ANNOUNCER: You’re listening to Short Wave from NPR.
BERLY MCCOY: Hey, Short Wavers, producer Berly McCoy in the host chair today. So every couple of years when I was growing up in the suburbs of St. Louis, Missouri, my family and I would gather in our basement, not for a party or game night, but to take cover from a potential tornado. As a kid living around Tornado Alley, I thought this was normal. If you’re unfamiliar, Tornado Alley is just a seasonally shifting section of the US that gets a high level of tornadoes. But I later learned that people who live outside of this area don’t experience nearly the same amount of tornadoes.
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SUSHMITA PATHAK: It is really the global hotspot of tornadoes.
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MCCOY: That’s Sushmita Pathak, a freelance science journalist who wrote about the science of tornadoes for the publication Eos. And she says the reason this region has at least 10 times more tornadoes than any other place in the world is clear.
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PATHAK: You have to blame geography for that.
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MCCOY: So tornadoes form from thunderstorms. And for them to do that, different types of winds need to blow at different temperatures in different directions.
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PATHAK: You need cold, dry air coming in from one direction.
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MCCOY: Like from the Rocky Mountains.
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PATHAK: You need warm, humid air coming in from one direction.
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MCCOY: Like the air coming up from the Gulf of Mexico.
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PATHAK: And you also need, like these strong gusts of wind that are a little bit higher up in the sky that kind of like exert that force.
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MCCOY: These winds moving at vastly different speeds and directions, are called wind shear, and they can lead to rotation within the thunderstorm.
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PATHAK: So now you have a spinning column, a rotating thunderstorm, and if the conditions are right, that rotating column can stretch to the ground. And that’s how you have a tornado.
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MCCOY: Technically, tornadoes can happen anywhere in the world and have been recorded on every continent except Antarctica. But even taking into account the cold air from the Rockies and warm air from the Gulf, scientists still wondered why there were so many more tornadoes in Tornado Alley than anywhere else around the globe, especially considering Tornado Alley is very similar to a section of South America, at least geographically.
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PATHAK: So the questions researchers really had was like, if there’s this like similarities in the geography of these two continents, why doesn’t Central and South America have a Tornado Alley?
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MCCOY: Today on the show, new research on why the US gets so many more tornadoes, and what the findings might mean for reducing them in the future. You’re listening to Short Wave, the science podcast from NPR.
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MCCOY: OK, Sushmita, let’s talk about the research you reported on. It was done by a scientist named Funing Li when he was at MIT, and Funing and his colleagues studied tornado occurrence by using this historical data to model and simulate the interaction between land and the atmosphere. Tell me about what they found.
PATHAK: So the starting point of their research was really like the geographic setup in Central North America and Central South America is kind of similar, but South America does not get as many tornadoes, even near as many tornadoes as North America does. And so they really wanted to figure out why there was this huge contrast. And what they found was it was because of a surprising new ingredient, which is the roughness of the land surface, many, many thousands of kilometers away from where the tornadoes were actually happening. So when the easterly winds are coming over the Gulf of Mexico towards central North America, Tornado Alley, that’s a relatively smooth surface, this vast expanse of ocean. So, you know, winds don’t really encounter any resistance. They can, you know, build up a lot of speed. And that’s really important for wind shear, which is very important for tornado formation. But in South America, what happens is those easterly winds are blowing over the Amazon rainforest. So you have a lot of vegetation, a lot of hills, so a very rough surface. And because of this rough surface, the winds get broken down. And so the tornado potential also gets suppressed.
MCCOY: Wow. So it really is this big picture, geographical reason for why there’s a different amount of tornadoes in North and South America.
PATHAK: Yeah.
MCCOY: And so it’s really just like if you have flat or you have not flat. That’s the big factor?
PATHAK: Yeah.
MCCOY: And way upstream, yeah, and upstream being different for North and South America?
PATHAK: Just the region near the Equator from where the easterly winds are coming into those regions.
MCCOY: OK. So is this like coming from the south in the Northern Hemisphere and coming from the north and the south?
PATHAK: Yes.
MCCOY: OK. OK.
