and Indiana on
   March 18, 1925, was on the ground continuously for 219 miles (352 km).
   However, without a modern damage survey, it is impossible to determine
   whether or not the deadly event was a single tornado or a series of
   violent tornadoes produced by the same storm. The longest modern-day
   damage path was caused by a tornado which was on the ground for
   160 miles (260 km) in northeastern North Carolina on November 22, 1992.

Appearance

   Tornadoes, depending on the environment in which they form, can have a
   wide range of colors. Tornadoes which form in a dry environment can be
   nearly invisible, marked only by swirling debris at the base of the
   funnel. Condensation funnels which pick up little or no debris can be
   grey to white. While travelling over a body of water as a waterspout,
   they can turn very white or even blue. Funnels which move slowly,
   ingesting a lot of debris and dirt, are usually darker, taking on the
   colour of debris. Tornadoes in the Great Plains can turn red because of
   the reddish tint of the soil, and tornadoes in mountainous areas can
   travel over snow-covered ground, turning brilliantly white in the
   process.
   These are two photographs of the Waurika, Oklahoma tornado of May 30,
   1976, taken at nearly the same time. In the top picture, the tornado is
   front-lit, with the sun behind the east-facing camera, so the funnel
   appears nearly white. In the lower image, where the camera is facing
   the opposite direction, the tornado is back-lit, with the sun behind
   the clouds.
   Enlarge
   These are two photographs of the Waurika, Oklahoma tornado of May 30,
   1976, taken at nearly the same time. In the top picture, the tornado is
   front-lit, with the sun behind the east-facing camera, so the funnel
   appears nearly white. In the lower image, where the camera is facing
   the opposite direction, the tornado is back-lit, with the sun behind
   the clouds.

   Lighting conditions are also a major factor in the appearance of a
   tornado. A tornado which is " back-lit", or viewed with the sun behind
   it, will appear to be very dark. The same tornado, viewed with the sun
   at the observer's back, may appear grey or brilliant white. Tornadoes
   which occur near the time of sunset can be many different colors,
   appearing in hues of yellow, orange, and pink.

   Dust kicked up by the winds of the parent thunderstorm, heavy rain and
   hail, and the darkness of night are all factors which can reduce the
   visibility of tornadoes, making them "invisible", in essence. Tornadoes
   occurring in these conditions are especially dangerous, since only
   radar observations, or possibly the sound of an approaching tornado,
   serve as any warning to those in the storm's path. Fortunately most
   significant tornadoes form under the storm's rain-free base, or the
   area under the thunderstorm's updraft, where there is little or no
   rain. In addition, most tornadoes occur between the hours of 4 and 8
   pm, when the bright sun can penetrate even the thickest clouds. Also,
   night-time tornadoes are often illuminated by frequent lightning.

   There is mounting evidence, including doppler radar images and
   eyewitness accounts, which suggest that most tornadoes have a clear,
   calm centre with extremely low pressure, akin to the eye found in
   tropical cyclones. This area would be clear (possibly full of dust),
   have relatively light winds, and be very dark, with the light blocked
   out by swirling debris on the outside of the tornado. Lightning is said
   to be the source of illumination for those who claim to have seen the
   interior of a tornado.

Rotation

   Tornadoes normally rotate in a cyclonic direction (counterclockwise in
   the northern hemisphere). Large-scale storms always rotate cyclonically
   because of the Coriolis effect; however, tornadoes are too small in
   scale to be directly affected by the rotation of the earth.
   Approximately 1 tornado in 100 rotates in an anticyclonic direction.
   Typically, only landspouts and gustnados also rotate anticyclonically.
   However, on very rare occasions, an anticyclonic supercell can develop,
   producing a tornado that is typical except for its direction of
   rotation.

Intensity and damage

   One of the earliest photographs of a tornado. Taken in Norton, Kansas
   on June 24, 1909.
   Enlarge
   One of the earliest photographs of a tornado. Taken in Norton, Kansas
   on June 24, 1909.

   Tornadoes vary in intensity regardless of shape, size, and location.
   While strong tornadoes are typically larger than weak tornadoes, there
   are several instances of F5 tornadoes with damage paths less than
   500 feet (150 m) wide.

