Tornadoes are destructive weather-related disasters that strike suddenly and frequently. They are violently rotating columns of air that descend from the base of large cumulonimbus clouds (thunderstorms) and make contact with the ground. When they touch down in populated areas they can cause a great deal of destruction.
Super cell Formation
In recent years tornadoes have claimed hundreds of thousands of lives and have devastated entire communities. The National Weather Service documents about 1000 touchdowns of tornadoes every year in the United States. Additionally, there may be as many as 1000 or more weaker tornadoes each year that touch down in remote areas and thus remain completely undocumented. Fortunately only a small percentage of tornadoes that touch down can be classified as violent.
The interaction and collision of different air masses can result in thunderstorms. The largest and most violent of tornadoes are generated by the "super cell" thunderstorms, that are associated with strong winds, heavy rains and hail. Necessary for super cell thunderstorm formation and tornado generation, is warm, moist air close to the ground - being forced upward into colder, dry air at about 10,000 feet or higher in the atmosphere.
Air Mass Interactions in Tornado Formation (graphic, U. K. Meteorological Office)
Such conditions can be triggered by the rapid passage of a cold, or a low pressure front, which can then generate strong vertical shear winds and updrafts which can often be visible as dark “wall clouds”. "Shearing" winds that change direction and increase in speed with height seem to be necessary for a thunderstorm to develop rotation and generate tornadoes. Wall clouds form in extremely strong updraft regions of the storm, where the air is being pulled from below at speeds of up to 100 mph. The winds thus generated can change directions, pick up additional speed with altitude and develop into cyclonic patterns, with counterclockwise rotation. These tornadic super cell thunderstorms suck additional warm, moist air from below and can form a "funnel cloud" - as rapid cooling and condensation occur. Often such funnels will grow in a downward direction and make make contact with the ground. Such tornado funnel touchdown can result in catastrophic damage if it occurs in populated areas.
Usually, the Gulf Stream supplies some of the warm, moist air that is needed for tornado formation in the U.S., while the Appalachian mountain region funnels cool air toward the coasts. However, near the southern U.S. coasts the effects of air mass interactions are not as strong. Thus tornadoes formed in Florida by thunderstorms, although frequent, lack the intensity of tornadoes that can form further inland.
Super cell and Tornado formation
Conditions become ideal for the formation of more intense tornadoes in the Midwest region. Warmer and moist air moving in from the Gulf of Mexico collides with the colder and dry air that funnels over the Rocky mountains. As the colder air cascades down to the central plains, it expands and starts a strong counterclockwise circulation, much like a hurricane does in the northern hemisphere - but in a more compact and intense way. Depending on local conditions, such funneling effect may gain momentum and develop into a tornado that may touch down with great intensity.
When the rotating funnel cloud makes contact with the ground, begins to suck debris and forms a visible dust cloud, then it is officially classified as a tornado. Its rotating winds may reach speeds of 200 miles or greater. Its intensity, as explained below (measured by the Fujita scale) is determined not by wind speed or funnel dimensions, but on the amount of damage the tornado causes when it makes touchdown.
Tornado Occurrence, Frequency and Seasonal Variation
Occurrence and Frequency - Tornadoes occur worldwide and can form at any time of the year when cumulonimbus clouds (thunderstorms) are present. As stated, about 1,000 tornadoes form every year in the United States. Tornadoes can form anywhere but more frequently occur in the plains region between Florida and the Rocky and Appalachian Mountains. As stated, most tornadoes occur in Florida but are usually weaker and short lived. The more severe tornadoes occur across the Central Plains, extending from Texas to Colorado to Oklahoma and South Dakota. About 40% of them occur yearly the regions of Central United States along a corridor which often is referred to as "Tornado Alley", which includes parts of Florida, Mississippi and Texas, but extends northward through the states of Oklahoma, Kansas, Nebraska and South Dakota, and east to Iowa, southern Ohio and northern Alabama. Also, dangerous tornadoes occur frequently in Indiana, Illinois, Kentucky and Tennessee. Sometimes the term “Dixie Alley” is used to describe the path that tornadoes follow in the southern states – more frequently along the lower Mississippi and upper Tennessee valleys.
Seasonal Variation - Tornadoes occur with higher frequency during certain times of the year - referred to as "tornado season" - which also shifts geographically. More tornadoes strike the southeastern states during the winter months. However, in early spring (between March to May), most occur in the central States. As temperatures rise during the summer months, more tornadoes are likely to develop further north in the region of central plains, where warm and cold air interactions become more frequent.
Areas with the greatest incidence of tornadoes with F2 or greater Intensity (After Meyer et al. 2005)
Tornado Intensity – The Fujita Damage Scale
The intensity of a tornado is usually described by the amount of damage it causes when it touches down in populated areas. Estimating a tornado’s maximum wind speed from damage to structures is a very subjective process and difficult to describe with the various available meteorological parameters. The size of a tornado is not necessarily an indication of its intensity. Large tornadoes can be weak and small tornadoes can be violent. Two percent of all tornadoes are classified as "violent" and account for about 70% of all tornado-related fatalities in the United States.
