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{{dablink|This article is about the weather phenomenon. For other uses, see [[Tornado (disambiguation)]].}}'''
[[Image:Dszpics1.jpg|thumb|300px| 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 [[transparency (optics)|translucent]] dust cloud, kicked up by the tornado's strong winds at the surface.]]
HEY JOSH AND COLIN HA HA

A '''tornado''' is a violently rotating column of air which is in contact with both a [[cumulonimbus cloud|cumulonimbus]] (or, in rare cases, [[cumulus cloud|cumulus]]) cloud base and the surface of the earth. Tornadoes can 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 winds of 110&nbsp;[[miles per hour|mph]] (175&nbsp;[[kilometres per hour|km/h]]) or less, are approximately 250&nbsp;[[foot (unit of length)|feet]] (75&nbsp;[[metre|meters]]) across, and travel a few [[mile]]s (several [[kilometre|kilometers]]) before dissipating. However, some tornadoes can have winds of more than 300&nbsp;mph (480&nbsp;km/h), be more than a mile (1.6&nbsp;km) across, and stay on the ground for dozens of miles (more than 100&nbsp;kilometers).<ref name="fastest wind"> {{cite web| url = http://cswr.org/dow/DOW.htm| title = Doppler On Wheels| accessdate = 2006-12-29| author=[http://cswr.org/ Center for Severe Weather Research]| year = 2006}}</ref><ref name="widest tornado">{{cite web| url = http://www.crh.noaa.gov/oax/archive/hallam/hallam.php| title = Hallam Nebraska Tornado | accessdate = 2006-09-08| author = Omaha/Valley, NE Weather Forecast Office | date = 2005-10-02}}</ref><ref name="SPC FAQ"> {{cite web| url = http://www.spc.noaa.gov/faq/tornado| title = The Online Tornado FAQ| accessdate = 2006-09-08| last = Edwards| first = Roger| date = 2006-04-04| publisher = [[Storm Prediction Center]]}}</ref>

Tornadoes have been observed on every continent except [[Antarctica]]; however, most of the world's tornadoes occur in the [[United States]].<ref name="Science News 1">{{cite web| url = http://www.sciencenews.org/articles/20020511/bob9.asp| title = Tornado Alley, USA| accessdate = 2006-09-20| last = Perkins| first = Sid| date = 2002-05-11| work = [[Science News]]| pages = 296-298}}</ref> Other areas which commonly experience tornadoes include [[New Zealand]], western and southeastern [[Australia]], south-central [[Canada]], northwestern and central [[Europe]], [[Italy]], south-central and eastern Asia, east-central [[South America]], and [[Southern Africa]].<ref name="EB tornado climatology"/>
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== Definitions ==
;Tornado
[[Image:Seymour Texas Tornado.jpg|thumb|250px|right|A tornado near [[Seymour, Texas]].]]
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...''"<ref name="Glossary of Meteorology">{{cite web| url = http://amsglossary.allenpress.com/glossary/browse?s=t&p=34| title = Glossary of Meteorology, Second Edition| accessdate = 2006-11-17| author = American Meteorological Society| year = 2000| publisher = [http://www.ametsoc.org/ American Meteorological Society]}}</ref>

;Condensation funnel
A tornado is not necessarily visible; however, the intense low pressure caused by the fast wind speeds (see [[Bernoulli's principle]]) and rapid rotation (due to [[Balanced flow|cyclostrophic]] balance) usually causes [[water vapor]] in the air to condense into a visible '''condensation funnel'''.<ref name="Science News 1"/> 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. <ref name="SPC FAQ"/>

;Tornado family
Occasionally a single storm may produce multiple tornadoes and [[mesocyclone]]s. 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. <ref>{{cite web| url = http://www.srh.noaa.gov/oun/severewx/glossary4.php#t| title = A Comprehensive Glossary of Weather Terms for Storm Spotters| accessdate = 2007-02-27| last = Branick| first = Michael| year = 2006| publisher = NOAA}}</ref>

;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.<ref name="Glossary of Meteorology"/><ref name="significant tornadoes"/><ref>{{cite web| url = http://ams.confex.com/ams/pdfpapers/81933.pdf| title = TORNADO OUTBREAK DAY SEQUENCES: HISTORIC EVENTS AND CLIMATOLOGY (1875-2003)| accessdate = 2007-03-20| author = Russell S. Schneider| coauthors = Harold E. Brooks, and Joseph T. Schaefer| year = 2004| format = PDF}}</ref>

== Etymology ==
The word "tornado" is an altered form of the [[Spanish language|Spanish]] word ''tronada'', which means "thunderstorm". This in turn was taken from the [[Latin language|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]].<ref name="etymology 1">{{cite web| url = http://www.etymonline.com/index.php?term=tornado| title = Online Etymology Dictionary| accessdate = 2006-09-20| last = Harper| first = Douglas| year = 2001| month = November}}</ref><ref name="etymology 2">{{cite book | title = Merriam Webster's Collegiate Dictionary | url = http://www.m-w.com | edition = 10th Edition| year = 1993 | publisher = Merriam-Webster, Incorporated | location = Springfield, MA | isbn = 0-87779-709-9 }}</ref> Tornadoes are also commonly referred to as ''twisters''.<ref name="TT"/>

