Tornado outbreaks are getting more violent

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Enlarge (credit: The National Severe Storms Laboratory)

A tornado may cause localized destruction, but the most severe problems come when a storm system spawns multiple tornadoes. This creates what’s called a tornado outbreak, which spreads destruction across a wider area. Now, a new study suggests that the most violent tornado outbreaks are on the rise. But the researchers behind the study see no indication that the rise in tornado outbreaks is connect with our warming climate.

It would make sense for a warming climate to influence tornado activity. After all, higher temperatures mean more energy in the atmosphere, potentially powering the storms. But past studies have produced mixed results when it comes to tornado activity. There’s not a significant trend in the number of tornadoes or the frequency of outbreaks (defined as six or more tornadoes that occur in rapid succession). At the same time, tornadoes are occurring in more of the year, and the number of tornadoes in outbreaks has become increasingly variable.

A team of researchers from Columbia University (Michael Tippett, Chiarra Lepore, and Joel Cohen) decided to look at this last figure more carefully. They collected data on the number of storms in outbreaks in the period between 1965 and 2015. While there was no trend in the number of outbreaks, the number of tornadoes per outbreak has gone up across that time period. Not only was the mean number of tornadoes per outbreak going up, but the more extreme outbreaks—the ones with the most storms—were increasing the fastest.

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