Tornado Activity

Hello Bloggers, today I wanted to write to you about Tornado Myths. When I was growing up my mother would always say "We can't get tornadoes here in the mountains", or "Tornadoes can't cross rivers", or "A tornado can never strike a big city". Well all that is complete utter bullshit! So thanx to www.wikipedia.org I have enclosed some Tornado myths and I hope this helps someone out there escape from the wrath of these dreadful beasts!!

A tornado is a violently rotating column of air which is in contact with both a cumulonimbus (or, in rare cases, a cumulus) cloud base and the surface of the earth. Tornadoes come in many sizes, 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 tornadoes have wind speeds of 110 mph (175 km/h) or less, are approximately 250 feet (75 m) across, and travel a few miles (several kilometers) before dissipating. However, some tornadoes attain wind speeds of more than 300 mph (480 km/h), stretch more than a mile (1.6 km) across, and stay on the ground for dozens of miles (more than 100 km).[1][2][3]

Although tornadoes have been observed on every continent except Antarctica, most occur in the United States.[4] Other areas where they commonly occur include south-central Canada, south-central and eastern Asia, east-central South America, Southern Africa, northwestern and central Europe, Italy, western and southeastern Australia, and New Zealand.[5]

Extremes

Tornado records

In terms of the most extreme tornado in recorded history, the honor 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 any damage 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).[8] It was also the second costliest tornado in history at the time, but has since been surpassed by several others non-normalized. When costs are normalized for wealth and inflation, it still ranks third today.[62]

The deadliest tornado in world history occurred in Bangladesh on April 26, 1989, killing approximately 1300 people.[46]
A map of the tornado paths in the Super Outbreak.
A map of the tornado paths in the Super Outbreak.

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 April 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.[63]

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 ± 20 mph (484 ± 32 km/h) in the F5 Moore, Oklahoma tornado. Though the reading was taken about 100 feet (30 m) above the ground, this is a testament to the power of the strongest tornadoes.[1]

Storms which produce tornadoes can feature intense updrafts (sometimes exceeding 150 mph, 240 km/h). Debris from a tornado can be lofted into the parent storm and carried a very long distance. 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) away, 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.[64]

Definitions
A tornado near Seymour, Texas.
A tornado near Seymour, Texas.

Tornado
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..."[6]

Condensation funnel
A tornado is not necessarily visible; however, the intense low pressure caused by the high wind speeds (see Bernoulli's principle) and rapid rotation (due to cyclostrophic balance) usually causes water vapor in the air to condense into a visible condensation funnel.[4] Strictly, the term tornado refers to the vortex of wind, not the condensation cloud.
A funnel cloud is a visible condensation funnel with no associated strong winds at the surface. Not all funnel clouds evolve into a tornado. However, many tornadoes are preceded by a funnel cloud as the mesocyclonic rotation descends toward the ground. Most tornadoes produce strong winds at the surface while the visible funnel is still above the ground, so it is difficult to tell the difference between a funnel cloud and a tornado from a distance.[3]

Tornado family
Occasionally a single storm produces multiple tornadoes and mesocyclones. This process is known as cyclic tornadogenesis. Tornadoes produced from the same storm are referred to as a tornado family. Sometimes multiple tornadoes from distinct mesocyclones occur simultaneously.[7]

Tornado outbreak
Occasionally, several tornadoes are spawned from the same large-scale storm system. While there is no single agreed-upon definition, multiple tornadoes spawned by the same storm system with no break in activity is considered a tornado outbreak. A period of several successive days with tornado outbreaks in the same general area (spawned by multiple weather systems) is a tornado outbreak sequence, occasionally called an extended tornado outbreak.[6][8][9]

[edit] 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.[10][11] Tornadoes are also commonly referred to as twisters.[12]

[edit] Types of tornadoes
A multiple-vortex tornado outside of Dallas, Texas on April 2, 1957.
A multiple-vortex tornado outside of Dallas, Texas on April 2, 1957.

[edit] True tornadoes

Multiple vortex tornado
A multiple vortex tornado is a type of tornado in which two or more columns of spinning air rotate around a common center. Multivortex structure can occur in almost any circulation, however it is very often observed in intense tornadoes.

Satellite tornado
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.[3]

A waterspout near the Florida Keys.
A waterspout near the Florida Keys.

Waterspout
A waterspout is officially defined by the U.S. National Weather Service simply as a tornado over water. Amongst researchers, however, they are typically divided into two categories: "fair weather" waterspouts, and tornadic waterspouts.

* "Fair weather" waterspouts are the less-severe (but far more common) variety, and are similar in dynamics to dust devils and landspouts.[13] They form at the bases of cumulus congestus cloud towers in tropical and semitropical waters.[13] They have relatively weak winds, smooth laminar walls, and typically travel very slowly, if at all.[13] They occur most commonly in the Florida Keys.[14]
* Tornadic waterspouts are more literally "tornadoes over water", and form the same way as mesocyclonic tornadoes. A tornado which develops in the traditional manner on land and crosses onto a body of water is also considered a tornadic waterspout. Since they form from severe thunderstorms and have the capacity to be far more intense, faster, and longer-lived than their fair weather cousins, they are considered to be far more dangerous.

A landspout near North Platte, Nebraska on May 22, 2004.
A landspout near North Platte, Nebraska on May 22, 2004.