== Types of tornadoes ==
[[Image:1957 Dallas multi-vortex 1 edited.JPG|thumb|250px|right|A ''multiple-vortex'' [[1957 Dallas Tornado Outbreak|tornado]] outside of [[Dallas, Texas]] on [[April 2]], [[1957]].]]
[[Image:Trombe.jpg|thumb|250px|right|A waterspout near the [[Florida Keys]].]]
[[Image:GID Landspout.jpg|thumb|250px|right|A landspout near [[North Platte, Nebraska]] on [[May 22]], [[2004]].]]
[[Image:Whirlwind9832.jpg|thumb|right|250px|Dust devil in [[Johnsonville, South Carolina]].]]
[[Image:Cold-Core Funnel Cloud.jpeg|thumb|250px|right|A cold-air vortex extending from a generic [[cumulus cloud]].]]
=== 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.<ref name="SPC FAQ"/>

;[[Waterspout]]
A '''waterspout''' is officially defined by the [[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 [[landspout]]s.<ref name="USA Today 1"/> They form from the bases of [[cumulus congestus]] (also called "convective cumulus") cloud towers in tropical and semitropical waters.<ref name="USA Today 1"/> They have relatively weak winds (F0 on the Fujita Scale), smooth [[laminar]] walls, and typically travel very slowly, if at all (since the cloud they are attached to is being formed by [[convection|convective action]] instead of the interaction between colliding [[weather front|fronts]]).<ref name="USA Today 1">{{cite web| url = http://www.usatoday.com/community/chat/0504tornb.htm | title = Tornado Chase 2000| accessdate = 2007-5-30| last = Zittel| first = Dave| publisher = [[USA Today]]}}</ref> They occur more commonly in the Florida Keys than anywhere on Earth.<ref name="USA Today 2">{{cite web| url = http://www.usatoday.com/weather/wspouts.htm | title = Waterspouts| accessdate = 2007-5-30| last = Golden| first = Joseph| publisher = [[USA Today]]}}</ref>
*Tornadic waterspouts are more literally "tornadoes over water", and form the same way as tornadoes. A tornado which spawns in the traditional manner on land and later crosses into a body of water would also be considered a tornadic waterspout. Since they form from [[severe thunderstorm]]s 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 third type, a [[winter waterspout]], is an extremely rare [[meteorological phenomenon]] in which a vortex resembling that of a [[waterspout]] forms under the base of a [[snow squall]]. As opposed to warm season waterspouts, winter waterspouts form in temperatures of -18°C (0°F) or colder.

;[[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". They share most of the characteristics with their water-based brethren, 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 possess strong winds and can cause serious damage.<ref name="SPC FAQ"/><ref name="Advanced Spotter Guide"> {{cite web| url = http://www.weather.gov/os/brochures/adv_spotters.pdf| title = Advanced Spotters' Field Guide| accessdate = 2006-09-20| author = Doswell, Moller, Anderson et al.| year = 2005| format = PDF| publisher = [http://www.commerce.gov/ US Department of Commerce]}}</ref>

=== Tornado-like circulations ===
;[[Gustnado]]
A '''gustnado''' (gust front tornado) is a small, vertical swirl associated with a [[Outflow boundary|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 diagnonally) and make contact with the ground. The result is a gustnado. <ref name="SPC FAQ"/><ref name="gustnado AMS"> {{cite web| url = http://amsglossary.allenpress.com/glossary/search?id=gustnado1 | title = Gustnado| accessdate = 2006-09-20| author = American Meteorological Society | authorlink = American Meteorological Society | work = Glossary of Meteorology}}</ref> 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]]
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 and fatalities, especially in [[arid]] areas.<ref name="Handy Weather Answer Book"> {{cite book | last = Lyons | first = Walter A | title = The Handy Weather Answer Book | edition = 2nd Edition | year = 1997 | publisher = Visible Ink press | location = [[Detroit, Michigan]] | isbn = 0-7876-1034-8 | pages = pgs. 175-200 | chapter = Tornadoes }}</ref><ref name="dust devil injury">{{cite web| url = http://www.srh.noaa.gov/ssd/techmemo/sr207.htm| title = SEVERE WEATHER CLIMATOLOGY FOR NEW MEXICO| accessdate = 2006-09-29| author = Charles H. Jones| coauthors = Charlie A. Liles| year = 1999}}</ref>

;[[Fire whirl]]
Tornado-like circulations occasionally occur near large, intense [[wildfire]]s 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.<ref name="significant tornadoes"/>

;[[Funnel cloud|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.<ref name="cold air funnel">{{cite web| url = http://www.crh.noaa.gov/fsd/science/faqsummer.php| title = FAQ's of Summer Weather | accessdate = 2007-02-28| last = Schumacher| first = Phil| year = 2005| publisher = National Weather Service, Sioux Falls, South Dakota}}</ref> 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).
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== Characteristics ==

[[Image:Binger Oklahoma Tornado.jpg|thumb|250px|right|A ''wedge tornado'', nearly a mile wide.]]
[[Image:Roping tornado.jpg|thumb|250px|right|A ''rope tornado'' in its dissipating stage.]]

