Tornado climatology

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Areas worldwide which experience the highest chance of seeing tornadoes, indicated by orange shading.

Intense tornado activity in the United States. The darker-colored areas denote the area commonly referred to as Tornado Alley.

[edit] 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.^[1] This area extends into Canada, particularly Ontario and the Prairie Provinces, however, activity is less than that of the US. The Netherlands has the highest average number of recorded tornadoes per area of any country (more than 20 annually), followed by the UK (at least 33 per year), 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 is about 179 deaths per year from tornadoes in Bangladesh, 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.^[2].

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

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The United States experiences by far the most tornadoes of any country, and has also suffered the most intense ones. Tornadoes are common in several states, particularly in the Midwest. More strong and violent tornadoes occur here than anywhere else in the world. While Florida has more reported tornadoes by land area than anywhere else in the world, most tornadoes which form here are resultant of non-rotating thunderstorms, or are spawned by hurricanes, both of which tend to be weaker than traditional tornadoes.

Some people mistakenly believe that tornadoes only occur out 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].

Tornadoes can occur west of the continental divide but they are infrequent and usually relatively weak with a short duration. Recently tornadoes have struck the Pacific coast town of Lincoln City, Oregon (1996), 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. 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.

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 they are on average a few F2 or stronger that touchdown 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 Pine Lake, Alberta, on July 14, 2000, where 12 died. The two worst tornadoes in terms of death toll on Canadian soil were the Edmonton Tornado of July 31, 1987, which measured F4, and the misnamed Regina Cyclone of June 30, 1912, which was likely an F4 or weak F5 tornado. The city of Windsor, Ontario was struck by strong tornadoes three times within a 27 year span (1946, 1953 and 1974) ranging in strength from a F2 to F4.

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 and Brazil. 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. ^{[citations needed]}

[edit] Frequency of occurrence

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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.

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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, 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.

[edit] Time of occurrence

Tornado occurrence is highly dependant on the time of day. ^[3] Austria, Finland, Germany, and the United States'^[4] 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.^[5][6] Despite this, there are several morning tornadoes reported, like the Seymour, Texas one in April 1980.

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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. July is the peak month in Austria, Finland, and Germany.^[7] 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 basically occur any time of year. 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, July, and August ) In the colder autumn and winter months, tornado activity is generally limited to the south, where it is possible for warm Gulf of Mexico air to penetrate.

The reason for the peak of tornado season in the spring has much to do with temperature patterns in the U.S. Tornadoes often form when cool, polar air travelling 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 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, occurences are found mostly in the Gulf states and Florida during winter. Interestingly, there is a second active tornado season of the year - sometime around late October/early 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 frequencies are higher along the Atlantic Coastal plain as oppposed to the Midwest. They usualy appear in late summer.

[edit] See also

• Tornado
• Tornado records

[edit] References