Editing Tornado

{{Short description|Violently rotating column of air}}
{{Use dmy dates|date=March 2025}}
{{For|the current tornado season|Tornadoes of {{CURRENTYEAR}}}}
{{About|the weather phenomenon}}
{{Pp-semi-indef|small=yes}}

{{Infobox weather type
|name               = Tornado
|image              = F5 tornado Elie Manitoba 2007.jpg
|caption            = The [[2007 Elie tornado]] approaching [[Elie, Manitoba]], Canada in June 2007.
|area of occurrence = North America (particularly in central and southeastern regions of the United States colloquially known as [[Tornado Alley]]), South Africa, much of Europe (except most of the Alps), western and eastern Australia, New Zealand, Bangladesh and adjacent eastern India, Japan, the Philippines, and southeastern South America (Uruguay and Argentina)
|season             = Primarily [[Spring (season)|spring]] and [[summer]], but can occur at any time of year with the right atmospheric conditions
|effect             = [[Enhanced Fujita scale|Wind damage]]
}}
{{Weather}}
{{Commons category|Tornadoes}}
{{Commons category|Pictures of tornadoes}}

A '''tornado''' is a violently rotating column of [[air]] that is in contact with the surface of [[Earth]] and a [[cumulonimbus cloud]] or, in rare cases, the base of a [[cumulus cloud]]. It is often referred to as a '''twister''', '''whirlwind''' or '''cyclone''',<ref>{{cite web |url=http://www.merriam-webster.com/dictionary/cyclone |title=merriam-webster.com |publisher=merriam-webster.com |access-date=2012-09-03 |archive-date=2017-07-09 |archive-url=https://web.archive.org/web/20170709022406/https://www.merriam-webster.com/dictionary/cyclone |url-status=live }}</ref> although the word [[cyclone]] is used in [[meteorology]] to name a weather system with a [[low-pressure area]] in the center around which, from an observer looking down toward the surface of the Earth, winds blow counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.<ref>{{Cite book|title=Essentials of Oceanography|last=Garrison|first=Tom|publisher=Cengage Learning|year=2012|isbn=978-0-8400-6155-3}}</ref> Tornadoes come in many shapes and sizes, and they are often (but not always) visible in the form of a [[funnel cloud|condensation funnel]] originating from the base of a cumulonimbus cloud, with a cloud of rotating [[debris]] and [[dust]] beneath it. Most tornadoes have wind speeds less than {{convert|110|mph|km/h|sigfig=2|abbr=off|sp=us|order=flip}}, are about {{convert|250|ft|-1|abbr=off|sp=us|order=flip}} across, and travel several kilometers (a few miles) before dissipating. The [[Tornado records#Highest winds observed in a tornado|most extreme]] tornadoes can attain wind speeds of more than {{convert|300|mph|km/h|sigfig=2|order=flip|sp=us}}, can be more than {{convert|2|mi|km|0|order=flip|sp=us}} in diameter, and can stay on the ground for more than {{convert|100|km|mi|abbr=on}}.<ref name="fastest wind">{{cite web|url=http://cswr.org/dow/DOW.htm|title=Doppler on Wheels |access-date=2009-12-13 |last=Wurman|first=Joshua |publisher=Center for Severe Weather Research |date=2008-08-29 |url-status=dead |archive-url=https://web.archive.org/web/20070205124033/http://www.cswr.org/dow/dow.htm |archive-date=2007-02-05 }}</ref><ref name="widest tornado">{{cite web|url=http://www.crh.noaa.gov/oax/archive/hallam/hallam.php|title=Hallam Nebraska Tornado|access-date=2009-11-15|work=[[National Weather Service]]|publisher=National Oceanic and Atmospheric Administration|date=2005-10-02|archive-date=2013-04-30|archive-url=https://web.archive.org/web/20130430223118/http://www.crh.noaa.gov/oax/archive/hallam/hallam.php|url-status=live}}</ref><ref name="SPC FAQ">{{cite web|url=http://www.spc.ncep.noaa.gov/faq/tornado/|title=The Online Tornado FAQ|access-date=2006-09-08|first=Roger|last=Edwards|author-link=Roger Edwards (meteorologist)|date=2006-04-04|work=[[Storm Prediction Center]]|publisher=National Oceanic and Atmospheric Administration|url-status=dead|archive-url=https://web.archive.org/web/20060929185156/http://www.spc.ncep.noaa.gov/faq/tornado/|archive-date=2006-09-29}} {{Pd-notice}}</ref>

Various types of tornadoes include the [[multiple-vortex tornado]], [[landspout]], and [[waterspout]]. Waterspouts are characterized by a spiraling funnel-shaped wind current, connecting to a large cumulus or cumulonimbus cloud. They are generally classified as non-[[supercell]]ular tornadoes that develop over bodies of water, but there is disagreement over whether to classify them as true tornadoes. These spiraling columns of air frequently develop in tropical areas close to the [[equator]] and are less common at [[high latitude]]s.<ref>{{cite web|url=http://www.erh.noaa.gov/btv/events/15Jan2009/overview.shtml|title=15 January 2009: Lake Champlain Sea Smoke, Steam Devils, and Waterspout: Chapters IV and V|author=National Weather Service|publisher=National Oceanic and Atmospheric Administration|date=2009-02-03|access-date=2009-06-21|author-link=National Weather Service|archive-date=2016-01-01|archive-url=https://web.archive.org/web/20160101224234/http://www.weather.gov/btv|url-status=live}}</ref> Other tornado-like phenomena that exist in nature include the [[gustnado]], [[dust devil]], [[fire whirl]], and [[steam devil]].

Tornadoes occur most frequently in North America (particularly in central and southeastern regions of the United States colloquially known as [[Tornado Alley]]; the United States has by far the most tornadoes of any country in the world).<ref>{{cite magazine|url=http://www.sciencenews.org/articles/20020511/bob9.asp|title=Tornado Alley, USA|date=11 May 2002|url-status=dead|archive-url=https://web.archive.org/web/20060825011156/http://www.sciencenews.org/articles/20020511/bob9.asp|archive-date=25 August 2006|accessdate=2006-09-20|last=Perkins|first=Sid|ref={{harvid|Perkins|2002}}|magazine=[[Science News]]}}</ref> Tornadoes also occur in [[South Africa]], much of Europe (except most of the Alps), western and eastern Australia, New Zealand, Bangladesh and adjacent eastern India, Japan, the Philippines, and southeastern South America (Uruguay and Argentina).<ref name="EB tornado climatology" /><ref>{{cite web|url=https://weather.com/storms/tornado/news/tornadoes-around-world-20140329|title=TORNADO CENTRAL, Where Tornadoes Strike Around the World, February 12, 2018|date=12 February 2018|access-date=30 September 2021|archive-date=30 September 2021|archive-url=https://web.archive.org/web/20210930215440/https://weather.com/storms/tornado/news/tornadoes-around-world-20140329|url-status=live}}</ref> Tornadoes can be detected before or as they occur through the use of [[pulse-Doppler radar]] by recognizing patterns in velocity and reflectivity data, such as [[hook echo]]es or [[debris ball]]s, as well as through the efforts of [[storm spotter]]s.<ref>{{Cite journal |last1=Coleman |first1=Timothy A. |last2=Knupp |first2=Kevin R. |last3=Spann |first3=James |last4=Elliott |first4=J. B. |last5=Peters |first5=Brian E. |date=2011-05-01 |title=The History (and Future) of Tornado Warning Dissemination in the United States |journal=Bulletin of the American Meteorological Society |language=EN |volume=92 |issue=5 |pages=567–582 |doi=10.1175/2010BAMS3062.1|doi-access=free |bibcode=2011BAMS...92..567C }}</ref><ref>{{Cite book |last=Ahrens |first=C. Donald |title=Meteorology today: an introduction to weather, climate, and the environment |date=2016 |publisher=Cengage Learning |isbn=978-1-305-11358-9 |edition=11th |location=Boston, MA, USA}}</ref>

== Tornado rating scales ==
{{See also|Research history of tornadoes}}
There are several scales for rating the strength of tornadoes. The [[Fujita scale]] rates tornadoes by damage caused and has been replaced in some countries by the updated [[Enhanced Fujita Scale]]. An F0 or EF0 tornado, the weakest category, damages trees, but not substantial structures. An [[List of F5 and EF5 tornadoes|F5 or EF5]] tornado, the strongest category, rips buildings off their foundations and can deform large [[skyscraper]]s. The similar [[TORRO scale]] ranges from T0 for extremely weak tornadoes to T11 for the most powerful known tornadoes.<ref>{{cite web|url=http://www.torro.org.uk/TORRO/ECSS_Slide_Show/2004%20SPAIN%20ECSS%20Post-FINAL%20slide%20show.html |title=Wind Scales: Beaufort, T&nbsp;– Scale, and Fujita's Scale |last=Meaden|first=Terrance |publisher=Tornado and Storm Research Organisation |year=2004 |access-date=2009-09-11 |url-status=dead |archive-url=https://web.archive.org/web/20100430211910/http://www.torro.org.uk/TORRO/ECSS_Slide_Show/2004%20SPAIN%20ECSS%20Post-FINAL%20slide%20show.html |archive-date=2010-04-30 }}</ref> The [[International Fujita scale]] is also used to rate the [[intensity of tornadoes]] and other wind events based on the severity of the damage they cause.<ref>{{cite web |title=The International Fujita (IF) Scale Tornado and Wind Damage Assessment Guide |url=https://www.essl.org/media/publications/IF-scale_v0.10.pdf |website=ESSL.org |publisher=[[European Severe Storms Laboratory]] |access-date=26 June 2022 |archive-date=28 April 2022 |archive-url=https://web.archive.org/web/20220428044530/https://www.essl.org/media/publications/IF-scale_v0.10.pdf |url-status=live }}</ref> Doppler [[weather radar|radar]] data, [[photogrammetry]], and ground swirl patterns ([[trochoid]]al marks) may also be analyzed to determine intensity and assign a rating.<ref name="EF SPC">{{cite web|title=Enhanced F Scale for Tornado Damage|work=Storm Prediction Center|date=2007-02-01|publisher=National Oceanic and Atmospheric Administration|url=http://www.spc.noaa.gov/efscale/ef-scale.html|access-date=2009-06-21|archive-date=2012-07-11|archive-url=https://web.archive.org/web/20120711091501/http://www.spc.noaa.gov/efscale/ef-scale.html|url-status=live}}</ref><ref>{{Cite journal | doi=10.1175/BAMS-D-11-00006.1|title = Tornado Intensity Estimation: Past, Present, and Future| journal=Bulletin of the American Meteorological Society| volume=94| issue=5| pages=641–653|year = 2013|last1 = Edwards|first1 = Roger| last2=Ladue| first2=James G.| last3=Ferree| first3=John T.| last4=Scharfenberg| first4=Kevin| last5=Maier| first5=Chris| last6=Coulbourne| first6=William L.| bibcode=2013BAMS...94..641E| s2cid=7842905 | doi-access=free}}</ref>

[[File:AllTdots.png|thumb|300px|Tornadoes are so common in the U.S. that plotting their mid-points alone generates a recognizable map of the U.S. ...all tornadoes in the US, 1950–2013, highest [[Fujita scale|F-scale]] on top, source [[NOAA]] [[Storm Prediction Center]].]]

== Etymology ==
The word ''tornado'' comes from the [[Spanish language|Spanish]] {{lang|es|tronada}} (meaning 'thunderstorm', past participle of ''tronar'' 'to thunder', itself in turn from the [[Latin]] ''tonāre'' 'to thunder').<ref name="etymology 1">{{OEtymD|tornado|access-date=2009-12-13}}</ref><ref name="etymology 2">{{cite book|title=Merriam Webster's Collegiate Dictionary|access-date=2009-12-13|url=https://archive.org/details/merriamwebstersc01merr|first=Frederick C.|last= Mish|edition=10|year=1993|publisher=Merriam-Webster, Incorporated|isbn=0-87779-709-9}}</ref> The [[Metathesis (linguistics)|metathesis]] of the r and o in the English spelling was influenced by the Spanish ''tornado'' (past participle of ''tornar'' 'to twist, turn,', from Latin ''tornō'' 'to turn').<ref name="etymology 1" /> The English word has been [[reborrowed]] into Spanish, referring to the same weather phenomenon.