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

Small, relatively weak [[landspout]]s 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&nbsp;mph (64&nbsp;km/h), the circulation is considered a tornado.<ref name="Advanced Spotter Guide"/> Large single-vortex tornadoes can look like large [[Wedge (mechanical device)|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. Even experienced storm observers may not be able to tell the difference between a low-hanging cloud and a wedge tornado from a distance.<ref name="wedge tornado">{{cite web| url = http://www.spc.noaa.gov/faq/tornado/binger.htm| title = Wedge Tornado | accessdate = 2007-02-28| last = Edwards| first = Roger}}</ref>

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.<ref name="rope tornado">{{cite web| url = http://www.spc.noaa.gov/faq/tornado/el_reno.htm| title = Rope Tornado | accessdate = 2007-02-28| last = Edwards| first = Roger}}</ref>

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.<ref name="Handy Weather Answer Book"/>

=== Size ===
In the United States, an average tornado is around 500&nbsp;feet (150&nbsp;m) across, and stays on the ground for 5&nbsp;miles (8&nbsp;km).<ref name="Handy Weather Answer Book"/> 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&nbsp;feet (2&nbsp;m) long.<ref name="Handy Weather Answer Book"/> On the other end of the spectrum, wedge tornadoes can have a damage path a mile (1.6&nbsp;km) wide or more. A [[Hallam, Nebraska Tornado Outbreak|tornado which affected Hallam, Nebraska]] on [[May 22]], [[2004]] was at one point 2.5&nbsp;miles (4&nbsp;km) wide at the ground.<ref name="widest tornado">"[http://www.crh.noaa.gov/oax/archive/hallam/hallam.php Hallam Nebraska Tornado]." ''[http://www.crh.noaa.gov/oax/ National Weather Service Weather Forecast Office, Omaha/Valley, NE]''. November 2, 2005.</ref>

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&nbsp;miles (352&nbsp;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.<ref name="significant tornadoes">{{cite book | last = Grazulis | first = Thomas P | title = Significant Tornadoes 1680-1991 | year = July | month = 1993 | publisher = The Tornado Project of Environmental Films | location = St. Johnsbury, VT | isbn = 1-879362-03-1 }}</ref> In fact, modern reanalysis of the path suggests that the tornado began 15 miles (24 km) further west than previously thought.<ref>{{cite web| url = http://apollo.lsc.vsc.edu/ams/AMS%20VP/Storm%20Conference/NESC%20Presentations/32ndNESC_Presentation/Banquet/Doswell.ppt| title = The Tri-State Tornado of 18 March 1925 Reanalysis Project| accessdate = 2007-04-07| last = Doswell| first = Dr. Charles A, III| format = Powerpoint Presentation}}</ref>

=== 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.<ref name="Handy Weather Answer Book"/>

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[[Image:Waurika Oklahoma Tornado Back and Front.jpg|thumb|250px|left|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.<ref name="PD tornado images">{{cite web| url = http://www.spc.noaa.gov/faq/tornado/torscans.htm | title = Public Domain Tornado Images| accessdate = 2006-10-20| last = Edwards| first = Roger| publisher = [[National Severe Storms Laboratory]]}}</ref>]]

Lighting conditions are also a major factor in the appearance of a tornado. A tornado which is "[[Backlighting (lighting design)|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.<ref name="target tornado">{{cite video | people = Lloyd, Linda Mercer | year = 1996 | title = Target: Tornado | medium = Videotape | location = [[Atlanta, Georgia]] | publisher = [[The Weather Channel (United States)|The Weather Channel Enterprises, Inc.]]}}</ref><ref name="TT">{{cite web| url = http://www.tornadoproject.com/cellar/tttttttt.htm| title = The Tornado Project's Terrific, Timeless and Sometimes Trivial Truths about Those Terrifying Twirling Twisters!| accessdate = 2007-03-21| author = The Tornado Project| year = 1999}}</ref>

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.<ref name="significant tornadoes"/> Also, night-time tornadoes are often illuminated by frequent lightning.
There is mounting evidence, including [[Doppler On Wheels|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 (cyclone)|eye]] found in [[tropical cyclone]]s. 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.<ref name="Science News 2">{{cite web| url = http://www.sciencenews.org/pages/sn_arc99/5_15_99/fob1.htm| title = Oklahoma Tornado Sets Wind Record| accessdate = 2006-10-20| author = R. Monastersky| date = 1999-05-15| work = Science News| pages = 308-309}}</ref><ref name="inside eyewitness">{{cite web| url = http://docs.lib.noaa.gov/rescue/mwr/058/mwr-058-05-0205.pdf| title = Seeing the Inside of a Tornado| accessdate = 2006-10-20| last = Justice| first = Alonzo A| year = 1930| month = May| format = PDF| work = [[Monthly Weather Review]]| publisher = [[American Meteorological Society]]| pages = 205-206}}</ref><ref>{{cite book | last = Hall | first = Roy S. | title = Tornadoes | year = 2003 | publisher = Greenhaven Press | location = Farmington Hills, MI | isbn = 0-7377-1473-5 | pages = 59-65 | chapter = Inside a Texas Tornado}}</ref>

