Tornado climatology

Tornadoes have been recorded on all continents except Antarctica and are most common in the middle latitudes where conditions are often favorable for convective storm development.[1] The United States has the most tornadoes of any country, as well as the strongest and most violent tornadoes. A large portion of these tornadoes form in an area of the central United States popularly known as Tornado Alley.[2] Canada experiences the second most tornadoes. Ontario and the prairie provinces see the highest frequency, particularly with southward extent. Other areas of the world that have frequent tornadoes include significant portions of Europe, South Africa, Philippines, Bangladesh, parts of Argentina, Uruguay, and southern and southeast Brazil, northern Mexico, New Zealand, and far eastern Asia.[3][4]

Areas worldwide with the highest frequency of tornadoes are indicated by orange shading.

The United States averaged 1,274 tornadoes per year in the last decade while Canada reports nearly 100 annually (largely in the southern regions). However, the UK has the most tornadoes per area per year, 0.14 per 1000 km², although these tornadoes are generally weak, and many other European countries have a similar number of tornadoes per area.[1]

The severity of tornadoes is commonly measured by the Enhanced Fujita Scale, which scales tornado intensity from EF0 to EF5 by wind speed and the amount of damage they do to human environments. These judgments are made after the tornado has dissipated and the damage trail is carefully studied by weather professionals.[5]

Tornadoes are most common in spring and least common in winter.[6] The seasonal transition during autumn and spring promotes the development of extratropical cyclones and frontal systems that support strong convective storms. Tornadoes are also common in landfalling tropical cyclones, where they are focused in the right poleward section of the cyclone. Tornadoes can also be spawned as a result of eyewall mesovortices, which persist until landfall.[7] However, favorable conditions for tornado development can occur any time of the year.

Tornado occurrence is highly dependent on the time of day, because of solar heating.[8] Worldwide, most tornadoes occur in the late afternoon, between 3 pm and 7 pm local time, with a peak near 5 pm.[9][10][11][12][13] Destructive tornadoes can occur at any time of day, as evidenced by the Gainesville Tornado of 1936 (one of the deadliest tornadoes in history) that occurred at 8:30 p.m. local time.[6]

Geography

The United States has the most tornadoes of any country. Many of these form in an area of the central (with some definitions including Southern) United States known as Tornado Alley.[14] This area extends into Canada, particularly the Prairie Provinces and Ontario; however, activity in Canada is less frequent and intense than that of the US. The high frequency of tornadoes in North America is largely due to geography, as moisture from the Gulf of Mexico is easily advected into the midcontinent with few topographic barriers in the way. The Rocky Mountains block Pacific-sourced moisture and buckle the atmospheric flow, forcing drier air at mid-levels of the troposphere due to downsloping winds and causing cyclogenesis downstream to the east of the mountains. Downsloping winds off the Rockies force the formation of a dry line when the flow aloft is strong, while the Gulf of Mexico fuels abundant low-level moisture. This unique topography allows for frequent collisions of warm and cold air, the conditions that breed strong, long-lived storms throughout the year. This area extends into Canada, particularly Ontario and the Prairie Provinces, and strong tornadoes can also occur in northern Mexico.

A large region of South America is characterized by storms that reach the level of supercells and produce intense hailstorms, floods, and tornadoes during the spring, summer, and early fall. The region recently appointed as the Tornado Corridor (South America) is considered as the second largest in the world in terms of the formation of extreme weather events. It covers most of central Argentina, southern Paraguay, southeastern Brazil, and Uruguay.

Bangladesh and surrounding areas of eastern India suffer from a couple tornadoes annually of similar severity to stronger tornadoes in the US. These occur with a greater recurrence interval (although over a smaller region), and tend to be under-reported due to the scarcity of media coverage of a developing country. The annual human death toll from tornadoes in Bangladesh is estimated at about 179 deaths per year, which is much greater than in the US. This is likely due to the density of population, poor quality of construction, lack of tornado safety knowledge and warnings, and other factors.[15]

Other areas of the world that have frequent strong tornadoes include Germany, the Czech Republic, Slovakia, Italy, Spain, China, and the Philippines. Australia, France, Russia, areas of the Middle East, Japan, and parts of Mexico have a history of multiple damaging tornado events.

