Editing Tornado (section)

==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>