History of tornado intensity measurements

   For many years, before the advent of home movies and doppler radar,
   scientists had nothing more than educated guesses as to the speed of
   the winds in a tornado. The only evidence indicating the wind speeds
   found in the tornado was the damage left behind by tornadoes which
   struck populated areas. Some thought they might exceed 500 mph, and
   perhaps even be supersonic.

   In the 1950s, however, evidence mounted that the actual wind speeds
   were much lower than this. On April 2, 1957, a slow moving tornado
   traversed the south and east parts of Dallas, Texas. Before this day,
   only a few photographs and motion pictures of tornadoes were known to
   exist. However, because of many factors, including the tornado's high
   visibility, slow forward motion, and proximity to an urban centre, it
   became (and still may be) the most filmed and photographed tornado in
   history. Frame-by-frame analysis of several pieces of footage taken
   that day showed that the debris flung about by the tornado was
   travelling at speeds up to 170 mph. Scientists had thought that faster
   wind speeds would produce the severe damage seen that day, so this
   tornado gave them their first real clue as to the range of tornado
   speeds.
   A diagram of the Fujita scale as it relates to the Beaufort scale and
   the Mach number scale.
   Enlarge
   A diagram of the Fujita scale as it relates to the Beaufort scale and
   the Mach number scale.

   In 1971, Dr. Tetsuya Theodore Fujita introduced the idea for a scale of
   tornado winds. With the help of colleague Allen Pearson, he created and
   introduced what came to be called the Fujita scale in 1973. The scale
   was based on a relationship between the Beaufort scale and the Mach
   number scale; the low end of F1 on his scale corresponds to the low end
   of B12 on the Beaufort scale, and the low end of F12 corresponds to the
   speed of sound at sea level, or Mach 1. In practice, tornadoes are only
   assigned categories F0 through F5.

   The TORRO scale, created by the Tornado and Storm Research Organisation
   (TORRO), was developed in 1974, and published a year later. The TORRO
   scale has 12 levels, which cover a broader range with tighter
   graduations. It ranges from a T0 for extremely weak tornadoes to T11
   for the most powerful known tornadoes. T0-T1 roughly correspond to F0,
   T2-T3 to F1, and so on. While T10+ would be approximately an F5, the
   highest tornado rated to date on the TORRO scale was a T8. There is
   some debate as to the usefulness of the TORRO scale over the Fujita
   scale—while it may be helpful for statistical purposes to have more
   levels of tornado strength, often the damage caused could be created by
   a large range of winds, rendering it hard to narrow the tornado down to
   a single TORRO scale category.

