Weather Explainers

The Science Behind Why Bridges And Overpasses Freeze Before Roads In Winter

The signs along roads are not lying: bridges and overpasses freeze faster than roads. Read on for a breakdown of why.

We have all seen the signs: “BRIDGE FREEZES BEFORE ROADWAY.”

Well, it's true, and I'm here to break down why it does just that.

Understanding the science behind this phenomenon can help you to not get caught off guard if your drive takes you over bridges, overpasses or elevated roads.

How it happens

Let's start with 'The how.' For example, let's assume that a rain-snow mix hits a road and a bridge, followed by a blast of sub-freezing temperatures.

When cold air hits, the bridge experiences that frigid air both above and below the bridge itself. This leads to significantly faster heat loss and causes any water on the bridge to freeze very quickly.

Regular roads have the ground below to serve as insulation, which helps to regulate temperatures. So, the road is only exposed to cold from above and freezes more slowly than the bridge, or it may not even freeze at all.

New Snow Or Ice Not Required

Snow doesn't need to be actively falling for a bridge to freeze over.

One example occurs when fog freezes into ice on a bridge. This is possible when the temperatures drop below freezing.

Also, bridges situated over cold spots like deep ravines or bodies of water can keep sub-freezing temperatures near the bridge. This means that ice from rain, snow or ice that fell days before can remain on the bridge, even if the skies are clear.

Also, be mindful of the shadows. If part of a bridge stays in the dark all day, icy patches can form. This can happen even if the air temperature is above freezing.

Why bridges freeze faster than roads. In this example, cold air surrounds the bridge from all sides, removing heat faster and causing freezing more quickly. Roads have the ground to serve as a regulator of temperatures, allowing roads to freeze more slowly.

Building Materials Matter

Bridges freezing isn’t just about the temperature of the air.

These structures are often made of steel or concrete, which lose heat quicker than the ground and the materials that make up regular roads.

If you want the technical term, like I often do, steel and concrete are good heat conductors.

Asphalt is not a good heat conductor, so it is able to hold onto its warmth more.

Driving At Night is Even More Dangerous

The final point I want to make is that driving at night on these bridges is even more dangerous.

At least during the day, you can often see the ice and potentially avoid icy lanes.

At night, a lane with ice looks almost identical to a dry lane. This black ice is super dangerous.

If you want more information about black ice and how to handle driving over it, click the link below.

(For More: Black Ice)

Stay safe traveling this winter.

Rob Shackelford is a meteorologist and climate scientist at weather.com. He received his undergraduate and master’s degrees from the University of Georgia studying meteorology and experimenting with alternative hurricane forecasting tools.