Tornadoes in the United States
Average annual tornado reports in the United States
Further information: Tornadoes in the United States

The United States averaged 1,274 tornadoes per year in the last decade. April 2011 saw the most tornadoes ever recorded for any month in the US National Weather Service's history, 875; the previous record was 542 in one month.[16] It has more tornadoes yearly than any other country and reports more violent (F4 and F5) tornadoes than anywhere else.

Tornadoes are common in many states but are most common to the west of the Appalachian Mountains and to the east of the Rockies. The Atlantic seaboard states – North Carolina, South Carolina, Georgia and Virginia – are also very vulnerable, as well as Florida. The areas most vulnerable to tornadoes are the Southern Plains and Florida, though most Florida tornadoes are relatively weak. The Southern United States is one of the worst-affected regions in terms of casualties.

More tornadoes occur in Texas than in any other US state, which is due to its large size, and its location on the southern end of Tornado Alley and the occurrence of tropical cyclone related tornadoes. The state which has the highest number of tornadoes per unit area is Florida, although most of the tornadoes in Florida are weak tornadoes of EF0 or EF1 intensity. A number of Florida's tornadoes occur along the edge of hurricanes that strike the state. The state with the highest number of strong tornadoes per unit area is Oklahoma. The neighboring state of Kansas is also a particularly notorious tornado state. It records the most EF4 and EF5 tornadoes in the country.

Tornado reports have been officially collated since 1950. These reports have been gathered by the National Climatic Data Center (NCDC), based in Asheville, North Carolina. A tornado can be reported more than once, such as when a storm crosses a county line and reports are made from two counties.

Common misconceptions

Some people mistakenly believe that tornadoes only occur in the countryside. This is hardly the case. While it is true that the plains states are tornado-prone, tornadoes have been reported in every U.S state, including Alaska and Hawaii. One likely reason tornadoes are so common in the central U.S is because this is where Arctic air, cold fronts that have not been "weakened" yet first collide with warm tropical air from the Gulf of Mexico. As these fronts head further east, they sometimes lose their strength as they travel over more warm air. For this reason, tornadoes are not as common on the East Coast as they are in the Midwest. However, they have happened on rare occasion, such as the F3 twister that struck Limerick Township, Pennsylvania near Philadelphia on 27 July 1994,[17] the F2 twister that struck the northern suburbs of New York City on 12 July 2006,[18] the EF2 twister in the borough of Brooklyn on 8 August 2007, or the F4 twister that struck La Plata, Maryland on 28 April 2002.

Another popular misconception is that tornadoes don't occur in mountainous areas, or that mountainous terrain will protect people from tornadoes. While the frequency of tornadoes is lower in mountainous areas compared to Tornado Alley, significant tornadoes can occur regularly in the southern Appalachian region of the United States [19] and have even been documented at high altitudes along the continental divide of Wyoming.[20] In addition, tornado outbreaks occasionally occur around the southern Appalachian mountains [21] with the 1974 Super Outbreak and the 2011 Super Outbreak directly affecting the southern Appalachian region.[22]

Tornadoes can occur west of the continental divide, but they are infrequent and usually relatively weak and short-lived. Recently, tornadoes have struck the Pacific coast town of Lincoln City, Oregon (1996), Sunnyvale, California (1998), Port Orchard, Washington (2018), and downtown Salt Lake City, Utah (1999 - see Salt Lake City Tornado). The California Central Valley is an area of some frequency for tornadoes, albeit of weak intensity. Though tornadoes that occur on the Western Seaboard are typically weak, more powerful and damaging tornadoes, such as the tornado that occurred on 22 May 2008 in Perris, California, can also occur.[23]

Tornadoes in Canada

Canada also experiences numerous tornadoes, although fewer than the United States. On average 62 are reported per year, but this number is expected to be higher due to undetected tornadoes in large expanses of underpopulated areas.[24] NOAA has a higher average 100 per year in Canada.[25] This causes tens of millions of dollars in damage. Most are weak F0 or F1 in intensity, but there are on average a few F2 or stronger that touch down each season.