   Research conducted in the late 1980s and 1990s suggested that, even
   with the implication of the Fujita scale, tornado winds were
   notoriously overestimated, especially in significant and violent
   tornadoes. Because of this, in 2006, the American Meteorological
   Society introduced the Enhanced Fujita Scale, to help assign realistic
   wind speeds to tornado damage. The scientists specifically designed the
   scale so that a tornado assessed on the Fujita scale and the Enhanced
   Fujita scale would receive the same ranking. The EF-scale is more
   specific in detailing the degrees of damage on different types of
   structures for a given wind speed. While the F-scale goes from F0 to
   F12 in theory, the EF-scale is capped at EF5, which is defined as
   "winds ≥ 200 mph (≥ 320 km/h)". In the United States, the Enhanced
   Fujita scale will be used for tornado damage assessments beginning
   February 2, 2007.
   An example of F0 damage. The only significant damage to structures in
   this picture was caused by falling tree branches. Even though
   well-built structures are typically unscathed by F0 tornadoes, falling
   trees and tree branches can injure and kill people, even inside a
   sturdy structure.
   Enlarge
   An example of F0 damage. The only significant damage to structures in
   this picture was caused by falling tree branches. Even though
   well-built structures are typically unscathed by F0 tornadoes, falling
   trees and tree branches can injure and kill people, even inside a
   sturdy structure.
   An example of F1 damage. F1 tornadoes cause major damage to mobile
   homes and automobiles, and can cause minor structural damage to
   well-constructed homes. This particular mobile home appears to be a
   double-wide, and it was still moved off its foundations, with its roof
   badly damaged. A mobile home or car is a very poor shelter, even during
   severe thunderstorms which do not contain a tornado.
   Enlarge
   An example of F1 damage. F1 tornadoes cause major damage to mobile
   homes and automobiles, and can cause minor structural damage to
   well-constructed homes. This particular mobile home appears to be a
   double-wide, and it was still moved off its foundations, with its roof
   badly damaged. A mobile home or car is a very poor shelter, even during
   severe thunderstorms which do not contain a tornado.
   An example of F2 damage. At this intensity, tornadoes have a more
   significant impact on well-built structures, damaging roofs, collapsing
   walls, and generating large amounts of flying debris. This wood-frame
   home was unroofed, with many outer walls collapsed or destroyed.
   Enlarge
   An example of F2 damage. At this intensity, tornadoes have a more
   significant impact on well-built structures, damaging roofs, collapsing
   walls, and generating large amounts of flying debris. This wood-frame
   home was unroofed, with many outer walls collapsed or destroyed.
   An example of F3 damage. Here, the roof and some inner walls of this
   brick building have been demolished. While taking shelter in a
   basement, cellar, or inner room improves your odds of surviving a
   tornado drastically, occasionally even this is not enough. F3 and
   stronger tornadoes only account for about 6% of all tornadoes in the
   United States, and yet since 1980 they have accounted for more than 75%
   of tornado-related deaths.
   Enlarge
   An example of F3 damage. Here, the roof and some inner walls of this
   brick building have been demolished. While taking shelter in a
   basement, cellar, or inner room improves your odds of surviving a
   tornado drastically, occasionally even this is not enough. F3 and
   stronger tornadoes only account for about 6% of all tornadoes in the
   United States, and yet since 1980 they have accounted for more than 75%
   of tornado-related deaths.
   An example of F4 damage. Above-ground structures are almost completely
   vulnerable to F4 tornadoes, which level well-built structures, toss
   heavy vehicles through the air, and uproot trees, turning them into
   flying missiles.
   Enlarge
   An example of F4 damage. Above-ground structures are almost completely
   vulnerable to F4 tornadoes, which level well-built structures, toss
   heavy vehicles through the air, and uproot trees, turning them into
   flying missiles.
   An example of F5 damage. These tornadoes cause incredible destruction,
   obliterating and sweeping away almost anything in their paths.
   Fortunately, they are extremely rare, and often only a small portion of
   the tornado's path contains F5 damage. While these tornadoes often
   destroy everything in their path, it is possible to survive. Some
   survived a direct hit by the Jarrell Tornado by lying down in a bathtub
   as the tornado swept the rest of the house away.
   Enlarge
   An example of F5 damage. These tornadoes cause incredible destruction,
   obliterating and sweeping away almost anything in their paths.
   Fortunately, they are extremely rare, and often only a small portion of
   the tornado's path contains F5 damage. While these tornadoes often
   destroy everything in their path, it is possible to survive. Some
   survived a direct hit by the Jarrell Tornado by lying down in a bathtub
   as the tornado swept the rest of the house away.

   The first observation which confirmed that F5 winds could occur
   happened on April 26, 1991. A tornado near Red Rock, Oklahoma was
   monitored by scientists using a portable Doppler radar, an experimental
   radar device that measures wind speed. Near the tornado's peak
   intensity, they recorded a wind speed of 115-120 m/s (257-268 mph or
   414-432 km/h). Though the portable radar had uncertainty of ± 5-10 m/s
   (± 11-22 mph or ± 18-36 km/h), this reading was probably within the F5
   range, confirming that tornadoes were capable of violent winds found
   nowhere else on earth.

   Eight years later, during the Oklahoma Tornado Outbreak of May 3, 1999,
   another scientific team was monitoring an exceptionally violent tornado
   (one which would eventually kill 36 people in the area near Moore,
   Oklahoma). At about 7 pm, they recorded one measurement of 318 mph ,
   50 mph faster than the previous record. Though this reading is just
   short of the theoretical F6 rating, the measurement was taken more than
   100 feet in the air, where winds are typically stronger than at the
   surface. In rating tornadoes, only surface wind speeds, or the wind
   speeds indicated by the damage resulting from the tornado, are taken
   into account.