Tornadoes' opposite phenomena are the widespread, straight-line [[derecho]]s ({{IPAc-en|d|ə|ˈ|r|eɪ|tʃ|oʊ}}, from {{langx|es|derecho}} {{IPA|es|deˈɾetʃo}}, 'straight'). A tornado is also commonly referred to as a "twister" or the old-fashioned colloquial term ''cyclone''.<ref name="TT" /><ref name="nssl faq">{{cite web| url =http://www.nssl.noaa.gov/primer/tornado/tor_faq.shtml| title =Frequently Asked Questions about Tornadoes| date =2009-07-20| publisher =National Severe Storms Laboratory| access-date =2010-06-22| archive-url =https://web.archive.org/web/20120523165229/http://www.nssl.noaa.gov/primer/tornado/tor_faq.shtml| archive-date =2012-05-23| url-status=dead}}</ref>

== Definitions ==
A tornado is 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 book|url=https://glossary.ametsoc.org/wiki/Tornado|title=Tornado|author=Glossary of Meteorology|edition=2|access-date=2021-03-06|publisher=American Meteorological Society|year=2020|archive-date=2021-05-08|archive-url=https://web.archive.org/web/20210508191421/https://glossary.ametsoc.org/wiki/Tornado|url-status=live}}</ref> For a vortex to be classified as a tornado, it must be in contact with both the ground and the cloud base. The term is not precisely defined; for example, there is disagreement as to whether separate touchdowns of the same funnel constitute separate tornadoes.<ref name="SPC FAQ"/> ''Tornado'' refers to the [[vortex]] of wind, not the condensation cloud.<ref name="Advanced Spotter Guide"/><ref name="tornado?">{{cite web|first=Charles A. III|last=Doswell|url=http://www.cimms.ou.edu/~doswell/a_tornado/atornado.html|title=What is a tornado?|access-date=2008-05-28|publisher=Cooperative Institute for Mesoscale Meteorological Studies|date=2001-10-01|archive-date=2018-07-03|archive-url=https://web.archive.org/web/20180703163826/http://www.cimms.ou.edu/~doswell/a_tornado/atornado.html|url-status=live}}</ref>

=== Funnel cloud ===
{{Main|Funnel cloud}}
[[File:A tornado near Anadarko, Oklahoma, on May 3, 1999.jpg|thumb|upright=1.35|A tornado near [[Anadarko, Oklahoma]], 1999. The ''funnel'' is the thin tube reaching from the cloud to the ground. The lower part of this tornado is surrounded by a [[transparency (optics)|translucent]] dust cloud, kicked up by the tornado's strong winds at the surface. The wind of the tornado has a much wider radius than the funnel itself.]]
A tornado is not necessarily visible; however, the intense low pressure caused by the high wind speeds (as described by [[Bernoulli's principle]]) and rapid rotation (due to [[cyclostrophic balance]]) usually cause [[water vapor]] in the air to condense into cloud droplets due to [[adiabatic cooling]]. This results in the formation of a visible funnel cloud or condensation funnel.<ref name="Renno">{{cite journal |first=Nilton O.|last=Renno |title=A thermodynamically general theory for convective vortices |journal=[[Tellus A]] |volume=60 |issue=4 |pages=688–99 |date=2008-07-03 |url=http://vortexengine.ca/misc/Renno_2008.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://vortexengine.ca/misc/Renno_2008.pdf |archive-date=2022-10-09 |url-status=live |doi=10.1111/j.1600-0870.2008.00331.x |access-date=2009-12-12 |bibcode=2008TellA..60..688R |hdl=2027.42/73164 |hdl-access=free }}</ref>

There is some disagreement over the definition of a funnel cloud and a condensation funnel. According to the ''Glossary of Meteorology'', a funnel cloud is any rotating cloud pendant from a cumulus or cumulonimbus, and thus most tornadoes are included under this definition.<ref>{{cite book |url=http://glossary.ametsoc.org/index.php?title=Special:AllPages/F |title=Funnel cloud |date=2000-06-30 |publisher=[[American Meteorological Society]] |edition=2 |access-date=2009-02-25 |archive-url=https://web.archive.org/web/20130205095929/http://glossary.ametsoc.org/index.php?title=Special%3AAllPages%2FF |archive-date=2013-02-05 |url-status=dead}}</ref> Among many meteorologists, the "funnel cloud" term is strictly defined as a rotating cloud which is not associated with strong winds at the surface, and condensation funnel is a broad term for any rotating cloud below a cumuliform cloud.<ref name="SPC FAQ"/>

Tornadoes often begin as funnel clouds with no associated strong winds at the surface, and not all funnel clouds evolve into tornadoes. Most tornadoes produce strong winds at the surface while the visible funnel is still above the ground, so it is difficult to discern the difference between a funnel cloud and a tornado from a distance.<ref name="SPC FAQ"/>

===Outbreaks and families===
{{Main|Tornado family|tornado outbreak|tornado outbreak sequence}}
Occasionally, a single storm will produce more than one tornado, either simultaneously or in succession. Multiple tornadoes produced by the same [[storm cell]] are referred to as a "tornado family".<ref>{{cite web|url=http://www.srh.noaa.gov/oun/severewx/glossary4.php#t|title=A Comprehensive Glossary of Weather Terms for Storm Spotters|access-date=2007-02-27|first=Michael|last=Branick|year=2006|publisher=National Oceanic and Atmospheric Administration |archive-url=https://web.archive.org/web/20030803230231/http://www.srh.noaa.gov/oun/severewx/glossary4.php#t |archive-date=2003-08-03}}</ref> Several tornadoes are sometimes spawned from the same large-scale storm system. If there is no break in activity, this is considered a tornado outbreak (although the term "tornado outbreak" has various definitions). 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 |archive-url=https://ghostarchive.org/archive/20221009/http://ams.confex.com/ams/pdfpapers/81933.pdf |archive-date=2022-10-09 |url-status=live|title=Tornado Outbreak Day Sequences: Historic Events and Climatology (1875–2003)|access-date=2007-03-20|first1=Russell S. |last1=Schneider |first2=Harold E. |last2=Brooks |first3=Joseph T. |last3=Schaefer  |name-list-style=amp |year=2004}}</ref>

== Characteristics ==

=== Size and shape ===
[[File:A Texas twister in Wichita County April 1964.jpg|thumb|[[1964 Wichita Falls tornado|This F5 rated tornado]] in [[Wichita Falls, Texas]] in April 1964, has a "rope" structure. This usually occurs when a tornado first forms or when a tornado ropes out and dissipates.]]
[[File:Greensburg tornado on the highway.jpg|thumb|286x286px|[[2007 Greensburg tornado|This nocturnal wedge EF5 tornado]] near [[Greensburg, Kansas]] in May 2007, has a "wedge" structure, these can reach widths of 1 mile or even 2 miles in rare occasions.]]
Most tornadoes take on the appearance of a narrow [[funnel]], a few hundred meters (yards) across, with a small cloud of debris near the ground. Tornadoes may be obscured completely by rain or dust. These tornadoes are especially dangerous, as even experienced meteorologists might not see them.<ref name="Handy Weather Answer Book"/>

Small, relatively weak landspouts may be visible only as a small swirl of dust on the ground. Although the condensation funnel may not extend all the way to the ground, if associated surface winds are greater than {{convert|40|mph|km/h|abbr=on|order=flip}}, the circulation is considered a tornado.<ref name="Advanced Spotter Guide"/> A tornado with a nearly cylindrical profile and relatively low height is sometimes referred to as a "stovepipe" tornado. Large tornadoes which appear wider than their cloud-to-ground height can look like large [[wedges]] stuck into the ground, and so are known as "wedge tornadoes" or "wedges".<ref name="wedge tornado"/> The "stovepipe" classification is also used for this type of tornado if it otherwise fits that profile. 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. Many, but not all major tornadoes are wedges.<ref name="wedge tornado">{{cite web|url=http://www.spc.noaa.gov/faq/tornado/binger.htm|title=Wedge Tornado|first=Roger|last=Edwards|author-link=Roger Edwards (meteorologist)|work=[[National Weather Service]]|publisher=National Oceanic and Atmospheric Administration|date=2008-07-18|access-date=2007-02-28|archive-date=2021-05-11|archive-url=https://web.archive.org/web/20210511042056/https://www.spc.noaa.gov/faq/tornado/binger.htm|url-status=live}}</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". When they rope out, the length of their funnel increases, which forces the winds within the funnel to weaken due to [[conservation of angular momentum]].<ref name="thebible">{{cite journal|pages=57–58|journal=Bible of Weather Forecasting|volume=1|issue=4|title=27.0.0 General Laws Influencing the Creation of Bands of Strong Bands|last=Singer|first=Oscar|date=May–July 1985 }}</ref> Multiple-vortex tornadoes can appear as a family of swirls circling a common center, or they 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|work=[[National Weather Service]]|publisher=National Oceanic and Atmospheric Administration|date=2008-07-18|title=Rope Tornado|access-date=2007-02-28|first=Roger|last=Edwards|author-link=Roger Edwards (meteorologist)|archive-date=2007-07-11|archive-url=https://web.archive.org/web/20070711024223/http://www.spc.noaa.gov/faq/tornado/el_reno.htm|url-status=live}}</ref>

In the United States, tornadoes are around {{convert|500|ft|m}} across on average.<ref name="Handy Weather Answer Book"/> However, there is a wide range of tornado sizes. Weak tornadoes, or strong yet dissipating tornadoes, can be exceedingly narrow, sometimes only a few feet or couple meters across. One tornado was reported to have a damage path only {{convert|7|ft|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 [[2004 Hallam tornado|tornado that affected Hallam, Nebraska]] on May 22, 2004, was up to {{convert|2.5|mi|km}} wide at the ground, and [[2013 El Reno tornado|a tornado in El Reno, Oklahoma]] on May 31, 2013, was approximately {{convert|2.6|mi|km}} wide, the widest on record.<ref name="widest tornado" /><ref>{{cite web|title=May 31–June 1, 2013 Tornado and Flash Flood Event: The May 31, 2013 El Reno, OK Tornado|url=http://www.srh.noaa.gov/oun/?n=events-20130531-elreno|date=July 28, 2014|access-date=December 25, 2014|work=National Weather Service Weather Forecast Office|publisher=National Oceanic and Atmospheric Administration|location=Norman, Oklahoma|archive-date=July 25, 2015|archive-url=https://web.archive.org/web/20150725013446/http://www.srh.noaa.gov/oun/?n=events-20130531-elreno|url-status=live}}</ref>

=== Track length ===
In the United States, the average tornado travels on the ground for {{convert|5|mi|km}}. However, tornadoes are capable of both much shorter and much longer damage paths: one tornado was reported to have a damage path only {{convert|7|ft|m}} long, while the record-holding tornado for path length—the [[Tri-State Tornado]], which affected parts of [[Missouri]], [[Illinois]], and [[Indiana]] on March 18, 1925—was on the ground continuously for {{convert|219|mi|km}}.<ref name="Handy Weather Answer Book" /> Many tornadoes which appear to have path lengths of {{convert|100|mi|km}} or longer are composed of 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|first=Thomas P.|last=Grazulis|title=Significant Tornadoes 1680–1991|date=July 1993|publisher=The Tornado Project of Environmental Films|location=St. Johnsbury, VT|isbn=1-879362-03-1}}</ref> A 2007 reanalysis of the path suggests that the tornado may have begun {{convert|15|mi|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|archive-url=https://web.archive.org/web/20070614014358/http://apollo.lsc.vsc.edu/ams/AMS%20VP/Storm%20Conference/NESC%20Presentations/32ndNESC_Presentation/Banquet/Doswell.ppt|url-status=dead|archive-date=2007-06-14|title=The Tri-State Tornado of 18 March 1925 |publisher=Reanalysis Project|access-date=2007-04-07|first=Charles A. III|last=Doswell |format=Powerpoint Presentation}}</ref>

===Appearance===
[[File:Waurika Oklahoma Tornado Back and Front.jpg|thumb|right|Photographs of the [[Waurika, Oklahoma]] tornado of May 30, 1976, taken at nearly the same time by two photographers. In the top picture, the tornado is lit by the sunlight focused from behind the [[camera]], thus the funnel appears bluish. In the lower image, where the camera is facing the opposite direction, the sun is behind the tornado, giving it a dark appearance.<ref name="PD tornado images">{{cite web|url=http://www.spc.noaa.gov/faq/tornado/torscans.htm|title=Public Domain Tornado Images|work=[[National Weather Service]]|access-date=2009-11-17|year=2009|first=Roger|last=Edwards|author-link=Roger Edwards (meteorologist)|publisher=National Oceanic and Atmospheric Administration|archive-date=2006-09-30|archive-url=https://web.archive.org/web/20060930133841/http://www.spc.noaa.gov/faq/tornado/torscans.htm|url-status=live}}</ref>]]Tornadoes can have a wide range of colors, depending on the environment in which they form. Those that form in dry environments can be nearly invisible, marked only by swirling debris at the base of the funnel. Condensation funnels that pick up little or no debris can be gray to white. While traveling over a body of water (as a waterspout), tornadoes can turn white or even blue. Slow-moving funnels, which ingest a considerable amount 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 white.<ref name="Handy Weather Answer Book" />

Lighting conditions are a major factor in the appearance of a tornado. A tornado which is "[[Backlighting (lighting design)|back-lit]]" (viewed with the sun behind it) appears 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="TT">{{cite web|first=Tim|last=Marshall |url=http://www.tornadoproject.com/cellar/tttttttt.htm |title=The Tornado Project's Terrific, Timeless and Sometimes Trivial Truths about Those Terrifying Twirling Twisters! |access-date=2008-11-09 |publisher=The Tornado Project |date=2008-11-09 |url-status=dead |archive-url=https://web.archive.org/web/20081016163615/http://www.tornadoproject.com/cellar/tttttttt.htm |archive-date=2008-10-16 }}</ref><ref name="target tornado">{{cite video|people=Linda Mercer Lloyd|date=1996|title=Target: Tornado|medium=Videotape|publisher=The Weather Channel}}</ref>

Dust kicked up by the winds of the parent thunderstorm, heavy rain and hail, and the darkness of night are all factors that can reduce the visibility of tornadoes. Tornadoes occurring in these conditions are especially dangerous, since only [[weather radar]] observations, or possibly the sound of an approaching tornado, serve as any warning to those in the storm's path. Most significant tornadoes form under the storm's ''updraft base'', which is rain-free,<ref>{{cite web|url=http://www.srh.noaa.gov/oun/stormspotting/basics.php |work=[[National Weather Service]]|publisher=National Oceanic and Atmospheric Administration|title=The Basics of Storm Spotting|date=2009-01-15|access-date=2009-11-17 |archive-url = https://web.archive.org/web/20031011045434/http://www.srh.noaa.gov/oun/stormspotting/basics.php |archive-date = 2003-10-11}}</ref> making them visible.<ref>{{cite journal | last1 =Peterson | first1 = Franklynn | last2 =Kwsselman | first2 = Judi R | title =Tornado factory – giant simulator probes killer twisters | journal = Popular Science | volume =213 | issue =1 | pages = 76–78 | date = July 1978| url = {{GBUrl|YwEAAAAAMBAJ|p=76}}}}</ref> Also, most tornadoes occur in the late afternoon, when the bright sun can penetrate even the thickest clouds.<ref name="significant tornadoes"/>

There is mounting evidence, including [[Doppler on Wheels]] mobile radar images and eyewitness accounts, that most tornadoes have a clear, calm center with extremely low pressure, akin to the [[eye (cyclone)|eye]] of [[tropical cyclone]]s. 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=https://www.sciencenews.org/archive/oklahoma-tornado-sets-wind-record?mode=magazine&context=1921|title=Oklahoma Tornado Sets Wind Record|access-date=2006-10-20|first=R.|last=Monastersky|date=1999-05-15|work=Science News|pages=308–09|archive-date=2015-04-02|archive-url=https://web.archive.org/web/20150402141112/https://www.sciencenews.org/archive/oklahoma-tornado-sets-wind-record?mode=magazine&context=1921|url-status=live}}</ref><ref name="inside eyewitness">{{cite journal |last=Justice |first=Alonzo A. |year=1930 |title=Seeing the Inside of a Tornado |journal=Monthly Weather Review |volume=58 |issue=5 |pages=205–06 |bibcode=1930MWRv...58..205J |doi=10.1175/1520-0493(1930)58<205:STIOAT>2.0.CO;2 |doi-access=free}}</ref><ref>{{cite book|first=Roy S.|last=Hall|isbn=0-7377-1473-5|title=Tornadoes|year=2003|publisher=Greenhaven Press|pages=59–65|chapter=Inside a Texas Tornado}}</ref>

