Tornadoes: Tip Sheet
BOULDER -- Parts of Oklahoma and Texas were ravaged by violent tornadoes on Monday, May 3, killing over 40 people. To help you file stories on these and other devastating twisters, here are some tornado facts and resources in print and on the World Wide Web. The National Center for Atmospheric Research (NCAR) is managed by the University Corporation for Atmospheric Research, a consortium of more than 60 universities offering Ph.D.s in atmospheric and related sciences. NCAR's primary sponsor is the National Science Foundation.
Facts on United States tornadoes
Frequency. As storm spotter and Doppler-radar networks improve and public awareness increases, the number of reported tornadoes also is rising. From 1953 to 1991 an average of 768 tornadoes were reported per year, but since 1990 records list over 1,000 tornadoes each year. The year 1992 produced both the annual record of 1,297 and the monthly record of 399, reported in June. The preliminary total for 1998 was 1,254.
Fatalities. U.S. tornadoes have killed over 3,700 people since 1953. Though fatalities have been dropping in recent decades, the latter 1990s have seen an upturn in tornado deaths. Tornadoes killed 67 people in 1997 and 129 in 1998, the most since 1974.
What causes tornadoes? The strongest tornadoes occur from "supercells," long-lived thunderstorms with large-scale cyclonic circulations that can persist for hours and extend well beyond the storm itself. These strong circulations often are associated with the most violent twisters, yet only a small percentage of supercells produces tornadoes. Recent data indicate that gust fronts and other wind boundaries near the ground are important factors in a tornado's eventual formation.
Where? 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.
When? Spring is tornado season, with about 50% of all reported tornadoes occurring from April through June. Most tornadoes strike between noon and sunset.
Path of destruction. The typical (median) 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 was 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.
Direction and wind speed. Thunderstorms and their attendant tornadoes typically move northeast or east, but tornado tracks can be erratic. Tornadoes move forward at speeds averaging 20 to 40 mph but ranging from nearly stationary to 60 mph or more. Measurements from Doppler radar and estimates based on motion pictures give top speeds in the range of 250-300 mph. Most tornadic winds are below 150 mph.
Sites on the World Wide Web
This page includes official outlooks, climatology, and statistics on tornadoes and severe storms. SPC is the federal entity in Norman, Oklahoma, that issues tornado watches. It was formerly the Severe Local Storms unit of the National Severe Storms Forecast Center.
This page brings Tom Grazulis's expertise in tornado climatology to the Internet. You can learn about tornado history, review safety tips, peruse a set of "top ten" lists, or order books, videos, and posters from the project, including their three-volume "Tornado Video Classics" series.
This British entity provides statistics and other information on tornadoes and severe storms in the United Kingdom and throughout Europe.
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Prepared for the web by Jacque Marshall
Last revised: Fri Apr 7 15:38:50 MDT 2000
Last revised: Thu May 6 13:24:20 MDT 1999