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A tornado—also popu la rly known as a twister—is an atmospheric phenomenon associated with a supercell thunderstorm or hurricane. It consists of a small rapidly and violently rotating column of air-or vortex-extending continuously from a convective cumuliform cloud to the ground. When the vortex is spinning but not touching the ground, it is called a funnel cloud, which eventually may extend to the ground evolving into a tornado. It becomes clearly visible in daylight as a funnel or tube cloud when it carries water vapor and debris lifted from the ground. It may be thin and rope-shaped in the case of weak tornadoes with speeds below 175 kilometers per hour (110 miles per hour). Sometimes the funnel is not visible except by signs such as whirling debris on the ground. A prolonged roar and hail or heavy rain happen during the event.
Most tornadoes are associated with rotating and long-lasting supercell thunderstorms. Tornado formation starts with a vortex in the base of the storm cloud, out of the wall cloud. Next, an organizing phase follows when wind intensity increases and the vortex extends to the ground. In its mature stage, the tornado reaches its maximum width and speed. After that it weakens, dimensions are reduced, and it adopts a rope-like form.
Tornadoes can last seconds or hours. Most tornadoes last from one to 20 minutes; however, some have been observed to last hours. The vortex may touch ground several times in different locations. The vortex has a diameter of 20-100 meters (20-100 yards) and travels at a translational average speed of 50-65 kilometers per hour (30-40 miles per hour), reaching maxima of 115 kilometers per hour (70 miles per hour), with a rotational speed of 480 kilometers per hour (300 miles per hour). Forward speed is not the only factor in damage; lifetime also contributes, as slow-moving tornadoes may be more dangerous than fast-moving ones. Maximum rotational speed is developed at the edge, decreasing to the center, so that major destruction takes place where rotational and translational speeds sum up. Indirect measurements indicate there is a pressure drop at the center of the tornado. Vortex rotation is commonly counterclockwise in the Northern Hemisphere. The average path length is 8 kilometers (5 miles) although some tornadoes have traveled for 100 miles, and the average path width is 300-400 meters (300-400 yards), and some have covered up to a mile.
Seventy-four percent of tornadoes are in the F0F1 range of strength level, the weak class, while less frequent, violent tornadoes cause 68 percent of fatalities. They are most likely to occur in the afternoon and move from southwest to northeast.
A single tornado can develop various smaller vortices, known as subvortices or suction vortices with higher speeds. A sequence of continuous tornados along a line of storms is called a tornado outbreak. When the tornado happens over water it is called a waterspout.
Tornadoes represent a major local hazard causing notable destruction, loss of lives, and injuries. Houses collapse, structures are uprooted, and pieces of debris become projectiles. About 1,000 tornadoes are reported every year across the United States, versus 30-50 in the United Kingdom. The number of tornadoes registered in the United States has increased with the implementation of the Doppler Radar Network by the National Weather Service, particularly F0 tornadoes, many of which were not formerly detected.
Tornadoes are frequent in central North America, including: The Canadian central provinces of Alberta, Saskatchewan, and Manitoba; northern Argentina; western and central Europe, South Africa; and eastern and southwestern Australia. About one-fourth of all significant tornadoes occur in Tornado Alley in the Central Plains region of the United States, which includes parts of Texas, Oklahoma, Kansas, Colorado, Nebraska, Iowa, South Dakota, and Minnesota. In the southern states peak tornado season is spring, while in the northern states it is summer, with maximum frequency in May-June.
The most deadly tornadoes happen, however, in areas where they are less frequent, particularly the southeast. In the period of 1950-99, the national year average was 89 deaths. The three deadliest tornadoes were the Tri-State (MO/IL/IN) tornado outbreak on March 18, 1925, which killed 689 people; Natchez, Minnesota, on May 6, 1840, with 317 victims; and St. Louis, Missouri, on May 27, 1896, causing 255 deaths. The three costliest tornadoes happened in Omaha, Nebraska, on May 6, 1975, with an estimated damage of $1.132 billion; Wichita Falls, Texas, on April 10, 1979, which reached $840 million; and Lubbock, Texas, on May 11, 1970, which caused damage of $530 million.
The F-scale (or Fujita scale) was proposed by Tetsuya “Theodor” Fujita in 1971 to categorize the intensity of tornadoes based on the structural damage caused to man-made structures, estimated once the tornado has passed. It comprises six categories, from F0 to F5, although a theoretical maximum F12 tornado is possible. From F0 to F1 a tornado is considered to be weak, strong if in the range F2-F3, and violent from categories F4 to F5. Although subjective in damage assessment, which causes overestimation of wind speeds, the F-scale has widespread use in the United States after being accepted as the official classification system.
Despite the limitations, it was decided to maintain and improve the scale in order to provide continuity to historical tornado records. An enhanced Fujita scale (EF-Scale) was developed for use in the United States after February 2007. Twenty-eight indicators are used and the degree of damage estimated up to eight levels, associating them to upper and lower wind speeds.
As damage is not necessarily associated to wind speed, however, Terence Meaden proposed the Tornado Intensity Scale in 1972, relating the levels to the well-established Beaufort wind intensity scale. As the speed can be measured directly-or, better, remotely estimated-this allows the determination of intensity even if the tornado causes no damage. The scale ranges from T0 to a maximum T10, each level representing a range of windspeeds. From T0 to T3 tornadoes are considered to be weak, from T4 to T7 are strong tornadoes and from T8 to T10 are violent tornadoes; a degree higher than T10 is possible.
U.S. emergency administration issues two differentiated levels of risk to alert population. A tornado watch indicates there is a high probability of a tornado in the area and recommends remaining alert to future evolution. A tornado warning indicates a tornado has been sighted or detected by radar in the area and recommends taking shelter in pre-designated places of safety.
- Howard Bluestein, Tornado Alley (Oxford University Press, 1999);
- Thomas P. Grazulis, The Tornado: Nature’s Ultimate Windstorm (University of Oklahoma Press, 2003);
- Long T. Phan and Emil Simiu, The Fujita Tornado Intensity Scale: A Critique Based on Observations of the Jarrell Tornado of May 27, 1997 (National Institute of Standards and Technology, 1998);
- Philip W. Suckling and Walker S. Ashley, “Spatial and Temporal Characteristics of Tornado Path Direction,” The Professional Geographer (v.58, 2006).