for National Geographic News
Tornadoes that form in midwinter—such as the outbreaks that killed at least 50 people across the southern U.S. Tuesday—are not necessarily stronger than storms that form in warmer months, experts say.
But they can be deadlier.
"The big problem is that the tornadoes themselves tend to be moving faster," said Harold Brooks, a research meteorologist at the National Severe Storms Laboratory (NSSL) in Norman, Oklahoma, an agency of the National Oceanic and Atmospheric Administration.
That's because the winds in the upper atmosphere that produce tornado-spawning thunderstorms are faster during the winter, he pointed out.
Such storms reduce the time that warnings are effective and allow people to get out of harm's way.
Data from Tuesday's storms are still being examined, and investigators haven't reached any conclusions about their intensity or movement, Brooks said.
But he said he would be "shocked" if the tornadoes weren't rated at least F3 on the Fujita scale, which is used to rank twister destructiveness.
Such tornadoes have winds ranging from 158 to 200 miles an hour (254 to 322 kilometers an hour) and can result in severe damage.
Only about 5 percent of tornadoes reach at least F3 intensity, but these stronger storms cause at least 70 percent of tornado fatalities, Brooks said.
F4 and F5 storms, the worst tornadoes ever observed, are even rarer. And there are doubts that F6 twisters—the highest on the scale—can even form.
Cause and Effect
Greg Carbin, NSSL's warning-coordination meteorologist, said unseasonably warm and moist conditions across the Mississippi and Tennessee River Valleys were a major factor in the formation of the tornadoes. (See photos of the tornadoes and their deadly effects.)
A weather phenomenon known as La Niña could also have contributed to the storms' formation.
La Niña is a cooling of tropical waters in the Pacific Ocean. The phenomenon is thought to contribute to hurricane formation during the summer months.
Carbin said that recent studies "indicate apparent increases of tornadoes during a La Niña cycle."
But he also noted that reliable data about tornadoes goes back only 50 to 75 years, and the link between tornado formation and La Niña is "tenuous at best."
"It would be nice to have 200 to 250 years of high-quality data," Carbin said. "Then we might be able to say with certainty whether a link exists."
Brooks added that he doesn't think global warming is affecting tornado formation.
"We really don't have a very good handle on what the expectations would be for tornadoes to change—or if they would change—as a result of global warming," he said.
"If I had to bet, I would say that we won't see a huge change in tornadoes."
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