Compared to a roadway, a bridge has more surface area to exchange energy with the atmosphere and thus will cool down to the air temperature — and freeze — quicker.

Q: Why do bridges ice before the road?

A: People living in a cold climate are used to seeing signs that say “bridge freezes before road.”

The fundamental reason is that a bridge hangs above the ground, while the roadway rests on the ground. Water on a road or bridge will freeze once the surface becomes cold enough. So the bridge must cool faster than the roadway.

"Weather Guys" Steve Ackerman and Jonathan Martin are professors in the University of Wisconsin-Madison department of atmospheric and oceanic sciences.

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