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MELISSA BLOCK, HOST:

Familiar as we are with tornadoes, we confess we'd never heard the name of this kind of storm system before. The derecho, from the Spanish word meaning straight ahead. So tornadoes are turning winds while derechos are moving in one direction fast.

Jeff Masters is a meteorologist with the weather website, Weather Underground, and he's going to tell us more about them.

Jeff, welcome to the program.

JEFF MASTERS: Thank you.

BLOCK: And, first, what causes a derecho?

MASTERS: Thunderstorms can pop up randomly during the summer and, if you get enough of these popping up in the same location, they have the opportunity to kind of work together and form a complex - kind of a greater whole among a bunch of separate thunderstorms. And, when they get organized like this, they form one of these - they're called mesoscale convective systems or, sometimes, they're called derechos if they happen to move in a straight line and stay together as one mass.

BLOCK: I've seen this described as having a bow echo shape and, on radar, it looks sort of like concentric semicircles, almost like a wave moving in one direction.

MASTERS: Yes. A lot of times, a derecho will have a squall line that has a bow shape to it and right at the point of that bow is where you don't want to be. That's where the most violent winds are.

BLOCK: This storm over the weekend caused damage across a huge swath of the country, from Indiana across Ohio, West Virginia into Maryland, Virginia, Washington, D.C. here and up to New Jersey. Is it typical for the path of a derecho to be that long?

MASTERS: The definition of a derecho is it does have to hold together for at least 240 miles, so they're very long-traveling types of thunderstorm systems and they're also defined as being able to generate winds in excess of 58 miles per hour, which is the threshold for a severe thunderstorm.

BLOCK: And I think, at least around here, the winds were much, much stronger than that, even.

MASTERS: We had over 30 of these thunderstorms that had wind gusts in excess of 80 miles per hour and that's equivalent to a weak tornado. This is kind of equivalent to having 30-plus tornadoes of the weak variety on the ground, so no wonder they caused so much damage.

BLOCK: Yeah. It was also moving at what seemed to be an extremely fast speed, at 60 miles per hour traveling across the country. Is that unusual?

MASTERS: That's pretty typical for a derecho. They do move quickly and they cover a lot of territory.

BLOCK: Are there certain parts of the country where these storms are likely to happen?

MASTERS: They're most likely in the Midwest and, sometimes, in the northern tier states from, say, Minnesota across into southern Canada, as well. They tend to form right along the northern edge of a intense heat wave, which exactly is what we had on Friday.

BLOCK: As you were watching this storm develop, Jeff, were you predicting that it would be as bad and as catastrophic as it has been?

MASTERS: No, because it's very unusual to have a derecho of this intensity. I don't think I've ever seen that sort of a death toll in a derecho event. That's more like what you'd get from a tornado outbreak. And the last time we had anything close to this was back in 2009, when we had a derecho that killed six people from Kansas to Kentucky. So, very unusual, very hard to predict and forecast these sorts of intense rare events.

BLOCK: Well, Jeff Masters, thanks very much for talking with us.

MASTERS: You're welcome, Melissa.

BLOCK: That's Jeff Masters, a meteorologist with the website, Weather Underground, telling us about derechos. Well, we were also curious where that name, derecho, comes from and it turns out that Dr. Gustavus Hinrichs penned the term in the 19th century. Hinrichs was a physics professor at the University of Iowa. The state hired him to study local severe weather and, in 1888, he published a paper in the American Meteorological Journal. He defined a new term to describe these severe windstorms.

Since tornado was derived from Spanish, Professor Hinrichs chose another Spanish word to describe these storms and he landed on the word, derecho, meaning straight ahead. It took nearly a century for the term to be used in the modern meteorological lexicon.

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