National Weather Service United States Department of Commerce


Mark F. Britt and Fred H. Glass
St. Charles, Missouri


Data Sources and Methodology

The Storm Prediction Center's (SPC, formerly the National Severe Storms Forecast Center) severe weather database was used for this study (Schaefer and Edwards, 1999). This database contains tornado statistics from 1950 to 1995. Statistics for 1996 to present were extracted from the U.S. Local Storm Events Database on the National Climatic Data Center's website for inclusion in this study.  Statistics previous to 1950 were taken from a extensive database of killer and significant tornadoes (F2-F5) compiled by Tom Grazulis (1993).

Since tornado warnings are issued and verified by county, the SPC database contains information on tornado segments rather than tornadoes. A segment is counted each time a tornado touches down or crosses a county line, therefore a single tornado which moved across three counties would be recorded as three separate tornado segments. After close examination of all tornado segments using the Severe Plot software (Hart 1993) and Storm Data, the authors were able to merge individual segments back into individual tornadoes.

Tornado intensity is determined by using the F-Scale (Fujita 1981), as listed in Storm Data. This study follows the accepted nomenclature that F2 and F3 tornadoes are strong and F4 are violent. Tornado times are in Central Standard Time (CST) and refer to the hour in which they occurred. For example, 14 refers to any tornado that occurred between 1400 to 1459 CST. A tornado day is a traditional 24-hour day (0000 to 2359 CST) in which at least one tornado occurred.

Data Limitations

Kelly et al. (1985) and Hales (1993) have documented a number of limitations inherent to the SPC database. They found that the number of severe weather reports increases in areas with greater population density and/or a greater concentration of trained spotters and well-educated people. Highway distributions, distance to an official reporting station, time-of-day, the warning office's motivation in verifying warnings, lack of appropriate measuring devices, intervening clouds, and topographic features are additional factors which affect severe weather reporting. Finally, the perception and motivation of the person witnessing the event must be considered. For example, a farmer may not feel obligated to report a tornado that touches down in an open field.

Ostby (1993) found that the occurrence of weak tornadoes (F0-F1) has shown a dramatic increase since 1980, while the number of strong and violent tornadoes has either remain steady or decreased. Reasons for this include improved verification efforts by local NWS offices and the marked increase in storm chasing. Since strong and violent  tornadoes produce a more stable long-term data set, these categories were the main focus of this study.


The authors would like to thank Doug Speheger from WFO Norman for his computerized help in deciphering the SPC database. Additional thanks are extended to Ron Przybylinski (SOO/WFO LSX) and Steven Thomas (MIC/WFO LSX) for support in this study.


Fujita, T., 1981: Tornadoes and Downbursts in the context of generalized planetary scales. J. Atmos. Sci., 38, 1511-1534.

Galway, J. G., 1977: Some Climatological Aspects of Tornado Outbreaks. Mon. Wea. Rev., 105, 477-484.

Grazulis, T. P., 1993: Significant Tornadoes 1680-1991.  A Chronology and Analysis of Events. Environmental Films, Tornado Project, St. Johsnbury, VT.

Hales, J.E., 1993: Biases in the severe thunderstorm data base: Ramifications and solutions. Preprints, 13th Conf. Wea. Forecasting and Analysis, Vienna, VA, AMS (Boston), 504-507.

Hart, J.A., 1993: SVRPLOT: A New Method of Accessing and Manipulating the NSSFC Severe Weather Data Base. Preprints, 17th Conf. On Severe Local Storms, St. Louis, AMS (Boston), 40-41.

Kelly, D.L., J.T. Schaefer, and C.A. Doswell, III, 1985: Climatology of Nontornadic Severe Thunderstorm Events in the United States. Mon. Wea. Rev. 113, 1997-2014.

National Oceanic and Atmospheric Administration, National Climatic Data Center Website:

Ostby, F. P., 1993: The Changing Nature of Tornado Climatology. Preprints, 17th Conf. On Severe Local Storms, St. Louis, AMS (Boston), 1-5.

Schaefer, J. T. and R. Edwards, 1999: The SPC Tornado/Severe Thunderstorm Database. Preprints, 11th Conf. On Applied Climatology, Dallas, AMS (Boston), 215-220.