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THEY are mimicking buses: you wait ages for a tornado and then lots come at once. US twisters are increasingly arriving in clusters, sometimes of more than 30 in a single day.
Global warming could be to blame. A warmer world ought to influence tornadoes but numbers have held steady for decades. What鈥檚 changed is that they now seem to turn up in groups, says James Elsner of Florida State University in Tallahassee.
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Elsner studied records of major US tornadoes between 1954 and 2013. He found there have been fewer 鈥渢ornado days鈥� per year in recent years, but just as many tornadoes. Before 1990 there were no days with 32 or more tornadoes. But every year since 2001 has had at least one such day, and in 2011 there were six (Climate Dynamics, ).
鈥淵ou鈥檒l see fewer days in which you鈥檙e threatened by tornadoes, but when you are, the threat will be greater,鈥� says Elsner.
Harold Brooks of the National Severe Storms Laboratory in Norman, Oklahoma, presented similar data at a 2012 meeting. He found the number of tornado days per year has dipped from 150 to 100 over 30 years.
This article appeared in print under the headline 鈥淭ornadoes now arrive in gangs鈥