Landspout
A landspout is an unofficial term for a tornado not associated with a mesocyclone. The name stems from their characterization as essentially a "fair weather waterspout on land". Waterspouts and landspouts share many defining characteristics, including relative weakness, short lifespan, and a small, smooth condensation funnel which often does not reach the ground. Landspouts also create a distinctively laminar cloud of dust when they make contact with the ground, owing to their differing mechanics from true mesoform tornadoes. Though usually weaker than classic tornadoes, they still produce strong winds and can potentially cause serious damage.[3][15]

[edit] Tornado-like circulations

Gustnado
A gustnado (gust front tornado) 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. They are formed when fast moving cold, dry outflow air from a thunderstorm is blown through a mass of stationary, warm, moist air near the outflow boundary, resulting in a "rolling" effect (often exemplified through a roll cloud). If low level wind shear is strong enough, the rotation can be turned horizontally (or diagonally) and make contact with the ground. The result is a gustnado.[3][16] They usually cause small areas of heavier rotational wind damage among areas of straight-line wind damage. It is also worth noting that since they are absent of any Coriolis influence from a mesocyclone, they seem to be alternately cyclonic and anticyclonic without preference.

Dust devil in Johnsonville, South Carolina.
Dust devil in Johnsonville, South Carolina.

Dust devil
A dust devil resembles a tornado in that it is a vertical swirling column of air. However, they form under clear skies and are rarely as strong as even the weakest tornadoes. They form when a strong convective updraft is formed near the ground on a hot day. If there is enough low level wind shear, the column of hot, rising air can develop a small cyclonic motion that can be seen near the ground. They are not considered tornadoes because they form during fair weather and are not associated with any actual cloud. However, they can, on occasion, result in major damage, especially in arid areas.[17][18]

Fire whirl
Tornado-like circulations occasionally occur near large, intense wildfires and are called fire whirls. They are not considered tornadoes except in the rare case where they connect to a pyrocumulus or other cumuliform cloud above. Fire whirls usually are not as strong as tornadoes associated with thunderstorms. However, they can produce significant damage.[8]

Cold air vortex
A cold air vortex or shear funnel is a tiny, harmless funnel cloud which occasionally forms underneath or on the sides of normal cumuliform clouds, rarely causing any winds at ground-level.[19] Their genesis and mechanics are poorly understood, as they are quite rare, short lived, and hard to spot (due to their non-rotational nature and small size).

[edit] Characteristics
A wedge tornado, nearly a mile wide.
A wedge tornado, nearly a mile wide.
A rope tornado in its dissipating stage.
A rope tornado in its dissipating stage.

[edit] Shape

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

Small, relatively weak landspouts may 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), the circulation is considered a tornado.[15] Large single-vortex tornadoes can look like large wedges stuck into the ground, and so are known as wedge tornadoes or wedges. A wedge can be so wide that it appears to be a block of dark clouds, wider than the distance from the cloud base to the ground. Even experienced storm observers may not be able to tell the difference between a low-hanging cloud and a wedge tornado from a distance.[20]

Tornadoes in the dissipating stage can resemble narrow tubes or ropes, and often curl or twist into complex shapes. These tornadoes are said to be roping out, or becoming a rope tornado. Multiple-vortex tornadoes can appear as a family of swirls circling a common center, or may be completely obscured by condensation, dust, and debris, appearing to be a single funnel.[21]

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.[17]

[edit] 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).[17] 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.[17] On the other end of the spectrum, wedge tornadoes can have a damage path a mile (1.6 km) wide or more. A tornado that affected Hallam, Nebraska on May 22, 2004 was at one point 2.5 miles (4 km) wide at the ground.[2]

In terms of path length, the Tri-State Tornado, which affected parts of Missouri, Illinois, and Indiana on March 18, 1925, was officially on the ground continuously for 219 miles (352 km). Many tornadoes which appear to have path lengths of 100 miles or longer are actually a family of tornadoes which have formed in quick succession; however, there is no substantial evidence that this occurred in the case of the Tri-State Tornado.[8] In fact, modern reanalysis of the path suggests that the tornado began 15 miles (24 km) further west than previously thought.[22]

[edit] 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 gray 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 color 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.[17]
These are two photographs of the Waurika, Oklahoma tornado of May 30, 1976, taken at nearly the same time by two different photographers. 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.
These are two photographs of the Waurika, Oklahoma tornado of May 30, 1976, taken at nearly the same time by two different photographers. 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.[23]

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 gray or brilliant white. Tornadoes which occur near the time of sunset can be many different colors, appearing in hues of yellow, orange, and pink.[24][12]

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 in the late afternoon, when the bright sun can penetrate even the thickest clouds.[8] Also, night-time tornadoes are often illuminated by frequent lightning. There is mounting evidence, including DOW mobile radar images and eyewitness accounts, which suggest that most tornadoes have a clear, calm center 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.[25][26][27]

[edit] Rotation

Tornadoes normally rotate cyclonically in direction (counterclockwise in the northern hemisphere, clockwise in the southern). While large-scale storms always rotate cyclonically due to the Coriolis effect, thunderstorms and tornadoes are so small that the direct influence of Coriolis effect is inconsequential, as indicated by their large Rossby numbers. Supercells and tornadoes rotate cyclonically in numerical simulations even when the Coriolis effect is neglected.[28][29] Low-level mesocyclones and tornadoes owe their rotation to complex processes within the supercell and ambient environment.[30]

Approximately 1% of tornadoes rotate in an anticyclonic direction. Typically, only landspouts and gustnados rotate anticyclonically, and usually only those which form on the anticyclonic shear side of the descending rear flank downdraft in a cyclonic supercell.[31] However, on rare occasions, anticyclonic tornadoes form in association with the mesoanticyclone of an anticyclonic supercell, in the same manner as the typical cyclonic tornado, or as a companion tornado—either as a satellite tornado or associated with anticyclonic eddies within a supercell.[32]

[edit] Sound and seismology

Tornadoes emit widely on the acoustics spectrum and multiple mechanisms cause the sound of a tornado. Various sounds of tornadoes have been reported throughout time, mostly related to familiar sounds for the earwitness and generally some variation of a whooshing roar. Among the popularly reported sounds are a freight train, rushing rapids or a waterfall, and a jet engine from close proximity, or combinations thereof. Many tornadoes are not audible from much distance; the nature and propagation distance of the audible sound depends on atmospheric conditions and topography.