=== Rotation ===
Tornadoes normally rotate [[cyclone|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 infinitesimal, as demonstrated physically by the [[Rossby number]]. Supercells and tornadoes would rotate cyclonically even without the Coriolis effect, as evidenced by their doing so in numerical simulations which neglect the Coriolis component.<ref name="Origin of Updraft Rotation in Supercells">{{cite journal| last = Davies-Jones| first = Robert| authorlink = Robert Davies-Jones| title = Streamwise Vorticity: The Origin of Updraft Rotation in Supercell Storms| journal = [[Journal of the Atmospheric Sciences]]| volume = 41| issue = 20| pages = 2991–3006| publisher = [[American Meteorological Society]]| date = October 1984| url = http://ams.allenpress.com/perlserv/?request=get-abstract&doi=10.1175%2F1520-0469(1984)041%3C2991%3ASVTOOU%3E2.0.CO%3B2| accessdate = 2007-04-13}}</ref><ref name="Rotation and Propagation of Simulated Supercells">{{cite journal| last = Rotunno| first = Richard| authorlink = Richard Totunno| coauthors = Joseph Klemp| title = On the Rotation and Propagation of Simulated Supercell Thunderstorms| journal = [[Journal of the Atmospheric Sciences]]| volume = 42| issue = 3| pages = 271-292| publisher = [[American Meteorological Society]]| date = February 1985| url = http://ams.allenpress.com/perlserv/?request=get-abstract&doi=10.1175%2F1520-0469(1985)042%3C0271%3AOTRAPO%3E2.0.CO%3B2| accessdate = 2007-04-13}}</ref> Low-level [[mesocyclone]]s and tornadoes owe their rotation to complex processes within the supercell and ambient environment.<ref name="Tornado Development and Decay within a Supercell">{{cite journal| last = Wicker| first = Louis J.| authorlink = Louis J. Wicker| coauthors = Robert B. Wilhelmson| title = Simulation and Analysis of Tornado Development and Decay within a Three-Dimensional Supercell Thunderstorm| journal = [[Journal of the Atmospheric Sciences]]| volume = 52| issue = 15| pages = 2675–2703| publisher = [[American Meteorological Society]]| date = August 1995| url = http://ams.allenpress.com/perlserv/?request=get-abstract&doi=10.1175%2F1520-0469(1995)052%3C2675%3ASAAOTD%3E2.0.CO%3B2| accessdate = 2007-04-13}}</ref>

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|RFD]] in a cyclonic supercell.<ref name="Recent Example of an anticyclonic tornado in El Reno, OK">{{cite web| url = http://www.weather.com/blog/weather/8_9262.html| title = weather.com - Blog: The Weather Channel on weather news, hurricanes, tornadoes & meteorology| accessdate = 2006-12-30| last = Forbes| first = Greg}}</ref> However, on rare occasions, [[anticyclonic tornado]]es 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 or a satellite tornado or associated with anticyclonic eddies within a supercell.<ref name="Sunnyvale Tornado">{{cite web| url = http://tornado.sfsu.edu/geosciences/StormChasing/Cases/Sunnyvale/Sunnyvale.html| title = Sunnyvale and Los Altos, CA Tornadoes May 4, 1998| accessdate = 2006-10-20| last = Monteverdi| first = John| date = 2003-01-25}}</ref>

=== Sound ===
The sound of a tornado is classically described as being similar to that of a speeding [[freight train]] rumbling in the distance.<ref name="freight train">{{cite web| url = http://www.redcross.org/services/disaster/0,1082,0_591_,00.html| title = Tornado| accessdate = 2007-04-26| author = [[American Red Cross]]| year = 2007}}</ref> A tornado's sound at close range, however, is different, due to direct effects of wind and debris. [[Storm chaser]] [[Tim Samaras]] described a tornado only 180&nbsp;meters (600&nbsp;ft) away as sounding "like a high-powered [[waterfall]] combined with a [[jet engine]]".<ref name="National Geographic News">{{cite web| url = http://news.nationalgeographic.com/news/2004/03/0315_040315_tornadoes.html| title = What Triggers Tornadoes?| accessdate = 2007-04-25| last = Drye| first = Willie | date = 2004-03-15}}</ref> Most people residing inside their homes at the time a tornado strikes give a similar description.<ref name="Taipei Times">{{cite web| url = http://www.taipeitimes.com/News/world/archives/2006/12/09/2003339743| title = Tornado Ravages London| accessdate = 2007-04-25| last = Palmere| first = William| date = 2006-12-09}}</ref> <div style="clear: both"></div>

== Life cycle ==
[[Image:Funnel cloud approaching the ground - NOAA.jpg|thumb|250px|right|A funnel cloud near [[Ardmore, Oklahoma|Ardmore]], [[Oklahoma]].]]
[[Image:Dimmit Sequence.jpg|thumb|250px|right|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|[[Tornadogenesis]]}}

=== Supercell relationship ===
{{seealso|Supercell}}
Tornadoes often develop from a class of thunderstorms known as ''[[supercell]]s''. Supercells contain [[mesocyclone]]s, an area of organized rotation a few miles up in the atmosphere, usually 1&ndash;6&nbsp;miles (2&ndash;10&nbsp;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.<ref name="Advanced Spotter Guide"/> 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.

=== 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&nbsp;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.

=== 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.<ref name="PD tornado images"/><ref name="target tornado"/><ref name="significant tornadoes"/>

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 [[wiktionary:cycle|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&mdash;however, most tornadoes follow a pattern similar to this one.<ref name ="tornadogenesis"> {{cite web| url = http://ams.allenpress.com/amsonline/?request=get-document&doi=10.1175%2F1520-0469(2003)060%3C0795:TRFTTO%3E2.0.CO%3B2| title = Tornadogenesis Resulting from the Transport of Circulation by a Downdraft: Idealized Numerical Simulations| accessdate = 2006-09-13| author = Markowski, Straka, and Rasmussen| date = 2002-10-14| work = [[Journal of the Atmospheric Sciences]]: Vol. 60, No. 6| pages = 28}}</ref>

