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A single tornado can cause a lot of damage, but even worse - tornado outbreaks. Just this week, a cluster of at least 18 tornadoes struck the southeast over two days. And in recent years, scientists have been seeing bigger clusters. As NPR's Christopher Joyce reports, they're struggling to figure out what's happening.
CHRISTOPHER JOYCE, BYLINE: When weather conditions are just right, sometimes you get more than just a tornado. You get an outbreak. Mathematician Michael Tippett at Columbia University tracks these outbreaks. He says while the number of tornadoes nationwide varies a lot year to year, the overall average is pretty steady.
MICHAEL TIPPETT: But the number of tornadoes in outbreaks is increasing.
JOYCE: And the number of tornadoes in the most extreme outbreaks is increasing the fastest. Scientists who study climate change suspect that warming temperatures may affect how many tornadoes we get, but Tippett's says he's not seeing a connection between climate change and these bigger outbreaks.
TIPPETT: It's not the expected signature of climate change - OK? - which could be either it's something else, or it could be that we don't really understand why climate change is doing.
JOYCE: All sorts of things influence weather. Two big ones are the circulation of warm water in the Pacific and Atlantic Oceans. That circulation changes over decades, which, in turn, alters long-term weather patterns. Writing in the journal Science, Tippett says those ocean changes may be implicated here, but there's no evidence yet. The only thing that seems to be changing are certain kinds of wind patterns near these clusters. No matter what the cause, these bigger outbreaks hurt people in their path. This week's killed five, and they affect the insurance industry. Bigger outbreaks usually mean more damage and more payouts. Christopher Joyce, NPR News.