===Rotation===
Tornadoes normally rotate [[cyclone|cyclonically]] (when viewed from above, this is counterclockwise in the [[northern hemisphere]] and clockwise in the [[southern hemisphere|southern]]). While large-scale storms always rotate cyclonically due to the [[Coriolis effect]], thunderstorms and tornadoes are so small that the direct influence of the Coriolis effect is negligible, as indicated by their large [[Rossby number]]s. Supercells and tornadoes rotate cyclonically in numerical simulations even when the Coriolis effect is neglected.<ref name="Origin of Updraft Rotation in Supercells">{{cite journal|first=Robert|last=Davies-Jones|author-link=Robert Davies-Jones|title=Streamwise Vorticity: The Origin of Updraft Rotation in Supercell Storms|journal=J. Atmos. Sci.|volume=41|issue=20|pages=2991–3006|doi=10.1175/1520-0469(1984)041<2991:SVTOOU>2.0.CO;2|bibcode = 1984JAtS...41.2991D|year=1984 |doi-access=free}}</ref><ref name="Rotation and Propagation of Simulated Supercells">{{cite journal|first1=Richard|last1=Rotunno|author2-link=Joseph B. Klemp|first2=Joseph|last2=Klemp|title=On the Rotation and Propagation of Simulated Supercell Thunderstorms|volume=42|issue=3|pages=271–92|journal=J. Atmos. Sci.|doi=10.1175/1520-0469(1985)042<0271:OTRAPO>2.0.CO;2|bibcode=1985JAtS...42..271R|year=1985|url=https://zenodo.org/record/1234689|doi-access=free|access-date=2019-08-01|archive-date=2019-08-01|archive-url=https://web.archive.org/web/20190801200317/https://zenodo.org/record/1234689|url-status=live}}</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|first1=Louis J.|last1=Wicker|first2=Robert B.|last2=Wilhelmson|title=Simulation and Analysis of Tornado Development and Decay within a Three-Dimensional Supercell Thunderstorm|pages=2675–703|journal=J. Atmos. Sci.|volume=52|issue=15|doi=10.1175/1520-0469(1995)052<2675:SAAOTD>2.0.CO;2|bibcode = 1995JAtS...52.2675W|year=1995|doi-access=free}}</ref>

Approximately 1&nbsp;percent of tornadoes rotate in an anticyclonic direction in the northern hemisphere. Typically, systems as weak as landspouts and gustnadoes can 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=anticyclonic tornado in El Reno, OK |publisher=The Weather Channel |date=2006-04-26 |access-date=2006-12-30 |first=Greg|last=Forbes |url-status=dead |archive-url=https://web.archive.org/web/20071011194725/http://www.weather.com/blog/weather/8_9262.html |archive-date=2007-10-11 }}</ref> 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 as 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 1998-05-04|access-date=2006-10-20|first=John|last=Monteverdi|date=2003-01-25|archive-date=2013-06-13|archive-url=https://web.archive.org/web/20130613194202/http://tornado.sfsu.edu/geosciences/StormChasing/cases/Sunnyvale/Sunnyvale.html|url-status=dead}}</ref>

=== Sound and seismology ===
[[File:Tornado infrasound sources.png|thumb|upright=1.2|An illustration of generation of infrasound in tornadoes by the [[Earth System Research Laboratories]]'s Infrasound Program]]
Tornadoes emit widely on the [[acoustics]] [[Audio frequency|spectrum]] and the sounds are caused by multiple mechanisms. Various sounds of tornadoes have been reported, mostly related to familiar sounds for the witness and generally some variation of a whooshing roar. Popularly reported sounds include a freight train, rushing rapids or waterfall, a nearby jet engine, or combinations of these. Many tornadoes are not audible from much distance; the nature of and the propagation distance of the audible sound depends on atmospheric conditions and topography.<ref name="SPC FAQ" />

The winds of the tornado vortex and of constituent turbulent [[eddy (fluid dynamics)|eddies]], as well as airflow interaction with the surface and debris, contribute to the sounds. Funnel clouds also produce sounds. Funnel clouds and small tornadoes are reported as 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".<ref name="tornado music">{{cite journal|first=Abdul|last=Abdullah|title=The "Musical" Sound Emitted by a Tornado"|journal=Mon. Wea. Rev.|volume=94|issue=4|pages=213–20|date=April 1966|doi=10.1175/1520-0493(1966)094<0213:TMSEBA>2.3.CO;2|bibcode=1966MWRv...94..213A|url=http://docs.lib.noaa.gov/rescue/mwr/094/mwr-094-04-0213.pdf|url-status=dead|archive-url=https://web.archive.org/web/20170921230829/https://docs.lib.noaa.gov/rescue/mwr/094/mwr-094-04-0213.pdf|archive-date=2017-09-21|citeseerx=10.1.1.395.3099}}</ref>

Since many tornadoes are audible only when very near, sound is not to be thought of as a reliable warning signal for a tornado. Tornadoes are also not the only source of such sounds in severe thunderstorms; any strong, damaging wind, a severe hail volley, or continuous thunder in a thunderstorm may produce a roaring sound.<ref name="sound obs">{{cite journal|first=David K.|last=Hoadley |title=Tornado Sound Experiences |journal=[[Storm Track]] |volume=6 |issue=3 |pages=5–9 |date=1983-03-31 |url=http://www.stormtrack.org/archive/0636.htm |url-status=dead |archive-url=https://web.archive.org/web/20120619210634/http://www.stormtrack.org/archive/0636.htm |archive-date=2012-06-19 }}</ref>

Tornadoes also produce identifiable inaudible [[infrasonic]] signatures.<ref name="tornado infrasonics">{{cite journal|first=A. J.|last=Bedard|title=Low-Frequency Atmospheric Acoustic Energy Associated with Vortices Produced by Thunderstorms|journal=Mon. Wea. Rev.|volume=133|issue=1|pages=241–63|date=January 2005|doi=10.1175/MWR-2851.1|bibcode = 2005MWRv..133..241B |s2cid=1004978|doi-access=free}}</ref>

Unlike audible signatures, tornadic signatures have been isolated; due to the long-distance propagation of low-frequency sound, efforts are ongoing to develop tornado prediction and detection devices with additional value in understanding tornado morphology, dynamics, and creation.<ref name="field programs history"/> Tornadoes also produce a detectable [[seismic]] signature, and research continues on isolating it and understanding the process.<ref name="tornado seismic signal">{{cite journal|first1=Frank|last1=Tatom |first2=Kevin R.|last2=Knupp |first3=Stanley J.|last3=Vitto  |name-list-style=amp |title=Tornado Detection Based on Seismic Signal|journal=J. Appl. Meteorol.|volume=34|issue=2|pages=572–82|doi=10.1175/1520-0450(1995)034<0572:TDBOSS>2.0.CO;2|bibcode = 1995JApMe..34..572T|year=1995 |doi-access=free}}</ref>

===Electromagnetic, lightning, and other effects===
Tornadoes emit on the [[electromagnetic spectrum]], with [[sferics]] and [[E-field]] effects detected.<ref name="field programs history"/><ref>{{cite journal|first1=John R.|last1=Leeman|first2=E. D.|last2=Schmitter|title=Electric signals generated by tornados|journal=Atmos. Res.|volume=92|issue=2|pages=277–79|date=April 2009|doi=10.1016/j.atmosres.2008.10.029|bibcode=2009AtmRe..92..277L}}</ref><ref name="in situ history">{{cite conference|first=Timothy M.|last=Samaras|author-link=Tim Samaras|title=A Historical Perspective of In-Situ Observations within Tornado Cores|book-title=Preprints of the 22nd Conf. Severe Local Storms|publisher=American Meteorological Society|date=October 2004|location=Hyannis, MA|url=http://ams.confex.com/ams/11aram22sls/techprogram/paper_81153.htm|access-date=2007-05-23|archive-date=2011-01-15|archive-url=https://web.archive.org/web/20110115081005/http://ams.confex.com/ams/11aram22sls/techprogram/paper_81153.htm|url-status=live}}</ref> There are observed correlations between tornadoes and patterns of lightning. Tornadic storms do not produce more lightning than other storms and some tornadic cells never produce lightning at all. More often than not, overall cloud-to-ground (CG) lightning activity decreases as a tornado touches the surface and returns to the baseline level when the tornado dissipates. In many cases, intense tornadoes and thunderstorms exhibit an increased and anomalous dominance of positive polarity CG discharges.<ref name="CG tor">{{cite journal|first1=Antony H.|last1=Perez |first2=Louis J.|last2=Wicker |first3=Richard E.|last3=Orville  |name-list-style=amp |title=Characteristics of Cloud-to-Ground Lightning Associated with Violent Tornadoes|journal=Weather Forecast|volume=12|issue=3|pages=428–37|doi=10.1175/1520-0434(1997)012<0428:COCTGL>2.0.CO;2|bibcode = 1997WtFor..12..428P|year=1997 |doi-access=free}}</ref>

[[Luminosity]] has been reported in the past and is probably due to misidentification of external light sources such as lightning, city lights, and [[power flash]]es from broken lines, as internal sources are now uncommonly reported and are not known to ever have been recorded. In addition to winds, tornadoes also exhibit changes in atmospheric variables such as [[temperature]], [[moisture]], and [[atmospheric pressure]]. For example, on June 24, 2003, near [[Manchester, South Dakota]], a probe measured a {{convert|100|mbar|hPa inHg|adj=on|lk=on}} pressure decrease. The pressure dropped gradually as the vortex approached then dropped extremely rapidly to {{convert|850|mbar|hPa inHg|abbr=on|lk=on}} 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.<ref name="Manchester">{{cite conference|first1=Julian J.|last1=Lee|first2=Timothy P.|last2=Samaras|first3=Carl R.|last3=Young|title=Pressure Measurements at the ground in an F-4 tornado|book-title=Preprints of the 22nd Conf. Severe Local Storms|publisher=American Meteorological Society|date=2004-10-07|location=Hyannis, Massachusetts|url=http://ams.confex.com/ams/11aram22sls/techprogram/paper_81700.htm|access-date=2007-07-06|archive-date=2011-06-09|archive-url=https://web.archive.org/web/20110609164901/http://ams.confex.com/ams/11aram22sls/techprogram/paper_81700.htm|url-status=live}}</ref>

== Life cycle ==
{{Further|Tornadogenesis}}
[[File:Tornado time lapse.webm|thumb|right|A [[timelapse]] of the life cycle of a tornado near [[Prospect Valley, Colorado]]]]

=== Supercell relationship ===
{{See also|Supercell}}
Tornadoes often develop from a class of thunderstorms known as supercells. Supercells contain [[mesocyclone]]s, an area of organized rotation a few kilometers/miles up in the atmosphere, usually {{convert|1|–|6|mi|km|abbr=in|order=flip}} 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.<ref>{{Cite web |title=Radar Signatures for Severe Convective Weather: Low-Level Mesocyclone, Print Version |url=http://www.faculty.luther.edu/~bernatzr/Courses/Sci123/comet/radar/severe_signatures/print_low_level_meso.htm |access-date=2022-06-03 |website=www.faculty.luther.edu}}</ref><ref>{{Cite web |last=US Department of Commerce |first=NOAA |title=Supercell Structure and Dynamics |url=https://www.weather.gov/lmk/supercell/dynamics |access-date=2022-06-03 |website=www.weather.gov |language=EN-US |archive-date=2022-05-26 |archive-url=https://web.archive.org/web/20220526012804/https://www.weather.gov/lmk/supercell/dynamics |url-status=live }}</ref>

Most tornadoes from supercells follow a recognizable life cycle which 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.<ref name="Advanced Spotter Guide" />

=== Formation ===
[[File:Tornado Formation of it's wall cloud from a Mesocyclone.svg|thumb|upright=1.3|Tornado formation of its wall cloud from a mesocyclone]]
As the [[mesocyclone]] lowers below the cloud base, it begins to take in cool, moist air from the downdraft region of the storm. The convergence of warm air in the updraft and cool air causes a rotating wall cloud to form. The RFD also focuses the mesocyclone's base, causing it to draw air from a smaller and smaller area on the ground. As the updraft intensifies, it creates an area of low pressure at the surface. This pulls the focused mesocyclone down, in the form of a visible condensation funnel. As the funnel descends, the RFD also reaches the ground, fanning outward and creating a gust front that can cause severe damage a considerable distance from the tornado. Usually, the funnel cloud begins causing damage on the ground (becoming a tornado) within a few minutes of the RFD reaching the ground.<ref name="Advanced Spotter Guide"/><ref>{{Cite web|title = How Tornadoes Form and Why They're so Unpredictable|url = http://news.nationalgeographic.com/2015/05/150511-tornadoes-storms-midwest-weather-science|archive-url = https://web.archive.org/web/20150514135021/http://news.nationalgeographic.com/2015/05/150511-tornadoes-storms-midwest-weather-science/|url-status = dead|archive-date = May 14, 2015|website = National Geographic News|access-date = 2015-05-11|first = Brian Clark|last = Howard|publisher = National Geographic |date= May 11, 2015}}</ref>  Many other aspects of tornado formation (such as why some storms form tornadoes while others do not, or what precise role downdrafts, temperature, and moisture play in tornado formation) are still poorly understood.<ref>{{Cite web |title=Tornado Types |url=https://www.nssl.noaa.gov/education/svrwx101/tornadoes/types/ |access-date=2023-03-28 |website=NOAA National Severe Storms Laboratory |language=EN-US |archive-date=2023-03-27 |archive-url=https://web.archive.org/web/20230327063522/https://www.nssl.noaa.gov/education/svrwx101/tornadoes/types/ |url-status=live }}</ref>

