Each year, the United States has about 1,200 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 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, one of Short Wave's producers. Read Sushmita's full article on tornadoes that she wrote for the publication Eos.Have other science weather stories you think we should cover on the show? Let us know by emailing [email protected]! Listen to every episode of Short Wave sponsor-free and support our work at NPR by signing up for Short Wave+ at plus.npr.org/shortwave.Learn more about sponsor message choices: podcastchoices.com/adchoicesNPR Privacy Policy
Chapter 1: Who Are the Hosts and What Is Tornado Alley?
You're listening to Shortwave from NPR. Hey, short wavers, producer Burleigh 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 U.S. 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.
It is really the global hotspot of tornadoes.
Chapter 2: What Geographic Factors Cause Tornado Alley to Have So Many Tornadoes?
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.
You have to blame geography for that.
Chapter 3: How Do Thunderstorms and Wind Shear Create Tornadoes?
So tornadoes form from thunderstorms. And for them to do that, different types of winds need to blow at different temperatures in different directions.
You need cold, dry air coming in from one direction. Like from the Rocky Mountains. You need warm, humid air coming in from one direction.
Like the air coming up from the Gulf of Mexico.
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.
These winds moving at vastly different speeds and directions are called wind shear. And they can lead to rotation within the thunderstorm.
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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Chapter 4: Why Does Tornado Alley Have More Tornadoes Compared to Similar Regions in South America?
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.
So the questions researchers really had was like, if there's this like similarities in geography of these two continents, why doesn't Central South America have a tornado alley?
Today on the show, new research on why the U.S. gets so many more tornadoes and what the findings might mean for reducing them in the future. You're listening to Shortwave, the science podcast from NPR.
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Okay, Sushmita, let's talk about the research you reported on. It was done by a scientist named Funing Lee 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.
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.
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Chapter 5: What New Research Explains the Tornado Frequency Difference Between North and South America?
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 winds don't really encounter any resistance. They can 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.
Chapter 6: How Does Land Surface Roughness Affect Tornado Formation?
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. Yeah. And so it's really just like if you have flat or you have not flat. That's the big factor. Yeah. And way upstream. Yeah. And upstream being different for North and South America?
Just the region near the equator from where the easterly winds are coming into those regions.
Okay, so is this like coming from the south in the northern hemisphere and coming from the north in this? 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. 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. Wow. And similarly, when they smoothed out the Amazon forest, tornado potential in Central South America went up.
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.
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Chapter 7: What Is the Role of Upstream Land Features in Tornado Formation?
Okay. And speaking of caveats, this, I mean, this research seems like it's pretty straightforward, pretty solid.
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 two miles, pretty two-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. So...
Okay, so it's like people hadn't thought to look this far upstream. And how far are we talking?
We're talking about hundreds of miles away from where the tornadoes are actually forming, from where the storms are actually forming.
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?
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. 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.
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.
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