The winds of both the tornado vortex and constituent turbulent eddies, as well as airflow interaction with the surface and debris, contribute to the sound of a tornado; this is evidenced by both funnel clouds and tornadoes having sounds, and the associated sounds differing. Funnel clouds and small tornadoes are reported as a whistling, whining, humming, or the buzzing of innumerable bees or electricity, or more or less harmonic, whereas many tornadoes are reported as a continuous, deep rumbling, or an irregular sound of “noise”.[33] It is important to note that many tornadoes do not produce any sound unless one is within very close proximity, so sound is not reliable forewarning of a tornado; and that not only tornadoes but also any strong, damaging wind, even a severe hail volley or continuous thunder within a thunderstorm may produce a roaring sound.[34]
An illustration of generation of infrasound in tornadoes by the Earth System Research Laboratory's Infrasound Program.
An illustration of generation of infrasound in tornadoes by the Earth System Research Laboratory's Infrasound Program.

In addition to the audible spectrum, tornadoes also produce identifiable infrasonic signatures.[35] Unlike audible signatures, tornadic signatures have been isolated; due to the long distance propagation of low-frequency sound, efforts are ongoing in developing tornado prediction and detection devices with additional value in understanding tornado morphology, dynamics, and tornadogenesis.[36] Tornadoes also produce a detectable seismic signature, although research continues on isolating the signature and understanding the process.[37]

[edit] Electromagnetic, lightning, and other effects

Tornadoes emit on the electromagnetic spectrum, for example, with sferics and E-field effects detected.[36][38] The effects vary, mostly with little observed consistency.

Correlations with patterns of lightning activity have also been observed, however, little in way of consistent correlations have been advanced. Tornadic storms do not necessarily contain more lightning than other storms, indeed some tornadic cells never contain lightning. More often that not, overall cloud-to-ground (CG) lightning activity decreases as a tornado reaches the surface and returns to the baseline level when the tornado lifts. In many cases, very intense tornadoes and thunderstorms exhibit an increased and anomalous dominance in positive polarity CG discharges.[39] Electromagnetics and lightning have little to nothing to do directly with what drives tornadoes (tornadoes are basically a thermodynamic phenomenon), though there are likely connections with the storm and environment affecting both phenomena.

Luminosity has been reported in the past, and is probably due to misidentification of external light sources such as lightning, city lights, and power flashes from broken lines, as internal sources are now uncommonly reported and are not known to ever been recorded.

In addition to winds, tornadoes also exhibit changes in atmospheric variables such as temperature, moisture, and pressure. For example, on June 24, 2003 near Manchester, South Dakota, a probe measured a 100 mb (hPa) (2.95 inHg) pressure deficit. The pressure dropped gradually as the vortex approached then dropped extremely rapidly to 850 mb (hPa) (25.10 inHg) in the core of the violent tornado before rising rapidly as the vortex moved away, resulting in a V-shape pressure trace. Temperature tends to decrease and moisture content to increase in the immediate vicinity of a tornado.[40]

[edit] Life cycle
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 best-observed violent tornadoes in history.
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 best-observed violent tornadoes in history.

Further information: Tornadogenesis

[edit] Supercell relationship

See also: Supercell

Tornadoes often develop from a class of thunderstorms known as supercells. Supercells contain mesocyclones, an area of organized rotation a few miles up in the atmosphere, usually 1–6 miles (2–10 km) across. Most intense tornadoes (EF3 to EF5 on the Enhanced Fujita Scale) develop from supercells. In addition to tornadoes, very heavy rain, frequent lightning, strong wind gusts, and hail are common in such storms.

Most tornadoes from supercells follow a recognizable life cycle.[15] The cycle begins when increasing rainfall 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 supercell's rotating mesocyclone towards the ground with it.

[edit] Tornado formation

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 becomes 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.

[edit] Maturity and demise

As the RFD completely wraps around and chokes off the tornado's air supply, the vortex begins to weaken, becoming thin and rope-like. This is the dissipating stage; often lasting no more than a few minutes, after which the tornado fizzles. During the dissipating stage the shape of the tornado becomes highly influenced by the winds of the parent storm, and can be blown into fantastic patterns.[23][24][8]

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 may be concentrated into a new area closer to the center of the storm. If a new mesocyclone develops, the cycle may start again, producing one or more new tornadoes. Occasionally, the old (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.[41]

[edit] Intensity and damage
An example of EF3 damage. Here, the roof and some inner walls of this brick building have been demolished.
An example of EF3 damage. Here, the roof and some inner walls of this brick building have been demolished.

Main article: Tornado intensity and damage

The Fujita scale and the Enhanced Fujita Scale rate tornadoes by damage caused. The Enhanced Fujita Scale was an upgrade to the older Fujita scale, with engineered (by expert elicitation) wind estimates and better damage descriptions, but was designed so that a tornado rated on the Fujita scale would receive the same numerical rating. An EF0 tornado will likely damage trees but not substantial structures, whereas an EF5 tornado can rip buildings off their foundations leaving them bare and even deform large skyscrapers. The similar TORRO scale ranges from a T0 for extremely weak tornadoes to T11 for the most powerful known tornadoes. Radar data, photogrammetry, and ground swirl patterns (cycloidal marks) may also be analyzed to determine intensity and award a rating.