PATHAK: Yes, and to test this sort of hypothesis and to arrive at their findings, the researchers tested this out in a global climate model, which is like a computer simulation of the Earth. You know, scientists use it all the time to learn about climate patterns and figure out how they’ll change in the future. So in this global climate model, you can sort of tweak different parameters to see how things will change. And so what the scientists did was they replaced the Gulf of Mexico with forests to make it kind of rougher. And when they did that, they saw that tornado potential in the US in Tornado Alley, it went down.
MCCOY: Wow.
PATHAK: And similarly, when they smoothed out the Amazon forest, tornado potential in Central and South America went up.
MCCOY: Wow.
PATHAK: And I say tornado potential here because the model cannot produce tornadoes exactly. It can only simulate the environment, the instabilities that lead to a tornado. So that’s just a small but important caveat.
MCCOY: OK. And speaking of caveats, this– I mean this research seems like it’s pretty straightforward, pretty solid?
PATHAK: Yeah, it is. It is pretty solid. And I asked another scientist who was not part of the research about limitations, and she was like, this feels like a pretty solid study. And actually, she studies how land cover in the vicinity of the storm, like a mile or 2 miles, 2-mile radius of the storm, changes tornadoes. And so she was actually surprised. She was actually very fascinated by this, that land cover and terrain so far away from the actual storm can also affect tornado potential.
MCCOY: OK, so it’s like people hadn’t thought to look this far upstream. And how far are we talking?
PATHAK: We’re talking about hundreds of miles away from where the tornadoes are actually forming, from where the storms are actually forming.
MCCOY: Wow. So the researchers did this work, solved kind of a big mystery in why Tornado Alley gets so many tornadoes. What could be some implications of their research?
PATHAK: There are a lot of implications because, you know, we as humans do a lot of things that can change the roughness of the terrain. When you cut down forests, it’s like smoothing out the surface. That’s already happening in the Amazon with deforestation. Or when you set up large-scale wind farms on a vast, flat expanse, it can make the overall surface rougher. That’s happening in the Midwest. So the authors of the study really want to bring attention to this point that there are a lot of different things that affect tornadoes and that lead to tornado genesis. And one thing that’s not been looked into till now is this large-scale surface roughness or the roughness of the terrain very, very far away from where the storms are forming. So this was the big takeaway.
MCCOY: And then I know climate change is linked to increases in extreme weather, so heavy, extreme rainfall, heat, drought. And I read that last year it was actually the second-most active tornado season in recorded history.
PATHAK: Wow.
MCCOY: Do scientists expect instances of tornadoes to change as our climate warms?
PATHAK: So the short answer is we don’t know how climate change will affect tornadoes. Now, research does suggest that we’ll have more storms in a warmer world. And there are some studies that suggest that climate change may be shifting Tornado Alley in the US towards the east. But tornadoes are very complex. They are so unpredictable, and they are so complex that it’s hard to attribute changes in tornadoes to climate change. It’s very easy to say, oh, this heat wave was caused because of climate change or this, like, really heavy spell of rain can be attributed to climate change. But you can’t do that very easily with tornadoes. And they’re also still kind of mysterious. Like, we don’t fully understand what factors have to come into play in what exact measures for a tornado to form. So all this, like, makes it hard to establish a clear, definite link between climate change and tornadoes. But that said, I think in order to understand how climate change will affect tornadoes, we have to first understand how tornadoes are formed and what leads to a tornado hotspot. And the findings of this study get us a little bit closer to that. And so I think– going forward, I think the broader takeaway is that when we think about how a changing climate will affect storms in general, including tornadoes, we should not only think about rising temperatures but also changes in terrain because that also affects storms and severe weather.
MCCOY: Sushmita, thank you so much for coming on the show to talk to us about tornadoes.
PATHAK: Thank you so much for having me.
MCCOY: Short Wavers, we’ll drop a link to Sushmita’s full article in our episode notes. And if you liked this episode, please follow us on the podcast platform you’re listening on. It really helps our show. This episode was produced by me, Berly McCoy and edited by our showrunner, Rebecca Ramirez and by Geoff Brumfiel. Tyler Jones checked the facts and Kwesi Lee was the audio engineer. Beth Donovan is our senior director and Collin Campbell is our senior vice president of podcasting strategy. I’m Berly McCoy. Thanks for listening to Short Wave from NPR.
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