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== Intensity and damage ==
[[Image:Stoughton damage.jpg|thumb|250px|right|An example of '''[[Enhanced Fujita Scale|EF3]]''' damage. Here, the roof and some inner walls of this brick building have been demolished.]]
{{main|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 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. <ref name ="width/length intensity relationship"> {{cite web| url = http://ams.allenpress.com/perlserv/?request=get-abstract&doi=10.1175%2F1520-0434%282004%29019%3C0310%3AOTROTP%3E2.0.CO%3B2| title = On the Relationship of Tornado Path Length and Width to Intensity| accessdate = 2007-04-06| author = Brooks, Harold E.| date = 2004-04-01| work = [[Weather and Forecasting]]: Vol. 19, No. 2| pages = 310–319}}</ref> In the case of violent tornadoes, only a small portion of the path is of violent intensity, most of the higher intensity from [[multiple vortex tornado|subvortices]]. <ref name="significant tornadoes"/>

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&mdash;violent tornadoes (stronger than '''EF4''', '''T8'''), account for less than 1% of all tornado reports.<ref name="Basic Spotter Guide">{{cite web| url = http://www.nws.noaa.gov/om/brochures/basicspot.pdf| title = Basic Spotters’ Field Guide| accessdate = 2006-11-01| author = Edwards, Moller, Purpura et al| year = 2005| format = PDF| publisher = [http://www.commerce.gov/ US Department of Commerce], [http://www.nws.noaa.gov National Weather Service]}}</ref> 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. <ref name ="intensity distribution"> {{cite web| url = http://www.essl.org/people/dotzek/pdf/ecss02p.pdf| title = Statistical modeling of tornado intensity distributions| accessdate = 2007-04-06| author = Dotzek, Nikolai, Jürgen Grieser, Harold E. Brooks| date = 2003-03-01| work = [[Atmospheric Research]]: Vol. 67-68| pages = 163-187}}</ref>
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== Climatology ==
{{main|Tornado climatology}}
[[Image:Globdisttornado.jpg|thumb|right|250px|Areas worldwide where tornadoes are most likely, indicated by orange shading.]]
[[Image:Tornado Alley.gif|thumb|right|250px|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).<ref>{{cite journal | author = Dr. Nikolai Dotzek | year = 2003 | month = March | title = An updated estimate of tornado occurrence in Europe | journal = Atmospheric Research | url = http://www.essl.org/people/dotzek/pdf/ecss02s.pdf | accessdate = 2007-03-11 }}</ref> This is mostly due to the unique geography of the continent. [[North America]] is a relatively large continent that extends from the [[tropics|tropical]] south into [[arctic]] areas, and has no major east-west mountain range to block air flow between these two areas. 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]].<ref name="Science News 1"/> This area extends into Canada, particularly [[Ontario]] and the [[Prairie Provinces]]. The [[Netherlands]] has the highest average number of recorded tornadoes per area of any country (more than 20 annually), followed by the [[United Kingdom|UK]] (around 50 per year), but most are small and result in minor damage. The UK experiences more tornadoes than any other European country.<ref name="UK Tornado numbers">{{cite web| url=http://news.bbc.co.uk/1/hi/england/london/6217514.stm| title = Six hurt as tornado hits London| accessdate = 2006-12-07| author = BBC News| year = 2006}}</ref>

[[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.<ref name="Bangladesh tornado">{{cite web| url = http://www.colorado.edu/hazards/qr/qr169/qr169.pdf| title = The April 2004 Tornado in North-Central Bangladesh: A Case for Introducing Tornado Forecasting and Warning Systems| accessdate = 2006-08-17| author = Paul, Bhuiyan| year = 2004}}</ref> Other areas of the world that have more frequent tornadoes include parts of [[Argentina]] and southern [[Brazil]], as well as [[South Africa]], [[Europe]], [[Australia]] and [[New Zealand]], and far eastern [[Asia]].<ref name="EB tornado climatology">{{cite web| url = http://www.britannica.com/eb/article-218357/tornado| title = Tornado: Global occurrence| accessdate = 2007-03-21| author = Encyclopædia Britannica| authorlink = Encyclopædia Britannica}}</ref>

Tornadoes can form in any month, providing the conditions are favorable. They are least common during the winter and most common in spring.<ref name="significant tornadoes"/> 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 [[landfall]]ing [[tropical cyclone]]s, which tend to occur in the late summer and fall.

Tornado occurrence is highly dependent on the time of day.<ref name="tornado time of day">{{cite web| url = http://ams.allenpress.com/pdfserv/10.1175%2F1520-0493(1978)106%3C1172:AATC%3E2.0.CO%3B2| title = An Augmented Tornado Climatology| accessdate = 2006-09-13| author = Kelly, Schaefer, McNulty, et al.| date = 1978-04-10| format = PDF| work = [[Monthly Weather Review]]| pages = 12}}</ref> Worldwide, most tornadoes occur in the late afternoon, between the hours of 3 and 7 pm local time, with a peak near 5 pm.<ref>{{cite web| url = http://www.britannica.com/eb/article-218362/tornado| title = Tornado: Diurnal patterns| accessdate = 2007-02-27| year = 2007| work = Encyclopædia Britannica Online| pages = pg. 6}}</ref><ref>{{cite journal | last = Holzer | first = A. M. | year = 2000 | title = Tornado Climatology of Austria | journal = Atmospheric Research | issue = 56 | pages = 203-211 | url = http://tordach.org/at/Tornado_climatology_of_Austria.html | accessdate = 2007-02-27 }}</ref><ref> {{cite journal | last = Dotzek | first = Nikolai | date = 2000-05-16 | title = Tornadoes in Germany| journal = Atmospheric Research | url = http://essl.org/people/dotzek/pdf/etss_1p.pdf | format = PDF | accessdate = 2007-02-27}}</ref><ref>{{cite web| url = http://www.saweather.co.za/References/SATornadoes.jsp | title = South African Tornadoes| accessdate = 2007-02-27| year = 2003| publisher = [[South African Weather Service]]}}</ref><ref>{{cite web| url = http://bangladeshtornadoes.org/climo/btorcli0.htm | title = Bangladesh Tornado Climatology| accessdate = 2007-02-27| last = Finch | first = Jonathan D. | coauthors = Dewan, Ashraf M}}</ref> However, destructive tornadoes can occur at any time of day. The [[Tupelo-Gainesville Outbreak|Gainesville Tornado]] of 1936, one of the deadliest tornadoes in history, occurred at 8:30 am local time.<ref name="significant tornadoes"/>

