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Stutzke’s Stats: Why bridges ice before roads

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Anytime snow or rain is in the forecast during the cold season, concerns about bridges and overpasses freezing up will quickly grab the attention of not only meteorologists but state and local highway departments, too. What is it about these structures that makes them so inviting for ice formation? Let's take a look!

The difference is what lies beneath the pavement, literally. Roads attached to the ground have temperatures that don't fluctuate as quickly thanks to the heat radiating from the ground itself acting as insolation and keeping pavement temperatures warm. Bridges, however, don't have this luxury as both the top and bottom of the pavement surface is exposed to colder temperatures, allowing the pavement to quickly drop in temperature. This makes it easier for things like snow and freezing rain to begin accumulating in this surface first, compared to other surfaces, sometimes, even grass!

This same phenomenon occurs with other elevated surfaces like powerlines, railings, and your car! All of these surfaces cool much faster than the ground itself making them more vulnerable to ice accumulation.

There is ONE element though that can actually help in both situations and that is sunshine! During the day, especially on sunny days, the darker color of the pavement helps attract heat from the sun warming the pavement temperature above freezing in most instances, even when the apparent temperature may be below freezing.  Darker objects will always attract and grab onto more of the suns energy because they have a low albedo, meaning they don't reflect back much energy from the sun.

Anytime you approach a bridge or overpass that appears "wet", especially when temperatures are below freezing, always approach with caution and be prepared for icy conditions leaving plenty of space between you and the vehicle in front of you.

Meteorologist Andrew Stutzke

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