Tornado climatology

Areas worldwide which experience the highest chance of seeing tornadoes, 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.

The United States averages about 1,200 tornadoes per year. The Netherlands has the highest average number of recorded tornadoes per area of any country (more than 20, or 0.0013 per sq mi (0.00048 per km²), annually), followed by the UK (around 33, or 0.00035 per sq mi (0.00013 per km²), per year), but most are small and cause minor damage. In absolute number of events, ignoring area, the UK experiences more tornadoes than any other European country, excluding waterspouts.

Tornadoes kill an average of 179 people per year in Bangladesh, the most in the world. This is due to their high population density, poor quality of construction, lack of tornado safety knowledge, as well as other factors. Other areas of the world that have frequent tornadoes include South Africa, parts of Argentina, Paraguay, and southern Brazil, as well as portions of Europe, Australia and New Zealand, and far eastern Asia.

Tornadoes are most common in spring and least common in winter. 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 are focused in the right poleward section of 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. Favorable conditions 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. The Gainesville Tornado of 1936, one of the deadliest tornadoes in history, occurred at 8:30 am local time.

Geography

The United States has the most tornadoes of any country, seeing about four times the activity estimated in all of Europe. Many of these form in an area of the central United States known as Tornado Alley. This area extends into Canada, particularly Ontario and the Prairie Provinces; however, activity in Canada is less than that of the US. The Netherlands has the highest average number of tornadoes per square mile in the world (at least 20 recorded per year). The UK also has a very high tornado density (more than 33 tornadoes reported annually), but most are small and result in minor damage.

Bangladesh and surrounding areas of eastern India suffer from tornadoes of equal severity to those in the US with more regularity than any other region in the world. However, these occur with greater recurrence interval, and tend to be under-reported due to the scarcity of media coverage in a third-world country. The annual human death toll from tornadoes in Bangladesh is 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 other factors.

Other areas of the world that have frequent strong tornadoes include parts of Argentina and southern Brazil, as well as South Africa. A fair number of weak and occasionally strong tornadoes occur annually in Germany, Italy, Spain, China and the Philippines. Australia, France, Russia, areas of the Middle East, and Japan have a history of multiple damaging tornado events.

Tornadoes in the USA

Further information: [[:Tornadoes in the United States]]

The United States reports about 1,200 tornadoes per annum; more than any other country. It also 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 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 USA is one of the worst affected regions in terms of casualties.

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 the most tornado-prone places in the nation, it should be noted that tornadoes have been reported in every U.S state, including Alaska and Hawaii. One likely reason why tornadoes are so common in the central U.S is because this is where Arctic air first collides with warm tropical air from the Gulf of Mexico where the cold front has not been "weakened" yet. As it heads further east, however, it is possible for the front to lose its strength as it travels over more warm air. Therefore, 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 F2 twister that struck the northern suburbs of New York City on July 12, 2006 [1], or the EF2 twister in parts of Brooklyn, New York on August 8, 2007.

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); 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 typically are weak, more powerful, damaging tornadoes can occur, such as a tornado that occurred on May 22, 2008 in Perris, California. http://articles.latimes.com/2008/may/23/local/me-weather23 More tornadoes occur in Texas than in any other US state.

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 F0 or F1 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 another particularly notorious tornado state. It records the most F4 and F5 tornadoes in the country. It should be mentioned that states such as Oklahoma and Kansas have much lower population densities than Florida and that tornadoes may go unreported.

Tornadoes in Canada

Canada also experiences numerous tornadoes, although fewer than the United States. In Canada, at least 80-100 tornadoes occur annually (with many more likely undetected in large expanses of unpopulated areas), causing 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 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 9, 2009, where 3 died, and the last killer tornado was on August 20, 2009 in Durham, Ontario. 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, Ontario was struck by strong tornadoes four times within a 61-year span (1946, 1953 and 1974 and 1997) ranging in strength from an F2 to F4. Canada's first official F5 tornado struck Elie, Manitoba on June 22, 2007. At least two other tornadoes in Saskatchewan in earlier parts of the 20th century are suspected as F5. Most common provinces for tornadoes are Manitoba, Alberta, Saskachewan and Ontario. But Windsor, Ontario and Winnipeg, Manitoba are the most tornado prone cities in Canada.

Tornadoes Outside North America

Tornadoes do occur throughout the world as well; the most tornado-prone region of the world (outside North America), as measured by number of reported tornadoes per unit area, is the Netherlands, followed by the United Kingdom (especially England). Bangladesh, India, Argentina, Italy, Australia, New Zealand, Germany, Estonia, and portions of Uruguay also have pockets of high tornadic activity. Occasional strong tornadoes occur in Russia, France, Spain, Japan, and portions of Paraguay, Brazil and Portugal (such as the F3/T7 of Castelo Branco on November 6, 1954, which killed 5 and injured 220). Tornadoes have recently hit South Africa and parts of Pakistan in 2001 as well, and on April 4, 2006, a rare F2 tornado hit northwestern Israel, causing significant damage and injuries. Approximately 170 tornadoes are reported per year on land in Europe. One notable tornado of recent years was the 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 July 23, 2004 leaving 6 people injured and more than 250 buildings massively damaged.

Frequency of occurrence

Tornadoes can form in any month, providing the conditions are favorable. 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 March 18, 1925, Tri-State Tornado that swept across southeastern Missouri, southern Illinois and southern Indiana, killing 695 people. The biggest tornado outbreak on record—with 148 tornadoes, including six F5 and 24 F4 tornadoes—occurred on April 3, 1974. It is dubbed the Super Outbreak. Another such significant storm system was the Palm Sunday tornado outbreak of 1965, which affected the United States Midwest on April 11, 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. Austria, Finland, Germany, and the United States' peak hour of occurrence is 5 p.m., with roughly half of all tornado occurrence between 3 p.m. and 7 p.m. local time, 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. July is the peak month in Austria, Finland, and Germany. 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 have occurred 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). Interestingly, 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.

Long-term trends

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. 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.

See also

• Tornado intensity and damage
• Tornado records
• List of tornadoes and tornado outbreaks
• List of F5 and EF5 tornadoes
• Tornadogenesis
• Tornado myths

References

Book reference

• Grazulis, Thomas P (July). Significant Tornadoes 1680–1991. St. Johnsbury, VT: The Tornado Project of Environmental Films. ISBN 1-879362-03-1. {{cite book}}: Check date values in: |date= (help); Unknown parameter |month= ignored (help)

External links

• When and Where Do Tornadoes Occur? (National Atlas of the United States)