== Prediction ==
[[Image:SPC severe outlook 04072006.png|thumb|250px|right|[[probability|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&nbsp;miles (40&nbsp;km) of any point within the enclosed area. The hashed area on the bottom map indicates a 10% or greater risk of an [[Fujita scale|F2]] or stronger tornado forming within 25&nbsp;miles (40&nbsp;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 (Australia)|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]].<ref>{{cite web| url = http://www.bom.gov.au/weather/nsw/sevwx/warning_service.shtml | title = Severe Thunderstorm Warning Service in NSW and the ACT| accessdate = 2006-10-25| author = Australian Government Bureau of Meteorology| year = 2006}}</ref>

;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.<ref>{{cite web| url = http://essl.org/ESWD| title = European Severe Weather Database | accessdate = 2006-10-25| author = European Severe Storms Laboratory}}</ref> 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.<ref>{{cite web| url = http://www.tordach.org/| title = TorDACH Homepage| accessdate = 2006-10-25| author = TorDACH}}</ref>

;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. [[Severe weather terminology (United States)|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]].

== Detection ==
[[Image:Tornado doppler signature.PNG|thumb|250px|right|A [[doppler radar|Doppler radar]] image indicating the likely presence of a [[2007 Central Florida Tornado|tornado]] over [[DeLand, Florida]]. Green colors indicate areas where the precipitation is moving towards the radar dish, while red areas are moving away. Strong [[mesocyclone]]s 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. 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 Forecast Office until after the storm was over. However, with the advent of [[weather radar]], areas near a weather service office could get advance warning of severe weather. With the discovery of the [[hook echo]] 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.<ref name="hook echo">{{cite web| url = http://chill.colostate.edu/tornado_1953/index.shtml | title = The First Tornadic Hook Echo Weather Radar Observations| accessdate = 2007-02-27| last = Kennedy| first = Patrick C.| date = [[2004]]-[[01-09]]| publisher = [[Colorado State University]]}}</ref>

In the mid [[1970s]], the US National Weather Service began increased efforts to train [[Skywarn]] spotters, consisting of local [[sheriff|sheriff's deputies]], [[State police|state troopers]], [[firefighter]]s, [[amateur radio operator]]s, [[civil defense]] (now [[emergency management]]) spotters, [[storm chaser]]s, 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..<ref name="WAF spotter history">{{cite web| url = http://ams.allenpress.com/perlserv/?request=get-abstract&doi=10.1175%2F1520-0434%281999%29014%3C0544%3ASSAPAS%3E2.0.CO%3B2| title = Storm Spotting and Public Awareness since the First Tornado Forecasts of 1948| accessdate = 2007-04-06| author = Doswell, Charles A. III, Alan R. Moller, Harold E. Brooks| date = 1999-08-01| work = [[Weather and Forecasting]]: Vol. 14, No. 4| pages = 544–557}}</ref> There are currently more than 230,000 trained Skywarn weather spotters across the United States.<ref name="NWS SKYWARN">{{cite web| url = http://www.weather.gov/skywarn/| title = National Weather Service SKYWARN| accessdate = 2007-02-27}}</ref> In Canada, a similar network of Volunteer Weather Watchers now exists to help spot severe weather, with more than 1,000 volunteers.<ref name="environment Canada detection">{{cite web| url = http://www.mb.ec.gc.ca/air/summersevere/ae00s10.en.html| title = Tornado Detection at Environment Canada| accessdate = 2007-03-16| author = Environment Canada| authorlink = Environment Canada| date = 2004-06-02}}</ref> In Europe, several nations are organizing spotter networks under the auspices of [http://www.skywarn.eu/ Skywarn Europe] and the [[Tornado and Storm Research Organisation (TORRO)]] has maintained a network of spotters in the [[United Kingdom]] since the 1970s.