   While scientists have long theorized that extremely low pressures might
   occur in the centre of tornadoes, there were no measurements to confirm
   it. A few home barometers had survived close passes by tornadoes,
   recording values as low as 24 in Hg (810 mbar), but these measurements
   were highly uncertain. However, on June 24, 2003, a group of
   researchers successfully dropped devices called "turtles" into an F4
   tornado, one of which measured a pressure drop of more than 100 mbar as
   the tornado passed directly overhead. Still, tornadoes are widely
   varied, so meteorologists are still conducting research to determine if
   these values are typical or not.

Typical intensity

   In the United States, F0 and F1 (T0 through T3) tornadoes account for
   80% of all tornadoes. The rate of occurrence drops off quickly with
   increasing strength—violent tornadoes (stronger than F4, T8), account
   for less than 1% of all tornado reports. Worldwide, strong tornadoes
   account for an even smaller percentage of total tornadoes. Violent
   tornadoes are extremely rare outside of the United States and
   Bangladesh.

   F5 tornadoes are exceptionally rare, occurring on average once every
   few years. The last confirmed F5 tornado anywhere in the world was the
   Moore, Oklahoma tornado, which killed 36 people on May 3, 1999.

Typical damage

   As stated in the lede section, a typical tornado has winds of 110 mph
   (175 km/h) or less, is approximately 250 feet (75 meters) across, and
   travels a mile (1.6 km) or so before dissipating. However, in reality,
   there is no such thing as a typical tornado.

   Two tornadoes that look almost exactly the same can produce drastically
   different effects. Also, two tornadoes which look very different can
   produce similar damage. This is due to the fact that tornadoes form by
   several different mechanisms, and also that they follow a life cycle
   which causes the same tornado to change in appearance over time. People
   in the path of a tornado should never attempt to determine its strength
   as it approaches. Between 1997 and 2005 in the United States, 38 people
   were killed by F1 tornadoes, and 3 were killed by F0 tornadoes. Even
   the weakest tornado can kill.
     * Weak tornadoes

   As stated in the previous section, an overwhelming majority of
   tornadoes are designated F1 or F0, also known as "weak" tornadoes.
   However, weak is a relative term for tornadoes, as even these can cause
   significant damage. F0 and F1 tornadoes are typically short-lived—since
   1980 almost 75% of tornadoes rated weak stayed on the ground for one
   mile or less. However, in this time, they can cause both damage and
   fatalities.

   F0 (T0-T1) damage is characterized by superficial damage to structures
   and vegetation. Well-built structures are typically unscathed,
   sometimes sustaining broken windows, with minor damage to roofs and
   chimneys. Billboards and large signs can be knocked down. Trees may
   have large branches broken off, and can be uprooted if they have
   shallow roots.

   F1 (T2-T3) damage has caused significantly more fatalities than that
   caused by F0 tornadoes. At this level, damage to mobile homes and other
   temporary structures becomes significant, and cars and other vehicles
   can be pushed off the road. Permanent structures can suffer major
   damage to their roofs.
     * Significant tornadoes

   F2 (T4-T5) tornadoes are the lower end of "significant", and yet are
   stronger than most tropical cyclones (though tropical cyclones affect a
   much larger area). Well-built structures can suffer serious damage,
   including roof loss and collapse of outer walls. Mobile homes, however,
   are almost totally destroyed. Vehicles can be lifted off the ground,
   and lighter objects can become small missiles, causing damage outside
   of the tornado's main path. Wooded areas will have a large percentage
   of their trees snapped or uprooted.

   F3 (T6-T7) damage is a serious risk to life and limb. Few parts of
   affected buildings are left standing; well-built structures lose outer
   and inner walls. Cars are lifted off the ground, and can be tossed
   through the air for some distance. Wooded areas will suffer almost
   total loss of vegetation.
     * Violent tornadoes

   F4 (T8-T9) damage typically results in a total loss of the affected
   structure. Well-built homes are reduced to a short pile of debris. Even
   heavy vehicles, including airplanes, trains, and large trucks, can
   become airborne, with other large projectiles being flung some
   distance.

