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Name: Ana
Status: Educator
Grade: 6-8
Location: NY
Country: United States
Date: Winter 2009-2010


Question:
Why will a bridge freeze before a road?



Replies:
This happens when the air is colder than freezing, but the ground is not frozen yet. The warmth (relatively speaking) of the ground prevents water on the road from freezing even though the air is below freezing. But a bridge is not in contact with the ground, so when the air around it is below freezing, the bridge surface can become cold enough for water to freeze.

Hope this helps,
Burr Zimmerman


Hi Ana,

When the weather gets below freezing, the roadway on a bridge will tend to freeze before the road that leads up to it, simply because both the top and the bottom of the bridge roadway is exposed to the cold. A normal road has only its top surface exposed to the cold; its underside is warmed by being in contact with the earth below, which (because of its vastly greater mass) takes much longer to cool.

Regards,

Bob Wilson


The ground is warmer than the air. The ground is a dark color and absorbs heat from the sun. Bridges and overpasses are surrounded by the air and is deprived of the heat of the ground.

Sincere regards,
Mike Stewart


The question is about a bridge freezing before a road. More precisely, a bridge surface freezing before a road surface. Two effects can cause bridges and roads to get cold: the surrounding air gets cold, and at night the surfaces can radiate heat to the cold night sky.

A roadway has a lot of thermal mass because it is part of the ground, and is cooled by the air only from one side - on top. Thus a road surface cools or warms slowly. Bridge decking is relatively thin and thus less massive, and easier to heat and cool. More important, bridge decking is cooled from both the bottom and the top by air circulating around it. Thus the bridge surface cools faster than the road surface.

It works the other way too, if there has been a long spell of very cold weather, the cold road surface will WARM slower than the cold bridge surface when warm weather comes.

Robert Erck


Hi,

A bridge would freeze earlier and faster than a regular road. There are at least three related reasons for this.

First, a typical road is built on top of a thick layer of earth whose average temperature at some depth is relatively constant during the year. During the winter when the surface of the road gets cold, the earth below it acts as a massive heating block that tends to warm up the surface, albeit mildly. This also explains why in colder climates water pipes are laid a few feet below the surface (in fact, below the ‘frost zone’) where the temperature remains above the freezing point and water pipes do not freeze.

Second, a typical bridge has its underside exposed to the same cold ambient temperature as its surface. Thus, when the weather gets cold, the bridge loses heat from both the top and the bottom surfaces, and thus freezes faster.

Thirdly, because many bridges are built at an elevation above the surrounding ground, cold air blowing (wind) past the bridge is likely to cool a bridge down faster than it cools the surrounding roads and grounds where wind velocity (thus the cooling effect) is reduced by obstacles such as trees, buildings, hills, etc.

Ali Khounsary, Ph.D.
Argonne National Laboratory



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