Tornado Trends over the past Thirty Years

Dr. Stanley Changnon has done extensive work in climatological studies that included tornado frequency. He studied these frequencies using tornado statistics across Illinois using data from storms that occurred between 1916 and 1980 (Changnon, 1982). He also guided work done at the Illinois State Water Survey on the significance of long-track tornadoes (Wilson and Morgan, 1971). One of the conclusions from their studies was that only half of all tornadoes are reported, and that half of those that were reported were really not tornadoes! This agrees with Grazulis (2001), who referred to a similar study by Iowa State University in 1978 that also concluded that only half of all tornadoes are actually counted! This paper looks at the reported frequencies of tornadoes and their characteristics over the contiguous United States since 1970. There was a significant increase in tornado occurrence during two periods in the last 33 years – in the early 1980s when National Weather Service (NWS) warning verification began, and in 1990 when the WSR-88D became operational. The frequency of tornadoes is also compared to fatalities and tornado days over the same period. Also, tornado frequency is compared to F-scale damage categories (Fujita, 1971). Finally, it will be seen that the number of strong and violent tornadoes has not varied much since 1970, and that long-track and very-long-track tornadoes as defined by the Illinois State Water Survey remain a very low percentage of all tornadoes reported.