1995-6 For release: March 1995 Contact: Joan Vandiver Frisch Manager, UCAR Media Relations tel. (303) 497-8607; fax 303-497-8610 E-mail: jfrisch@ucar.edu Dial-an-Expert and Topic Summary on Tornadoes The eastern two-thirds of the United States is home to the greatest concentration of tornadoes on Earth, and spring is tornado season. To help you file stories on these devastating storms, here is some information selected from the book Significant Tornadoes 1680-1991, by Thomas Grazulis and from recent issues of Weatherwise magazine. Frequency--As storm spotter networks improve and public awareness increases, the number of reported tornadoes also is rising. The number of officially counted tornadoes is increasing. From 1953 to 1991 an average of 768 tornadoes were reported per year, but since 1990 records list over 1000 tornadoes each year. 1992 produced both the yearly record of 1,293 and the monthly record of 399, reported in June. The 1,076 tornadoes in 1994 were the fewest in five years. Fatalities--Tornadoes have killed over 3700 people since 1953. Though fatalities have been dropping in recent years, tornadoes killed 69 people in 1994, the most deaths since 1984. Injuries and damage--Tornadoes injured 1,139 people and caused an estimated $481 million in damage in 1994. What causes tornadoes?--The exact mechanism is not yet known. 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 tornadoes, yet less than half of all supercells produce 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 state in the United States, 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. When--Spring is tornado season with 57% of tornadoes reported between 1962 and 1991 occurring in April, May, and June. Most tornadoes strike during the warmest part of the day, 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. Grazulis lists the longest track length recorded for a single tornado as 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 one mile. Usually the area of destruction is less than one square mile. Direction and wind speed--Thunderstorms and their attendant tornadoes typically move northeast., but tornado tracks can be erratic. Tornadoes move forward at speeds averaging 20 to 40 mph. Measurements of rotational wind speed made with Doppler radar and estimates based on motion pictures give top speeds in the range of 250-300 mph. Most tornadoes have wind speeds of less than 150 mph. For further information and interviews about recent tornado research, here are the names and telephone numbers of some experts at the National Center for Atmospheric Research (NCAR), at University Corporation for Atmospheric Research (UCAR) member institutions, and at the National Oceanic and Atmospheric Administration (NOAA). - Howard Bluestein, University of Oklahoma: synoptic meteorology; mesoscale meteorology; severe thunderstorms; tornadoes. (405) 325-3006 - Grant Darkow, University of Missouri: convective storms; tornadoes; mesoscale meteorology. (314) 882-6593 - Tom Grazulis, The Tornado Project, St. Johnsbury, VT: author of Significant Tornadoes 1680-1991; history, videos of tornadoes. (802) 748- 2505 - Joseph Klemp, NCAR, Boulder, CO: tornadoes; winds. (303) 497-8902 - Erik Rasmussen, NOAA's National Severe Storms Laboratory, Norman, OK: observations and theory of tornadoes and severe thunderstorms; tornadogenesis. (405) 366-0520 - Richard Rotunno, NCAR, Boulder, CO: dynamic meteorology, microscale to synoptic scale. (303) 497-8904 - Edward Szoke, NCAR: mesoscale meteorology; tornadogenesis; storm initiation. (303) 497-8961 - Morris Weisman, NCAR: storm modeling and observation; mesoscale convective systems. (303) 497-8901 - James Wilson, NCAR: mesoscale meteorology; short period forecasting; nowcasting with Doppler radar; microbursts. (303) 497-8418 NCAR is managed by UCAR, a consortium of 61 member institutions that grant the Ph.D. in atmospheric, oceanic, or related sciences. NCAR is sponsored by the National Science Foundation.