For example, the tornado frequency of Southwestern Ontario is about half that of the most tornado-prone areas of the central US plains. The last multiple tornado-related deaths in Canada were caused by a tornado in Ear Falls, Ontario on 9 July 2009, where 3 died, and the last killer tornado was on 3 August 2018 in Alonsa, Manitoba.[26] The two deadliest tornadoes on Canadian soil were the Regina Cyclone of 30 June 1912 (28 fatalities) and the Edmonton Tornado of 31 July 1987 (27 fatalities). Both of these storms were rated an F4 on the Fujita scale. The city of Windsor was struck by strong tornadoes four times within a 61-year span (1946, 1953, 1974, 1997) ranging in strength from an F2 to F4. Windsor has been struck by more significant tornadoes than any other city in Canada. Canada's first official F5 tornado struck Elie, Manitoba on 22 June 2007.[27] Tornadoes are most frequent in the provinces of Alberta, Saskatchewan, Manitoba, and Ontario.

Europe

Europe has about 300 tornadoes per year[28] – much more than estimated by Alfred Wegener in his classic book Wind- und Wasserhosen in Europa ("Tornadoes and Waterspouts in Europe"). They are most common in June–August, especially in the inlands – rarest in January–March. Strong and violent tornadoes (F3–F5) do occur, especially in some of the interior areas and in the south – but are not as common as in parts of the US. As in the US, tornadoes are far from evenly distributed. Europe has some small "tornado alleys" – probably because of frontal collisions as in the south and east of England,[29] but also because Europe is partitioned by mountain ranges like the Alps. Parts of Styria (Steiermark) in Austria may be such a tornado alley, and this county has had at least three F3 tornadoes since 1900.[30] F3 and perhaps one F4 tornado have occurred as far north as Finland.

Since 1900, deadly tornadoes have occurred in Austria, Belgium, Cyprus, Finland, France, Germany, Italy (such as the F5/T10 of Udine-Treviso on 24 July 1930, which killed 23 people,[31]), Lithuania, Malta, the Netherlands, Poland, Ukraine, Portugal (such as the F3/T7 of Castelo Branco on 6 November 1954, which killed 5 and injured 220), Romania, Russia and the United Kingdom. The 1984 Ivanovo–Yaroslavl outbreak, with more than 400 fatalities and 213 injured, was the century's deadliest tornado or outbreak in Europe. It included at least one F5 and one F4. Europe's perhaps deadliest tornado ever (and probably one of the World's deadliest tornadoes) hit Malta in 1551 (or 1556) and killed about 600.

One notable tornado of recent years was the Birmingham Tornado (UK) which struck Birmingham, United Kingdom, in July 2005. A row of houses was destroyed, but no one was killed. A strong F3 (T7) Tornado hit the small town Micheln in Saxony-Anhalt, Germany on 23 July 2004 leaving 6 people injured and more than 250 buildings massively damaged.[32] Another notable Tornado was a strong wedge Multivortex Tornado that hit Bützow in Germany, on 5 May 2015. The Tornado was nearly a mile wide. It was rated as an F3/T7 with windspeeds up to 205 mph. Luckily nobody died, but the Tornado left 30 people injured and damage of 40 million Euro.

The northern French town of Hautmont was hit by an EF4 tornado in August 2008. The tornado swept through the town, damaging many buildings and resulting in the death of three people.[2] The tornado also affected Maubeuge, Neuf-Mesnil and Boussières-sur-Sambre.[33]

Asia

Bangladesh and the eastern parts of India are very exposed to destructive tornadoes causing higher deaths and injuries. Bangladesh, Philippines, and Japan have the highest number of reported tornadoes in Asia.

