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Why do bridges get icy?

As a wintry mix tracks through the area this morning, you’ll hear us mention bridges and overpasses frequently. These are areas of concern when temperatures get to near freezing, especially when precipitation of any kind is falling. Many times the roads will seem fine only to cross a bridge where the black ice will send you into a fishtail.

There are temperatures at a few different levels that need to be taken into account for icy roads. One is the air temperature, measured at 2 meters above the surface. That’s about 6 feet 5 inches. These are the temperatures that you see on the map during the forecast and gives us a good idea of what temperatures feel like for us but a lot of differences can occur over those 6 feet towards the surface.

There is a network of road sensors that tells us what the actual pavement temperature is. If the pavement temperature is 32.0° or less, ice will develop on the roads. Conversely, an air temperature of 32 or less doesn’t guarantee icy roads since they may be warmer than the pavement temperature. Why is this?

We can go even deeper to look at soil temperatures. The 4 inch soil temperatures are hovering a couple of degrees above freezing and are slower to respond to everyday changes to weather conditions above ground. The warm soil and subsurface help to keep pavement temperatures warm, even if air temperatures are at or slightly below freezing. Therefore, pavement only loses heat from one side while being warmed from the bottom.

Bridges and overpasses do not have the luxury of being warmed from the underside. The cold wind can swirl above and below them, speeding the rate of heat loss on both sides. Temperatures will almost always be colder on bridges than regular roads. That’s why, even if the roads seem fine, be aware that slick spots develop on bridges and overpasses quicker.

Brandon Libby


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