   F5 (T10+) damage is almost always total. F5 tornadoes demolish
   well-built houses and sweep the foundation clean. The official
   description of this damage states that "incredible phenomena will
   occur". The damage they cause is an extreme hazard to life and
   limb—since 1950 in the United States, only 50 tornadoes (0.1% of all
   reports) have been designated F5, and yet these have been responsible
   for more than 1000 deaths and 11,000 injuries (21.5% and 13.6%,
   respectively). In recorded history, F5 tornadoes have performed awesome
   displays of power, including twisting skyscrapers, levelling entire
   communities, and stripping asphalt from the ground.

Prediction

   The following organizations provide official or de facto official
   tornado and severe convective storm forecasts and warnings.
   Probabilistic maps issued by the Storm Prediction Center during the
   heart of the April 6-8, 2006 Tornado Outbreak. The top map indicates
   the risk of general severe weather (including large hail, damaging
   winds, and tornadoes), while the bottom map specifically shows the
   percent risk of a tornado forming within 25 miles (40 km) of any point
   within the enclosed area. The hashed area on the bottom map indicates a
   10% or greater risk of an F2 or stronger tornado forming within
   25 miles (40 km) of a point.
   Enlarge
   Probabilistic maps issued by the Storm Prediction Centre during the
   heart of the April 6-8, 2006 Tornado Outbreak. The top map indicates
   the risk of general severe weather (including large hail, damaging
   winds, and tornadoes), while the bottom map specifically shows the
   percent risk of a tornado forming within 25 miles (40 km) of any point
   within the enclosed area. The hashed area on the bottom map indicates a
   10% or greater risk of an F2 or stronger tornado forming within
   25 miles (40 km) of a point.

Australia

   Severe thunderstorm warnings are provided to Australia by their Bureau
   of Meteorology. The country is in the middle of an upgrade to Doppler
   radar systems, with their first benchmark of installing six new radars
   reached in July 2006.

Canada

   In Canada, weather forecasts and warnings, including tornadoes, are
   produced by the Meteorological Service of Canada division of
   Environment Canada.

Europe

   The European Union founded a project in 2002 called the European Severe
   Storms virtual Laboratory, or ESSL, which is meant to fully document
   tornado occurrence across the continent. The ESTOFEX (European Storm
   Forecast Experiment) arm of the project also issues one day forecasts
   for severe weather likelihood.

Germany, Austria, and Switzerland

   An organization known as TorDACH was founded in 1997 to collect
   information regarding tornadoes, waterspouts, and downbursts from
   Germany, Austria, and Switzerland. A secondary goal is collect all
   severe weather information. This project is meant to document fully
   severe weather activity in these three countries.

Japan

   In Japan, predictions and study of tornadoes in Japan are handled by
   the Japan Meteorological Agency.

United Kingdom

   In the United Kingdom, the Tornado and Storm Research Organisation
   (TORRO) makes experimental predictions.

United States

   In the United States, generalized severe weather predictions are issued
   by the Storm Prediction Centre, based in Norman, Oklahoma. For the next
   one, two, and three days, respectively, they will issue categorical and
   probabilistic forecasts of severe weather, including tornadoes. There
   is also a more general forecast issued for the four to eight day
   period. Just prior to the expected onset of an organized severe weather
   threat, SPC issues severe thunderstorm and tornado watches, in
   collaboration with local National Weather Service offices. Warnings are
   issued by local National Weather Service offices when a severe
   thunderstorm or tornado is occurring or imminent.

Detection

   The National Weather Service trains Skywarn spotters, consisting of
   local sheriff's deputies, state troopers, firefighters, amateur radio
   operators, storm chasers, and ordinary citizens, to spot key features
   of storms which indicate severe hail, strong winds, and tornadoes. When
   severe weather is anticipated, local weather service offices request
   that these spotters be on the lookout for severe weather, and report
   any possible tornadoes immediately, so the office can issue a timely
   warning.

