As you travel, do you play games in the car? One game many kids enjoy is finding certain types of road signs. If you live in an area with lots of bridges or overpasses, you may have seen signs that indicate that bridges freeze before roads. Have you ever wondered why?
At first, it may not seem to make sense. Why would bridges freeze before roads? A bridge or an overpass is just like a road, right? The air temperature feels the same if you’re standing on the road or the bridge.
Bridges do indeed freeze before roads, and there’s a good reason why. It all has to do with science!
As the air temperature decreases, the surface of both roads and bridges will begin to cool down. Bridges will cool more quickly — and ice will form faster on bridges — for a couple of reasons.
First, cold air surrounds the surface of a bridge from above and below. This means that bridges lose heat from both sides. Bridges have no way to trap heat, so they will ice rapidly as soon as the temperature decreases to the freezing point.
Roads, on the other hand, only lose heat from their surface. The ground below roads helps to trap in heat and keep roads from getting icy unless the temperature drops several degrees below freezing.
Another reason bridges freeze before roads is because they’re usually made of steel and concrete. Both steel and concrete conduct heat very well. Any heat a bridge manages to trap will be transferred to its surface quickly, where it will be lost to the air around it.
Roads are made of asphalt, which does not conduct heat very well. The heat trapped within and below a road will tend to stay there longer, lengthening the time it takes a road to freeze.
If you’re riding with your parents during the winter, help them keep an eye on the temperature. If it dips below freezing, remind them that bridges and overpasses will freeze quickly. If it has been raining or snowing recently, bridges and overpasses may ice over in a hurry. If necessary, slow down and use a little extra caution to prevent an accident!