Tornado climatology

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Tornado climatology is the study of where and when tornadoes occur as well as associated physical reasons for this distribution. Research in this developing area of science has shown many patterns in tornado formation. Tornadoes are a rare weather phenomenon involving a violently rotating column of air in contact with both a cumiliform cloud and the surface.^[1] They can cause death and destruction almost anywhere on the globe; however, there are areas which are far more likely to experience them.

Contents 1 Geography 2 Tornadoes in the USA 2.1 The Southern Plains 2.2 Central and Northern Plains States, Midwest and Great Lakes States 2.3 The Southern States 2.4 Florida 2.5 The Northeast 2.6 Rocky Mountain Region 2.7 Southwestern States 2.8 Pacific Northwest 2.9 Common misconceptions 3 Tornadoes in Canada 4 Tornadoes Outside North America 5 Frequency of occurrence 6 Time of occurrence 6.1 Diurnality 6.2 Seasonality 7 See also 8 References 9 External links

[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.^[2] 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.^[3].

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 occur 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.

[edit] Tornadoes in the USA

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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. It is important to note that sometimes a tornado can be reported more than once, such as when a storm crosses a county line and reports are made from two counties.

[edit] The Southern Plains

The most tornadically active region of all is the southern part of the Great Plains. According to figures from the NCDC, Texas had 7,565 tornado reports for the period 1950 to 2006 inclusive, followed by Kansas (3,298) and Oklahoma (3,263). Oklahoma and Florida vie with each other for the most tornadoes per unit area, but Florida tornadoes only rarely approach the intensity of the Plains storms. Oklahoma has reported 98 F4/F5 tornadoes over the same period, Texas reported 84, and Kansas 74. Florida, by contrast, has only reported four and none of these were rated F5. Kansas has reported more F5/EF5 tornadoes over the period 1950 - 2007 than any other state. The first EF5 storm under the Enhanced Fujita Scale was the Greensburg, Kansas Tornado of May 2007. Other notable tornadoes to ravage the Southern Plains include the Glazier-Higgins-Woodward tornadoes in April 1947, the Goliad Tornado in May 1902, the Waco Tornado in May 1953, the Wichita Falls Tornado in April 1979, the Andover, Kansas Tornado Outbreak in April 1991 and the Jarrell Tornado of May 1997. The 1999 Oklahoma tornado outbreak of May 1999, was the costliest in US history.

Southern and eastern parts of Texas are vulnerable to tropical cyclones, which can generate many tornadoes and contribute to the state's tornado count. Eastern Texas can be considered part of the South rather than as part of the southern Plains. East Texas is prone to the more violent, supercell generated tornadoes common to the Plains and neighbouring southern states. However, it is central and northern parts of Texas which are most at risk from tornadoes.

Texas has reported more tornado fatalities than any other state, although the large population of the state should be taken into account here.

[edit] Central and Northern Plains States, Midwest and Great Lakes States

The Midwest and the states around the Great Lakes are also tornado prone. Nebraska is fifth overall for sheer numbers of tornadoes, whilst Indiana has had 88 violent tornado reports from the 1950 - 2006 period, more than any state except Oklahoma. Iowa reported 83, almost as many as Texas. All this region is vulnerable, from western Pennsylvania to eastern Wyoming, and from Nebraska to the Canadian border. The deadliest tornado in US history, the Tri-State Tornado, struck Missouri, Illinois, and Indiana in March 1925. St. Louis, Missouri and neighbouring East St. Louis, Illinois have been hit more than once by violent tornadoes, the most notorious of which was the St. Louis Tornado of May 1896. The New Richmond Tornado of May 1899 and the Flint, Michigan tornado of June 1953 also rank amongst the deadliest tornadoes in US history. The region was badly hit by the Palm Sunday Tornado Outbreak in April 1965 and by the Super Outbreak of April 1974.

According to NCDC figures for the 1950 to 2006 period, Nebraska reported 2,440 tornadoes followed by Iowa (2,185), Illinois (2,086), Missouri (1,922), South Dakota (1,487), Minnesota (1,477), Indiana (1,327), North Dakota (1,216), Wisconsin (1,185), Michigan (981), and Ohio (916).

[edit] The Southern States

The Southern USA has suffered more tornado fatalities than any other part of the country. Part of the reason for this has been that housing in this region is often of a weaker construction than elsewhere in the US. However, the region encounters more long-tracked violent tornadoes than any part of the USA. Some areas experience repeated damaging tornado events, such as the Tennessee Valley in northern Alabama. The state of Alabama has reported more F5 tornadoes than any state except for Kansas. For the period 1950 to 2006, three hundred and fifty eight people were killed by tornadoes in Alabama, ranking the state third nationwide behind Texas (521) and neighboring Mississippi (404). Fourth is Arkansas (336) and fifth is Tennessee with 271 fatalities.

Tornado disasters to affect the southern USA include the Great Natchez Tornado of May, 1840 - second deadliest on record in the US, behind only the Tri-State Tornado. Other outbreaks included the Tupelo-Gainesville tornado outbreak of April 1936, the 1908 Southeast tornado outbreak of April 1908 and the Candlestick Park Tornado of 1966. Alabama and Kentucky were very badly affected by the Super Outbreak of 1974. The 2008 Super Tuesday tornado outbreak was the deadliest tornado outbreak in the USA in 23 years. 58 people lost their lives with Tennessee, Arkansas, and Kentucky particularly badly affected.