=== Maturity ===
[[File:Stovepipe Tornado near Yuma, Colorado.jpg|thumb|A mature stovepipe tornado near Yuma, Colorado.]]
Initially, the tornado has a good source of warm, moist air [[inflow (meteorology)|flowing inward]] to power it, and it grows until it reaches the "mature stage". This can last from a few minutes to more than an hour, and during that time a tornado often causes the most damage, and in rare cases can be more than {{convert|1|mi|km|abbr=in|order=flip}} across. The low pressured atmosphere at the base of the tornado is essential to the endurance of the system.<ref>{{Cite web|url=http://www.spc.noaa.gov/faq/tornado/#The%20Basics|title=The Online Tornado FAQ|date=March 2016|website=www.spa.noaa.gov|publisher=Roger Edwards, Storm Prediction Center|access-date=27 October 2016|archive-date=2 March 2012|archive-url=https://web.archive.org/web/20120302230203/http://www.spc.noaa.gov/faq/tornado/#The%20Basics|url-status=live}} {{Pd-notice}}</ref> Meanwhile, the RFD, now an area of cool surface winds, begins to wrap around the tornado, cutting off the inflow of warm air which previously fed the tornado.<ref name="Advanced Spotter Guide"/>
The flow inside the funnel of the tornado is downward, supplying water vapor from the cloud above. This is contrary to the upward flow inside hurricanes, supplying water vapor from the warm ocean below. Therefore, the energy of the tornado is supplied from the cloud above.
<ref>
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=== Dissipation ===
[[File:Tornado Roping Out in Eastern Colorado.jpg|thumb|A tornado dissipating or "roping out" near the town of [[Eads, Colorado]].]]
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 ends. During this 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="significant tornadoes"/><ref name="PD tornado images"/><ref name="target tornado"/> Even though the tornado is dissipating, it is still capable of causing damage. The storm is contracting into a rope-like tube and, due to [[conservation of angular momentum]], winds can increase at this point.<ref name="thebible"/>

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. Sometimes, in intense supercells, tornadoes can develop [[wikt: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 and possibly feed a new mesocyclone. 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.<ref>{{Cite web |title=Family of tornadoes - Glossary of Meteorology |url=https://glossary.ametsoc.org/wiki/Family_of_tornadoes |archive-url=http://web.archive.org/web/20220816201555/https://glossary.ametsoc.org/wiki/Family_of_tornadoes |archive-date=2022-08-16 |access-date=2025-04-16 |website=glossary.ametsoc.org |language=en}}</ref>

Although 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.<ref name="tornadogenesis">{{cite journal|doi=10.1175/1520-0469(2003)060<0795:TRFTTO>2.0.CO;2|title=Tornadogenesis Resulting from the Transport of Circulation by a Downdraft: Idealized Numerical Simulations|first1=Paul M.|last1=Markowski|first2=Jerry M.|last2=Straka|first3=Erik N.|last3=Rasmussen|journal=J. Atmos. Sci. | volume= 60 |issue= 6|pages=795–823|year=2003|bibcode = 2003JAtS...60..795M |doi-access=free}}</ref>

==Types==

===Multiple vortex===
{{Main|Multiple-vortex tornado}}
A ''multiple-vortex tornado'' is a type of tornado in which two or more columns of spinning air rotate about their own axes and at the same time revolve around a common center. A multi-vortex structure can occur in almost any circulation, but is very often observed in intense tornadoes. These vortices often create small areas of heavier damage along the main tornado path.<ref name="SPC FAQ" /><ref name="Advanced Spotter Guide" /> This is a phenomenon that is distinct from a [[satellite tornado]], which is a smaller tornado that 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 circulation, and is much smaller than the main funnel.<ref name="SPC FAQ" />

===Waterspout===
{{Main|Waterspout}}

[[File:Trombe.jpg|thumb|left|A waterspout near the [[Florida Keys]] in 1969.]]

A ''waterspout'' is defined by the [[National Weather Service]] as a tornado over water. However, researchers typically distinguish "fair weather" waterspouts from tornadic (i.e. associated with a mesocyclone) waterspouts. Fair weather waterspouts are less severe but far more common, and are similar to [[dust devil]]s and [[landspout]]s. They form at the bases of [[cumulus congestus]] clouds over tropical and subtropical waters. They have relatively weak winds, smooth [[laminar flow|laminar]] walls, and typically travel very slowly. They occur most commonly in the [[Florida Keys]] and in the northern [[Adriatic Sea]].<ref name="USA Today 1">{{cite news|url=https://www.usatoday.com/community/chat/0504tornb.htm|title=Tornado Chase 2000|access-date=2007-05-19|first=Dave|last=Zittel|date=2000-05-04|newspaper=USA Today|url-status=dead|archive-url=https://web.archive.org/web/20070104170204/http://www.usatoday.com/community/chat/0504tornb.htm|archive-date=2007-01-04}}</ref><ref name="USA Today 2">{{cite news|url=https://www.usatoday.com/weather/wspouts.htm|title=Waterspouts are tornadoes over water|date=2007-11-01|access-date=2007-05-19|first=Joseph|last=Golden|newspaper=USA Today|archive-date=2012-09-07|archive-url=https://web.archive.org/web/20120907131453/http://www.usatoday.com/weather/wspouts.htm|url-status=live}}</ref><ref>{{cite book|first1=Thomas P.|last1=Grazulis |first2=Dan|last2=Flores |title=The Tornado: Nature's Ultimate Windstorm|publisher=University of Oklahoma Press|location=Norman OK|year=2003|page=256|isbn=0-8061-3538-7}}</ref> In contrast, tornadic waterspouts are stronger tornadoes over water. They form over water similarly to mesocyclonic tornadoes, or are stronger tornadoes which cross over water. Since they form from [[severe thunderstorm]]s and can be far more intense, faster, and longer-lived than fair weather waterspouts, they are more dangerous.<ref>{{cite web|url=http://www.srh.noaa.gov/mfl/?n=waterspouts|title=About Waterspouts|access-date=2009-12-13|date=2007-01-04|publisher=National Oceanic and Atmospheric Administration|archive-date=2009-09-13|archive-url=https://web.archive.org/web/20090913020610/http://www.srh.noaa.gov/mfl/?n=waterspouts|url-status=live}}</ref> In official tornado statistics, waterspouts are generally not counted unless they affect land, though some European weather agencies count waterspouts and tornadoes together.<ref name="SPC FAQ"/><ref name="ESWD definitions">{{cite web|url=http://essl.org/cgi-bin/eswd/eswd.cgi?action=showdefinitions&lang=en_0|title=European Severe Weather Database definitions|date=2012-01-02|access-date=2012-06-11|archive-date=2012-07-08|archive-url=https://web.archive.org/web/20120708073411/http://essl.org/cgi-bin/eswd/eswd.cgi?action=showdefinitions&lang=en_0|url-status=live}}</ref>

===Landspout===
{{Main|Landspout}}
A ''landspout'', or ''dust-tube tornado'', is a tornado not associated with a mesocyclone. The name stems from their characterization as a "fair weather waterspout on land". Waterspouts and landspouts share many defining characteristics, including relative weakness, short lifespan, and a small, smooth condensation funnel that often does not reach the surface. Landspouts also create a distinctively laminar cloud of dust when they make contact with the ground, due to their differing mechanics from true mesoform tornadoes. Though usually weaker than classic tornadoes, they can produce strong winds which could cause serious damage.<ref name="SPC FAQ"/><ref name="Advanced Spotter Guide"/>

===Similar circulations===

====Gustnado====
{{Main|Gustnado}}

A ''gustnado'', or ''gust front tornado'', is a small, vertical swirl associated with a [[Outflow boundary|gust front]] or [[downburst]]. Because they are not connected with a cloud base, there is some debate as to whether or not gustnadoes are 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 vertically or diagonally and make contact with the ground. The result is a gustnado.<ref name="SPC FAQ"/><ref name="gustnado AMS">{{cite web|url=http://glossary.ametsoc.org/wiki/Gustnado|title=Gustnado|access-date=2006-09-20|publisher=American Meteorological Society|work=Glossary of Meteorology|date=June 2000|archive-date=2007-09-30|archive-url=https://web.archive.org/web/20070930181123/http://amsglossary.allenpress.com/glossary/search?p=1&query=gustnado|url-status=live}}</ref> They usually cause small areas of heavier rotational wind damage among areas of straight-line wind damage.<ref>{{Cite web |title=NOAA's National Weather Service - Glossary |url=https://forecast.weather.gov/glossary.php?word=gustnado |access-date=2025-05-06 |website=forecast.weather.gov}}</ref>

====Dust devil====
{{Main|Dust devil}}

[[File:Dust devil.jpg|right|thumb|A dust devil in [[Arizona]]]]

A ''dust devil'' (also known as a whirlwind) resembles a tornado in that it is a vertical swirling column of air. However, they form under clear skies and are no stronger than 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 clouds. However, they can, on occasion, result in major damage.<ref name="Handy Weather Answer Book">{{cite book|first=Walter A.|last=Lyons|title=The Handy Weather Answer Book|edition=2nd|year=1997|publisher=Visible Ink press|location=[[Detroit|Detroit, Michigan]]|isbn=0-7876-1034-8|pages=[https://archive.org/details/handyweatheransw00lyon/page/175 175–200]|chapter=Tornadoes|chapter-url=https://archive.org/details/handyweatheransw00lyon/page/175}}</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|access-date=2006-09-29|first1=Charles H.|last1=Jones|first2=Charlie A.|last2=Liles|year=1999|archive-date=2018-10-21|archive-url=https://web.archive.org/web/20181021111504/http://www.srh.noaa.gov/ssd/techmemo/sr207.htm|url-status=live}}</ref>

====Fire whirls====
{{Main|Fire whirl}}

Small-scale, tornado-like circulations can occur near any intense surface heat source. Those that occur near intense [[wildfire]]s 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. They can, however, produce significant damage.<ref name="significant tornadoes"/>

====Steam devils====
{{Main|Steam devil}}

A ''steam devil'' is a [[rotating]] [[updraft]] between {{convert|50|and|200|m|ft|adj=mid|wide}} that involves steam or smoke. These formations do not involve high wind speeds, only completing a few rotations per minute. Steam devils are very rare. They most often form from smoke issuing from a power plant's smokestack. [[Hot springs]] and deserts may also be suitable locations for a tighter, faster-rotating steam devil to form. The phenomenon can occur over water, when cold arctic air passes over relatively warm water.<ref name="Handy Weather Answer Book"/>

{{clear}}

==Intensity and damage==
{{Main|Tornado intensity and damage}}
{| class="wikitable" style="float: right;"
|-
|+ Tornado rating classifications<ref name="significant tornadoes"/><ref>
{{Cite web |url=http://www.tornadoproject.com/fscale/fscale.htm |title=The Fujita Scale of Tornado Intensity |access-date=2013-05-08 |archive-url=https://web.archive.org/web/20111230005516/http://www.tornadoproject.com/fscale/fscale.htm |archive-date=2011-12-30 |url-status=dead }} 
</ref>
|-
! style="background: #e0e0ff;" | '''F0<br />EF0'''
! style="background: #e0e0ff;" | '''F1<br />EF1'''
! style="background: #e0e0ff;" | '''F2<br />EF2'''
! style="background: #e0e0ff;" | '''F3<br />EF3'''
! style="background: #e0e0ff;" | '''F4<br />EF4'''
! style="background: #e0e0ff;" | '''F5<br />EF5'''
|-
| colspan="2" style="background: #fd0;"    | Weak
| colspan="2" style="background: #f3943f;" | Strong
| colspan="2" style="background: #ea5297;" | Violent
|-
| colspan="2" | <!-- blank -->
| colspan="4" style="background: #ffce44;" | Significant
|-
| colspan="3" | <!-- blank -->
| colspan="3" style="background: #e84e0f;" | Intense
|}

The [[Fujita scale]], [[Enhanced Fujita scale]] (EF), and [[International Fujita scale]] rate tornadoes by damage caused. The EF scale was an update to the older Fujita scale, by [[expert elicitation]], using engineered wind estimates and better damage descriptions. The EF scale was designed so that a tornado rated on the Fujita scale would receive the same numerical rating, and was implemented starting in the United States in 2007. An EF0 tornado will probably damage trees but not substantial structures, whereas an [[List of F5 and EF5 tornadoes|EF5]] tornado can rip buildings off their foundations leaving them bare and even deform large [[skyscraper]]s. The similar [[TORRO scale]] ranges from a T0 for extremely weak tornadoes to T11 for the most powerful known tornadoes. [[Pulse-Doppler radar|Doppler]] [[weather radar]] data, [[photogrammetry]], and ground swirl patterns ([[cycloid]]al marks) may also be analyzed to determine intensity and award a rating.<ref name="SPC FAQ"/><ref>{{cite web|url=http://www.crh.noaa.gov/news/display_cmsstory.php?wfo%3Dcys%26storyid%3D28445%26source%3D2 |archive-url=https://web.archive.org/web/20100528014750/http://www.crh.noaa.gov/news/display_cmsstory.php?wfo=cys&storyid=28445&source=2 |url-status=dead |archive-date=2010-05-28 |title=Goshen County Tornado Given Official Rating of EF2 |work=[[National Weather Service]] |publisher=National Oceanic and Atmospheric Administration |access-date=2009-11-21 }}</ref><ref>{{cite conference|first1=David C.|last1=Lewellen |first2=M. I.|last2= Zimmerman |date=2008-10-28|title=Using Simulated Tornado Surface Marks to Decipher Near-Ground Winds|conference=24th Conf. Severe Local Storms|conference-url=http://ams.confex.com/ams/24SLS/techprogram/program_508.htm|publisher=American Meteorological Society|url=http://ams.confex.com/ams/pdfpapers/141749.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://ams.confex.com/ams/pdfpapers/141749.pdf |archive-date=2022-10-09 |url-status=live|access-date=2009-12-09}}</ref>

[[File:Shown here May 22, 2013, is an aerial view of homes destroyed by a tornado in Moore, Okla 130522-F-IE715-379.jpg|thumb|On May&nbsp;20, 2013, a [[2013 Moore tornado|large tornado]] of the highest category, EF5, ravaged [[Moore, Oklahoma]].]]

