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U.S. Tornado Clusters Are on the Rise: Study

Researchers find fewer days with tornadoes now than 50 years ago but the same number of twisters -- meaning they're clustering together on terrifying days.
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Tornadoes are touching down in clusters more often than 50 years ago, a new study reports. On some days, more than 30 twisters strike the United States. Even as storms spawn more tornadoes, there are fewer days on which tornadoes occur, according to the study, published Thursday in the journal Science. Since the 1970s, the number of days with at least one EF-1 tornado has dropped from a mean (or average) of 150 to 100.

While it's clear that something about tornados in the United States is changing, there is no strong evidence that climate change is to blame, lead study author Harold Brooks, senior scientist at the National Oceanic and Atmospheric Administration's Severe Storms Laboratory in Norman, Oklahoma, told Live Science.

A similar analysis published in August concluded that tornadoes are both growing stronger and increasingly arriving in clusters, even as the total number of days with tornadoes held steady. Brooks and his co-authors looked at tornadoes of EF-1 intensity and greater between 1954 and 2013, using official tornado records from U.S. Storm Prediction Center. The yearly tornado total has remained stable over time, the researchers discovered. The mean annual rate of tornadoes rated as EF-1 and greater has held steady at 495 per year since the mid-20th century, though the total count can swing wildly. The record high, in 2011, was 898 tornados, and the record low, of 311 tornadoes, was recorded in 2002.

But now there are long dry spells between days of terror when huge numbers of tornadoes add to the yearly count. For example, in 1973, most of the year's tornadoes were spread among 187 tornado days, and only two days had more than 30 tornadoes. But in 2011, there were nine days that had more than 30 tornados, with only 110 tornado days.

— Becky Oskin, LiveScience

This is a condensed version of a report from LiveScience. Read the full report. Email Becky Oskin or follow her@beckyoskin. Follow us@livescience, Facebook &Google+.