Tornadoes vary in intensity regardless of shape, size, and location, though strong tornadoes are typically larger than weak tornadoes. The association with track length and duration also varies, although longer track tornadoes tend to be stronger.[42] In the case of violent tornadoes, only a small portion of the path is of violent intensity, most of the higher intensity from subvortices.[8]

In the United States, EF0 and EF1 (T0 through T3) tornadoes account for 80% of all tornadoes. The rate of occurrence drops off quickly with increasing strength—violent tornadoes (stronger than EF4, T8), account for less than 1% of all tornado reports.[43] Outside of the United States, areas in south-central Asia, and perhaps portions of southeastern South America and southern Africa, violent tornadoes are extremely rare. This is apparently mostly due to the lesser number of tornadoes overall, however, as research has found that tornado intensity distributions are fairly similar worldwide. A few significant tornadoes occur annually in Europe, Asia, southern Africa, and southeastern South America, respectively.[44]


[edit] Climatology

Main article: Tornado climatology

Areas worldwide where tornadoes are most likely, indicated by orange shading.
Areas worldwide where tornadoes are most likely, indicated by orange shading.
Intense tornado activity in the United States. The darker-colored areas denote the area commonly referred to as Tornado Alley.
Intense tornado activity in the United States. The darker-colored areas denote the area commonly referred to as Tornado Alley.

The United States has the most tornadoes of any country, seeing about four times the activity estimated in all of Europe (not including waterspouts).[45] This is mostly due to the unique geography of the continent. North America is a relatively large continent that extends from the tropical south into arctic areas, and has no major east-west mountain range to block air flow between these two areas. In the middle latitudes, where most tornadoes of the world occur, the Rocky Mountains block moisture and atmospheric flow, allowing drier air at mid-levels of the troposphere, and causing cyclogenesis downstream to the east of the mountains. The desert Southwest also feeds drier air and the dry line, while the Gulf of Mexico fuels abundant low-level moisture. This unique topography allows for many collisions of warm and cold air; creating the conditions necessary to breed strong, long-lived storms which occur many times a year. A large portion of these tornadoes form in an area of the central United States known as Tornado Alley.[4] This area extends into Canada, particularly Ontario and the Prairie Provinces. Strong tornadoes also occasionally occur in northern Mexico.

The United States averages about 1,200 tornadoes per year. The Netherlands has the highest average number of recorded tornadoes per area of any country (more than 20, or 0.0013 per sq mi (0.00048 per km²), annually), followed by the UK (around 33, or 0.00035 per sq mi (0.00013 per km²), per year), but most are small and result in minor damage. In absolute number of events, ignoring area, the UK experiences more tornadoes than any other European country, excluding waterspouts.[45]

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 tend to be under-reported due to the scarcity of media coverage in third-world countries. 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.[46] Other areas of the world that have more frequent tornadoes include South Africa, parts of Argentina, Paraguay, and southern Brazil, as well as portions of Europe, Australia and New Zealand, and far eastern Asia.[5]

Tornadoes can form in any month, providing the conditions are favorable. They are least common during the winter and most common in spring.[8] 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. Tornadoes can also be caused by landfalling tropical cyclones, which tend to occur in the late summer and fall.

Tornado occurrence is highly dependent on the time of day, because of solar heating.[47] Worldwide, most tornadoes occur in the late afternoon, between the hours of 3 and 7 pm local time, with a peak near 5 pm.[48][49][50][51][52] However, destructive tornadoes can occur at any time of day. The Gainesville Tornado of 1936, one of the deadliest tornadoes in history, occurred at 8:30 am local time.[8]

[edit] Prediction
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.
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.

Weather forecasting is handled regionally by many national and international agencies. For the most part, they are also in charge of the prediction of conditions conducive to tornado development.

Australia

Severe thunderstorm warnings are provided to Australia by the 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.[53]

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.[54] In Germany, Austria, and Switzerland, an organization known as TorDACH collects 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 fully document severe weather activity in these three countries.[55]

United Kingdom

In the United Kingdom, the Tornado and Storm Research Organisation (TORRO) makes experimental predictions. The Met Office provides official forecasts for the UK.

United States

In the United States, generalized severe weather predictions are issued by the Storm Prediction Center, 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.

Other areas

In Japan, predictions and study of tornadoes in Japan are handled by the Japan Meteorological Agency. In Canada, weather forecasts and warnings, including tornadoes, are produced by the Meteorological Service of Canada, a division of Environment Canada.

[edit] Detection
A Doppler radar image indicating the likely presence of a tornado over DeLand, Florida. Green colors indicate areas where the precipitation is moving towards the radar dish, while red areas are moving away. In this case the radar is in the bottom right corner of the image. Strong mesocyclones show up as adjacent areas of bright green and bright red, and usually indicate an imminent or occurring tornado.
A Doppler radar image indicating the likely presence of a tornado over DeLand, Florida. Green colors indicate areas where the precipitation is moving towards the radar dish, while red areas are moving away. In this case the radar is in the bottom right corner of the image. Strong mesocyclones show up as adjacent areas of bright green and bright red, and usually indicate an imminent or occurring tornado.

Rigorous attempts to give warning for tornadoes began in the United States in the middle of the 20th century. The first public tornado warnings were issued in 1950 and the first tornado watches and convective outlooks were issued in 1952. Before the 1950s, the only method of detecting a tornado was by someone seeing it on the ground. Often, news of a tornado would not reach the local Weather Bureau Office until after the storm was over. However, with the advent of weather radar, areas near a local office could get advance warning of severe weather. With the confirmation that hook echoes are indeed associated with tornadoes in 1953, meteorologists gained the ability to detect thunderstorms likely producing tornadoes from dozens of miles away by recognizing the radar signatures associated with many tornadoes.[56]

Storm spotting

In the mid 1970s, the US National Weather Service began increased efforts to train Skywarn spotters, consisting of local sheriff's deputies, state troopers, firefighters, ambulance drivers, amateur radio operators, civil defense (now emergency management) spotters, storm chasers, and ordinary citizens, to spot key features of storms which indicate severe hail, damaging winds, and tornadoes, as well as damage itself and flash flooding. When severe weather is anticipated, local weather service offices request that these spotters be on the lookout for severe weather, and report any tornadoes immediately, so that the office can issue a timely warning. In most cases, spotters are trained by the NWS on behalf of their respective organization, and report to these organizations, who are responsible for activating public warning systems such as sirens and the Emergency Alert System as well as for forwarding the report to the NWS.[57]

There are currently more than 230,000 trained Skywarn weather spotters across the United States.[58] In Canada, Canwarn, a similar network of Volunteer Weather Watchers now exists to help spot severe weather, with more than 1,000 volunteers.[59] In Europe, several nations are organizing spotter networks under the auspices of Skywarn Europe[60] and the Tornado and Storm Research Organisation (TORRO) has maintained a network of spotters in the United Kingdom since the 1970s.