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.<ref name="width/length intensity relationship">{{Cite journal |last=Brooks |first=Harold E. |date=2004-04-01 |title=On the Relationship of Tornado Path Length and Width to Intensity |journal=Weather and Forecasting |language=en |volume=19 |issue=2 |pages=310–319 |bibcode=2004WtFor..19..310B |doi=10.1175/1520-0434(2004)019<0310:OTROTP>2.0.CO;2 |issn=0882-8156 |doi-access=free}}</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, 80% of tornadoes are EF0 and EF1 (T0 through T3) tornadoes. The rate of occurrence drops off quickly with increasing strength—less than 1% are violent tornadoes (EF4, T8 or stronger).<ref name="Basic Spotter Guide">{{cite web|url=https://www.weather.gov/media/grr/brochures/nwsbasicspottersfieldguide.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://www.weather.gov/media/grr/brochures/nwsbasicspottersfieldguide.pdf |archive-date=2022-10-09 |url-status=live|title=basic Spotters' Field Guide|publisher=National Oceanic and Atmospheric Administration, National Weather Service}}</ref> Current records may significantly underestimate the frequency of strong (EF2-EF3) and violent (EF4-EF5) tornadoes, as damage-based intensity estimates are limited to structures and vegetation that a tornado impacts. A tornado may be much stronger than its damage-based rating indicates if its strongest winds occur away from suitable damage indicators, such as in an open field.<ref>{{Cite journal | doi=10.1175/WAF-D-14-00152.1| title=A Multiscale Overview of the el Reno, Oklahoma, Tornadic Supercell of 31 May 2013| journal=Weather and Forecasting| volume=30| issue=3| pages=525–552| year=2015| last1=Bluestein| first1=Howard B.| last2=Snyder| first2=Jeffrey C.| last3=Houser| first3=Jana B.| bibcode=2015WtFor..30..525B| doi-access=free}}</ref><ref name="EFunderstimate">{{cite journal |last1=Wurman |first1=Joshua |last2=Kosiba |first2=Karen |last3=White |first3=Trevor |last4=Robinson |first4=Paul |title=Supercell tornadoes are much stronger and wider than damage-based ratings indicate |journal=Proceedings of the National Academy of Sciences |date=6 April 2021 |volume=118 |issue=14 |page=e2021535118 |doi=10.1073/pnas.2021535118 |pmid=33753558 |pmc=8040662 |bibcode=2021PNAS..11821535W |doi-access=free }}</ref> Outside [[Tornado Alley]], and North America in general, violent tornadoes are extremely rare. This is apparently mostly due to the lesser number of tornadoes overall, as research shows that tornado intensity distributions are fairly similar worldwide. A few significant tornadoes occur annually in Europe, Asia, southern Africa, and southeastern South America.<ref name="intensity distribution">{{Cite journal|title=Statistical modeling of tornado intensity distributions |last1=Dotzek |first1=Nikolai |date=2003-03-01 |journal=Atmos. Res. |volume=67 |pages=163–87 |bibcode=2003AtmRe..67..163D |last2=Grieser |first2=Jürgen |last3=Brooks |first3=Harold E.|doi=10.1016/S0169-8095(03)00050-4|citeseerx=10.1.1.490.4573 }}</ref>
{{clear}}

==Climatology==
{{Main|Tornado climatology}}
[[File:Globdisttornado.jpg|thumb|right|Areas worldwide where tornadoes are most likely, indicated by orange shading]]

The United States has the most tornadoes of any country, nearly four times more than estimated in all of Europe, excluding waterspouts.<ref name="European tornado climatology">{{cite journal|first=Nikolai|last=Dotzek |date=2003-03-20 |title=An updated estimate of tornado occurrence in Europe |journal=Atmos. Res. |volume=67–68 |pages=153–161 |doi=10.1016/S0169-8095(03)00049-8 |bibcode=2003AtmRe..67..153D|citeseerx=10.1.1.669.2418 }}</ref> This is mostly due to the unique geography of the continent. North America is a large continent that extends from the [[tropics]] north 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 buckle the [[prevailing winds|atmospheric flow]], forcing drier air at mid-levels of the [[troposphere]] due to downsloped winds, and causing [[cyclogenesis|the formation of a low pressure area]] downwind to the east of the mountains. Increased westerly flow off the Rockies force the formation of a [[dry line]] when the flow aloft is strong,<ref>{{cite web|author=Huaqing Cai |title=Dryline cross section |date=2001-09-24 |access-date=2009-12-13 |url=http://www.atmos.ucla.edu/~caihq/pic/fig23.html |publisher=University of California Los Angeles |url-status=dead |archive-url=https://web.archive.org/web/20080120180130/http://www.atmos.ucla.edu/~caihq/pic/fig23.html |archive-date=2008-01-20 }}</ref> while the [[Gulf of Mexico]] fuels abundant low-level moisture in the southerly flow to its east. This unique topography allows for frequent collisions of warm and cold air, the conditions that breed strong, long-lived storms throughout the 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">{{harvnb|Perkins|2002|pages=296–98}}</ref> This area extends into Canada, particularly [[Ontario]] and the [[Prairie Provinces]], although southeast [[Quebec]], the interior of [[British Columbia]], and western [[New Brunswick]] are also tornado-prone.<ref>{{cite web|url=http://www.pnr-rpn.ec.gc.ca/air/summersevere/ae00s02.en.html |title=Tornadoes |work=Prairie Storm Prediction Centre |publisher=Environment Canada |date=2007-10-07 |access-date=2009-12-13 |url-status=dead |archive-url=https://web.archive.org/web/20010309011307/http://www.pnr-rpn.ec.gc.ca/air/summersevere/ae00s02.en.html |archive-date=2001-03-09 }}</ref> Tornadoes also occur across northeastern Mexico.<ref name="SPC FAQ"/>

The United States averages about 1,200 tornadoes per year, followed by Canada, averaging 62 reported per year.<ref name="Canada62">{{cite web|last1=Vettese|first1=Dayna|title=Tornadoes in Canada: Everything you need to know|url=https://www.theweathernetwork.com/news/articles/tornadoes-in-canada-everything-you-need-to-know/25876/|website=The Weather Network|access-date=26 November 2016|archive-date=27 November 2016|archive-url=https://web.archive.org/web/20161127152429/https://www.theweathernetwork.com/news/articles/tornadoes-in-canada-everything-you-need-to-know/25876/|url-status=live}}</ref> NOAA's has a higher average 100 per year in Canada.<ref name="Canada100">{{cite web|title=U.S. Tornado Climatology|url=https://www.ncdc.noaa.gov/climate-information/extreme-events/us-tornado-climatology/trends|website=NOAA|access-date=26 November 2016|archive-date=9 December 2016|archive-url=https://web.archive.org/web/20161209095334/https://www.ncdc.noaa.gov/climate-information/extreme-events/us-tornado-climatology/trends|url-status=live}}</ref> The Netherlands has the highest average number of recorded tornadoes per area of any country (more than 20, or {{convert|0.00048|/km2|/sqmi|abbr=on|disp=x|, }} annually), followed by the UK (around 33, {{convert|0.00013|/km2|/sqmi|abbr=on|disp=x|, }} per year), although those are of lower intensity, briefer<ref>{{cite journal|first1=J.|last1=Holden|first2=A.|last2=Wright|title=UK tornado climatology and the development of simple prediction tools|journal=Q. J. R. Meteorol. Soc.|volume=130|pages=1009–21|date=2003-03-13|url=http://www.geog.leeds.ac.uk/people/j.holden/paper80.pdf |doi=10.1256/qj.03.45|access-date=2009-12-13|archive-url = https://web.archive.org/web/20070824151103/http://www.geog.leeds.ac.uk/people/j.holden/paper80.pdf |archive-date = 2007-08-24|bibcode = 2004QJRMS.130.1009H|issue=598 |citeseerx=10.1.1.147.4293|s2cid=18365306}}</ref><ref>{{cite web|title=Natural Disasters: Tornadoes|work=BBC Science and Nature|publisher=BBC|date=2002-03-28|url=http://www.bbc.co.uk/science/hottopics/naturaldisasters/hurricanes.shtml |access-date=2009-12-13 |archive-url = https://web.archive.org/web/20021014233047/http://www.bbc.co.uk/science/hottopics/naturaldisasters/hurricanes.shtml |archive-date = 2002-10-14}}</ref> and cause minor damage.<ref name="European tornado climatology"/>

[[File:Tornado Alley.gif|thumb|left|Intense tornado activity in the United States. The darker-colored areas denote the area commonly referred to as [[Tornado Alley]].]]
Tornadoes kill an average of 179 people per year in Bangladesh, the most in the world.<ref name="Bangladesh tornado"/> Reasons for this include the region's high population density, poor construction quality, and lack of tornado safety knowledge.<ref name="Bangladesh tornado">{{cite web|url=http://www.colorado.edu/hazards/research/qr/qr169/qr169.pdf|title=The April 2004 Tornado in North-Central Bangladesh: A Case for Introducing Tornado Forecasting and Warning Systems|access-date=2009-12-13|author1=Bimal Kanti Paul|author2=Rejuan Hossain Bhuiyan|date=2005-01-18|url-status=dead|archive-url=https://web.archive.org/web/20100606181213/http://www.colorado.edu/hazards/research/qr/qr169/qr169.pdf|archive-date=2010-06-06}}</ref><ref>{{cite web|url=http://bangladeshtornadoes.org/bengaltornadoes.html|title=Bangladesh and East India Tornadoes Background Information|access-date=2009-12-13|date=2008-04-02|first=Jonathan|last=Finch|archive-date=2009-09-01|archive-url=https://web.archive.org/web/20090901203558/http://bangladeshtornadoes.org/bengaltornadoes.html|url-status=live}}</ref> Other areas of the world that have frequent tornadoes include South Africa, the [[La Plata Basin]] area, portions of Europe, Australia and New Zealand, and far eastern Asia.<ref name="EB tornado climatology">{{cite encyclopedia|url=http://www.britannica.com/eb/article-218357/tornado|title=Tornado: Global occurrence|access-date=2009-12-13|encyclopedia=Encyclopædia Britannica Online|year=2009|archive-date=2007-03-17|archive-url=https://web.archive.org/web/20070317211343/http://www.britannica.com/eb/article-218357/tornado|url-status=live}}</ref><ref>{{cite web|url=http://www.extremwetter.ch/thesis.pdf|first=Michael|last=Graf|title=Synoptical and mesoscale weather situations associated with tornadoes in Europe|access-date=2009-12-13|date=2008-06-28|archive-url=https://web.archive.org/web/20160303195103/http://www.extremwetter.ch/thesis.pdf|archive-date=2016-03-03|url-status=dead}}</ref>

Tornadoes are most common in spring and least common in winter, but tornadoes can occur any time of year that favorable conditions occur.<ref name="significant tornadoes"/> Spring and fall experience peaks of activity as those are the seasons when stronger winds, wind shear, and atmospheric instability are present.<ref name="LouieLouie">{{cite web|url=http://www.crh.noaa.gov/lmk/?n=supercell/dynamics|title=Structure and Dynamics of Supercell Thunderstorms|work=[[National Weather Service]]|publisher=National Oceanic and Atmospheric Administration|date=2008-08-28|access-date=2009-12-13|archive-date=2009-11-17|archive-url=https://web.archive.org/web/20091117152717/http://www.crh.noaa.gov/lmk/?n=supercell/dynamics|url-status=live}}</ref> Tornadoes are focused in the right front quadrant of [[landfall (meteorology)|landfalling]] tropical cyclones, which tend to occur in the late summer and autumn. Tornadoes can also be spawned as a result of [[eyewall mesovortices]], which persist until landfall.<ref name="AOML FAQ L6">{{cite web|work=[[Atlantic Oceanographic and Meteorological Laboratory]], Hurricane Research Division |url=http://www.aoml.noaa.gov/hrd/tcfaq/L6.html |title=Frequently Asked Questions: Are TC tornadoes weaker than midlatitude tornadoes? |publisher=National Oceanic and Atmospheric Administration |date=2006-10-04 |access-date=2009-12-13 |url-status=dead |archive-url=https://web.archive.org/web/20090914103006/http://www.aoml.noaa.gov/hrd/tcfaq/L6.html |archive-date=2009-09-14 }} {{Pd-notice}}</ref> Tornados can even form during snow squalls events with no rain present.<ref>{{Cite journal |last=Sills |first=David |date=2016-11-07 |title=A Unique Cold-Season Supercell Produces an EF1 'Snownado' |url=https://ams.confex.com/ams/28SLS/webprogram/Paper300220.html |language=English |publisher=AMS}}</ref>

Tornado occurrence is highly dependent on the time of day, because of [[solar radiation|solar heating]].<ref name="tornado time of day">{{Cite journal|title=An Augmented Tornado Climatology|last1=Kelly |last2=Schaefer |last3=McNulty|year=1978|journal=Mon. Wea. Rev.|volume=106|issue=8|pages=1172–1183|doi=10.1175/1520-0493(1978)106<1172:AATC>2.0.CO;2 |display-authors=1|bibcode=1978MWRv..106.1172K|last4=Doswell|last5=Abbey|doi-access=free}}</ref> Worldwide, most tornadoes occur in the late afternoon, between 15:00 (3&nbsp;pm) and 19:00 (7&nbsp;pm) local time, with a peak near 17:00 (5&nbsp;pm).<ref>{{cite encyclopedia|url=http://www.britannica.com/eb/article-218362/tornado|title=Tornado: Diurnal patterns|access-date=2009-12-13|year=2007|encyclopedia=Encyclopædia Britannica Online|page=G.6|archive-date=2008-05-02|archive-url=https://web.archive.org/web/20080502195232/http://www.britannica.com/eb/article-218362/tornado|url-status=live}}</ref><ref>{{cite journal|first=A. M.|last=Holzer|year=2000|title=Tornado Climatology of Austria|journal=Atmos. Res.|volume=56|issue=1–4|pages=203–11|url=http://tordach.org/at/Tornado_climatology_of_Austria.html|access-date = 2007-02-27 |archive-url = https://web.archive.org/web/20070219045706/http://tordach.org/at/Tornado_climatology_of_Austria.html |archive-date = 2007-02-19|bibcode=2001AtmRe..56..203H|doi=10.1016/S0169-8095(00)00073-9}}</ref><ref>{{cite journal|first=Nikolai|last=Dotzek |date=2000-05-16 |title=Tornadoes in Germany |journal=Atmos. Res. |volume=56 |issue=1 |pages=233–51 |doi=10.1016/S0169-8095(00)00075-2|bibcode=2001AtmRe..56..233D }}</ref><ref>{{cite web|url=http://www.weathersa.co.za/References/Tornado.jsp |title=South African Tornadoes|access-date=2009-12-13|year=2003|publisher=[[South African Weather Service]] |archive-url = https://web.archive.org/web/20070526105238/http://www.weathersa.co.za/References/Tornado.jsp |archive-date = 2007-05-26}}</ref><ref>{{cite web|first1=Jonathan D.|last1=Finch|first2=Ashraf M.|last2=Dewan|url=http://bangladeshtornadoes.org/climo/btorcli0.htm|title=Bangladesh Tornado Climatology|date=2007-05-23|access-date=2009-12-13|archive-date=2011-07-25|archive-url=https://web.archive.org/web/20110725035701/http://www.bangladeshtornadoes.org/climo/btorcli0.htm|url-status=live}}</ref> Destructive tornadoes can occur at any time of day. The [[Tupelo-Gainesville tornado outbreak|Gainesville Tornado]] of 1936, one of the deadliest tornadoes in history, occurred at 8:30&nbsp;am local time.<ref name="significant tornadoes"/>

