Ever see the signs? Why do bridges freeze before roads?


We drive by roads signs all the time and if you’re a law-abiding citizen, you know what they mean… but sometimes it’s not always apparent WHY a sign may be used. Ever noticed the “bridges ice before roads” signs?

Here’s the science behind the hazard…. We start with the roads and bridges at the same temperature. In this case, 38 degrees. As they lose heat – either by cooling in the overnights or from a cold front, the roads lose heat slower than the bridges. The ground underneath the road acts as insulation to help keep warmth in a little longer.

The bridge allows cold air to flow all around it… especially underneath it where there isn’t any insulation. This results in a quicker heat loss which means colder temperatures on the bridge and icing first. It’s also about what the bridge is made of…most bridges are made of cement which are good heat conductors, bringing their heat to the surface and making it easier to release. Also, bridges could be over water which naturally may keep a colder air mass below it, allowing for the temperature of the bridge to be much lower than the road already.

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