What Is a Derecho? A Meteorologist Explains

Hurricanes and strong tornadoes make headlines, but fewer people know about the unique and destructive weather force known as a derecho. What are derechos, and why are they so devastating? Ask Science talked with Midwest meteorologist Vince Condella to find out.

Sabrina Stierwalt, PhD
9-minute read
Episode #386
The Quick And Dirty
  • Derechos differ from tornadoes and hurricanes because of their straight-line winds as opposed to rotating winds.
  • Derechos are particularly destructive because, unlike tornadoes, their paths can be hundreds of miles wide.
  • Wind speeds from an August 10, 2020 derecho that devastated Iowa are estimated to have reached 140 mph.
  • The word "derecho" is taken from Spanish, meaning "direct" or "straight ahead."
  • Derechos are most common across the Midwest and Plains states where conditions are right for moist air on the surface and cool, dry air aloft.

Earlier this month, a large, intense storm system known as a derecho devastated large swaths of the U.S. Midwest with winds reaching an estimated 140 miles per hour.

The damage was so widespread that NASA shared side-by-side satellite images of Iowa soybean and corn crops before and after the storm to illustrate the vast extent of the destruction. Agricultural economists have estimated the damage at somewhere around $4 billion. More than 1 million people lost power and some even lost their lives. 

I asked meteorologist Vince Condella to tell us exactly what a derecho is, how they form, how common they are, and why they're so destructive. As Chief Meteorologist for a Milwaukee, Wisconsin TV station for over 34 years, Vince is intimately familiar with the volatile weather patterns of the Upper Midwest. He grew up in the Chicago suburbs and was inspired to pursue meteorology thanks to what he calls "the dynamic Midwest weather." He has degrees in atmospheric science and meteorology from Purdue University and the University of Wisconsin-Madison.

What is a derecho and where does the name come from?

So, what is a derecho? 140 mph winds sound like "hurricane-force" winds to me. Are derechos and hurricanes at all the same? What about tornadoes?

"It's really a totally different phenomenon than a hurricane," said Vince, "but nonetheless, it represents some amazing destruction over a large swath of area, which is exactly what happens in a hurricane, as well. But a hurricane is a tropical system and derechos develop from a cluster of thunderstorms.

Derecho is taken from a Spanish word, meaning direct or straight ahead.

"['Derecho' is] taken from a Spanish word, meaning 'direct' or 'straight ahead,' which is very descriptive of the type of winds in a derecho. They are straight-line winds. Now, as best as can be found, the term was originated back in the late 1800s by a professor at the University of Iowa, Dr. Gustavus Hinrichs. He was writing about an event in Iowa, and he used that word 'derecho' ... because he wanted to differentiate [this straight-line wind damage] from wind damage from tornadoes. Tornadoes spin, but derechos are straight-ahead winds. And so that's the difference between the two."

Where do derechos happen?

Another key difference between a derecho and a hurricane is where they tend to crop up. Tropical coastal regions usually bear the brunt of hurricanes, but derechos are most common in the U.S. Midwest. Here’s Vince:

"Derechos can happen anywhere, although the prime location is really anywhere in the Midwest. That covers a lot of territory, but [you'll find derechos happening] from the U.S.-Canada border all the way down into the Southern Plains and even as far south as Texas and Oklahoma. But they are very common where this one occurred back on August 10th—right through Iowa, Wisconsin, Illinois, and over into Indiana. That is a real lane or alley for derecho formation. And so this one that occurred on August 10th is following a very similar flight path, if you will, of previous ones. They develop... It's really a perfect setup in the Upper Midwest because what we need for any sort of a strong storm is warm and humid air at the surface and cooler air aloft. And by 'aloft,' I'm talking about 15-20,000 feet up. So, you know, let's say we go three miles above us, even on a really hot summer day, three miles straight up, the air is much cooler.

Suddenly, the warm air at the surface wants to go up. The cold air aloft wants to come down, and you've got this overturning and lifting in the atmosphere.