The United Kingdom has the highest incidence of tornadoes per unit area of land in the world.<ref>{{Cite web|title=TORRO {{!}} Research ~ Tornadoes ~ Background|url=https://www.torro.org.uk/research/tornadoes/background|access-date=2022-01-20|website=www.torro.org.uk|archive-date=2022-01-20|archive-url=https://web.archive.org/web/20220120123040/https://www.torro.org.uk/research/tornadoes/background|url-status=live}}</ref> Unsettled conditions and weather fronts transverse the British Isles at all times of the years, and are responsible for spawning the tornadoes, which consequently form at all times of the year. The United Kingdom has at least 34 tornadoes per year and possibly as many as 50.<ref>{{cite web|url=http://www.torro.org.uk/tfaq.php#faq4|title=Tornado FAQ's|website=www.torro.org.uk|access-date=2017-03-12|archive-date=2017-03-13|archive-url=https://web.archive.org/web/20170313124451/http://www.torro.org.uk/tfaq.php#faq4|url-status=dead}}</ref> Most tornadoes in the United Kingdom are weak, but they are occasionally destructive. For example, the [[2005 Birmingham tornado|Birmingham tornado of 2005]] and the [[2006 London tornado|London tornado of 2006]] both registered F2 on the Fujita scale and both caused significant damage and injury.<ref>{{Cite news|url=https://www.bbc.co.uk/news/education-33136737|title=UK's 'tornado alley' identified|first=Sean|last=Coughlan|date=15 June 2015|newspaper=BBC News|access-date=22 June 2018|archive-date=22 December 2018|archive-url=https://web.archive.org/web/20181222133510/https://www.bbc.co.uk/news/education-33136737|url-status=live}}</ref>

===Associations with climate and climate change===
[[File:Tornado US annual count.svg|thumb|U. S. annual count of confirmed tornadoes. The count uptick in 1990 is coincident with the introduction of doppler weather radar.]]
Associations with various [[climate]] and environmental trends exist. For example, an increase in the [[sea surface temperature]] of a source region (e.g. Gulf of Mexico and [[Mediterranean Sea]]) increases atmospheric moisture content. Increased moisture can fuel an increase in [[severe weather]] and tornado activity, particularly in the cool season.<ref name="Edwards GoM">{{cite conference|first1=Roger|last1=Edwards|first2=Steven J.|last2=Weiss|author-link1=Roger Edwards (meteorologist)|title=Comparisons between Gulf of Mexico Sea Surface Temperature Anomalies and Southern U.S. Severe Thunderstorm Frequency in the Cool Season|book-title=18th Conf. Severe Local Storms|publisher=American Meteorological Society|date=1996-02-23|url=http://www.spc.noaa.gov/publications/edwards/sstsvr.htm|access-date=2008-01-07|archive-date=2008-05-03|archive-url=https://web.archive.org/web/20080503230451/http://www.spc.noaa.gov/publications/edwards/sstsvr.htm|url-status=live}}</ref>

Some evidence does suggest that the [[Southern Oscillation]] is weakly correlated with changes in tornado activity, which vary by season and region, as well as whether the [[ENSO]] phase is that of [[El Niño]] or [[La Niña]].<ref name="AGU ENSO tor">{{cite conference|first1=Ashton Robinson|last1=Cook|first2=Joseph T.|last2=Schaefer|title=The Relation of El Nino Southern Oscillation (ENSO) to Winter Tornado Outbreaks|book-title=19th Conf. Probability and Statistics|publisher=American Meteorological Society|date=2008-01-22|access-date=2009-12-13|url=http://ams.confex.com/ams/88Annual/techprogram/paper_134378.htm|archive-date=2008-12-06|archive-url=https://web.archive.org/web/20081206065655/http://ams.confex.com/ams/88Annual/techprogram/paper_134378.htm|url-status=live}}</ref> Research has found that fewer tornadoes and hailstorms occur in winter and spring in the U.S. central and southern plains during El Niño, and more occur during La Niña, than in years when temperatures in the [[Pacific]] are relatively stable. Ocean conditions could be used to forecast extreme spring storm events several months in advance.<ref>{{Cite journal|url= http://nature.com/|journal= Nature|title= El Niño brings fewer tornados|volume= 519|date= 26 March 2015|access-date= 27 March 2016|archive-date= 19 July 2016|archive-url= https://web.archive.org/web/20160719125259/http://www.nature.com/|url-status= live}}</ref>

Climatic shifts may affect tornadoes via [[teleconnection]]s in shifting the jet stream and the larger weather patterns. The climate-tornado link is confounded by the forces affecting larger patterns and by the local, nuanced nature of tornadoes. Although it is reasonable to suspect that [[global warming]] may affect trends in tornado activity,<ref name="agw tstm">{{cite journal|first1=Robert J.|last1=Trapp |first2=NS|last2=Diffenbaugh |first3=H. E.|last3=Brooks |first4=M. E.|last4=Baldwin|first5=E. D.|last5=Robinson|first6=J. S.|last6=Pal  |name-list-style=amp |title=Changes in severe thunderstorm environment frequency during the 21st century caused by anthropogenically enhanced global radiative forcing|journal=Proc. Natl. Acad. Sci. U.S.A.|volume=104|issue=50|pages=19719–23|date=2007-12-12|doi=10.1073/pnas.0705494104|bibcode=2007PNAS..10419719T|pmc=2148364|doi-access=free }}</ref> any such effect is not yet identifiable due to the complexity, local nature of the storms, and database quality issues. Any effect would vary by region.<ref name="IPCC4-WGI">{{cite book|first=Susan|last=Solomon |title=Climate Change 2007 – The Physical Science Basis |series=Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change |publisher=Cambridge University Press for the [[Intergovernmental Panel on Climate Change]] |year=2007 |location=Cambridge, UK and New York |access-date=2009-12-13 |url=http://ipcc-wg1.ucar.edu/wg1/wg1-report.html |isbn=978-0-521-88009-1 |display-authors=etal |url-status=dead |archive-url=https://web.archive.org/web/20070501031449/http://ipcc-wg1.ucar.edu/wg1/wg1-report.html |archive-date=2007-05-01 }}</ref>

==Detection==
[[File:Tornado1857.jpg|thumb|right|Path of a tornado across Wisconsin on August 21, 1857]]
{{Main|Convective storm detection}}
Rigorous attempts to warn of tornadoes began in the United States in the mid-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 reach a local weather office after the storm. However, with the advent of weather radar, areas near a local office could get advance warning of severe weather. The first public [[tornado warning]]s were issued in 1950 and the first [[tornado watch]]es and [[convective outlooks]] came about in 1952. In 1953, it was confirmed that [[hook echo]]es were associated with tornadoes.<ref>{{cite web|year=2008|title=The First Tornadic Hook Echo Weather Radar Observations|publisher=Colorado State University|url=http://www.chill.colostate.edu/w/CHILL_history#The_First_Tornadic_Hook_Echo_Weather_Radar_Observations|access-date=2008-01-30|archive-date=2008-08-20|archive-url=https://web.archive.org/web/20080820093538/http://www.chill.colostate.edu/w/CHILL_history#The_First_Tornadic_Hook_Echo_Weather_Radar_Observations|url-status=live}}</ref> By recognizing these radar signatures, meteorologists could detect thunderstorms probably producing tornadoes from several miles away.<ref name="hook echoes">{{cite journal|first=Paul M.|last=Markowski|author-link=Paul Markowski|title=Hook Echoes and Rear-Flank Downdrafts: A Review|journal=Mon. Wea. Rev.|volume=130 |issue=4|pages=852–76|doi=10.1175/1520-0493(2002)130<0852:HEARFD>2.0.CO;2|date=April 2002|bibcode = 2002MWRv..130..852M |s2cid=54785955 |doi-access=free}}</ref>

=== Radar ===
{{See also|Pulse-Doppler radar|weather radar}}
[[File:NapervilleWoodridgeRadarGraphic.png|thumb|right|[[2021 Naperville–Woodridge tornado|A 2021 EF3 tornado in Illinois]] is displayed across various [[NEXRAD]] data types. [[Weather radar#Polarization|Dual-polarization]] and [[Weather radar#Velocity|Doppler velocity]] products have greatly improved forecasters' ability to detect tornadoes while they are ongoing or imminent when no visual confirmation is available.]]
Today most developed countries have a network of weather radars, which serves as the primary method of detecting hook signatures that are likely associated with tornadoes. In the United States and a few other countries, Doppler weather radar stations are used. These devices measure the velocity and radial [[direction (geometry, geography)|direction]] (towards or away from the radar) of the winds within a storm, and so can spot evidence of rotation in storms from over {{convert|100|mi|km|abbr=in|order=flip}} away. When storms are distant from a radar, only areas high within the storm are observed and the important areas below are not sampled.<ref name="airbusradar">{{cite web|url=http://www.skybrary.aero/bookshelf/books/163.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.skybrary.aero/bookshelf/books/163.pdf |archive-date=2022-10-09 |url-status=live|title=Flight Briefing Notes: Adverse Weather Operations Optimum Use of Weather Radar|page=2|author=Airbus|publisher=SKYbrary|date=2007-03-14|access-date=2009-11-19 |author-link=Airbus}}</ref> Data resolution also decreases with distance from the radar. Some meteorological situations leading to tornadogenesis are not readily detectable by radar and tornado development may occasionally take place more quickly than radar can complete a scan and send the batch of data. [[Weather radar#Velocity|Doppler weather radar]] systems can detect [[mesocyclone]]s within a supercell thunderstorm. This allows meteorologists to predict tornado formations throughout thunderstorms.<ref>{{Cite web|url=http://www.nssl.noaa.gov/tools/radar/|title=Research tools: Radar|website=www.nssl.noaa.gov|publisher=NOAA National Severe Storms Laboratory|access-date=October 14, 2016|archive-url=https://web.archive.org/web/20161014053017/http://www.nssl.noaa.gov/tools/radar/|archive-date=2016-10-14|url-status=dead}}</ref>

===Storm spotting===
{{citation needed span|In the mid-1970s, the U.S. [[National Weather Service]] (NWS) increased its efforts to train [[Storm spotting|storm spotters]] so they could spot key features of storms that indicate severe hail, damaging winds, and tornadoes, as well as storm damage and [[flash flood]]ing. The program was called [[Skywarn]], and the spotters were local sheriff's deputies, state troopers, firefighters, ambulance drivers, [[amateur radio operator]]s, [[civil defense]] (now [[emergency management]]) spotters, [[storm chasing|storm chasers]], and ordinary citizens. When severe weather is anticipated, local weather service offices request these spotters to look out for severe weather and report any tornadoes immediately, so that the office can warn of the hazard.|date=December 2021}}

Spotters usually are trained by the NWS on behalf of their respective organizations, and report to them. The organizations activate public warning systems such as [[Civil defense siren|sirens]] and the [[Emergency Alert System]] (EAS), and they forward the report to the NWS.<ref name="spotter history">{{cite journal|first1=Charles A. III|last1=Doswell|first2=Alan R.|last2=Moller|first3=Harold E.|last3=Brooks|title=Storm Spotting and Public Awareness since the First Tornado Forecasts of 1948|journal=Weather Forecast.|volume=14|issue=4|pages=544–57|year=1999|doi=10.1175/1520-0434(1999)014<0544:SSAPAS>2.0.CO;2|bibcode = 1999WtFor..14..544D|url=http://www.flame.org/~cdoswell/publications/stormspotting_99.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.flame.org/~cdoswell/publications/stormspotting_99.pdf |archive-date=2022-10-09 |url-status=live|citeseerx=10.1.1.583.5732}}</ref>
There are more than 230,000 trained Skywarn weather spotters across the United States.<ref name="NWS SKYWARN">{{cite web|url=http://www.weather.gov/skywarn/|date=2009-02-06|author=National Weather Service|publisher=National Oceanic and Atmospheric Administration|title=What is SKYWARN?|access-date=2009-12-13|author-link=National Weather Service|archive-date=2009-12-10|archive-url=https://web.archive.org/web/20091210075145/http://www.weather.gov/skywarn/|url-status=live}}</ref>

In Canada, a similar network of volunteer weather watchers, called [[Canwarn]], helps 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 |access-date=2009-12-13 |publisher=Environment Canada |date=2004-06-02 |url-status=dead |archive-url=https://web.archive.org/web/20100407123441/http://www.mb.ec.gc.ca/air/summersevere/ae00s10.en.html |archive-date=2010-04-07 }}</ref> In Europe, several nations are organizing spotter networks under the auspices of [[Skywarn Europe]]<ref>{{cite web|date=2009-05-31 |url=http://www.skywarn.eu/ |title=Skywarn Europe |author=European Union |access-date=2009-12-13 |author-link=European Union |url-status=dead |archive-url=https://web.archive.org/web/20090917011719/http://www.skywarn.eu/ |archive-date=2009-09-17 }}</ref> and the Tornado and Storm Research Organisation (TORRO) has maintained a network of spotters in the United Kingdom since 1974.<ref>{{cite web|url=http://www.torro.org.uk/site/history.php|title=A Brief History|first=Terence|last=Meaden|year=1985|publisher=Tornado and Storm Research Organisation|access-date=2009-12-13|archive-date=2015-06-26|archive-url=https://web.archive.org/web/20150626221845/http://www.torro.org.uk/site/history.php|url-status=dead}}</ref>