The single deadliest tornado in recorded history struck the Manikganj District of Bangladesh on April 26, 1989, killing an estimated 1,300 people, injuring 12,000, and leaving approximately 80,000 people homeless.[34] Five other recorded tornadic events have killed more than 500 people in Bangladesh, most recently on 13 May 1996 when a tornado swept through the Jamalpur and Tangail districts, killing more than 600.[34]

China occasionally experiences destructive tornadoes. In June 2016, a storm producing multiple tornadoes and hail struck a densely populated area of farms and factories near the city of Yancheng in Jiangsu province, about 800 kilometers (500 miles) south of Beijing, China, killing at least 78 people and destroying buildings. Nearly 500 people were injured, 200 of them critically, the official Xinhua News Agency reported. Throughout China, an estimate 100 tornadoes may occur per year with a few exceeding F4 in intensity, with activity most prevalent in eastern regions.[35]

South America

South America has its own tornado alley, composed of central and northern Argentina, southern and southeast Brazil, Uruguay, and part of Paraguay, and is considered the second highest frequency tornado region in the world. Argentina has areas with high tornadic activity, and the strongest tornadoes in the southern hemisphere like the F5 in San Justo, and the tornado outbreak in Buenos Aires with more than 300 tornadoes registered in less than 24 hours.

This region is favorable for tornadoes and severe thunderstorms, due to the large size of the Pampas Plain where the cold air from Patagonia and Antarctica collides with warm, moist air from areas of Brazil, northern Argentina and Paraguay, and dry air from the Andes.[36]

South American Tornadoes

On 16 September 1816, one of the first tornadoes recorded in South America destroyed the town of Rojas (240 kilometres (150 mi) west of Buenos Aires)[37] An EF4 tornado struck the city of Encarnación, Paraguay, on 20 September 1926 and killed over 300 people, making it the second deadliest tornado in South America. On 21 April 1970, an F4 struck Fray Marcos in Uruguay and killed 11, making it the strongest in Uruguay's history.[38] On 10 January 1973, an F5 struck the city of San Justo, Argentina, 105 km (65 mi) north of the city of Santa Fe. The San Justo tornado is considered the worst tornado ever to occur in the Southern Hemisphere, with winds that exceeded 400 kilometres per hour (250 mph).[39] On 6 May 1992, Estación López (in Spanish), in Buenos Aires Province, was devastated by an EF4 tornado that caused 4 deaths among the 150 residents. The province of Buenos Aires (Argentina) was impacted by the largest tornado outbreak in South American history on 13 April 1993. More than 300 tornadoes were recorded over 24 hours with intensities ranging from F1 to F3. On 28 October 1978, an EF4 tornado with winds of 270 km/h (170 mph) hit the city of Morteros in the province of Córdoba, killing 5 people. The 26 December 2003 Córdoba Tornado (in Spanish) struck 6 km (3.7 mi) west of Córdoba's city center. It was rated an F3 with winds exceeding 300 km/h (190 mph), killing 5 and injuring hundreds.[40] An EF4 tornado destroyed the town of San Pedro, Misiones, on the night of 7 September 2009, killing 11 people. The same tornado hit the nearby town of Guaraciaba, Brazil, killing 6. The neighboring towns of Veloso Santo and Santa Cecilia were seriously damaged and were declared in a state of emergency.[41]

In Brazil, one of the most remarkable events occurred in 24 May 2005 when an F3(EF3) multiple-vortex tornado[42] struck the industrial district of the city of Indaiatuba in the state of São Paulo, southeastern region of the country. This episode was documented in video by a surveillance camera which showed the multiple-vortex structure of the tornado. No fatality was reported with this event in Indaiatuba.

Africa

Tornadoes do occur in extreme southern Africa (including the countries of South Africa, Lesotho, and Swaziland). In October 2011 (i.e. in the spring), two people were killed and nearly 200 were injured after a tornado formed, near Ficksburg in the Free State; more than 1,000 shacks and houses were flattened.[43] There is also the seasonal incidence of tornadoes in the coast of western Africa. These occur during the onset of rainy season when tumultuous winds accompanied by sheets of rain as well as spectacular thunder and lightning batter the coast.[44] The tornadoes, however, were welcomed by settlers in the region since it dissipates extreme heat and humidity during the last days of the dry season. These tornadoes are often embedded in the African squall lines and produced by the severe thunderstorms that they bring.[45] There are experts that attribute the formation of the tornado to the large hail, supported by wind shears in the northern part of the squall lines that veer and increase in height.[46] These tornadoes damage crops, diminishing the beneficial effect of its rains.

