New research suggests that tornado outbreaks aren't independent of each other, which in turn means they're a staggering 100 times more likely than we thought—but that stormy grey cloud may just have a silver lining.


Tornadoes are usually assumed to be independent, which is to say that their formation doesn't influence the formation of others. In turn, that should mean that, given a particular set of conditions, the number of tornadoes you'd expect on a given day would be independent of how many other tornadoes are occurring

But a team of researchers from Florida State University thought that seemed a little simplistic—so they examined daily tornado counts in the US between 1994 and 2012 to see if it was really true. When they plotted the frequency distribution of tornado days against how many tornadoes took place on that day, they had a shock: the relationship actually follows a power law.


The data means that—like earthquakes—tornadoes aren't entirely independent of each other. James Elsner, one of the researchers, explains to Environmental Research Web:

"Because the exponent is less than two it tells us that average statistics (for example, there are an average of 4.5 tornadoes per tornado day) are practically worthless for estimating the probability of rare events, like the tornado outbreak of 27 April 2011."

Turns out, the odds of having 145 tornadoes in a single day are actually one in 10,000, not the one in 10 million that chance—and accepted wisdom—would suggest. In fact, Elsner predicts in Environmental Research Letters that such a dramatic event should occur approximately every 71 years in the US.

This all sounds like real bad news. But there's some hope, because understanding how the existence of tornadoes can influence the formation of others means we might be safer than before. "More accurate risk assessments might lead to practical measures such as safe rooms and building codes in areas like the US South or Midwest," explains Elsner.


But in the shorter term, just knowing that tornadoes breed tornadoes means that prior warning for violent and extreme tornado outbreaks should be better than it's been in the past. And that's quite the silver lining. [Environmental Research Letters via Environmental Research Web]

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