Storm spotters are required because radar systems such as [[NEXRAD]]  detect signatures that suggest the presence of tornadoes, rather than tornadoes as such.<ref name="NSSLsearch">{{cite web|url=http://www.nssl.noaa.gov/primer/tornado/tor_detecting.html|title=Detecting Tornadoes: What Does a Tornado Look Like?|author=National Severe Storms Laboratory|publisher=National Oceanic and Atmospheric Administration|date=2006-11-15|access-date=2009-12-13|author-link=National Severe Storms Laboratory|archive-url=https://web.archive.org/web/20120523170026/http://www.nssl.noaa.gov/primer/tornado/tor_detecting.html|archive-date=2012-05-23|url-status=dead}}</ref> Radar may give a warning before there is any visual evidence of a tornado or an imminent one, but [[ground truth]] from an observer can give definitive information.<ref>{{cite web|url=http://www.stormeyes.org/tornado/verf/|title=Proposals For Changes in Severe Local Storm Warnings, Warning Criteria and Verification|year=2003|access-date=2009-12-13|first1=Roger|last1=Edwards|first2=Elke|last2=Edwards|archive-date=2009-06-28|archive-url=https://web.archive.org/web/20090628132309/http://www.stormeyes.org/tornado/verf/|url-status=live}}</ref> The spotter's ability to see what radar cannot is especially important as distance from the radar site increases, because the radar beam becomes progressively higher in altitude further away from the radar, chiefly due to curvature of Earth, and the beam also spreads out.<ref name="airbusradar"/>

===Visual evidence===
[[File:Wall cloud12 - NOAA.jpg|thumb|right|A rotating [[wall cloud]] with [[rear flank downdraft]] clear slot evident to its left rear]]
Storm spotters are trained to discern whether or not a storm seen from a distance is a supercell. They typically look to its rear, the main region of [[updraft]] and inflow. Under that updraft is a rain-free base, and the next step of [[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.<ref name="Basic Spotter Guide"/>

Evidence of a supercell is based on the storm's shape and structure, and cloud tower features such as a hard and vigorous updraft tower, a persistent, large [[overshooting top]], a hard anvil (especially when backsheared against strong upper level [[wind]]s), and a corkscrew look or [[striation (meteorology)|striations]]. Under the storm and closer to where most tornadoes are found, evidence of a supercell and the likelihood of a tornado includes inflow bands (particularly when curved) such as a "beaver tail", and other clues such as strength of inflow, warmth and moistness of inflow air, how outflow- or inflow-dominant a storm appears, and how far is the front flank precipitation core from the wall cloud. Tornadogenesis is most likely at the interface of the updraft and [[rear flank downdraft]], and requires a balance between the outflow and inflow.<ref name="Advanced Spotter Guide">{{cite web|url=http://www.crh.noaa.gov/oax/skywarn/adv_spotters.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.crh.noaa.gov/oax/skywarn/adv_spotters.pdf |archive-date=2022-10-09 |url-status=live|title=Advanced Spotters' Field Guide|publisher=National Oceanic and Atmospheric Administration|access-date=2009-12-13|date=2003-01-03}}</ref>

Only wall clouds that rotate spawn tornadoes, and they usually precede the tornado between five and thirty minutes. Rotating wall clouds may be a visual manifestation of a low-level 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 afterwards; 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 develop without wall clouds, under flanking lines and on the leading edge. Spotters watch all areas of a storm, and the [[cloud base]] and surface.<ref name="NSSL tornadoes">{{cite web|title=Questions and Answers about Tornadoes|work=A Severe Weather Primer|publisher=National Severe Storms Laboratory|date=2006-11-15|url=http://www.nssl.noaa.gov/primer/tornado/tor_basics.html|access-date=2007-07-05|archive-url=https://web.archive.org/web/20120809070939/http://www.nssl.noaa.gov/primer/tornado/tor_basics.html|archive-date=2012-08-09|url-status=dead}}</ref>

==Extremes==
{{Main|Tornado records}}
[[File:Pilger, NE tornadoes.jpg|thumb|266x266px|Twin EF4 tornadoes near [[Pilger, Nebraska]] in 2014]]
The tornado which holds most records in history was the [[Tri-State Tornado]], which roared through parts of [[Missouri]], [[Illinois]], and [[Indiana]] on March 18, 1925. It was likely an F5, though tornadoes were not ranked on any scale in that era. It holds records for longest path length ({{convert|219|mi|km|disp=semicolon}}), longest duration (about 3.5 hours), and fastest forward speed for a significant tornado ({{convert|73|mph|km/h|disp=semicolon|abbr=on}}) anywhere on Earth. In addition, it is the deadliest single tornado in United States history (695 dead).<ref name="significant tornadoes"/> The tornado was also the costliest tornado in history at the time (unadjusted for inflation), but in the years since has been surpassed by several others if population changes over time are not considered. When costs are normalized for wealth and inflation, it ranks third today.<ref name="tornado damage cost">{{cite journal|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|journal=Weather Forecast.|volume=16|pages=168–176|access-date=2007-02-28|first1=Harold E.|last1=Brooks|first2=Charles A. III|last2=Doswell|date=2000-10-01|issue=1|author-link1=Harold E. Brooks|doi=10.1175/1520-0434(2001)016<0168:ndfmti>2.0.co;2|bibcode=2001WtFor..16..168B|doi-access=free|archive-date=2007-02-08|archive-url=https://web.archive.org/web/20070208202421/http://www.nssl.noaa.gov/users/brooks/public_html/damage/tdam1.html|url-status=live}}</ref>

The deadliest tornado in world history was the [[Daultipur-Salturia Tornado]] in Bangladesh on April 26, 1989, which killed approximately 1,300 people.<ref name="Bangladesh tornado"/> Bangladesh has had at least 24 tornadoes in its history that killed more than 100 people, almost half of the [[Tornado records|rest of the world]].<ref>{{cite web |last1=Finch |first1=Jonathan |last2=Dewan |first2=Ashraf |title=Bangladesh Tornado Climatology |url=https://bangladeshtornadoes.org/climo/btorcli0.htm |website=bangladeshtornadoes.org |access-date=13 February 2025}}</ref><ref>{{cite web |title=List of 86 tornados in Bengal for 1838-2001 |url=https://bangladeshtornadoes.org/climo/TORCLIMB.htm |website=bangladeshtornadoes.org}}</ref>

One of the most extensive [[tornado outbreak]]s on record was the [[1974 Super Outbreak]], which affected a large area of the central United States and extreme southern [[Ontario]] on April 3 and 4, 1974. The outbreak featured 148 tornadoes in 18 hours, many of which were violent; seven were of F5 intensity, and twenty-three peaked at F4 strength. Sixteen tornadoes were on the ground at the same time during its peak. More than 300&nbsp;people, possibly as many as 330, were killed.<ref name="super outbreak">{{cite web|url=https://repository.library.noaa.gov/view/noaa/18850|title=Tornado Outbreak of April 3–4, 1974; Synoptic Analysis|access-date=2009-12-13|first1=Lee R.|last1=Hoxit|first2=Charles F.|last2=Chappell|date=1975-11-01|format=PDF|publisher=[[National Oceanic and Atmospheric Administration]]|archive-date=2020-09-30|archive-url=https://web.archive.org/web/20200930030334/https://repository.library.noaa.gov/view/noaa/18850|url-status=live}}</ref>

While direct measurement of the most violent tornado wind speeds is nearly impossible, since conventional [[anemometer]]s would be destroyed by the intense winds and flying debris, some tornadoes have been scanned by [[Doppler on Wheels|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&nbsp;±&nbsp;20&nbsp;mph (484&nbsp;±&nbsp;32&nbsp;km/h) in the F5 [[1999 Bridge Creek – Moore tornado|Bridge Creek-Moore, Oklahoma]], tornado which killed 36 people.<ref>[http://newsok.com/anatomy-of-may-3s-f5-tornado/article/3365909 Anatomy of May 3's F5 tornado] {{Webarchive|url=https://web.archive.org/web/20100523054215/http://newsok.com/anatomy-of-may-3s-f5-tornado/article/3365909 |date=2010-05-23 }}, The Oklahoman Newspaper, May 1, 2009</ref> The reading was taken about {{convert|100|ft|m}} above the ground.<ref name="fastest wind"/>

Storms that produce tornadoes can feature intense updrafts, sometimes exceeding {{convert|150|mi/h|km/h|abbr=on}}. 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 {{convert|80|mi|km}} from the town, a sack of flour was found {{convert|110|mi|km}} away, and a cancelled check from the Great Bend bank was found in a field outside of [[Palmyra, Nebraska]], {{convert|305|mi|km}} to the northeast.<ref name="tornado project oddities">{{cite web|url=http://www.tornadoproject.com/oddities/oddities.htm|title=Tornado Oddities |access-date=2009-12-13 |first1=Thomas P.|last1=Grazulis |date=2005-09-20 |url-status=dead |archive-url=https://web.archive.org/web/20090507004425/http://www.tornadoproject.com/oddities/oddities.htm |archive-date=2009-05-07 }}</ref> Waterspouts and tornadoes have been advanced as an explanation for instances of [[Rain of animals|raining fish and other animals]].<ref>{{cite news|url=https://www.usatoday.com/weather/resources/askjack/archives-tornado-history.htm|title=Q: You've probably heard the expression, "it's raining cats and dogs." Has it ever rained animals?|first=Emily|last=Yahr|date=2006-02-21|newspaper=USA Today|access-date=2009-12-13|archive-date=2010-05-24|archive-url=https://web.archive.org/web/20100524040429/http://www.usatoday.com/weather/resources/askjack/archives-tornado-history.htm|url-status=live}}</ref>

== Safety ==
{{Main|Tornado preparedness}}
[[File:Birmingham tornado 2005 damage.jpg|thumb|right|Damage from the [[Birmingham tornado of 2005]]. An unusually strong example of a tornado event in the [[United Kingdom]], the Birmingham Tornado resulted in 19 injuries, mostly from falling trees.]]
Though tornadoes can strike in an instant, there are precautions and preventative measures that can be taken to increase the chances of survival. Authorities such as the [[Storm Prediction Center]] in the United States advise having a pre-determined plan should a tornado warning be issued. When a 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|date=2008-07-16|access-date=2009-11-17|first=Roger|last=Edwards|author-link=Roger Edwards (meteorologist)|work=[[National Weather Service]]|publisher=National Oceanic and Atmospheric Administration|archive-date=2009-08-25|archive-url=https://web.archive.org/web/20090825014040/http://www.spc.noaa.gov/faq/tornado/safety.html|url-status=live}}</ref> In tornado-prone areas, many buildings have underground [[storm cellar]]s, which have saved thousands of lives.<ref>{{cite web|url=http://www.srh.noaa.gov/hun/preparedness/brochures/storm_shelter.pdf |title=Storm Shelters|access-date=2009-12-13|work=[[National Weather Service]]|publisher=National Oceanic and Atmospheric Administration|date=2002-08-26|archive-url = https://web.archive.org/web/20060223072127/http://www.srh.noaa.gov/hun/preparedness/brochures/storm_shelter.pdf |archive-date = 2006-02-23}}</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, 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 one of the worst places to take shelter during tornadoes, as the constricted space can be subject to increased wind speed and funneling of debris underneath the overpass.<ref name="highway overpasses"/>

==Myths and misconceptions==
{{Main|Tornado myths}}
Folklore often identifies a green sky with tornadoes, and though the phenomenon may be associated with severe weather, there is no evidence linking it specifically with tornadoes.<ref>{{cite journal |last=Knight |first=Meredith |url=http://www.scientificamerican.com/article.cfm?id=fact-or-fiction-if-sky-is-green-run-for-cover-tornado-is-coming |title=Fact or Fiction?: If the Sky Is Green, Run for Cover – A Tornado Is Coming |journal=Scientific American |date=2011-04-18 |access-date=2012-09-03 |archive-date=2012-10-14 |archive-url=https://web.archive.org/web/20121014160521/http://www.scientificamerican.com/article.cfm?id=fact-or-fiction-if-sky-is-green-run-for-cover-tornado-is-coming |url-status=live }}</ref> It is often thought that opening windows will lessen the damage caused by the tornado. While there is a large drop in [[atmospheric pressure]] inside a strong tornado, the pressure difference is unlikely to cause significant damage. Opening windows may instead increase the severity of the tornado's damage.<ref name="tornado project myths"/> A violent tornado can destroy a house whether its windows are open or closed.<ref name="tornado project myths">{{cite web|url=http://www.tornadoproject.com/safety/myths.htm|title=Myths and Misconceptions about Tornadoes|access-date=2007-02-28|first=Tim|last=Marshall|publisher=The Tornado Project|date=2005-03-15|archive-date=2013-06-08|archive-url=https://web.archive.org/web/20130608030946/http://www.tornadoproject.com/safety/myths.htm|url-status=live}}</ref><ref name="tornado myths">{{cite book|first=Thomas P.|last=Grazulis|title=The tornado : nature's ultimate windstorm|url=https://archive.org/details/tornadonaturesul0000graz|url-access=registration|year=2001|publisher=University of Oklahoma Press|isbn=0-8061-3258-2|chapter=Tornado Myths|ref={{harvid|Grazulis|2001}}|accessdate=2009-11-20}}</ref>
Another commonly held misconception is that highway overpasses provide adequate shelter from tornadoes. This belief is partly inspired by widely circulated video captured during the [[April 26, 1991 tornado outbreak|1991 tornado outbreak]] near [[Andover, Kansas]], where a news crew and several other people took shelter under an overpass on the [[Kansas Turnpike]] and safely rode out a tornado as it passed nearby.<ref name=noaa-overpass>{{cite web|author=National Weather Service Forecast Office|location=Dodge City, Kansas|title=Overpasses and Tornado Safety: Not a Good Mix|url=http://www.crh.noaa.gov/ddc/?n=over|work=Tornado Overpass Information|publisher=NOAA|access-date=24 March 2012|archive-url=https://web.archive.org/web/20120107153850/http://www.crh.noaa.gov/ddc/?n=over|archive-date=7 January 2012|url-status=dead}}</ref> However, a highway overpass is a dangerous place during a tornado, and the subjects of the video remained safe due to an unlikely combination of events: the storm in question was a weak tornado, the tornado did not directly strike the overpass,<ref name=noaa-overpass/> and the overpass itself was of a unique design. Due to the [[Venturi effect]], tornadic winds are accelerated in the confined space of an overpass.<ref>{{cite web|url=http://www.ncdc.noaa.gov/oa/climate/severeweather/tornadosafety.html|title=Tornado Myths, Facts, and Safety|publisher=National Climatic Data Center|date=2006-08-17|author=Climate Services and Monitoring Division|access-date=2012-03-27|archive-url=https://web.archive.org/web/20120314063308/http://www.ncdc.noaa.gov/oa/climate/severeweather/tornadosafety.html|archive-date=2012-03-14|url-status=dead}}</ref> Indeed, in the [[1999 Oklahoma tornado outbreak]] of May 3, 1999, three highway overpasses were directly struck by tornadoes, and at each of the three locations there was a fatality, along with many life-threatening injuries.<ref name="highway overpass danger">{{cite news|url=https://www.usatoday.com/weather/resources/basics/tornado-underpass.htm|archive-url=https://web.archive.org/web/20050408091759/http://www.usatoday.com/weather/resources/basics/tornado-underpass.htm|url-status=dead|archive-date=2005-04-08|title=Overpasses are tornado death traps|access-date=2007-02-28|first=Chris|last=Cappella|date=2005-05-17|newspaper=USA Today}}</ref> By comparison, during the same tornado outbreak, more than 2,000 homes were completely destroyed and another 7,000 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/?n=safety-overpass |title=Highway Overpasses as Tornado Shelters|access-date=2007-02-28|date=2000-03-01|work=[[National Weather Service]]|publisher=National Oceanic and Atmospheric Administration |archive-url = https://web.archive.org/web/20000616093920/http://www.srh.noaa.gov/oun/papers/overpass.html |archive-date = 2000-06-16}}</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 in a basement, under a staircase, or under a sturdy piece of furniture such as a workbench further increases the chances of survival.<ref name="tornado project myths"/><ref name="tornado myths"/>