Climatology

   Areas worldwide which experience the highest chance of seeing
   tornadoes, indicated by orange shading.
   Enlarge
   Areas worldwide which experience the highest chance of seeing
   tornadoes, indicated by orange shading.
   Intense tornado activity in the United States. The darker-colored areas
   denote the area commonly referred to as Tornado Alley.
   Enlarge
   Intense tornado activity in the United States. The darker-colored areas
   denote the area commonly referred to as Tornado Alley.

Geography

   The United States has the most tornadoes of any country, seeing about
   four times the activity estimated in all of Europe. Many of these form
   in an area of the central United States known as Tornado Alley. This
   area extends into Canada, particularly Ontario and the Prairie
   Provinces, however, activity is less than that of the US. The
   Netherlands has the highest average number of recorded tornadoes per
   area of any country (more than 20 annually), followed by the UK (at
   least 33 per year), but most are small and result in minor damage.

   Bangladesh and surrounding areas of eastern India suffer from tornadoes
   of equal severity to those in the US with more regularity than any
   other region in the world, however these occur with greater recurrence
   interval, and tend to be under-reported due to the scarcity of media
   coverage in a third-world country. The annual human death toll is about
   179 deaths per year from tornadoes in Bangladesh, which is much greater
   than in the US. This is likely due to the density of population, poor
   quality of construction, lack of tornado safety knowledge, and other
   factors..

   Other areas of the world that have more frequent strong tornadoes
   include parts of Argentina and southern Brazil as well as South Africa.
   A fair number of weak and occasionally strong tornadoes occurs annually
   in Germany, Italy, and China. Australia, France, Spain, Russia, areas
   of the Middle East, and Japan have a history of multiple damaging
   tornado events.

Frequency of occurrence

   Tornadoes can form almost every month, providing the conditions are
   favorable. Though they are scarce during the Winter months, they are
   abundant in Spring. Although Autumn isn't a common season for tornado
   outbreaks, they are significantly spotted, although in way lower
   numbers than in Spring. Since Autumn and Spring are transitional
   periods (warm to cool and vice versa)there are more chances of cooler
   air meeting with warmer air, resulting in thunderstorms. Summer can
   provide some storms too, but these are mostly afternoon showers.
   Significant numbers of tornadoes in the summertime are due to hurricane
   landfalls in the Gulf Coast states and their remnants traveling
   northward. Also, sometimes, a cold front may drop from the north,
   resulting in a possible thunderstorm.

Time of occurrence

   Tornado occurrence is highly dependant on the time of day. Austria,
   Finland, Germany, and the United States' peak hour of occurrence is 5
   p.m., with roughly half of all tornado occurrence between 3 p.m. and 7
   p.m. local time.

Extremes

   Tornadoes are the most violent weather events in the world. As such,
   they have been recorded to produce some incredible phenomena.

   In terms of the most extreme tornado in history, the honour undoubtedly
   goes to the Tri-State Tornado which roared through parts of Missouri,
   Illinois, and Indiana on March 18, 1925. This tornado, likely an F5
   (though this was before the era where tornadoes were ranked on the
   Fujita scale), set (and still holds) records for the deadliest single
   United States tornado (695 dead), longest path length (219 miles,
   352 km), longest duration (about 3.5 hours), and fastest forward speed
   for a significant tornado (73 mph, 117 km/h). It was also the second
   costliest tornado in history at the time, but has since been surpassed
   by several others non-normalized, it still ranks third when normalized
   for wealth and inflation.

   The deadliest tornado in world history occurred on April 26, 1989 in
   Bangladesh, killing approximately 1300 people.

   The most extensive tornado outbreak on record, in almost every
   category, was the Super Outbreak, which affected a large area of the
   Central United States and extreme southern Ontario in Canada on April 3
   and 4, 1974. Not only did this outbreak feature an incredible 148
   tornadoes in only 18 hours, but an unprecedented amount of them were
   violent; six of the tornadoes were of F5 intensity, and 24 were of F4
   intensity. More than 300 people, possibly as many as 330, were killed
   by tornadoes during this outbreak.

