More than 100 people are still missing after a cluster of tornadoes suddenly tore through several mid-American states last week, particularly ravaging Western Kentucky. Beyond the death toll—74 confirmed so far—the details of the storm were stunning: Roughly three dozen tornadoes flattened towns and factories across the states, with one traveling more than 200 miles. President Joe Biden called the breakout of tornadoes one of the worst in American history—and talk soon turned to climate change and a “new era of superstorms.”
But how normal is a storm like this actually? And what does this really tell us about what’s in store in the future? Deanna Hence is an atmospheric scientist and an assistant professor at the University of Illinois Urbana-Champaign, where she teaches about hazardous weather and its connections to climate and global change. She says she gets basic questions on how weather events can be so devastating very often, and she works to dispel myths about how these weather phenomena occur and how to mitigate the damage. We spoke by phone Monday. Our conversation has been condensed and edited.
Aymann Ismail: It was reported that at least 36 tornadoes touched down in Kentucky and neighboring states during this storm. Can that be normal?
Deanna Hence: Tornado outbreaks do occur. I wouldn’t say it’s common, but they’re not necessarily rare either. There was the tornado outbreak that happened across the Southeast in 2011. Numerous tornadoes crossed in Mississippi, Alabama, and Georgia. These events do happen. Thankfully, they don’t happen super frequently. But they happen. How we define these types of “outbreaks” has varied a bit over the years, but multiple tornadoes happening in a single event does occur. On that front, this one was a particular blockbuster.
President Biden said that this was one of the worst tornado events in American history, and one agency called it a once-in-a-generation catastrophe. Is that accurate?
These types of multistate, very long-lived, very long-track type of tornado events, they don’t happen that frequently. When we have the misfortune of those tornadoes taking a direct hit on a densely populated area, the potential for human loss of life is incredible. But just even last year, there’s a tornado that went through downtown Nashville, Tennessee. That was essentially one very strong storm that lifted and dropped tornadoes repeatedly. But because it hit downtown Nashville pretty head-on, the capacity and potential for damage was greater, in terms of just sheer number of people affected.
It’s not that I disagree—these multistate types of outbreaks are not common. But like I said, these types of huge outbreaks happen every 10 to 20 years. I’m a little hesitant to go that far. I don’t think I feel comfortable saying it’s one way or the other right now.
Are tornadoes like this happening more frequently than before? Are they being affected in any way by climate change?
There are certain ingredients that you need to form tornadoes overall. Some of those ingredients support the formation of the thunderstorm that carries the tornado. And then some of those ingredients actually help create the spin within that thunderstorm to allow the tornado to form. The first ingredient is what we call atmospheric instability, and that is related to both how much moisture there is in the atmosphere and to the difference in temperature at different heights. The second ingredient is that you need a certain type of change of wind with height—we call that vertical wind shear. That allows for the creation of spin within the thunderstorm itself. Storms that support tornadoes need that vertical wind shear to get that spin.
It does appear that the chances for those optimal tornado-forming ingredients to come together, especially atmospheric instability, do seem to be increasing. But there are still remaining questions on how things like vertical wind shear will change, because that has a lot to do with things like where the jet stream is and what it’s doing. So there has been some indication that the ingredient potential has been going up, but that doesn’t necessarily translate into tornadoes happening, because a lot of things have to come together in a particular way for a tornado outbreak to happen. Even if some of those things increase with climate change, will they come together to create more tornadoes? That part, a whole bunch of scientists are still working very, very hard to figure out.
Even with that uncertainty, are there things we can learn and do differently to prepare for storms like this?
No matter what the tornado occurrence does, there’s always this question about how will human vulnerability change in the future: Where do people live? How do they build their homes? Mobile manufactured housing tends to be a lot more vulnerable to tornadoes. So, a lot of atmospheric scientists are working with social scientists to try to figure out the increase in vulnerability and the increase in risk of harm and death and property loss, because that is changing as well. This is very important to keep in mind. These climactic patterns shift, but human behavior shifts as well. And so, how those come together to increase vulnerability and risk are extremely important to figure out.
