Tornados are behaving strangely: The number of tornado outbreaks per year is fairly constant, but the number of tornados per outbreak has skyrocketed. And scientists aren’t entirely sure why.

In an effort to learn more, researchers looked at meteorological factors related to tornado outbreaks , and then dug into the data to see whether these factors had changed over time, said study lead researcher Michael Tippett, an associate professor of applied physics and applied mathematics at Columbia University.

The analyses did yield a result, but an unexpected one, Tippett said.  [The Top 5 Deadliest Tornado Years in US History]

“The meteorological factors that are related with tornado outbreaks have also become more extreme,” Tippett told Live Science in an email. “The surprising finding was that the change in meteorological factors did not have the expected signature of climate change.”