You’ve probably seen the bright yellow road signs warning drivers to watch for black ice on bridges and overpasses during winter weather. Those signs are important because icy bridge surfaces are a major hazard for drivers, especially those driving trucks.
As soon as temperatures drop below freezing, bridges become icy. There are a few reasons for this. One reason is the fact that cold air surrounds the surface of a bridge, both above and below it.
This means that bridges lose heat from both sides, which causes ice to form rapidly as the temperature drops below freezing.
Another reason is the fact that bridges are generally made of steel and concrete, which are very good conductors of heat. This allows the heat within a bridge to rise to the surface and freeze as it meets the frigid air above and below it.
The ground beneath a road is also an excellent insulator, which helps to keep roads from becoming icy as quickly as they do on bridges.
It’s because of this that roads tend to hold onto their temperatures a bit longer than bridges do. This is because the ground below the roads acts as a heat conductor, keeping the cold air that reaches the road from directly contacting it. As a result, it takes less time for the ground to drop to freezing. This explains why the roads around your house aren’t always icy as much as those on main highways.