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‘Ask Evan’: “Why do bridges freeze before road surfaces?”

This week’s ‘Ask Evan’ comes from Justin C from Lebanon County.  Justin asks, “I see those signs that say bridges freeze before road surfaces. Why does that happen?”

On a cold, rainy day, ice forms more quickly on bridges and overpasses for two reasons.  One, the freezing wind strikes the bridge above and below and on both sides.  That means the bridge is losing heat from every side.  Roads, however, only lose heat from its surface.

Even while the temperature on the actual road surface is dropping, the heat underneath the road keeps it warm enough to prevent icing as temperatures in the atmosphere drop below freezing.  Bridges have no way to trap any heat, so they will continually lose heat and freeze shortly after temperatures in the atmosphere hit the freezing point.

Secondly, most bridges today are built with steel and concrete, both of which are good heat conductors.  Because these materials conduct heat, any heat that the bridge has moves through the bridge to the surface where the heat is lost because of the air flow around the bridge.  Roads on the other hand are mostly made from asphalt, which is a poor conductor of heat, and that lessens the rate of heat loss from the road.

The bottom line is that a bridge will follow the air temperature very closely.  If the air temperature falls below freezing, a bridge’s surface will fall below freezing very quickly.   Rain or snow, therefore, will freeze and stick to the bridge.

If you have a question you’d like to ask send me an email to AskEvan@FOX43.com.  Put ‘Ask Evan’ in the subject line.  Or, reach out to me on my Facebook page, Evan Forrester FOX43.  Whatever the topic, I’ll do my best to find the answers you are looking for.