Hurricanes and other tropical storms can also generate tornadoes. Louisiana and the coastal regions of Mississippi and Alabama are most at risk from these storms. The Atlantic seaboard states can be affected too.

According to NCDC figures for the 1950 to 2006 period, Mississippi reported 1,787 tornadoes, followed by Arkansas (1,644), Louisiana (1,608), Alabama (1,579), Georgia (1,324), North Carolina (1,042), Tennessee (892), South Carolina (819), Kentucky (710) and Virginia (565). West Virginia, by contrast, is one of the least vulnerable states of all with just 120 tornadoes reported over the period.

[edit] Florida

Florida is one of the most tornado prone states, with only Texas, Kansas and Oklahoma reporting more storms. During the period 1950 to 2006, Florida reported 2,884 tornadoes according to NCDC figures. However, Florida tornadoes are usually weak in comparison with those that strike the Plains and the Southern states - there have been only four reports of F4 strength tornadoes since 1950 and none of F5 strength.

Florida experiences more thunderstorms than any other state but fewer supercell storms. Florida tornadoes are more often spawned by the frequent ordinary thunderstorms which occur over the state. Hurricanes and other tropical storms can generate large numbers of tornadoes. Non-supercell tornadoes are rarely as strong as supercell-generated storms.

Florida's most violent tornadoes generally occur during the winter months when the state is most vulnerable to invasions of cold air which help generate such storms. In recent years there have been some particularly disastrous tornado events. During the night of 22/23 February 1998, an F3 tornado struck Kissimmee and killed 25 people. Later that night, another F3 tornado struck Seminole and Volusia counties and killed 13 people (see Kissimmee Tornado Outbreak). On February 2, 2007, an EF3 tornado struck Lake County with 21 fatalities resulting - see 2007 Central Florida Tornadoes.

[edit] The Northeast

The Northeastern USA to the east of the Appalachians is much less vulnerable to violent tornadoes but is by no means immune. One of the most extraordinary tornadoes in history struck Worcester, Massachusetts. This F4 tornado struck the town - not far west of Boston - on 9 June 1953 and killed 90 people. The tornado was generated by the same storm system that delivered a devastating tornado to the town of Flint, Michigan.

Areas further south - notably Maryland - and areas to the west of the Appalachians are more vulnerable to tornadoes. NCDC figures for the period 1950 to 2006 show that Pennsylvania reported 697 tornadoes, followed by New York State (358), Maryland (269), Massachusetts (153), New Jersey (144), Maine (101), New Hampshire (86), Connecticut (82), Delaware (58), Vermont (37), Rhode Island (9), and the District of Columbia (1).

[edit] Rocky Mountain Region

Of the states around the Rocky Mountains, Colorado reports by far the greatest numbers of tornadoes. Eastern Colorado, both climatically and physically, has much more in common with the neighboring Plains states of Kansas and Nebraska than with the mountainous areas further west. The same can be said, to a lesser extent, of eastern Wyoming. Tornadoes are less frequent in mountainous areas.

Of the states in this region - according to NCDC figures for the 1950 to 2006 period - Colorado reported 1,617 tornadoes, followed by Wyoming with 560, Montana (345), Idaho (175), and Utah (114).

[edit] Southwestern States

In the Southwestern United States, New Mexico reported 485 tornadoes during the 1950 to 2006 period (NCDC figures), California reported 355, Arizona reported 209, and Nevada reported 75. New Mexico borders the notoriously tornado prone states of Texas and Oklahoma, hence the noticeably higher figures. Arizona and New Mexico experience regular summer thunderstorms during their Monsoon season. These are sometimes tornadic but rarely produce violent tornadoes.

[edit] Pacific Northwest

The Pacific Northwest is perhaps the least vulnerable region. NCDC figures for the 1950 to 2006 period show that Washington State reported just 96 tornadoes for the entire period and that Oregon reported 91. However, Portland, Oregon and neighboring Vancouver, Washington were hit by a deadly tornado, the remarkable Portland-Vancouver Tornado, on 5 April 1972. This storm was the deadliest to hit the United States that year.

[edit] 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. 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.

[edit] 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.^{[citation needed]} 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 Pine Lake, Alberta, on July 14, 2000, where 12 died. The two deadliest tornadoes 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 F5 tornado. 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 ever tornado officially rated F5 occurred in Elie, Manitoba on June 22, 2007.^[4] At least two other tornadoes in Saskatchewan in earlier parts of the 20th century are suspected as F5.

[edit] 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 (F3/T7 of Castelo Branco the 6 November 1954 who made 5 dead and 220 injured). 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

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.

[edit] Time of occurrence

[edit] Diurnality

Tornado occurrence is highly dependent on the time of day. ^[5] Austria, Finland, Germany, and the United States'^[6] 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^[7][8], 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.

[edit] 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.^[9] 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 December to early March), 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. 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.

[edit] See also

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

[edit] References

[edit] External links

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