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. 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. 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.

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 km, although these tornadoes are generally weak, and many other European countries have a similar number of tornadoes per area.

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.

Tornadoes are most common in spring and least common in winter. 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. 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. Worldwide, most tornadoes occur in the late afternoon, between 3 pm and 7 pm local time, with a peak near 5 pm. 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 a.m. local time.

## 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. 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.

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

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. 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 July 27 1994, the F2 twister that struck the northern suburbs of New York City on July 12 2006, the EF2 twister in the borough of Brooklyn on August 8 2007, or the F4 twister that struck La Plata, Maryland on April 28 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 and have even been documented at high altitudes along the continental divide of Wyoming. In addition, tornado outbreaks occasionally occur around the southern Appalachian mountains  with the 1974 Super Outbreak and the 2011 Super Outbreak directly affecting the southern Appalachian region.

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.

## 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. NOAA has a higher average 100 per year in Canada. 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 July 3 2009, where 3 died, and the last killer tornado was on August 3 2018 in Alonsa, Manitoba. The two deadliest tornadoes on Canadian soil were the Regina Cyclone of June 30 1912 (28 fatalities) and the Edmonton Tornado of July 31 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 June 22 2007. Tornadoes are most frequent in the provinces of Alberta, Saskatchewan, Manitoba, and Ontario.

## Europe

Europe has about 700 tornadoes per year – 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, 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. F3 and perhaps one F4 tornado have occurred as far north as Finland.

Since 1900, deadly tornadoes have occurred in Austria, Belgium, Cyprus, Czech Republic, Finland, France, Germany, Italy (such as the F5/T10 of Udine-Treviso on July 24 1930, which killed 23 people,), Lithuania, Malta, the Netherlands, Poland, Ukraine, Portugal (such as the F3/T7 of Castelo Branco on November 6 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 2005 Birmingham tornado 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 June 23 2004 leaving 6 people injured and more than 250 buildings massively damaged.

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. The tornado also affected Maubeuge, Neuf-Mesnil and Boussières-sur-Sambre.

A F4 tornado hit the southeastern part of the Czech Republic in June 2021, leaving 6 fatalities, more than 200 injuries and several damaged villages, the overall damage would reach 15 billion CZK (approx. 693.9 million USD).

## 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 ever recorded 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. Five other recorded tornadic events have killed more than 500 people in Bangladesh, most recently on May 13 1996 when a tornado swept through the Jamalpur and Tangail districts, killing more than 600.

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 98 people and destroying buildings. Nearly 800 people were injured, 200 of them critically, the official Xinhua News Agency reported. Again in May of 2021, Hubei and Jiangsu experienced a large tornado which resulted in the death of at least 12 people and the injury of more than 300. Throughout China, an estimate 100 tornadoes may occur per year with a few exceeding F4 in intensity, with activity most prevalent in eastern regions.

## 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.