Damaged wind turbines have become important clue in solving a tornado's strength

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Wind turbine damage new key to help determine tornado strength

Meteorologist and forensic engineer Tim Marshall, who is part of the team who brought the Enhanced Fujita Scale, says damage from wind turbines may now play an added role in helping determine the wind speeds inside a tornado. 

GREENFIELD, Iowa — One of the lasting images of the deadly tornadoes that swept through Iowa earlier this week were the scenes of toppled and burning wind turbines just outside of town as the tornado approached Greenfield.

Storm survey teams with the National Weather Service office in Des Moines would later rate the tornado a strong EF-4, and one of the meteorologists and forensic engineers who helped refine the Enhanced Fujita Scale told FOX Weather those wind turbines were an important clue.

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"These turbines are designed for maybe 120 (mph) or so," Tim Marshall told FOX Weather on Friday. "So I'm saying that somewhere between 120-140 mph is probably what it takes to knock these out. And what happened is that if you saw the video there, that (they) just buckled like, like toothpicks - just broken right over. And this is an indication of a very strong tornado."

In Greenfield’s case, meteorologists eventually found evidence winds reached up to an incredible 175-185 mph — good enough for a strong EF-4 category rating, but not quite enough to end the nation’s 11-year drought of EF-5 tornadoes.

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"We are looking at the specifics," Marshall said. "That is, the details of how the house is anchored down. And so if we have something like a concrete slab, we're looking for those anchor bolts and seeing what the spacing was seeing if they had washers and nuts on those bolts. Those are the details that we look for to determine (whether) EF-4 or EF-5 (strength)."

The last EF-5 tornado, which struck Moore, Oklahoma in May 2013, the damage was even more extensive than in Greenfield.

00:53

Iowa wind turbine goes up in flames after tornado

FOX Weather video shows damage to a wind turbine near Carl, Iowa on Tuesday after a tornado was seen in the area on the ground.

"In Moore, Oklahoma, we had several of these homes that were built on slabs that were completely wiped clean," Marshall said. "And I'm talking about that bottom plate that was bolted down that I mentioned — it was gone, and there was nothing in there. The commode was gone, the tub was gone, and carpeting was gone. So anytime you see something that literally scours the ground, those are for high-end events."

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But Marshall says they are always applying what they learn to keep the tornado rating scale up to date. As wind power becomes more popular in the U.S., one big addition in the next iteration will be using damaged wind turbines as factors in determining a tornado rating.

"What we're doing right now is we're adding wind turbines to the EF scale," Marshall said. "So the next iteration, you're going to see wind turbines in there."