Bridge Before Road Freezing
Country: United States
Date: Winter 2009-2010
Why will a bridge freeze before a road?
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,
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.
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.
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.
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,
Ali Khounsary, Ph.D.
Argonne National Laboratory
Click here to return to the Engineering Archives
Update: June 2012