Earlier you said large steel-frame buildings are especially vulnerable. Is that what you’re talking about here?
It’s specifically these big-span, big-roof buildings. If you think of your typical box store, like a Home Depot—not throwing shade at them, they just come to mind because that was one of the buildings that was very badly damaged in the Nashville tornado. There was a Lowe’s that was very badly damaged during the Southeast tornado outbreak. These are these buildings that have only so much support to hold the roof up. So these types of buildings are fairly prone to collapse in on themselves when they’re hit with extremely strong winds. If one of those kinds of buildings takes a direct hit from a tornado, they don’t typically tend to fare well.
Are you worried at all about alarmist rhetoric around storms like this, and how does that affect your work?
Well, it’s very understandable that people are scared, right? We just had this major, traumatic event. One thing we always need to keep in mind is that we as scientists follow the evidence, and sometimes— especially in the aftermath of a big event like in Kentucky—it takes time. I think it’s important for folks to let us do our work. It’s not like we’re going to forget about it. We’ll keep working and do our best to try to talk about both what the issues are and what we can do about them, with the best evidence that we can collect.
Part of what is fueling that fear this time is that the death toll seems so high. What was it about this particular storm that made it this deadly? You called the cluster that hit Nashville a high-casualty event, which killed about two dozen people. The casualty count in Kentucky is expected to rise in the next few days, and it’s already at least four times Nashville’s.
Part of it certainly was that these were very powerful. Tornadoes are ranked on a 1-through-5 scale, and one of these maxed out. The National Weather Service is out doing damage surveys, but we’re already seeing that entire towns were leveled. That tornado was both particularly violent, and it was very wide. It could create a damage swath that covered a decently big area. But I think another aspect of this is, at least from what I understand of where the casualties are so far, the types of structures that some of these tornadoes happened to hit. As I mentioned, some structures simply are more vulnerable to tornadoes than others. Add in the fact that these tornadoes happened at night, which made them difficult to see.
There was a decently strong tornado that went about 45 miles south of Urbana-Champaign, through Mattoon, Illinois. There’s some fairly significant damage to barns and everything, but that tornado happened to go through a less densely populated area. That may be the difference between why that event had no casualties, at least reported so far, versus some of these other ones.
Many stories also described panic as the biggest storms hit, and people who weren’t sure what to do or how seriously to take the warning. What would you do in a storm like this?
That will depend on what kind of structure they’re inside of. Ideally, in a solidly constructed home, there would be either an interior room that has no windows—a closet, a bathroom, something like that—because you want to put as many walls between yourself and the outdoors. Tornadoes can throw things hard enough to puncture walls, to say nothing of glass. Ideally, you would get underground, say in a basement, but there are plenty of parts of the country where basements aren’t really much of a thing. So an interior room of your house is safest. If you live in a multistory building, getting to the lowest floor in an interior room or hallway is your best bet. Where this becomes more of a difficult thing is when you’re dealing with things like mobile and manufactured homes, where you would actually have to evacuate that structure for a well-built structure. Hopefully the community that you lived in would have, say, a concrete or framed structure where you can shelter in emergencies, because the likelihood that those homes will withstand a direct hit is very low. And with some of the different businesses and things like that, that’s going to depend on their structure and how those buildings are built. But a lot of the same ideas apply.
What do people like me usually get wrong about tornadoes?
There are so many myths about what tornadoes can do or can’t do that we work pretty hard to dispel, because those kinds of myths can really mean the difference between whether someone takes shelter or not. Lots of folks believe that tornadoes can’t cross water, or that they can’t go over mountains, or they can’t form over big cities with high-rises.
One of the hardest things about this line of work is that the weather can be both absolutely just devastatingly beautiful and also just actually devastating. And having to deal with both the beauty of these phenomena while they are actively hurting and killing people, it’s a very weird emotional place to be. So I can understand why anyone is both fascinated and terrified, and both excited and feel filled with dread about the weather. I think I’m an expert in weather, but I’m not an expert in any other kind of science. I just try to approach those things with a learning attitude, just as much as I hope people will approach me with that.