   While it is nearly impossible to directly measure the most violent
   tornado wind speeds (conventional anemometers would be destroyed by the
   intense winds), some tornadoes have been scanned by mobile doppler
   radar units, which can provide a good estimate of the tornado's winds.
   The highest wind speed ever measured in a tornado, which is also the
   highest wind speed ever recorded on the planet, is 301 mph (484 km/h)
   in the F5 Moore, Oklahoma tornado. Though there was an uncertainty in
   the measurement of about 20 mph, and the reading was taken about
   100 feet (30 m) above the ground, this is a testament to the power of
   the strongest tornadoes.

   Storms which produce tornadoes can feature intense updrafts (sometimes
   exceeding 150 mph, 240 km/h). As such, debris from a tornado can be
   lofted into the parent storm, and be carried for very long distances. A
   tornado which affected Great Bend, Kansas in November, 1915 was an
   extreme case, where a "rain of debris" occurred 80 miles (130 km) from
   the town, a sack of flour was found 110 miles (177 km), and a cancelled
   check from the Great Bend bank was found in a field outside of Palmyra,
   Nebraska, 305 miles (491 km) to the northeast.

Tornado safety

Precautions

   Though tornadoes can strike in an instant, there are precautions and
   preventative measures that you can take in order to increase the
   chances of surviving a tornado.

   In tornado-prone areas, many buildings have storm cellars on the
   property. These underground refuges have saved thousands of lives.

   Prepare a plan of action before a tornado threatens. Tornado drills
   test the effectiveness of the plan and make others aware of it.

   Some countries have meteorological agencies which distribute tornado
   forecasts and increase levels of alert of a possible tornado (such as
   watches and warnings in the United States and Canada). Weather radios
   (where available) provide alarm when increased tornadic threat is
   detected for your local area. Being aware of the different terminology
   used by your weather organization and monitoring data sources (which
   can also include the Internet, television, and radio) for information
   can increase the warning you have for a tornado.

If a tornado approaches while you are...

     * Outside

   Seek shelter immediately inside a sturdy building. If such a building
   is out of your reach, a ditch or culvert can be used as a last resort.
     * Driving... and the tornado is quite distant

   Try to remain calm and drive in a direction that takes you away from
   where the tornado is moving. While the tornado is a threat, so are
   traffic accidents induced by panic.
     * Driving... and the tornado is near or other circumstances prevent
       escape

   Look for safe shelter. If you find it, park your car away from travel
   lanes and hastily move into the shelter. Highway overpasses are not
   safe shelter. Vehicles are extremely dangerous locations to be caught
   in during a tornado, but making alert decisions can save your life.
     * Inside a house or other building

   If inside a weak structure, such as a shed or mobile home, try to
   relocate to a stronger shelter if possible. If inside a stronger
   structure, seek shelter in the basement or tornado shelter. If the
   structure does not have a such options, or if the tornado is so close
   that it will not allow you to reach it, go to the innermost part of the
   building (commonly under the stairs, a bathroom, or an interior hallway
   in a house, or a bathroom, stairwell, or interior office in a larger
   building), preferably on the lowest level. Whatever location you go to,
   attempt to seek protection under a sturdy object such as a workbench or
   heavy table, and cover yourself (with objects like blankets, pillows,
   cushions, and even clothing) for additional protection.
     * In all cases:

   Get as low as possible, and attempt to protect your head and vital
   organs as best you can. Remain in your shelter until you are certain
   the tornado has moved past, and be careful of debris when exiting your
   shelter.

Myths and misconceptions

   One of the most persistant myths associated with tornadoes is that
   opening windows will lessen the damage caused by the tornado. While it
   is true that there is a large drop in atmospheric pressure inside a
   strong tornado, it is unlikely that the pressure drop would be enough
   to cause the house to explode. In fact, some research indicates that
   opening windows may indeed increase the severity of the tornado's
   damage. Regardless of the validity of the explosion claim, however,
   time would be better spent seeking shelter before a tornado than
   opening windows.

