Shifting Twisters

WooooOOOOOO!

A loud siren begins to wail. It’s warning people to seek shelter—quickly! Moments later, a spinning funnel-shaped cloud descends from the dark, stormy sky above. It’s a tornado! The rotating column of air begins to move over the ground. Its strong, violent winds leave a trail of devastation in its wake, uprooting trees, tossing cars like toys, and ripping roofs off houses.

Tornadoes are the most destructive storms on the planet. About 1,300 occur each year in the United States—more than any other country. Canada, in second place, averages only about 100 annually. In the U.S., an area in the Great Plains known as Tornado Alley experiences the most tornadoes. But over the past 30 years, scientists have noticed a shift in tornado patterns. Fewer twisters are spawning in Tornado Alley and more are showing up in the Southeast (see A New Tornado Alley?).

That shift has big implications for people in the Southern U.S., who aren’t used to dealing with tornadoes. Scientists are studying what could be causing the uptick in twisters in this region—and what it means for the people living there.

WooooOOOOOO!

A loud siren wails. It’s warning people to seek shelter—quickly! Moments later, a funnel-shaped cloud drops from the dark, stormy sky. It’s a tornado! The spinning column of air moves over the ground. Its strong, violent winds leave a trail of destruction behind. They uproot trees, toss cars like toys, and rip roofs off houses.

Tornadoes are the most destructive storms on the planet. About 1,300 occur each year in the United States. That’s more than any other country. Canada is in second place, with only about 100 each year. In the U.S., an area in the Great Plains experiences the most tornadoes. It’s known as Tornado Alley. But over the past 30 years, scientists have noticed a shift in tornado patterns. Fewer twisters are forming in Tornado Alley, and more are showing up in the Southeast (see A New Tornado Alley?).

That shift spells big trouble for people in the Southern U.S. They aren’t as used to dealing with tornadoes. What could be causing the increase in twisters in this region? And what does it mean for the people living there? Scientists are looking for answers.

There’s a reason so many tornadoes have historically spawned in the Great Plains: The area has unique weather conditions that fuel twisters. “There is a specific set of ingredients that come together quite often in the region,” says Victor Gensini, an atmospheric scientist at Northern Illinois University.

Winds tend to carry moist air northward from the Gulf of Mexico into the Great Plains. Drier air flows over the Rocky Mountains from the west and forms a layer like a cap on top of this moist air. It’s like putting a lid on a pot of boiling water, says Gensini. This allows energy to build up in the atmosphere until a severe thunderstorm develops.

Sometimes a large rotating thunderstorm called a supercell forms. This happens when winds blow at different speeds or directions at different heights, creating wind shear that causes a storm to rotate around its center. This spinning storm can spawn a funnel cloud that stretches to the ground as a tornado.

So many tornadoes have formed in the Great Plains for a reason. The area has unique weather conditions that fuel twisters. “There is a specific set of ingredients that come together quite often in the region,” says Victor Gensini. He’s an atmospheric scientist at Northern Illinois University.

Winds tend to carry moist air northward from the Gulf of Mexico into the Great Plains. Drier air flows over the Rocky Mountains from the west. It forms a layer like a cap on top of this moist air. It’s like putting a lid on a pot of boiling water, says Gensini. This allows energy to build up in the atmosphere. Finally, a severe thunderstorm develops.

Sometimes a large rotating thunderstorm forms. It’s called a supercell. This happens when winds blow at different speeds or directions at different heights. Because of this wind shear, the storm begins to rotate around its center. This spinning storm can form a funnel cloud. The funnel stretches to the ground as a tornado.

Although tornadoes are still most common in the central U.S., the number that occur there is smaller than it was a few decades ago. Meanwhile, the frequency of tornadoes in the Southeast has increased. One possible cause is climate change. Warmer weather has led the southern Great Plains region to become hotter and drier. With less moisture in the air, fewer thunderstorms can develop.

But something else may be going on, such as a natural shift in tornado patterns. Official records go back only to the 1950s, so scientists don’t know if such changes have occurred in the past. “We can definitely tell that where tornadoes are happening is changing,” says Gensini. “But we can’t really point our finger at the cause of the change.”

Whatever the reason, the trend spells danger. Even though Tornado Alley still experiences more tornadoes, the Southeast suffers the most injuries and deaths from these storms. That’s partly because the Southeast has a denser population. And many people live in homes that aren’t designed to withstand tornadoes. And unlike the vast, open Great Plains, where a twister is easily visible, the Southeast is full of buildings and trees that can make a tornado hard to spot until it’s too late.

Although the risk is increasing in the Southeast, Gensini points out that tornadoes have occurred in all 50 states. So everyone should know what to do in case a twister strikes: Find shelter in a basement or lowest level of a permanent structure, in an interior room away from windows. Do not remain in a mobile home. If appropriate shelter is not available, lie flat in the nearest ditch and cover your head with your hands. “The best thing that you can do is create an action plan ahead of time,” says Gensini.

Tornadoes are still most common in the central U.S. But the number there is smaller than it was a few decades ago. And the number of tornadoes in the Southeast has increased. One possible cause is climate change. Because of warmer weather, the southern Great Plains region has become hotter and drier. Less moisture is in the air, so fewer thunderstorms can develop.

But something else may be going on. It could be a natural shift in tornado patterns. Have such changes happened in the past? Scientists don’t know, because official records go back only to the 1950s. “We can definitely tell that where tornadoes are happening is changing,” says Gensini. “But we can’t really point our finger at the cause of the change.”

No matter the reason, the trend spells danger. Tornado Alley still experiences more tornadoes. But the Southeast suffers the most injuries and deaths from these storms. That’s partly because the Southeast has a denser population. And many homes aren’t designed to withstand tornadoes. Also, a twister is easy to spot on the vast, open Great Plains. But the Southeast is full of buildings and trees. That can make a tornado hard to see until it’s too late.

The risk is increasing in the Southeast. But Gensini points out that tornadoes have occurred in all 50 states. So everyone should know what to do if a twister strikes. Find shelter in a basement or lowest level of a permanent structure. Then go into an inner room away from windows. Do not remain in a mobile home. If you can’t find shelter, lie flat in the nearest ditch. Then cover your head with your hands. “The best thing that you can do is create an action plan ahead of time,” says Gensini.