Pattern of US tornado activity has changed over last 45 years

Science : For decades, the US has averaged nearly 500 tornados per year. However, a pair of recent studies has revealed that whereas the average number of days with at least one damaging tornado was higher in the 1970s, there are now more days during which multiple tornados occur. Harold Brooks of the National Oceanic and Atmospheric Administration (NOAA) National Severe Storms Laboratory in Norman, Oklahoma, and his colleagues performed one study, and James Elsner of Florida State University in Tallahassee and his colleagues performed the other. Both groups used data from NOAA’s National Weather Service, which began tracking tornados in 1953 and recording their scale in the 1970s. The researchers had to discard the F0 rating because of the inconsistency in the reporting of minor tornados, but the remaining data set was found to be relatively unbiased. Although the change in tornado patterns may be tied to changing climate patterns, the actual connections are still not clear. Moisture, land temperatures, and differences between polar and midlatitude temperatures are all root causes of tornado formation, but exactly how they interact and affect each other is unknown.