Nowadays, most developed countries have a network of [[weather radar]]s, which remains the main method of detecting signatures likely associated with tornadoes. In the United States and a few other countries, [[Doppler radar|Doppler weather radar]] stations are used as the main detection method for tornadic thunderstorms. 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 Satellite]]s (GOES), which aid in the detection of tornadic storms.<ref name="environment Canada detection"/>

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== Extremes ==
{{main|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 '''EF5''' (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&nbsp;miles, 352&nbsp;km), longest duration (about 3.5 hours), and fastest forward speed for a significant tornado (73&nbsp;mph, 117&nbsp;km/h).<ref name="significant tornadoes"/> 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.<ref name="tornado damage cost">{{cite web| url = http://www.nssl.noaa.gov/users/brooks/public_html/damage/tdam1.html | title = Normalized Damage from Major Tornadoes in the United States: 1890-1999| accessdate = 2007-02-28| last = Brooks| first = Harold E. | coauthors = Doswell, Charles A, III | year = 2000| month = September}}</ref>

The [[Daultipur-Salturia Tornado|deadliest tornado]] in world history occurred in [[Bangladesh]] on [[April 26]], [[1989]], killing approximately 1300 people.<ref name="Bangladesh tornado"/>
[[Image:Super Outbreak Map.PNG|thumb|left|250px|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 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.<ref name="super outbreak">{{cite web| url = http://www.ncdc.noaa.gov/oa/climate/extremes/1999/april/TornOut.pdf| title = Tornado Outbreak of April 3-4, 1974; Synoptic Analysis| accessdate = 2007-03-02| author = Hoxit, Lee R| coauthors = Chappell, Charles F| year = 1975| month = October| format = PDF| publisher = National Oceanic and Atmospheric Administration}}</ref>

While it is nearly impossible to directly measure the most violent tornado wind speeds (conventional [[anemometer]]s would be destroyed by the intense winds), some tornadoes have been scanned by mobile [[doppler radar|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&nbsp;[[±]]&nbsp;20&nbsp;mph (484&nbsp;±&nbsp;32&nbsp;km/h) in the '''F5''' Moore, Oklahoma tornado. Though the reading was taken about 100&nbsp;feet (30&nbsp;m) above the ground, this is a testament to the power of the strongest tornadoes.<ref name="fastest wind"/>

Storms which produce tornadoes can feature intense updrafts (sometimes exceeding 150&nbsp;mph, 240&nbsp;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&nbsp;miles (130&nbsp;km) from the town, a sack of [[flour]] was found 110&nbsp;miles (177&nbsp;km) away, and a cancelled check from the Great Bend bank was found in a field outside of [[Palmyra, Nebraska]], 305&nbsp;miles (491&nbsp;km) to the northeast.<ref name="tornado project oddities">{{cite web| url = http://www.tornadoproject.com/oddities/oddities.htm| title = Tornado Oddities| accessdate = 2007-02-28| last = Grazulis| first = Thomas P. | year = 1999}}</ref>
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== 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.<ref name="tornado safety">{{cite web| url = http://www.spc.noaa.gov/faq/tornado/safety.html| title = Tornado Safety| accessdate = 2007-02-28| last = Edwards| first = Roger | publisher = Storm Prediction Center}}
</ref> In tornado-prone areas, many buildings have [[storm cellar]]s on the property. These underground refuges have saved thousands of lives.<ref>{{cite web| url = http://www.srh.noaa.gov/hun/preparedness/brochures/storm_shelter.pdf| title = Storm Shelters| accessdate = 2007-02-28| author = National Weather Service, Huntsville Alabama| year = 2002| month = August| format = PDF}}</ref>

Some countries have meteorological agencies which distribute tornado forecasts and increase levels of alert of a possible tornado (such as [[tornado watch]]es and [[Tornado warning|warnings]] in the United States and Canada). [[Weather radio]]s 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).<ref name="highway overpasses"/>

== Myths and misconceptions ==
[[Image:Saltlaketornado.jpeg|thumb|right|250px|[[Salt Lake City Tornado]], [[August 11]], [[1999]]. This tornado disproved several myths, including the idea that tornadoes cannot occur in areas like [[Utah]].]]
{{main|Tornado myths}}
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.<ref name="tornado myths">{{cite book | last = Grazulis | first = Thomas P | title = The Tornado: Nature's Ultimate Windstorm | year = 2001 | publisher = University of Oklahoma Press | location = Norman, OK | isbn = 0-8061-3258-2 | chapter = Tornado Myths }}</ref><ref name="tornado project myths">{{cite web| url = http://www.tornadoproject.com/myths/myths.htm| title = Myths and Misconceptions about Tornadoes| accessdate = 2007-02-28| author = The Tornado Project| year = 1999}}</ref>

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 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.<ref name="highway overpass danger">{{cite web| url = http://www.usatoday.com/weather/resources/basics/tornado-underpass.htm| title = Overpasses are tornado death traps| accessdate = 2007-02-28| last = Cappella| first = Chris | date = 2005-05-17| publisher = [[USA Today]]}}</ref> 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.<ref name="highway overpasses">{{cite web| url = http://www.srh.noaa.gov/oun/papers/overpass.html| title = Highway Overpasses as Tornado Shelters| accessdate = 2007-02-28| last = Miller| coauthors = Doswell, Brooks et al| year = 1999| month = October| publisher = National Weather Service, Norman, Oklahoma}}</ref>

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.<ref name="tornado myths"/><ref name="tornado project myths"/>

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 "[[spirit]]s". 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 [[Tornado climatology#Geography|Tornado climatology]]).<ref name="tornado myths"/><ref name="tornado project myths"/><ref name="Handy Weather Answer Book"/>
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== Continuing research ==
[[Image:Tornado with DOW.jpg|thumb|right|250px|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.<ref name="VORTEX book">{{cite web| url = http://www.nssl.noaa.gov/noaastory/book.html| title = VORTEX: Unraveling the Secrets| accessdate = 2007-02-28| author = National Severe Storms Laboratory| authorlink = National Severe Storms Laboratory| year = 2006}}</ref> Scientists do have a fairly good idea of the development of thunderstorms and [[mesocyclone]]s, 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.