There are areas which people believe to be protected from tornadoes, whether by being in a city, near a major river, hill, or mountain, or even protected by [[supernatural]] forces.<ref>{{cite web|title=Tornado Myths & Tornado Reality|url=http://www.hprcc.unl.edu/nebraska/tornado-myths.html|first=Kenneth F.|last=Dewey|publisher=High Plains Regional Climate Center and [[University of Nebraska–Lincoln]]|date=2002-07-11|access-date=2009-11-17|archive-url = https://web.archive.org/web/20080611013128/http://www.hprcc.unl.edu/nebraska/tornado-myths.html |archive-date = June 11, 2008|url-status=dead}}</ref> Tornadoes have been known to cross major rivers, climb mountains,<ref name="Tornadoes in mountains">{{cite web|url=http://tornado.sfsu.edu/RockwellPassTornado/index.html|title=Tornado, Rockwell Pass, Sequoia National Park, 2004-07-07|first1=John|last1=Monteverdi|first2=Roger|last2=Edwards|first3=Greg|last3=Stumpf|first4=Daniel|last4=Gudgel|date=2006-09-13|access-date=2009-11-19|archive-url=https://web.archive.org/web/20150819041516/http://tornado.sfsu.edu/RockwellPassTornado/index.html|archive-date=2015-08-19|url-status=dead}}</ref> affect valleys, and have damaged [[List of tornadoes striking downtown areas|several city centers]]. As a general rule, no area is safe from tornadoes, though some areas are more susceptible than others.<ref name="Handy Weather Answer Book"/><ref name="tornado project myths"/><ref name="tornado myths"/>

==Ongoing research==
{{main|History of tornado research}}
[[File:Tornado with DOW.jpg|thumb|right|A [[Doppler on Wheels]] unit observing a [[Tornadoes of 2004#May 12|tornado]] near [[Attica, Kansas]]]]
Meteorology is a relatively young science and the study of tornadoes is newer still. Although researched for about 140 years and intensively so 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|access-date=2007-02-28|author=National Severe Storms Laboratory|publisher=National Oceanic and Atmospheric Administration|date=2006-10-30|author-link=National Severe Storms Laboratory|archive-url=https://web.archive.org/web/20121103055533/http://www.nssl.noaa.gov/education/|archive-date=2012-11-03|url-status=dead}}</ref> Meteorologists have a fairly good understanding of the development of [[thunderstorm]]s and mesocyclones,<ref name="Extreme Weather">{{cite book|title=Extreme Weather|first=Michael H.|last=Mogil|year=2007|publisher=Black Dog & Leventhal Publisher|location=New York|isbn=978-1-57912-743-5|pages=[https://archive.org/details/extremeweatherun0000mogi/page/210 210–11]|url=https://archive.org/details/extremeweatherun0000mogi/page/210}}</ref><ref>{{cite web|title=Mesocyclone Climatology Project |publisher=University of Oklahoma |first=Kevin|last=McGrath |date=1998-11-05 |access-date=2009-11-19 |url=http://mesocyclone.ou.edu/ |url-status=dead |archive-url=https://web.archive.org/web/20100709035137/https://mesocyclone.ou.edu/ |archive-date=2010-07-09 }}</ref> and the meteorological conditions conducive to their formation. However, the step from [[supercell]], or other respective formative processes, to [[tornadogenesis]] and the prediction of tornadic vs. non-tornadic mesocyclones is not yet well known and is the focus of much research.<ref name="LouieLouie"/>

Also under study are the low-level mesocyclone and the [[vortex stretching|stretching]] of low-level [[vorticity]] which tightens into a tornado,<ref name="LouieLouie"/> in particular, 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.<ref name="Seymour 2001 32">{{Cite book |last = Seymour |first = Simon |title = Tornadoes |publisher = [[HarperCollins]] |year = 2001 |location = New York City |page = [https://archive.org/details/tornadoes00seym/page/32 32] |isbn = 0-06-443791-4 |url = https://archive.org/details/tornadoes00seym/page/32 }}</ref>

In particular, the role of [[downdraft]]s, particularly the [[rear-flank downdraft]], and the role of [[baroclinic]] boundaries, are intense areas of study.<ref>{{harvnb|Grazulis|2001|pages=[https://archive.org/details/tornadonaturesul0000graz/page/63 63]–65}}</ref>

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 [[:wikt:mesovortex|mesovortices]] within linear thunderstorm structures and within tropical cyclones.<ref name="tornado forecasting">{{cite web|last=Rasmussen|first=Erik|author-link=Erik N. Rasmussen|url=http://cimms.ou.edu/~erik/Tornadoes/Forecasting/Detailed/Detailed.htm|title=Severe Storms Research: Tornado Forecasting|access-date=2007-03-27|date=2000-12-31|publisher=Cooperative Institute for Mesoscale Meteorological Studies|archive-url = https://web.archive.org/web/20070407031255/http://cimms.ou.edu/~erik/Tornadoes/Forecasting/Detailed/Detailed.htm |archive-date = April 7, 2007|url-status=dead}}</ref>

Meteorologists still do not know the exact mechanisms by which most tornadoes form, and occasional tornadoes still strike without a tornado warning being issued.<ref>{{cite web|url=http://www.epa.gov/naturalevents/tornadoes.html|title=Tornadoes|date=2009-09-30|access-date=2009-11-20|author=United States Environmental Protection Agency|archive-date=2012-05-12|archive-url=https://web.archive.org/web/20120512005833/http://www.epa.gov/naturalevents/tornadoes.html|url-status=live}}</ref> 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. [[Mathematical model|Numerical modeling]] also provides new insights as observations and new discoveries are integrated into our physical understanding and then tested in [[computer simulation]]s 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.<ref>{{harvnb|Grazulis|2001|pages=[https://archive.org/details/tornadonaturesul0000graz/page/65 65]–69}}</ref>

Research programs, including field projects such as the [[VORTEX projects]] (Verification of the Origins of Rotation in Tornadoes Experiment), 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="field programs history">{{cite journal|first=Howard|last=Bluestein|title=A History of Severe-Storm-Intercept Field Programs|journal=Weather Forecast.|pages=558–77|volume=14|issue=4|doi=10.1175/1520-0434(1999)014<0558:AHOSSI>2.0.CO;2|year=1999|bibcode = 1999WtFor..14..558B |doi-access=free}}</ref> 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]].<ref name="NSSLsearch"/><ref>{{cite web|url=http://www.ncar.ucar.edu/research/meteorology/storms/tornadoes.php|title=Tornadoes|author=National Center for Atmospheric Research|year=2008|access-date=2009-11-20|publisher=University Corporation for Atmospheric Research|author-link=National Center for Atmospheric Research|archive-url=https://web.archive.org/web/20100423151801/http://www.ncar.ucar.edu/research/meteorology/storms/tornadoes.php|archive-date=2010-04-23|url-status=dead}}</ref> The pace of research is partly constrained by the number of observations that can be taken; gaps in information about the wind, pressure, and moisture content throughout the local atmosphere; and the computing power available for simulation.<ref>{{cite news|url=https://www.npr.org/templates/story/story.php?storyId=125767948|title=Scientists Chase Tornadoes to Solve Mysteries|newspaper=NPR.org|date=2010-04-09|access-date=2014-04-26|archive-date=2014-04-26|archive-url=https://web.archive.org/web/20140426215245/http://www.npr.org/templates/story/story.php?storyId=125767948|url-status=live}}</ref>

Solar storms similar to tornadoes have been recorded, but it is unknown how closely related they are to their terrestrial counterparts.<ref>{{cite web |url=http://www.physorg.com/news/2012-03-huge-tornadoes-sun.html |title=Huge tornadoes discovered on the Sun |publisher=Physorg.com |access-date=2012-09-03 |archive-date=2024-06-04 |archive-url=https://web.archive.org/web/20240604063415/https://phys.org/news/2012-03-huge-tornadoes-sun.html |url-status=live }}</ref>

==Gallery==
===Photos===
<gallery>
File:Rope Tornado near Yuma, Colorado.jpg|Rope Tornado near [[Yuma, Colorado]] on August 8, 2023.
File:Tornado near Punkin Center, CO.jpg|A wall cloud with tornado South of [[Limon, Colorado]].
File:NorthSeaTornadoEF0.jpg| F0 tornado in its final stages over the North Sea near Vrångö, Sweden on July 17, 2011.
File:Tornado Roping Out near Merino, CO.jpg| A tornado in its roping out stage near the town of Merino, Colorado during a [[Tornado outbreak sequence of June 20–26, 2023#June_21_event|historic tornado outbreak]] that produced 37 tornadoes across the state.
</gallery>

===Videos===
<gallery>
File:Andover Tornado, April 29, 2022, City Hall East Camera.webm|The [[2022 Andover tornado]] seen from the Andover City Hall camera.
File:Tornado time lapse.webm|Time-lapse of a tornado's life cycle near [[Prospect Valley, Colorado]] on June 19, 2018.
</gallery>
===Radar===
<gallery>
File:Tornadic classic supercell radar.gif|A radar reflectivity image of a classic tornadic supercell near [[Oklahoma City, Oklahoma]] on May 3, 1999.
File:NEXRAD radar of an EF2 tornado in Kansas on March 13, 2024.png|Classic hook echo can be seen for this [[Tornado_outbreak_of_March_13–15,_2024|Kansas EF2 tornado]] in 2024
File:05june-dow7-wide.gif|A [[Doppler on Wheels]] image of a tornadic thunderstorm near [[La Grange, Wyoming]] (USA) captured during the [[VORTEX2]] project. In the velocity image on the left, Blues/green represent winds moving towards the radar, and reds/yellows indicate winds moving away from the radar. In the reflectivity image on the right, the main body of the storm can be seen, with the appendage on the bottom of the storm being a hook echo.
</gallery>

==See also==
{{portal|Tornadoes|Weather}}

{{Div col|colwidth=25em}}
* [[Cultural significance of tornadoes]]
* [[Cyclone]]
* [[Derecho]]
* [[List of tornadoes and tornado outbreaks]]
**[[List of F5 and EF5 tornadoes]]
**[[List of F4 and EF4 tornadoes]]
***[[List of F4 and EF4 tornadoes (2020–present)]]
* [[List of tropical cyclone-spawned tornadoes]]
* [[List of tornadoes with confirmed satellite tornadoes]]
* [[Secondary flow]]
* [[Skipping tornado]]
* [[Space tornado]]
* [[Tornado preparedness]]
* [[Tornadoes of {{#time: Y }}]]
* [[Tropical cyclone]]
* [[Hypercane]]
* [[Typhoon]]
* [[Vortex]]
* [[Whirlwind]]
{{Div col end}}

==References==
{{Reflist}}

==Further reading==
* {{cite book|first=Howard B.|last=Bluestein|title=Tornado Alley: Monster Storms of the Great Plains|url=https://archive.org/details/tornadoalleymons0000blue|url-access=registration|publisher=Oxford University Press|location=New York |year=1999|isbn=0-19-510552-4}}
* {{cite book|first=Marlene|last=Bradford|year=2001|title=Scanning the Skies: A History of Tornado Forecasting|publisher=University of Oklahoma Press|location=Norman, OK|isbn=0-8061-3302-3|url=https://archive.org/details/scanningskieshis0000brad}}
* {{cite book|first=Thomas P.|last=Grazulis|date=January 1997|title=Significant Tornadoes Update, 1992–1995|publisher=Environmental Films|location=St. Johnsbury, VT|isbn=1-879362-04-X}}
* {{cite journal|last=Pybus|first=Nani|title='Cyclone' Jones: Dr. Herbert L. Jones and the Origins of Tornado Research in Oklahoma|journal=Chronicles of Oklahoma|volume=94|date=Spring 2016|pages=4–31|url=https://gateway.okhistory.org/ark:/67531/metadc1725816/m1/|access-date=May 5, 2022}} Heavily illustrated.

==External links==
{{Sister project links |wikt= |commons= |b= |n= |q= |s= |v= |voy=Tornado safety |species=no |d=no |display=Tornado }}
* [http://www.ncdc.noaa.gov/stormevents/ NOAA Storm Events Database 1950–present]
* [http://www.essl.org/ESWD European Severe Weather Database]
* [http://celebrating200years.noaa.gov/breakthroughs/tornadowarnings/welcome.html Tornado Detection and Warnings]
* [http://www.ejssm.org/ Electronic Journal of Severe Storms Meteorology]
* [https://web.archive.org/web/20080624204058/http://www.nssl.noaa.gov/edu/safety/tornadoguide.html NOAA Tornado Preparedness Guide]
* {{usurped|1=[https://web.archive.org/web/20070110234159/http://www.tornadohistoryproject.com/ Tornado History Project – Maps and statistics from 1950 to present]}}
* "[http://scitation.aip.org/content/aip/magazine/physicstoday/article/67/9/10.1063/PT.3.2514 What we know and don’t know about tornado formation]", ''Physics Today'', September 2014
* [http://www.ncdc.noaa.gov/billions U.S. Billion-dollar Weather and Climate Disasters]

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