"So we've got warm air at low levels, cool air at upper levels, and that is the ingredient you need for severe weather because those two air masses want to reverse themselves. Warm air wants to rise. Cool air is heavier, more dense. It wants to sink. So suddenly, the warm air at the surface wants to go up. The cold air aloft wants to come down, and you've got this overturning and lifting in the atmosphere. And you combine that with wind sheer—very strong winds that change with height—and suddenly you've got the recipe for strong thunderstorms and that can lead to derechos."

Is there a "derecho season?" How common are derechos?

Even though thunderstorms tend to happen year-round, there is a season, much like for hurricanes, when all of the ingredients are just right for their formation. And although devastating tornadoes and hurricanes seem to make the news more often, derechos are not all that uncommon.

This part of the world would experience a derecho every one to two years. They're not what we would call rare, but they are very noteworthy.

"If we look at it climatologically, the season is June, July, and August. Those are the three months where derechos are most likely to happen," said Vince. "And typically, in the Upper Midwest, this part of the world would experience a derecho every one to two years. They're not what we would call rare, but they are very noteworthy. I mean, consider this particular one on August 10th—it had a length of 700 miles. Derechos can last even longer than that. And so, this is really the prime season for them—June, July, August. That's when we have that warm humid air at the surface that cold air aloft. and in between you get a decent wind shear, and what happens is strong thunderstorms develop.

"On August 10th, developing way back in, let's say, South Dakota, a couple of strong thunderstorms continued to develop into what is called a 'mesoscale convective system', meaning one thunderstorm spawns, a second one spawns, a third one, and suddenly you've got these families of thunderstorms clumping together to form this unique system.

"And so this huge blob of strong thunderstorms continues to develop and expand, develop and expand, and suddenly they are producing strong winds and damage in a length 100, 200 miles wide and just racing across the Midwest. It's just an amazing phenomenon. It just really shows the power of the atmosphere, how one strong thunderstorm, if conditions are just right, can develop another and another and another. And they all become like this internal combustion engine, this mesoscale convective system that can then just race across hundreds of miles of real estate and cause tremendous amounts of damage."

So while derechos aren't exactly rare—certain parts of the Midwest expect a few every summer—the August 10th, 2020 storm that tore through large parts of Iowa was particularly devastating. But why?

"I think the thing that was so devastating about the August 10th derecho was the strong winds over such a broad area. There have been some estimates now of winds on the southwest side of Cedar Rapids, Iowa, of up to 140 miles per hour. That is stunning! But a lot of the wind damage in Iowa [was] estimated between a hundred and 120 miles per hour. That's amazing. That's the equivalent of an EF2 tornado, maybe even an EF3 tornado.

This derecho was hundreds of miles wide and just barreling across the state of Iowa.

"But, you know, a tornado might be a half-mile wide. Some really strong tornadoes could be as much as a mile wide. Well, this derecho was hundreds of miles wide and just barreling across the state of Iowa, in particular. And it damaged so much of the corn crop. Here's the corn crop, ripe growing, reaching its peak almost in terms of its growth for the year. And here comes this strong wind that literally flattened the corn and the soybeans and caused a tremendous amount of damage to the crops as well as infrastructures like buildings, like the power grid, and all of that. And so I think that just the sheer speed of these winds— I can't even imagine. Here is this giant snowplow of 100 to 120 mile per hour winds just rolling across hundreds of miles of real estate."

What's it like to experience a derecho?

I asked Vince if he had ever experienced a derecho in his years as a meteorologist.

"We had something similar that I had firsthand experience with here where I live in Southeast Wisconsin. Back in 1998, at the end of the month of May, a very strong derecho formed—very similar path, just a little bit farther north than this most recent one. The one that affected Milwaukee so strongly started as a tornado-producing thunderstorm in South Dakota and then raced across Minnesota, Iowa into Wisconsin.

