Why do bridges freeze before other road surfaces?


NEW YORK STATE (WROC) — ‘Tis the season for cold weather, and a lot of it. We’re now entering the month of December and temperatures are beginning to hover closer and closer to the freezing mark more often. Cold air means roads can become slippery as ice has the tendency to form over roadways.

Odds are, living in New York, you’ve come across road signs that look like this:

Image courtesy: MattHrusc from Pixabay

The scientific reason that bridges and underpasses freeze before other roads is simple: Access to cold air.

Roads on the ground—like the streets in your neighborhood or on main highways—tend to hold onto their temperatures a bit longer than bridges. One reason is that it only contacts the cold directly above it. The ground beneath it acts as an insulator, so it’s able to hold in heat energy much more efficiently. Insulators keep objects from transferring heat from one object to another; in this case, the other object to the road would be the air directly above it. This, in turn, helps keep the road warmer.

Bridges are much more exposed. They have both the air above it, and the air flowing underneath it. This allows for the bridge to lose its heat and get colder much quicker, and therefore freeze over much faster.

Bridges are also good conductors, meaning they’re better at transferring heat through physical contact because of how they’re made. Many bridges and overpasses are made out of materials like steel and concrete, which happen to be good conductors of heat. Objects that are good conductors of heat tend to lose their heat much faster than other objects when it’s cold.

Mini science lesson: The second law of thermodynamics tells us that heat always flows from hot to cold objects. This means objects can only transfer their heat from one place to another. Think of it like trying to get toothpaste back in the tube: you can’t. It only goes in one direction, and heat works the same way.

All in all, bridges have more surface area for the air to work with, and therefore more opportunity to lose heat than roads do. Roads only lose heat from one side; the immediate surface above it, while bridges lose heat from 2 sides; both above and below it.

It’s because of this that bridge surfaces will ice much more rapidly as soon as temperatures approach the freezing mark. Cold air is also denser than warm air, which means it tends to hang around low lying areas, and bridges host the perfect place for colder air to settle.

Roads can still freeze at the surface. It just takes a little bit longer for the cold to form ice.

Also worth mentioning is that surfaces can still be icy even if the thermometer reads above 32°, because the air is often colder closer to the surface than it is around eye-level. It’s usually around 2 meters up or so where we forecast and reference our temperatures, since that’s where we mostly feel it.

It can be difficult to spot icy surfaces especially while driving, or if there’s black ice, which is why it’s important to slow down when crossing a bridge during winter. It’s especially important to slow down before crossing the bridge if possible. Hitting the brakes over icy surfaces can result in losing control of the vehicle, and can even cause an accident.

Copyright 2021 Nexstar Media Inc. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.

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