In 1971, Professor Tetsuya Fujita of the University of Chicago and Allen Pearson, then director of NSSFC (National Severe Storm Forecast Center - predecessor organization to the Storm Prediction Center in Kansas City, Missouri), introduced a scale that would rate the intensity of the tornado, as well as a measure of its path length and width. This is the Fujita Scale (also known as the Fujita-Pearson Scale) presently in use in estimating the strength of a tornado. The Fujita scale does not link damage to wind speed, but it is simple enough to use as a measure of intensity.
The scale rates the intensity of a tornado, not by the appearance of its funnel but on the damage it caused after it passed over man-made structures. Since the scale is based on a subjective interpretation of damage, the designation of intensity may vary depending on how a survey is conducted and the experience of surveyors in separating tornado damage from other wind damage. Often, complex combinations of strong down bursts or other lateral winds may accompany tornado generating thunderstorms - so their damage may be difficult to separate from that of the tornado. The Fujita scale has applied retroactively to rate past tornadoes from 1950 onward in the United States, and as well as in rating some prior events. Part of the Fujita scale is as follows:
Taken Thursday night, April 3,2008 South of Ft Stockton , TX . Lariat # 2 - Sandridge Energy.
THE FUJITA TORNADO SCALE
F-Scale Number Intensity Phase Wind Speed Type of Damage Done
F0 Gale tornado, 40-72 mph. Some damage to chimneys; breaks branches off trees; pushes over shallow-rooted trees; damages signboards.
F1 Moderate tornado, 73-112 mph. The lower limit is the beginning of hurricane wind speed; peels surface off roofs; mobile homes pushed off foundations or overturned; moving autos pushed off the roads; attached garages may be destroyed.
F2 Significant tornado, 113-157 mph. Considerable damage. Roofs torn off frame houses; mobile homes demolished; boxcars pushed over; large trees snapped or uprooted; light object missiles generated.
F3 Severe tornado, 158-206 mph. Roof and some walls torn off well constructed houses; trains overturned; most trees in forest uprooted
F4 Devastating tornado, 207-260 mph. Well-constructed houses leveled; structures with weak foundations blown off some distance; cars thrown and large missiles generated.
F5 Incredible Damage, 261-318 mph. Strong frame houses lifted off foundations and carried considerable distances to disintegrate; automobile sized missiles fly through the air in excess of 100 meters; trees debarked; steel reinforced concrete structures badly damaged.
F6 Inconceivable tornado, 319-379 mph. These winds are very unlikely. The small area of damage they might produce would probably not be recognizable along with the mess produced by F4 and F5 wind that would surround the F6 winds. Missiles, such as cars and refrigerators would do serious secondary damage that could not be directly identified as F6 damage. If this level is ever achieved, evidence for it might only be found in some manner of ground swirl pattern, for it may never be identifiable through engineering studies.
The Enhanced Fujita Scale (EF)
Essentially, the above given Fujita scale relates damage level associated with a range of wind speeds – which are estimated but often cannot be verified with actual measurements. Frequently, the wind speeds for strong tornadoes are greatly overestimated. To compensate for this problem an Enhanced Fujita Scale (EF) was formulated and implemented in the U.S. as of 1 February 2007.
This Enhanced F-scale is also related to a set of wind estimates (not measurements) based on damage, but it uses also three-second wind gusts estimated at the point of damage, based on a judgment of 8 levels of damage to 28 indicators of different types of structures. The 3-second gust in the EF scale is not the same wind as in standard surface observations. Standard measurements are taken by weather stations in areas of open exposure, using a directly measured “one minute mile" speed. Recently, new mobile doppler radar units mounted on trucks have been used and wind speeds of up to 315 mph have been recorded within tornadoes.
Major Historical Tornadoes
Documentation of historical tornadoes begun in the 1880s, when John Finley - of what was then the U.S. Army Signal Corps - was sent to conduct surveys of tornadoes in the Midwest. Finley began his work by compiling a historical record of 600 tornadoes from 1794 to 1881. Also, he organized a system of volunteer tornado reporters. Finley’s compilation of historical tornadoes in the United States, combined with more recent events of the 20th Century were combined in a catalog published by Grazulis (1993), entitled “Significant Tornadoes, 1680 – 1995” (see reference below). Deadly tornadoes occur nearly every year in the U.S. Information for all the tornadoes and severe weather events -- including killer tornadoes -- is provided in NOAA’s National Climatic Data Center (NCDC) publication “Storm Data”.
A photo of the April 11, 1965 (Palm Sunday) double tornado touchdowns at Elkhart Indiana (NOAA archives)
Grazulis, T.P., 1993. “Significant Tornadoes, 1680-1995”. Environmental Films, St. Johnsbury, VT, 1326 pp.
TORNADO NUMBERS 1950 - 1994. Storm Prediction Center. Retrieved on 2007-04-26.
Tornado Climatology. National Climatic Data Center (January 29, 2007). Retrieved on 2007-04-26.
F5 and EF5 Tornadoes of the United States Southeast tornadoes are nation's deadliest. USA Today (May 20, 2005). Retrieved on 2007-10-16.
Supecell Cloud - Web Photo
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