Storm spotters are needed because radar currently does not detect a tornado itself, rather, indications of a tornado. Radar warnings may give lead time before there is any visual evidence of a tornado or imminent tornado, but ground truth from an observer on location can report what is actually happening, either verifying the threat or determining that a tornado is not imminent. The spotter's ability to see what radar cannot is especially important as distance from the radar site increases, for the radar beam becomes progressively higher in altitude further away from the radar, chiefly due to curvature of the Earth, and the beam also spreads out. Therefore, when far away from a radar, only high in the storm is observed and the important areas are not sampled, and data resolution also suffers. Additionally, some meteorological situations leading to tornadogenesis are not readily detectable by radar and on occasion tornado development may occur more quickly than radar can complete a scan and send the batch of data.

Visual evidence

When a storm is first sighted from a distance, storm spotters are trained to discern whether a storm is a supercell, and typically look to the rear of a thunderstorm, where the main updraft and inflow region is. Under this updraft is often found a rain-free base (RFB), and the next step towards tornadogenesis is the formation of a rotating wall cloud. The vast majority of intense tornadoes occur with a wall cloud on the backside of a supercell.[43]

Evidence that a storm is a supercell is given by its shape and structure, as well as cloud tower features, such as hardness and vigorousness of the updraft tower, a persistent, large overshooting top, a hard anvil (especially when backsheared against strong upper level winds), and a corkscrew look or striations. Underneath the storm and closer to where most tornadoes are found, evidence of a supercell and likelihood of a tornado may be in the form of inflow bands (particularly when curved) such as a "beaver tail", as well as other environmental and storm clues such as strength of and warmth and moistness of inflow air, how outflow or inflow dominant a storm appears, and how far the front flank precipitation core is from the wall cloud. Tornadogenesis is most likely at the interface of the updraft and front flank downdraft, but a balance between the outflow and inflow is necessary for tornado formation.[15]

Not all wall clouds rotate, it is the ones that do rotate that spawn tornadoes, generally preceding tornado formation by five to thirty minutes. Rotating wall clouds are the visual manifestation of a mesocyclone. Barring a low-level boundary, tornadogenesis is highly unlikely unless a rear flank downdraft occurs, which is usually visibly evidenced by evaporation of cloud adjacent to a corner of a wall cloud. A tornado often occurs as this happens or shortly after; first, a funnel cloud dips and in nearly all cases by the time it reaches halfway down, a surface swirl has already developed, signifying a tornado is on the ground before condensation connects the surface circulation to the storm. Tornadoes may also occur without wall clouds, under flanking lines, and on the leading edge. Spotters are trained to watch all areas of a storm, and both the cloud base and surface for cues on what is occurring.[61]

Radar

Today, most developed countries have a network of weather radars, which remains the main method of detecting signatures likely associated with tornadoes. In the United States and a few other countries, Doppler radar stations are used. These devices are capable of measuring the velocity and radial direction (towards or away from the radar) of the winds in a storm, and so can spot evidence of rotation in storms from more than a hundred miles away.

Additionally, most populated areas of the earth are now visible from the Geostationary Operational Environmental Satellites (GOES), which aid in the nowcasting of tornadic storms.[59]

[edit] Extremes

Main article: Tornado records

In terms of the most extreme tornado in recorded history, the honor 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 any damage 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).[8] It was also the second costliest tornado in history at the time, but has since been surpassed by several others non-normalized. When costs are normalized for wealth and inflation, it still ranks third today.[62]

The deadliest tornado in world history occurred in Bangladesh on April 26, 1989, killing approximately 1300 people.[46]
A map of the tornado paths in the Super Outbreak.
A map of the tornado paths in the Super Outbreak.

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 April 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.[63]

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 ± 20 mph (484 ± 32 km/h) in the F5 Moore, Oklahoma tornado. Though the reading was taken about 100 feet (30 m) above the ground, this is a testament to the power of the strongest tornadoes.[1]

Storms which produce tornadoes can feature intense updrafts (sometimes exceeding 150 mph, 240 km/h). Debris from a tornado can be lofted into the parent storm and carried a very long distance. 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) away, 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.[64]

[edit] Safety

Though tornadoes can strike in an instant, there are precautions and preventative measures that people can take to increase the chances of surviving a tornado. Authorities such as the Storm Prediction Center advise having a tornado plan. When a tornado warning is issued, going to a basement or an interior first-floor room of a sturdy building greatly increases chances of survival.[65] In tornado-prone areas, many buildings have storm cellars on the property. These underground refuges have saved thousands of lives.[66]

Some countries have meteorological agencies which distribute tornado forecasts and increase levels of alert of a possible tornado (such as tornado watches and warnings in the United States and Canada). Weather radios provide an alarm when a severe weather advisory is issued for the local area, though these are mainly available only in the United States.

