A 75-Foot Challenge: Making a Tornado

If you thought stopping a tornado was next to impossible, imagine trying to create one. Ned Kahn tried to make a 75-foot twister for an exposition in Hanover, Germany. The problems he encountered reveal how the subtlest changes in the atmosphere can determine whether a tornado forms.

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ROBERT SIEGEL, Host:

This is ALL THINGS CONSIDERED from NPR News. I'm Robert Siegel.

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And I'm Melissa Block. More than 700 tornadoes have been reported in the U.S. so far this year, so you might think twisters form pretty easily. Well, they don't. NPR's Jon Hamilton learned that it's nearly as hard to start a tornado as it is to stop one.

JON HAMILTON: A few years ago, the artist Ned Kahn was hired to create a seven-story tornado. He's made a career of exhibits that explore the science and beauty of natural phenomena. This tornado was for Expo 2000 in Hanover, Germany. Kahn had made smaller versions for museums, using fans to move the air and vapor to make the vortex visible. But this project was big. It cost about $10 million.

NED KAHN: So everyone was all excited about this, and we finally fire the thing up and absolutely nothing happens. There's just a big mess of fog and there's nothing. And it was one of the profound anticlimaxes of my life.

HAMILTON: But not very surprising when you consider how many things have to go just right for a real tornado to start. Ned Kahn's advisor on the project was John Snow, who's now a meteorologist at the University of Oklahoma in Norman. Snow says nature doesn't usually succeed the first time, either.

JOHN SNOW: Tornadoes are really rare events. Only one in a thousand thunderstorms are actually produced as a tornado force. And maybe only one in a thousand of those turns out to be an extreme form of tornado.

HAMILTON: Kahn needed to produce a tornado every time. So he visited Snow in his lab where there was a tornado simulator.

KAHN: And he basically said, well, you know, these things are so complicated that there's really no way to mathematically predict what this one is going to be like.

HAMILTON: Snow says he wasn't sure Kahn could create a reliable tornado machine on such an enormous scale.

SNOW: I wished him good luck, because I knew that would be a real, real challenge right from the start.

HAMILTON: Even a small tornado requires a lair of warm wet air and a massive updraft to get going. Snow says that in nature, this only happens during certain kinds of thunderstorms.

SNOW: You can sort of think of them as a big vacuum cleaner moving across the landscape, sucking up this warm, moist air into the updraft.

HAMILTON: So for the German exhibit, Kahn decided to build something like a vacuum cleaner, but seven stories high. He designed a cylindrical building that was open to the sky and added a huge fan.

KAHN: The fan at the top was a 200-horsepower motor with these beautiful fiberglass cast fan blades. The whole thing was about 15 feet in diameter, and could move about a million cubic feet of air a minute.

HAMILTON: But Snow knew from his lab work that it takes a lot more than rising air to produce a tornado.

SNOW: You also need to have the right type of spin in the air. So you have to look at the way the winds are directed - some coming from one direction, some coming from another - to give you that shear.

HAMILTON: Tornadoes usually get their shear or spin from colliding weather patterns. You can't exactly recreate that inside a building. So Khan came up with a different technique to get the air rotating.

KAHN: The whole building was essentially something like the Guggenheim Museum in New York - kind of a spiral ramp tracing up this basically open cylindrical space. And all along the underside of this ramp, we mounted smaller turbine blowers, about three feet in diameter.

HAMILTON: 50 of them. That added some spin to Kahn's rising column of air. He had hoped it would create a vortex spiraling upward. But when the exhibit was completed it produced nothing, which is what often happens in nature. Snow says that even in perfect conditions, sometimes a tornado starts, sometimes it doesn't.

SNOW: Seeing a tornado form, you almost want to cheer it on as it rotates up.

HAMILTON: Ned Khan wasn't finding much to cheer about. He'd spent weeks trying different settings on the computers that controlled the fans. His deadline was approaching, and still, nothing. Eventually he asked engineers to bypass the whole computer system so he could make manual adjustments.

KAHN: It was late at night. The film crew that was documenting the whole thing had left days before that. It was just me and the poor architects that were completely stressed out. So they were all down in some meeting room - and I'm sure they were in there, you know, planning who was going to, you know, sell all the computers. And I still, at the point, was completely depressed that we weren't even close.

HAMILTON: In desperation, Khan adjusted the fans one more time using only his intuition. Then he sprinkled Styrofoam beads on the floor and switched on the system.

KAHN: I'm watching all these little Styrofoam beads, and I see a couple of them start moving, and a couple more and a couple more. Then all of the sudden, I see this beautiful, six-foot diameter spiral of Styrofoam beads swirling around on the floor.

HAMILTON: Within minutes, he'd got his tornado, and it kept spinning in Hanover for four months.

Jon Hamilton, NPR News.

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