why do tornadoes form mostly in the great plains

Most tornadoes are found in the Great Plains of the central United States an ideal environment for the formation of severe thunderstorms. In this area, known as Tornado Alley, storms are caused when dry cold air moving south from Canada meets warm moist air traveling north from the Gulf of Mexico. Tornadoes can form at any time of year, but most occur in the spring and summer months along with thunderstorms. May and June are usually the peak months for tornadoes. Which two states have the highest number of tornadoes per year per 10,000 sq miles? Notice that the location with the highest number of thunderstorms does not match the location with the highest number of tornadoes. Why do so many tornadoes form in this part of south central North America? The Great Plains are conducive to the type of thunderstorms (supercells) that spawn tornadoes. It is in this region that cool, dry air in the upper levels of the atmosphere caps warm, humid surface air. This situation leads to a very unstable atmosphere and the development of severe thunderstorms. Look at the map above. The blue lines show the paths of tornadoes produced by storms on April 3rd 4th, 1974.


Why are the lines all parallel and pointing in a southwest to north direction?
Tornado at beginning of life, Cordell, Oklahoma, May 22, 1981 Tornadoes occur more frequently and are more intense in the Great Plains than in any other region on earth. Plains geography, with fringing mountain ranges running predominantly north to south, allows cold dry air from the north to collide with warm moist tropical air from the south. The jet stream, which seasonally migrates south across the Great Plains, is another essential ingredient in the creation of tornadoes. The highest frequency of tornadoes is in central Oklahoma, but incidence is also high in a zone from northeast Texas through Kansas to eastern Nebraska and Iowa. This region is commonly known as "tornado alley. " The tornado season for North America peaks in May; however, there is considerable geographic variation in the timing of peak occurrence. For example, tornadoes in Texas and Oklahoma peak in April, in Nebraska in June, and in the Southern Prairie Provinces in late July and early August. Tornadoes develop on the backside of particularly strong thunderstorms and hang down from the cloud as if the thunderstorm had a tail.


These tornado-producing thunderstorms can occur singly or in a line known as a squall line, which often produces multiple tornadoes. Great Plains tornadoes commonly occur in late afternoon, when surface temperatures are the highest, providing the daily maximum uplift to the moist air near the ground. Most tornadoes move from the southwest toward the northeast; however, they may travel in any direction, and in some rare instances they remain stationary or reverse direction. Tornado intensity is described by the fscale. F0 and F1 tornadoes are weak tornadoes, with top wind speeds reaching 112 miles per hour, a typical width of 50 to 100 feet, and a typical life span of only a few minutes. They account for 68 percent of all Great Plains tornadoes. F2 and F3 tornadoes are strong tornadoes, with winds as high as 206 miles per hour, a width of 100 to 400 feet, and a life span of more than twenty minutes. They account for 30 percent of all Great Plains tornadoes. F4 and F5 violent tornadoes are the strongest, with wind speeds that can exceed 300 miles per hour, a funnel as wide as a mile, and a duration of several hours.


Violent tornadoes represent only 2 percent of all tornadoes in the Great Plains, but they account for more than 90 percent of tornado-caused deaths. For the period 1950 97, the annual average number of F2 to F5 tornadoes was twenty-nine for Texas, seventeen for Oklahoma, eleven for Kansas, seven for Nebraska, five for South Dakota, three for North Dakota, and about one per year for each of the Prairie Provinces. Average annual tornado incidence per 10,000 square miles from 1970 to 1997 Tornado data have been collected only since the early 1950s. Since that time the data indicate that there has been an increasing trend in tornadoes in the Great Plains. However, this increase has been observed only in the weak F0 and F1 tornadoes; there has been no increase in the number of days with tornadoes. Scientists have concluded that almost all of the apparent increase is due to enhanced public awareness and better reporting methods during the last few decades. See also FOLKWAYS:. Kenneth F. Dewey University of Nebraska-Lincoln

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