Unless the tornado is far away and highly visible, meteorologists advise that drivers park their vehicles far to the side of the road (so as not to block emergency traffic), and find a sturdy shelter. If no sturdy shelter is nearby, getting low in a ditch is the next best option. Highway overpasses are extremely bad shelter during tornadoes (see next section).[67]

Salt Lake City Tornado, August 11, 1999. This tornado disproved several myths, including the idea that tornadoes cannot occur in areas like Utah.
Salt Lake City Tornado, August 11, 1999. This tornado disproved several myths, including the idea that tornadoes cannot occur in areas like Utah.


One of the most persistent myths associated with tornadoes is that opening windows will lessen the damage caused by the tornado. While 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. Some research indicates that opening windows may actually increase the severity of the tornado's damage. Regardless of the validity of the explosion claim, time would be better spent seeking shelter before a tornado than opening windows. A violent tornado can destroy a house whether its windows are open or closed.[68][69]

Another commonly held belief is that highway overpasses provide adequate shelter from tornadoes. On the contrary, a highway overpass is a dangerous place during a tornado. In the Oklahoma Tornado Outbreak of May 3, 1999, three highway overpasses were directly struck by tornadoes, and at all three locations there was a fatality, along with many life-threatening injuries. The small area under the overpasses created a kind of wind tunnel, increasing the wind's speed, making the situation worse.[70] By comparison, 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.[67]

An old belief is that the southwest corner of a basement provides the most protection during a tornado. The safest place is the side or corner of an underground room opposite the tornado's direction of approach (usually the northeast corner), or the central-most room on the lowest floor. Taking shelter under a sturdy table, in a basement, or under a staircase increases chances of survival even more.[68][69]

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". Tornadoes have been known to cross major rivers, climb mountains,[71] and affect valleys. As a general rule, no area is "safe" from tornadoes, though some areas are more susceptible than others.[68][69][17] (See Tornado climatology).

[edit] Continuing research
A Doppler On Wheels unit observing a tornado near Attica, Kansas.
A Doppler On Wheels unit observing a tornado near Attica, Kansas.

Meteorology is a relatively young science and the study of tornadoes even more so. Although studied for about 140 years and intensively for around 60 years, there are still aspects of tornadoes which remain a mystery.[72] Scientists do have a fairly good idea of the development of thunderstorms and mesocyclones, and the meteorological conditions conducive to their formation; however, the step from supercell (or other respective formative processes) to tornadogenesis and predicting tornadic vs. non-tornadic mesocyclones is not yet well understood and is the focus of much research.

Also under study are the low-level mesocyclone and the stretching of low-level vorticity which tightens into a tornado, namely, what are the processes and what is the relationship of the environment and the convective storm. Intense tornadoes have been observed forming simultaneously with a mesocyclone aloft (rather than succeeding mesocyclogenesis) and some intense tornadoes have occurred without a mid-level mesocyclone. In particular, the role of downdrafts, particularly the rear-flank downdraft, and the role of baroclinic boundaries, are intense areas of study.

Reliably predicting tornado intensity and longevity remains a problem, as do details affecting characteristics of a tornado during its life cycle and tornadolysis. Other rich areas of research are tornadoes associated with mesovortices within linear thunderstorm structures and within tropical cyclones.[73]

Scientists still do not know the exact mechanisms by which most tornadoes form, and occasional tornadoes still strike without a tornado warning being issued, especially in under-developed countries. Analysis of observations including both stationary and mobile (surface and aerial) in-situ and remote sensing (passive and active) instruments generates new ideas and refines existing notions. Numerical modeling also provides new insights as observations and new discoveries are integrated into our physical understanding and then tested in computer simulations which validate new notions as well as produce entirely new theoretical findings, many of which are otherwise unattainable. Importantly, development of new observation technologies and installation of finer spatial and temporal resolution observation networks have aided increased understanding and better predictions.

Research programs, including field projects such as VORTEX, deployment of TOTO (the TOtable Tornado Observatory), Doppler On Wheels (DOW), and dozens of other programs, hope to solve many questions that still plague meteorologists.[36] Universities, government agencies such as the National Severe Storms Laboratory, private-sector meteorologists, and the National Center for Atmospheric Research are some of the organizations very active in research; with various sources of funding, both private and public, a chief entity being the National Science Foundation.


Tornado myths
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As a powerful force of nature, tornadoes have become a source of some persistent urban legends and common misconceptions. These urban legends are typically in the form of folk wisdom on how to find safe shelter from a tornado, or how to minimize property damage.
Contents
[hide]

* 1 Using highway overpasses as shelter
* 2 Tornado behavior
* 3 Opening windows or doors
* 4 Mobile home and trailer parks
* 5 Safest location in a house
* 6 Escaping a Tornado
* 7 Tornadoes in rough terrain and crossing rivers
* 8 Tornadoes in urban areas
* 9 Tornadoes in far-northern latitudes or in winter
* 10 References
* 11 See also
* 12 External links

[edit] Using highway overpasses as shelter

Myth: Highway overpasses are adequate shelter if a tornado approaches while you are on a road.

Sensational footage taken by a television crew hiding from a tornado under an overpass during the 1991 Andover, Kansas Tornado Outbreak helped to convince some that bridges are good shelters when a tornado is nearby. The members of the television crew (and several other travelers) survived by huddling high underneath the bridge and bracing themselves against support columns while a weak tornado appeared to pass directly over the bridge.

In reality, when directly hit by tornadoes, the confined spaces beneath overpasses increase the speed of the winds due to the Venturi effect, and thus make them potentially less safe (somewhat like being in a windtunnel). In the case of the Andover tornado footage, it was discovered that the tornado did not pass directly over the bridge, but instead over the ground slightly south of the bridge and camera crew, exposing them to much weaker winds.

[edit] Tornado behavior

Myth: Tornadoes sometimes "skip houses".