"What's interesting about the one that occurred in 1998 is that it occurred overnight. And so, even the time of day is not dependent on these derechos. By the time the derecho hit Milwaukee, it was three o'clock in the morning here. [At the TV station where I worked], we had wind gusts estimated at 100 miles per hour. And it was absolutely stunning."

Can we predict derechos?

Unfortunately, there's a limit to how far in the future we can predict a derecho because we can’t be certain which thunderstorms will come together to form one. 

"Based on the climatology—meaning based on the time of year (June, July, August)—and based on certain atmospheric setups, the storm prediction center can say, and local national weather service offices can also say, You know what? Conditions are lining up that this could produce a derecho-type event. So we need to be aware of it.

Weather Service personnel can see the conditions coming together, but that's no guarantee.

"So, if certain conditions are in place, the National Weather Service offices can give a heads up. But [derechos don't] always happen. And so it's one of those things where, you know, okay, all the conditions are here. Maybe one out of 10 times, it may form the derecho. So it's really kind of hit-and-miss. Weather Service personnel can see the conditions coming together, but that's no guarantee. So they can be ready for it, but until [a derecho] starts to form, then okay, now they know it's happening.

"So once it starts to form, then the warnings can go out down the line upwind of these strong storms. So, for example, if they see this big line of storms creating some damage and starting to form this derecho in Western Iowa—and they can clock the speed: 'Okay, it's moving forward at a certain speed'—then they can start giving warnings all the way down the line—into Eastern Iowa, into Northern Illinois—then they can give four-, five-, six-hour warnings that yep, this thing is definitely a derecho. Yes, it will hold together. But until [a derecho] actually starts to form, there's really no way to know for sure."

As we’ve discussed here in previous episodes, it can help to turn to our local weather experts when it comes to tricky predictions like these. Local meteorologists are the most familiar with the weather phenomena that occur in their region.

"The storm prediction center in Norman, Oklahoma, they're overseeing all potential severe weather threats. But then some of the local offices of the National Weather Service, they're the ones that are more in tune with perhaps starting to see some of the local damage and then issuing some of those warnings.

They're all challenging for a meteorologist to forecast, but the speed of things makes a big difference and that's what's so devastating about derechos.

"What's important in a case like a derecho, for example, is to make sure that the power companies all the way down the line have advanced warning. Like, 'Alright, here's what we expect. These potentially 100-plus-mile-per-hour winds are due in your area in five hours', you know, just to give the power companies a heads-up so they can start to assemble their crews and get them ready to respond. That kind of advanced warning is really important. It's very different from a tornado. A tornado is a much smaller scale event, very strong damaging winds, but a much smaller scale of only a half-mile to maybe a mile. Derechos are straight-line winds instead of twisting winds. And they extend for hundreds of miles. And that's what makes them so devastating.

"So, different weather events on different scales move at different speeds. And obviously, they're all challenging for a meteorologist to forecast, but the speed of things makes a big difference and that's what's so devastating about things like derechos and tornadoes. Boy, if we could give people two- and three-day warnings on a derecho or a tornado, wouldn't that be something? That would really save lives."

I told Vince it seemed like there was never a dull moment in the life of a midwestern meteorologist. 

"It is the antithesis of a boring job. That's for sure. Every day is very unique. Every day is different. Different parts of the country have more challenging weather than others, but it's all challenging if you ask me. The atmosphere— you know, nature throws us a lot of curveballs."

You can find Vince Condella on Instagram and on his website, where he posts everything from music to photography. He also has his own fan page on Facebook and a fantastic YouTube playlist where he breaks down different weather phenomena in relatable ways. 

Please note that archive episodes of this podcast may include references to Ask Science. Rights of Albert Einstein are used with permission of The Hebrew University of Jerusalem. Represented exclusively by Greenlight.

About the Author

Sabrina Stierwalt, PhD

Dr Sabrina Stierwalt earned a Ph.D. in Astronomy & Astrophysics from Cornell University and is now a Professor of Physics at Occidental College.