Oceania

Australia has about 16 tornadoes per year – excluding waterspouts, which are common.[47] In New Zealand, a tornado hit the northern suburbs of Auckland on 3 May 2011, killing one and injuring at least 16 people.

Frequency of occurrence

Tornadoes can form in any month, providing the conditions are favorable. For example, a freak tornado hit South St. Louis County Missouri on 31 December 2010, causing pockets of heavy damage to a modest area before dissipating. The temperature was unseasonably warm that day. They are least common during the winter and most common in spring. Since autumn and spring are transitional periods (warm to cool and vice versa) there are more chances of cooler air meeting with warmer air, resulting in thunderstorms. Tornadoes in the late summer and fall can also be caused by hurricane landfall.

Not every thunderstorm, supercell, squall line, or tropical cyclone will produce a tornado. Precisely the right atmospheric conditions are required for the formation of even a weak tornado. On the other hand, 700 or more tornadoes a year are reported in the contiguous United States.

On average, the United States experiences 100,000 thunderstorms each year, resulting in more than 1,200 tornadoes and approximately 50 deaths per year. The deadliest U.S. tornado recorded is the 18 March 1925, Tri-State Tornado that swept across southeastern Missouri, southern Illinois and southern Indiana, killing 695 people. The biggest tornado outbreak on record—with 353 tornadoes over the course of just 3 1/2 days, including four EF5 and eleven EF4 tornadoes—occurred starting on 25 April 2011 and intensifying on 26 April and especially the record-breaking day of 27 April before ending on 28 April. It is referred to as the 2011 Super Outbreak. Previously, the record was 148 tornadoes, dubbed the 1974 Super Outbreak. Another such significant storm system was the Palm Sunday tornado outbreak of 1965, which affected the United States Midwest on 11 April 1965. A series of continuous tornado outbreaks is known as a tornado outbreak sequence, with significant occurrences in May 1917, 1930, 1949, and 2003.

Time of occurrence

Diurnality

Tornado occurrence is highly dependent on the time of day. [8] Austria, Finland, Germany, and the United States'[48] peak hour of occurrence is 5 pm, with roughly half of all tornado occurrence between 3 p.m. and 7 p.m. local time,[49][50] due to this being the time of peak atmospheric heating, and thus the maximum available energy for storms; some researchers, including Howard B. Bluestein of the University of Oklahoma, have referred to this phenomenon as "five o'clock magic." Despite this, there are several morning tornadoes reported, like the Seymour, Texas one in April 1980.

Seasonality

The time of year is a big factor of the intensity and frequency of tornadoes. On average, in the United States as a whole, the month with the most tornadoes is May, followed by the months June, April, and July. There is no "tornado season" though, as tornadoes, including violent tornadoes and major outbreaks, can and do occur anywhere at any time of year if favorable conditions develop. Major tornado outbreaks have occurred in every month of the year.

July is the peak month in Austria, Finland, and Germany.[51] On average, there are around 294 tornadoes throughout the United States during the month of May, and as many as 543 tornadoes have been reported in the month of May alone (in 2003). The months with the fewest tornadoes are usually December and January, although major tornado outbreaks can and sometimes do occur even in those months. In general, in the Midwestern and Plains states, springtime (especially the month of May) is the most active season for tornadoes, while in the far northern states (like Minnesota and Wisconsin), the peak tornado season is usually in the summer months (June and July). In the colder late autumn and winter months (from early December to late February), tornado activity is generally limited to the southern states, where it is possible for warm Gulf of Mexico air to penetrate.

