Pod Insights Short Wave Tornado Alley: Home Of Extreme Winds About this episode Each year, the United States has about 1,200 https://www.nssl.noaa.gov/education/svrwx101/tornadoes/ tornadoes. Many of them happen in tornado alley, a very broad swath of the U. S. that shifts seasonally. This area gets at least ten times more tornadoes than the rest of the world. Science writer Sushmita Pathak https://sushmita.journoportfolio.com/ says that huge difference can be chalked up to one word: geography. But there's a slice of South America with similar geographical features that gets comparatively fewer tornadoes, so what gives? Sushmita wades into the research weeds with guest host Berly McCoy https://www.npr.org/people/985775371/berly-mccoy, one of Short Wave's producers.
Read Sushmita's full article https://eos.org/articles/the-surprising-factor-making-the-united-states-a-tornado-hot-spot on tornadoes that she wrote for the publication Eos https://eos.org/articles/the-surprising-factor-making-the-united-states-a-tornado-hot-spot.
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0:16 Speaker 01 You're listening to Short Wave from NPR. 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.
0:56 Speaker 02 It is really the global hotspot of tornadoes.
0:59 Speaker 01 That's Sushmina Partak, 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.
1:14 Speaker 02 You have to blame geography for that.
1:17 Speaker 01 So tornadoes form from thunderstorms. And for them to do that, different types of winds need to blow at different temperatures in different directions.
1:26 Speaker 02 You need cold, dry air coming in from one direction.
1:29 Speaker 01 Lake from the Rocky Mountains.
1:31 Speaker 02 You need warm, humid air coming in from one direction.
1:34 Speaker 01 Like the air coming up from the Gulf of Mexico.
1:36 Speaker 02 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.
1:45 Speaker 01 These winds, moving at vastly different speeds and directions, are called wind shear. And they can lead to rotation within the thunderstorm.
1:53 Speaker 02 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.
2:10 Speaker 01 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.
2:37 Speaker 02 So the question researchers really had was like, if there's this similarities in geography of these two continents, why doesn't Central South America have a tornado alley?
2:52 Speaker 01 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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4:35 Speaker 01 Okay, so Shmita, let's talk about the research you reported on. It was done by a scientist named Fooning Lee when he was at MIT. And Fooning 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.
4:54 Speaker 02 So the starting point of the 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.
5:35 Speaker 02 So when the easterly winds are coming over the Gulf of Mexico towards Central North America tornado alley, that's a relatively smooth surface, like 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.
6:16 Speaker 02 And because of this rough surface, the winds get broken down. And so the tornado potential also gets suppressed.
6:25 Speaker 01 Wow, so it really is like this big picture geographical reason for why there's a different amount of tornadoes in North and South America.
6:33 Speaker 02 Yeah.
6:33 Speaker 01 And so it's really just like if you have flat or you have not flat. That's the big factor?
6:39 Speaker 02 Yeah.
6:40 Speaker 01 And way upstream, yeah. And upstream being different for
6:45 Speaker 01 North and South America?
6:46 Speaker 02 Uh, just the, the region near the equator from where the easterly winds are coming into those regions.
6:52 Speaker 01 Okay, so is it, is this like coming from the south in the northern hemisphere and coming from the north and this other, okay, okay.
6:58 Speaker 02 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. Uh, 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, when they did that, they saw that tornado potential in the US in tornado alley, it went down.
7:34 Speaker 01 Wow.
7:35 Speaker 02 And similarly, when they smoothed out the Amazon forest, uh, tornado potential in central South America went up.
7:45 Speaker 01 Wow.
7:45 Speaker 02 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.
7:59 Speaker 01 Okay, and speaking of caveats, this, I mean, this research seems like it's pretty straightforward, pretty solid.
8:04 Speaker 02 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, this, this feels like a pretty solid study. And actually she studies how uh, land cover uh, in the vicinity of the storm, like a mile or two miles, two mile radius of the storm changes tornadoes. And so she was actually surprised. Um, she was actually like very fascinated by this, that you know, land cover and terrains so far away from the, the actual storm can also affect tornado potential, so.
8:38 Speaker 01 Okay, so it's like people hadn't thought to look this far upstream. And how far are we talking?
8:43 Speaker 02 We're talking about hundreds of miles away from where the, where the tornadoes are actually forming, from where the storms are actually forming.
8:51 Speaker 01 Wow. So the researchers did this work, solved kind of a big mystery in why tornado alley get so many tornadoes. What, what could be some implications of their research?
9:03 Speaker 02 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, like 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.
9:25 Speaker 01 Hmm.
9:26 Speaker 02 That's happening in the Midwest. Uh, so the authors of the study really want to, um, you know, bring attention to this point that, you know, 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.
9:55 Speaker 01 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 was actually the second most active tornado season in recorded history.
10:08 Speaker 02 Wow.
10:09 Speaker 01 Do scientists expect instances of tornadoes to change as our climate warms?
10:15 Speaker 02 Um, so the short answer is we don't know how climate change will affect tornadoes.
10:19 Speaker 01 Hmm.
10:20 Speaker 02 Um, now, research does suggest that we have more storms in a warmer world and there are some studies that suggest that climate change may be shifting tornado alley in the U S 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.
10:46 Speaker 01 Hmm.
10:46 Speaker 02 It's, you know, very easy to say, oh, this, 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.
11:20 Speaker 01 Hmm.
11:20 Speaker 02 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, you know, get us a little bit closer to that. And so I think going forward, I think the broader takeaway is that, you know, 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.
11:59 Speaker 01 Sushmita, thank you so much for coming on the show to talk to us about tornadoes.
12:03 Speaker 02 Thank you so much for having me.
12:07 Speaker 01 Short Wavers will drop a link to Sushmita's full article in our episode notes. And if you like this episode, please follow us on the podcast platform you're listening on. It really helps our show.
12:20 Speaker 01 This episode was produced by me, Burleigh McCoy, and edited by our showrunner Rebecca Ramirez and by Jeff Brumfield. Tyler Jones checked the facts. And Quacey Lee was the audio engineer. Beth Donovan is our senior director and Colin Campbell is our senior vice president of podcasting strategy. I'm Burleigh McCoy. Thanks for listening to Short Wave from NPR.
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