It is true that a house that is in between two destroyed homes can be "untouched", but this is not the result of a tornado "skipping" as was previously thought. After the Super Outbreak, Dr. Fujita studied many films of tornadoes from that day. Included in his review was damage and tornado film footage of F4 and F5 tornadoes. Fujita concluded that the multiple vortices of an F-4 or F-5, which are highly volatile but small vortices that dance around the main funnel, are responsible for making tornadoes appear to "skip houses".

The way it works is that the main funnel, moving along in a general straight path, will miss several houses that are just to the left or just to the right of it. However, the "multiple vortices", which are dancing in circles around the main funnel, do hit these houses, since they swing out and around, in a circle around the funnel. But because they are constantly spinning in circles around the main funnel as it moves forward, then a multi-vortex may hit one house as the main funnel passes by, but that same multi-vortex may already have moved to other side of the main funnel by the time the main funnel passes the next house, making it appear that the main funnel "skipped over" a house.

Myth: I don't have to worry about skinny tornadoes, only the fat ones are strong

A lethal myth. In the first place all tornadoes are dangerous, and should never be dismissed as "not powerful". Secondly, although large tornadoes are generally more powerful, this is not always the case. There have been many instances where "classic" funnels (normal size) or even skinny funnels were deadly F-4 or F-5 tornadoes, where-as a large 1/2 mile wide "wedge" tornado (which make up a lot of F-4 or F-5's) might be an F-3. So the width of a tornado is not a good indicator of how powerful it is, and all tornadoes should be taken very seriously.

[edit] Opening windows or doors

Myth: Most tornado damage is due to the low pressure in the tornado causing the house to explode. Opening your windows or doors while a tornado approaches will equalize atmospheric pressure and help prevent property damage.

Since windows are typically the most fragile external feature of a house, they are in more danger from flying debris. Opening them during an active tornado wastes time and effort that could be spent on other, more useful protective measures. Homes do not "explode" when hit by a tornado, though it often appears so. Commonly, a tornado will break the windows first, allowing strong winds to enter the home. These winds may then push on the underside of the roof upwards, blowing it off. Without the roof, the walls lose structural support and will often fall outwards. Observing the wreckage after the collapse may give the impression the house was pushed apart from the inside. Other ways a house may be perceived to have been "blown apart" is from the winds pushing up against the roof where it meets the walls, ripping the roof off, and causing the walls to collapse.

Flying debris or wind from a tornado will break the windows anyway, so opening them only wastes valuable time and is even counterproductive to the soundness of the structure. It is the debris and wind that breaks windows, not the difference in pressure.

As a note, this also applies to homes or structures that are hit by a hurricane. Studies from the National Hurricane Center suggest that closed containers do not explode during high wind scenarios. But rather, an opening, such as a broken window, will allow the hurricane force winds to enter a room and subsequently destroy an entire building.

[edit] Mobile home and trailer parks

Myth: Twisters are attracted to mobile homes and/or trailer parks.

Trailer parks consist of low-cost mobile homes with less structural integrity than traditional houses. A weak storm that leaves little damage to well-built structures might devastate a trailer park. Mobile homes do not attract tornadoes; they are just more susceptible to damage from them.

Myth: In a trailer or mobile home, the best place to be is in a closet or bathroom

You do not want to be anywhere inside a mobile home or trailer when a tornado strikes. Unlike a house, a mobile home is easily ripped apart by even the weakest tornadoes. So if there is no underground shelter nearby, a ditch or low lying area would be better than staying inside a mobile home.

[edit] Safest location in a house

Myth: During a tornado, the southwest corner of a building is the safest.

An unfortunately fatal belief, and for a long time it was considered sound advice but without any proof of safety compared to any other parts of a building. After the increase in tornado research during the turn of the millennium, the U.S. National Weather Service has now adopted the advice that the central-most-room on the lowest level of a structure is the safest, with centrally-located rooms in an underground level being far safer than any above-ground location. In reality, a tornado can hit any part of a building thereby making any part of the exterior subject to damage from rapidly changing winds.

Some of the worst places during a tornado are in a room with many windows, any room with an exterior wall, or a large theater-like room such as a church or indoor basketball court. The best places are small rooms like closets or bathrooms. Bathrooms are considered particularly safe as the plumbing fixtures strengthen the walls and anchor them to the ground, while a bathtub can provide some degree of protection from flying debris. The void space underneath a stairwell is also a recommended shelter, as the stairway itself braces and strengthens the walls.

Bank vaults are probably the safest above-ground shelters from tornadoes; in a number of cases, small towns have been entirely swept away by violent tornadoes, but the vault at the local bank was left undamaged. Other potential shelters in commercial buildings include restaurant walk-in freezers and interior stairwells.

[edit] Escaping a Tornado

Myth: The best thing to do when a twister approaches, is to get in a car and drive away

Actually, it can be true in many cases, but it may depend. This can be the worst thing to do, since a car is the last place you want to be "caught" when a tornado strikes. This is where you must use great judgment; if a tornado is far enough away (several miles), and you can judge the direction that it is moving, then driving away from it would be acceptable. But if the tornado is too close and only minutes away, or if you cannot judge its direction of movement, then an automobile is a horrible place to be, since a tornado can easily pick up a car and turn it into a flying missile. In that case, the best place to be is in a basement or storm cellar, or if those do not exist, then interior closet or bathroom. Also, even if the tornado is very far away and you can easily judge its direction of movement, if you live in a city it's best never to attempt to drive away since the possibilities are good that you may get caught in a traffic jam.

[edit] Tornadoes in rough terrain and crossing rivers

Myth: Tornadoes cannot form near rivers or cross them.

Myth: Tornadoes cannot follow terrain into steep valleys.

Myth: Tornadoes cannot travel over steep hills or mountains.

During the Super Outbreak, a tornado formed near Sayler Park section of Cincinnati, Ohio (near the Ohio River). It was among the six F5s of the outbreak. The city of Cairo, Illinois, which lies at the confluence of the Ohio and Mississippi Rivers, was also hit by a tornado that day.

