eli5: “Bridge may be icy” but why?



Does it make a difference if the bridge is over water or a field or etc.? Expand my brain please.

In: Earth Science

On regular roads, the Earth is insulating the road, which does a lot to reduce ice build up. Because air can pass under the bridge, they’re a lot colder than the roads they connect to. This means the bridge might be icy while the road is not.

The ground, and the rest of the planet, acts like a giant baked potato. When it’s hot (or at least, warmer than the air), for instance when the weather has recently turned cold, it retains its warmth for a while. A bridge is like a small slice of that potato – it cools much faster. So, while the air may be cold enough to freeze water, the bridge will cool to that temperature faster than the normal ground. This happens because it has much less material to retain heat, while it is exposed on all sides to the cold air.

The reason the bridge gets icy first is due to a number of factors. The ground is a massive sink of heat, in comparison a bridge does not. The smaller volume makes it a lot easier to heat or cool. The bridge has the additional surface area of its sides and bottom. Things with greater surface area will cool and heat faster than some thing of the same volume, imagine waffles vs a cake.

The points to consider are its volume, area and stored energy.

There may be additional information that I have not covered. If you have any questions, please feel free to ask.

Solid ground acts as a good insulator, warming a road from below even as there is cold air coming in contact above. A bridge is surrounded by cold air top and bottom, and this freezes over sooner than the road will.