By Lauren Casey

PHILADELPHIA (CBS) — Fewer days, more outbreaks. Since the 1950’s, the number of days on which a tornado occurs somewhere in the United States has decreased. But on days when tornadoes do form, there are more of them and global climate change may be playing a role.

“Instead of one or two going through a state with a couple of storms, you’re looking at multiple tornadoes, five, ten or more than that over a much broader area, several hundred square miles,” said Meteorologist Sean Sublette, with Climate Central.

In 2011, the United States experienced its largest tornado outbreak ever recorded.

From April 25th to the 28th, the super outbreak, spawned over 350 tornadoes from Texas to New York, killing more than 320 people.

This type of multi-day, multi-state tornado outbreak is increasing, and a rise in average global temperatures may be playing a role.

“You’ve got to have heat and you’ve got to have spin,” Sublette says.

These are the two main ingredients in tornado formation.

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“So in a warming world, you’re certainly going to have more heat available, but the spin is still an open question, the spin is referred to as shear, in the atmosphere you have to have winds going different direction and speeds as you travel upward,” said Sublette.

And the higher heat availability may be a factor in an increasing number of outbreak days.

In a 2014 study, tornado researchers at Florida State University looked at days with at least four, eight, 16 and 32 tornadoes. They found that while days with at least four tornadoes were on the decline, the number of days with 16 and 32 tornadoes has increased. And the odds of a day with at least 32 tornadoes has more than doubled.

“Whenever the weather is really bad, people want to know, is this climate change, and strictly speaking, you can’t say yes or no. Having said that, the entire atmosphere is a little bit different now than it used to be so we can measure how much climate change impacted a certain event,” Sublette says.

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