The Tri-state tornado of 1925 crossed the Mississippi river and the Wabash river, and possibly several other small bodies of water.

The F5 tornado of May 3, 1999 crossed the Canadian River in Oklahoma before it hit Moore, Oklahoma.

The Windsor - Tecumseh, Ontario Tornado of 1946 crossed the Detroit River from River Rouge, Michigan into downtown Windsor, Ontario, where the river is roughly 3/4 of a mile wide. The F3 tornado that struck on July 2, 1997 also crossed the river into Windsor.

During the Super Outbreak, after destroying three schools, the Monticello tornado crossed over a 60-foot bluff and the Tippecanoe River and damaged several homes.

During the Super Outbreak, the Huntsville tornado crossed Monte Sano mountain (1,650 feet) and gained in intensity as it descended the mountain.

During the same outbreak, an F4 tornado caused damage in Murphy, NC after crossing a 3,000-foot ridge, and F2 tornadoes were confirmed in Roanoke, VA and Great Smoky Mountains National Park, NC. Tornadoes formed elsewhere in West Virginia, western Virginia, southwestern North Carolina, and north Georgia - regions of four states that are in the ranges of the Appalachian mountains.

Appalachia has been struck by other destructive tornado outbreaks: during the "Enigma" outbreak (Feb. 19, 1884), at least one major tornado family struck the mountains of SW North Carolina. On May 1, 1929, a destructive tornado outbreak swept from SW to NE up the Appalachians from Alabama to Maryland, spawning destructive tornadoes at Rye Cove, VA, Morgantown, WV, and in a series moving from Rappahannock County, VA to Frederick, MD. In 1944, a devastating tornado outbreak swept from NW to SE through parts of Ohio, West Virginia, Pennsylvania, Maryland and Virginia, with the worst damage seen in mountainous areas between Pittsburgh and Washington, DC. And in May 1985, several large tornadoes associated with a wide outbreak crossed the Alleghenies in central Pennsylvania.

High altitudes are not necessarily an impediment to tornado formation - the 1999 Salt Lake City Tornado in Utah formed at elevations of over 4000 feet and produced F2 damage in the downtown area. Farther north, a 1989 tornado shredded timber and left a mile-wide path of F4 damage over extremely rugged terrain in the Teton Wilderness in Wyoming, crossing the continental divide at an elevation of over 11,000 feet. In 2004, a tornado was photographed near Rockwell Pass in the Sierra Nevada of California at nearly 12,000 feet. However, it should be noted (for other climatological reasons) that it is a rare occurrence for tornadoes to form west of the Rocky Mountains.

[edit] Tornadoes in urban areas

Myth: You're safe from a tornado in a big city.

Closely related to the "terrain" story (See Salt Lake City tornado just above), it is commonly believed that a tornado will dissipate in an urban area because of the tall skyscrapers. The May 3, 1999 tornado outbreak which struck urban Oklahoma City, and the tornado that ripped through the heart of downtown Fort Worth, Texas in March of 2000 are just two of many examples that negate this belief. While urban areas seem to be less susceptible to tornado strikes than rural areas, it is simply a matter of percentage of land area covered by these types of regions. Urban areas take up a relatively tiny surface area compared with the sprawling suburbs and the thousands of rural communities. Downtown Dallas is no less likely to have a tornado cross through it than an empty field in southern Oklahoma. While it is true that the typical urban building is a much more rugged structure than many comparable rural structures, it is not to be assumed that there is an increased measure of safety.

[edit] Tornadoes in far-northern latitudes or in winter

Tornadoes can, and do, form in extreme northern or southern latitudes. Tornadoes that form in winter are rare, but have also been documented, when warm air meets a strong storm front, causing a tornado that becomes a brilliant white (instead of a dirt-brown) from picking up snow on the ground.

* The Edmonton Tornado of 1987, a powerful F4, struck the Metropolitan Edmonton, Alberta area, which is located 53.57 degrees north. See article for more in-depth information.
* A funnel cloud was sighted on Upper Garry Lake, Northwest Territories, the most northerly funnel cloud on record in Canada. August 10, 1973.
* Yellowknife Tornado of 1978. A tornado touches down near Yellowknife, Northwest Territories toppling a tower and destroying a transmission tower at Rae-Edzo. It is the third tornado in 16 years there.
* White Point Beach, Nova Scotia Tornado of January 30, 1954. A great deal of hail and lightning along the coast, touched down near Liverpool, Nova Scotia during mid-winter at a latitude of almost 44 degrees north.
* November 1989 Tornado Outbreak. A late-season tornado touches down on November 16, in Mont-Saint-Hilaire. It was rated F2.
* Sudbury, Ontario Tornado (August 20, 1970). A strong F3 tornado strikes the Northern Ontario mining city of Sudbury, Ontario and its suburbs, with Lively, Ontario being the hardest hit in the early morning hours. This is the first recorded major tornado in Northern Ontario. The terrain is not too hilly (actually fairly flat for Northern Ontario standards), but its strength is unusual for its location. Tornadoes are thought to be more common in Northern Ontario than prevalence maps indicate, this is mostly because many go unreported as they travel through very remote forest, far away from any roads or human settlements. Damage is usually be assessed from the air, after the fact and the number of reported tornadoes each year in the north are increasing in recent years, as a result of better technology to confirm them. However, larger, severe tornadoes are more common in the Great Plains and Canadian Prairies.

[edit] References

* Tornado Myths, Facts, and Safety. National Oceanic and Atmospheric Administration (17 August 2006). Retrieved on August 23, 2006.
* Tornado Myths. Indiana Department of Homeland Security (14 July 2005). Retrieved on August 23, 2006.

Thanx again to my favourite website www.wikipedia.org