News Release 96-023
Tornado Season Strikes!
May 22, 1996
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Spring is tornado season, with about 50 percent of all reported tornadoes occurring between April and June. Most tornadoes strike between noon and sunset, with the eastern two thirds of the United States home to the greatest concentration of tornadoes on earth.
The National Science Foundation (NSF) supports research in several fields of atmospheric sciences related to the study of tornadoes.
NSF researchers have found that the strongest tornadoes develop in "supercells," long-lived thunderstorms with large scale cyclonic circulations that can persist for hours and extend well beyond the storm itself. These strong circulations are often associated with the most violent twisters, yet only a small number of supercells produce tornadoes, says Jewel Prendeville of NSF.s lower atmospheric facilities section. Recent data indicate that gust fronts and other wind boundaries near the ground are important factors in a tornado's eventual formation.
As storm-spotter and Doppler-radar networks improve and public awareness increases, the number of reported tornadoes is rising. From 1953 to 1991, an average of 768 tornadoes reported per year, but since 1990, more than 1,000 tornadoes have been reported annually. The year 1992 produced both the annual record of 1,293 and the monthly record of 399, reported in June. The 1995 total was 1,233.
Tornadoes have killed more than 3,700 people since 1953. Though fatalities have been dropping in recent years, tornadoes killed 69 people in 1994--the most deaths since 1984--and 29 people in 1995.
Tornadoes have been reported in every U.S. state, but they are most concentrated in .Tornado Alley, which runs north from central Texas through Oklahoma and Kansas into eastern Nebraska, South Dakota, and Iowa. Other areas of high frequency include the Corn Belt of Illinois and Indiana and the Deep South states of Mississippi, Alabama, and Georgia.
The typical track length is about one mile for all tornadoes and 23 miles for the strongest ones. The longest track length recorded for a single tornado is the 219-mile track of the Great Tri-State Tornado of March 18, 1925. The typical track width is only 48 yards, but it can range to more than a mile.
Thunderstorms and their attendant tornadoes typically move northeasterly or easterly, but tornado tracks can be erratic. Tornadoes move forward at speeds ranging from nearly stationary to 60 mph or more, and averaging 20 to 40 mph. Measurements from Doppler radar give top speeds in the range of 250-300 mph. Most tornadic winds are below 150 mph.
Cheryl L. Dybas, NSF, (703) 292-8070, email: firstname.lastname@example.org
Jewel Prendeville, NSF, (703) 292-8521, email: email@example.com
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