The reason for the peak period for tornado formation being in the spring has much to do with temperature patterns in the U.S. Tornadoes often form when cool, polar air traveling southeastward from the Rockies overrides warm, moist, unstable Gulf of Mexico air in the eastern states. Tornadoes therefore tend to be commonly found in front of a cold front, along with heavy rains, hail, and damaging winds. Since both warm and cold weather are common during the springtime, the conflict between these two air masses tends to be most common in the spring. As the weather warms across the country, the occurrence of tornadoes spreads northward. Tornadoes are also common in the summer and early fall because they can also be triggered by hurricanes, although the tornadoes caused by hurricanes are often much weaker and harder to spot. Winter is the least common time for tornadoes to occur, since hurricane activity is virtually non-existent at this time, and it is more difficult for warm, moist maritime tropical air to take over the frigid Arctic air from Canada, occurrences are found mostly in the Gulf states and Florida during winter (although there have been some notable exceptions). There is a second active tornado season of the year, late October to mid-November. Autumn, like spring, is a time of the year when warm weather alternates with cold weather frequently, especially in the Midwest, but the season is not as active as it is during the springtime and tornado frequencies are higher along the Atlantic Coastal plain as opposed to the Midwest. They usually appear in late summer.

U. S. Annual January – December Tornado Count 1976–2017 from NOAA National Climatic Data Center

The reliable climatology of tornadoes is limited in geographic and temporal scope; only since 1976 in the United States and 2000 in Europe have thorough and accurate tornado statistics been logged.[52][53] However, some trends can be noted in tornadoes causing significant damage in the United States, as somewhat reliable statistics on damaging tornadoes exist as far back as 1880. The highest incidence of violent tornadoes seems to shift from the Southeastern United States to the southern Great Plains every few decades. Also, the 1980s seemed to be a period of unusually low tornado activity in the United States, and the number of multi-death tornadoes decreased every decade from the 1920s to the 1980s, suggesting a multi-decadal pattern of some sort.[54]

See also

References
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  37. es:1816
  38. "Tornados en Uruguay". 6 November 2011.
  39. San Justo tornado
  40. es:Tornado de Córdoba
  41. http://www.tandar.cnea.gov.ar/eventos/seminariosGIyA/2012/diapositivas/20120605-Diferentes_formas_de_las_trazas_de_danios_en_tornados-Agosta+Altinger.pdf
  42. de Lima Nascimento, Ernani; Held, Gerhard; Gomes, Ana Maria (15 July 2019). "A Multiple Vortex Tornado in Southeastern Brazil". Monthly Weather Review. 142 (9): 3017–3037. doi:10.1175/MWR-D-13-00319.1. ISSN 0027-0644.
  43. "SOUTH AFRICA: Tornadoes kill two, leave thousands homeless". Los Angeles Times. 3 October 2011.
  44. Brooks, George (2003). Eurafricans in Western Africa: Commerce, Social Status, Gender, and Religious Observance from the Sixteenth to the Eighteenth Century. Athens, OH: Ohio University Press. pp. 8. ISBN 978-0852554890.
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  46. Galvin, J.F.P. (2016). An Introduction to the Meteorology and Climate of the Tropics. Hoboken, NJ: Wiley Blackwell. p. 277. ISBN 9781119086222.
  47. Gary, Stuart (4 June 2010). "How common are tornadoes in Australia?". www.abc.net.au.
  48. Encyclopædia Britannica. Tornadoes. Retrieved on 2006-10-25.
  49. A. M. Holzer. Tornado Climatology of Austria. Archived 19 February 2007 at the Wayback Machine Retrieved on 2006-10-25.
  50. N. Dotzek. Tornadoes in Germany. Retrieved on 2006-10-25.
  51. Jenni Teittinen. A Climatology of Tornadoes in Finland. Retrieved on 2006-10-25.
  52. Grazulis, pg. 194
  53. "ESSL [ESWD Project and Data Use]". European Severe Storms Laboratory. Archived from the original on 31 July 2012. Retrieved 15 February 2009.
  54. Grazulis, 196–198

Book references
  • Grazulis, Thomas P (July 1993). Significant Tornadoes 1680–1991. St. Johnsbury, VT: The Tornado Project of Environmental Films. ISBN 978-1-879362-03-1.

Further reading
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