   Another commonly held belief is that highway overpasses provide
   adequate shelter from tornadoes. On the contrary, a highway overpass is
   a very dangerous place to be during a tornado. During the Oklahoma
   Tornado Outbreak of May 3, 1999, three highway overpasses were directly
   struck by tornadoes, and at all three there was a fatality, along with
   many life-threatening injuries. During the same tornado outbreak, more
   than 2000 homes were completely destroyed, with another 7000 damaged,
   and yet only a few dozen people died in their homes. Similarly, an old
   belief was that the southwest corner of a basement provides the most
   protection during a tornado. In actuality, the northeast corner of a
   building is the safest, and taking shelter under a sturdy table, in a
   basement, or under a staircase increases chances of survival even more.

   Finally, there are areas which people believe to be protected from
   tornadoes, whether by a major river, a hill or mountain, or even
   protected by " spirits". All of these are untrue assumptions, as
   tornadoes have been known to cross major rivers, climb mountains, and
   affect valleys. As a general rule, no area is "safe" from tornadoes,
   though some areas are more susceptible than others (see the Geography
   section).

Continuing research

   Though scientists have learned much from years of research, there are
   still many things about tornadoes which remain a mystery. In fact,
   scientists still don't know the exact method by which most tornadoes
   form. Research programs, including VORTEX, deployment of TOTO (the
   TOtable Tornado Observatory), and dozens of other programs, hope to
   solve many questions that still plague meteorologists.

Social implications of tornadoes

   Salt Lake City Tornado, August 11, 1999. (Orange fireball is substation
   exploding)
   Enlarge
   Salt Lake City Tornado, August 11, 1999. (Orange fireball is substation
   exploding)

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

   Some individuals and hobbyists, known as storm chasers, enjoy pursuing
   thunderstorms and tornadoes to explore their many visual and scientific
   aspects. Attempts have been made by some storm chasers from educational
   and scientific institutions to drop probes in the path of oncoming
   tornadoes in an effort to analyze the interior of the storms, but only
   about five drops have been successful since around 1990.

   Due to the relative rarity and large scale of destructive power that
   tornadoes possess, their occurrence or the possibility that they may
   occur can often create what could be considered sensationalism in their
   reporting. This results in so-called weather wars, in which competing
   local media outlets, particularly TV news stations, engage in
   continually escalating technological one-upsmanship and drama in order
   to increase their market share. This is especially evident in
   tornado-prone markets, such as those in the Great Plains.

   According to Environment Canada, the chances of being killed by a
   tornado are 12 million to 1 (12,000,000:1). One may revise this yearly
   and/or regionally, but the probability may be factually stated to be
   low. Tornadoes do cause millions of dollars in damage, both economic
   and physical, displacement, and many injuries every year.^[ citations
   needed]

Cultural significance

Tornadoes as a metaphor

   Cyclone as metaphor for political revolution; the farm woman taking
   shelter is labeled 'Democratic Party'. Puck magazine (1894)
   Enlarge
   Cyclone as metaphor for political revolution; the farm woman taking
   shelter is labeled 'Democratic Party'. Puck magazine (1894)

   The tornado has been used by cartoonists for over 100 years as a
   metaphor for political upheaval. For example, according to political
   interpretations of The Wonderful Wizard of Oz, the tornado takes
   Dorothy to a utopia, the Land of Oz, and kills the Wicked Witch of the
   East, who had oppressed a little people, the Munchkins.. The storm
   cellar has also been used as a metaphor for seeking safety, as shown in
   the cartoon from 1894 at right.

   A 1960s advertising campaign for the household cleaner, Ajax, claimed
   the product "Cleans like a white tornado".

   Tornadoes in dreams are sometimes said to be associated with fear,
   chaos, and upheaval. It is alleged that the location where one is
   during a tornado dream, e.g. at home, can help to determine the
   meaning.^[ citations needed]

Motion pictures with a tornado theme

     * The Wizard of Oz, 1939.
     * Mr. and Mrs. Bridge, 1990.
     * Night of the Twisters (TV), 1996.
     * Tornado! (TV), 1996.
     * Twister, 1996.
     * Atomic Twister (TV), 2001.
     * The Day After Tomorrow, 2004.
     * Perfect Disaster: Super Tornado (Discovery Channel), premiered on
       March 19, 2006.
     * Category 7: The End of the World, 2005.

   Retrieved from " http://en.wikipedia.org/wiki/Tornado"
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