New Tornado Rating System Scales Back Winds The National Weather service says the Tornado that struck Greensburg, Kan., had top winds of at least 205 miles per hour. If the exact same tornado had struck the same place last year, they would have rated wind speeds much higher. What changed?
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New Tornado Rating System Scales Back Winds

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New Tornado Rating System Scales Back Winds

New Tornado Rating System Scales Back Winds

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This is ALL THINGS CONSIDERED from NPR News. I'm Jacki Lyden. Time now for Science Out of the Box.

(Soundbite of music)

This week, we considered the work of meteorologists.

According to the National Weather Service, the tornado that destroyed the town of Greensburg, Kansas, last week carried winds of up to 205 miles per hour. If that same tornado had struck the same place last year, the weather service would have rated wind speeds much higher.

Frank Morris of member station KCUR explains how the rating system has changed.

FRANK MORRIS: For residents of the town like Richard Ingleton(ph), the tornado was an overwhelming force of nature.

Mr. RICHARD INGLETON (Resident, Greensburg, Kansas): It hit hard. It's something to hear and ripped that wood right off the walls and the dust and your ears pop and it's a scary deal.

MORRIS: Minutes later, Ingleton knew the storm had pulverized his house and much of the town. But rating it had to wait until the next morning.

(Soundbite of metal clanging)

MORRIS: As rescuers searched for survivors in the first salvage efforts got underway, Dan McCarthy and the rest of the damage assessment team rolled into town. What they found was pretty stunning.

Mr. DAN McCARTHY (Member, Damage Assessment Team): And then as you look up to the path going down Main Street or just to the west of Main Street, many homes were swept off their foundations. And that was a telling part of the whole story.

MORRIS: That damage earned the Greensburg tornado the weather service's highest rating - EF5.

Unidentified Man: (Unintelligible) with that many additional hail reports in the last few minutes over Stevens County there, east of Highway 81?

MORRIS: In Norman, Oklahoma, at the National Oceanic and Atmospheric Administration Storm Prediction Center, meteorologists are tracking a long line of thunderstorms. Images flash on dozens of screens. If there's a thunderstorm or tornado anywhere in the country, they track it in this room. They can tell how big hale is while it's still in the clouds, and discern the shape of raindrops. But they can't measure winds in the tornado. That's based on damage and it's not too precise.

Mr. JOE SCHAEFER (Director, Storm Prediction Center): So the conversion from damage to wind speeds is really just a guess.

MORRIS: Joe Schaefer directs the Storm Prediction Center. He worked with Ted Fujita who devised the original Fujita or F-scale based on a mathematical formula.

Mr. SCHAEFER: Ted Fujita got a little carried away. Since then, the wind engineering community has learned a lot about what it takes to destroy buildings. Wind speeds went down.

MORRIS: Meteorologists and engineers had known for a while that wind speeds were out of whack. So last year, they came up with the enhanced Fujita Scale. While an F5 rating under the old system carried wind speed estimates of over 260 to almost 320 miles per hour, an EF5 rating allows wind speeds no higher than 234 miles an hour. The damage is roughly consistent, though. So an EF5 tornado is just as bad as an F5.

Schaeffer says the new estimates are based on empirical evidence, just like far earlier efforts. He tells a story of a professor in the 1840s who noticed that chickens sometimes survived tornadoes but come out completely bald. So the man shot fresh chicken carcasses from a cannon, measured the muzzle velocity and found that at 180 miles per hour, the feathers stay on. But at something like 300 miles per hour, you can't find the chicken. Presto, he had a tornado wind speed range fairly close to the ones used now.

Ms. SCHAEFFER: So the guy with this chicken was - it's humbling, the thank yous, doing as good as we can now.

MORRIS: There are mobile Doppler radars that can discern wind speeds precisely some place within tornados. But they don't necessarily measure the winds that matter, those close to the ground. Schaeffer says he hopes that lowering the wind speeds associated with tornadoes will encourage people to build houses able to hold up against them.

And though he doesn't think most people would want to live in a place that could withstand a monstrous Greensburg-type tornado, the only building left standing after a direct hit from that one was the grain elevator, which is made of thick, heavily reinforced concrete and has practically no windows.

For NPR News, I'm Frank Morris in Kansas City.

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