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Why Do Bridges and Overpasses Freeze Before Roads?
BRIDGE FREEZES BEFORE ROAD. BRIDGE MAY BE ICY. WATCH FOR ICE ON BRIDGE.
You’ve probably seen these signs before. And you might even be familiar with what happens when you ignore them in bad weather (it often involves a frightening skid across a slippery bridge or overpass). Have you ever wondered why bridges ice up before regular road surfaces? Here’s a look at the science behind the phenomenon.
Bridges are exposed to the cold from all sides
As the temperature drops below freezing, the only part of a road that’s exposed to the cold air is the surface. The ground underneath insulates the road and retains heat, which helps the surface temperature stay above freezing for a longer period of time.
Compare that to a bridge, which is exposed to freezing air from all sides. The cold surrounds the bridge, hitting the surface and the underside at the same time. There’s no way for the bridge to retain heat, meaning it’s more likely to match the surrounding atmosphere and freeze quickly.
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Bridges conduct heat instead of retaining it
Roads are often built with asphalt, which conducts heat poorly — encouraging it to stick around instead of letting it pass through. So, when the weather gets cold, the temperature of a road will often be warmer than the air above it.
On the other hand, most bridges are built with concrete and steel, and both of these materials are effective heat conductors. What does that mean? Instead of storing up heat, bridges allow it to quickly pass through. When the temperature drops, any heat the bridge has will quickly dissipate into the surrounding air.
The result: It’s easy to be lulled into a false sense of security as you drive down a warm road in freezing temperatures — only to be surprised by an icy bridge.
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Source: HowStuffWorks
A longtime editor/writer and recently transplanted Hoosier, Caleb Cook lives in Xenia, Ohio. His favorite activities are reading and listening to music, although he occasionally emerges from the heap of books and vinyl records in his basement to stand blinking in the sunlight. Once fully acclimated to the outside world again, he can be observed hanging out with his wife, attempting a new recipe in the kitchen, attending movies, walking the dog, or wandering into a local brewery to inquire about what’s on tap. See more articles by Caleb.