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 [[mesovortice]]s within linear thunderstorm structures and with tropical cyclones.<ref name="tornado life cycle">{{cite web| url = http://cimms.ou.edu/~erik/Tornadoes/Forecasting/Detailed/Detailed.htm| title = Tornado Life Cycle| accessdate = 2007-03-27| last = Rasmussen| first = Erik| date = 2000-12-31| work = Severe Storms Research at Oklahoma University}}</ref>

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 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. Research programs, including [[VORTEX]], deployment of [[TOtable Tornado Observatory|TOTO]] (the TOtable Tornado Observatory), [[Doppler On Wheels]] (DOW), and dozens of other programs, hope to solve many questions that still plague meteorologists.<ref name="tornado research">{{cite web| url = http://cimms.ou.edu/~erik/Tornadoes/Forecasting/Detailed/Detailed.htm| title = Tornado Forecasting| accessdate = 2007-02-28| last = Markowski| first = Paul | date = 2000-12-31}}</ref>. Importantly, development of new observation technologies and installation of finer spatial and temporal resolution observation networks have aided increased understanding and better predictions.

== See also ==
{{sisterlinks|tornado}}
* [[Cultural significance of tornadoes]]
* [[Cyclone]]
* [[Derecho]]
* [[Downburst]]
* [[Emergency Alert System]]
* [[History of tropical cyclone-spawned tornadoes]]
* [[Tornado Emergency]]
* [[List of tornadoes and tornado outbreaks]]
** [[List of F5 tornadoes]]
** [[List of tornadoes striking downtown areas]]
** [[List of tornado-related deaths at schools]]
** [[List of Canadian tornadoes]]
* [[Severe weather]]
* [[Tornadoes of 2007]]
* [[Weather]]
{{WeatherPortal}}
<div style="clear:both;"></div>

== References ==
<div class="references-small" style="column-count:2;-moz-column-count:2;">
<references/>
</div>

== Further reading ==
*{{cite book
|last=Grazulis
|first=Thomas P
|year=1997
|month=January
|authorlink=Thomas P. Grazulis
|title=Significant Tornadoes Update, 1992-1995
|publisher=Environmental Films
|location=St. Johnsbury, VT
|isbn=1-879362-04-X
}}

*{{cite book
|last=Bradford
|first=Marlene
|year=2001
|title=Scanning the Skies: a History of Tornado Forecasting
|publisher=University of Oklahoma Press
|address=Norman, OK
|isbn=0-8061-3302-3
}}

*{{cite book
|last=Bluestein
|first=Howard B
|year=1999
|title=Tornado Alley: Monster Storms of the Great Plains
|publisher=Oxford University Press
|address=New York, NY
|isbn=0-19-510552-4
}}

== External links ==
;General
*[http://www.spc.noaa.gov/climo/torn/monthlytornstats.html United States Year to date tornado reports.]
*[http://www.srh.weather.gov/srh/jetstream/mesoscale/tornado.htm NWS Jet Stream Online Weather School]
*[http://www.tornadoarchive.com Tornado Newspaper Articles Archive] Free archive of more than 50,000 newspaper articles detailing tornadoes through out history.
*[http://www.tornadohistoryproject.com The Tornado History Project] Searchable database of tornadoes overlaid on a Google Map
*[http://climate.engin.umich.edu/tornadopaths/ Fly Tornado Paths] Fly tornado paths in your area using Google Maps and Google Earth
*[http://www4.ncdc.noaa.gov/cgi-win/wwcgi.dll?wwEvent~Storms NOAA Storm Database 1950-2005]

;Regional prediction
*[http://www.spc.noaa.gov Storm Prediction Center] (United States)
*[http://www.torro.org.uk/ The Tornado and Storm Research Organisation] ([[United Kingdom]])
*[http://www.essl.org/ European Severe Storms Laboratory]

;Research
*[http://www.nssl.noaa.gov/researchitems/tornadoes.shtml Tornado research and education] ([[National Severe Storms Laboratory]])
*[http://www.hprcc.unl.edu/nebraska/U_S_SEVERE.html U.S. Severe Weather and Meteorology and Climatology]
*[http://www.ejssm.org/ Electronic Journal of Severe Storms Meteorology]
*[http://magma.nationalgeographic.com/ngm/0404/feature1/index.html Chasing Tornadoes] ([[National Geographic Magazine]])

;Images and Video
*[http://www.spc.noaa.gov/faq/tornado/torscans.htm Tornado images] ([[Public Domain]])
*[http://www.photolib.noaa.gov/nssl/index.html National Severe Storms Laboratory Photo Album]
*[http://www.facethewind.com/videos/attica.wmv Video of a tornado forming and lifting a house off the ground (WMV)]

;Safety and Preparedness
*[http://www.nssl.noaa.gov/NWSTornado/ NOAA Tornado Preparedness Guide]
*[http://tornadoproject.com/safety/safety.htm Tornado Safety] (The Tornado Project)
*[http://www.spc.noaa.gov/faq/tornado/school.html Tornado Preparedness Tips for School Administrators] (NOAA / SPC)
*[http://www.srh.noaa.gov/oun/papers/overpass.html Highway Overpasses as Tornado Shelters: Fallout from the May 3, 1999 Tornado Outbreak] (National Weather Service, Norman, Oklahoma)

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Revision as of 06:32, 18 May 2007

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