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Brainstorm Answer: Bridge Freezes Before Road
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Dave Thurlow, Host
 
Hi, I'm Dave Thurlow from the Mount Washington Observatory. Here's a recording of me and Weather Notebook Producer Bryan Sejvar driving around town, yakin' away about what else, the weather.

Bryan: "Hey what's that sign....?"

Dave: "mmm...."

Now notice we ended with a question, a question that somehow turned into last month's Weather Notebook Brainstorm. It's a question with answers, like this one from David Cass, of Hermon, NY, who listens to WSLU. He said there's three reasons a bridge will freeze before the road:

David: "First, the road loses heat only from its upper surface while the bridge also loses heat from the sides and the bottom."

That's right, air surrounds a bridge, but only sits on top of the road.

David: "Second, even though it's losing heat to the atmosphere, the road surface is rewarmed by the ground beneath it for a period of time. The bridge on the other hand does not have a similar source of residual heat."

Yup!

David: "And finally bridges are generally made of steel, which is a good conductor of heat so it conducts the heat well from the interior of the bridge to the surface where the heat is then lost to the air. On the other hand the asphalt and the gravel of the road are poor conductors of heat reducing the roads rate of heat loss."

Well put, Dave. I think you got it. Tune in for a brand new brainstorm, tomorrow, on The weather Notebook, which is underwritten by Subaru, the beauty of all wheel drive even on frozen bridges with major support provided by the National Science Foundation.