|FORECASTING ICING ON ROADS
METEOROLOGIST JEFF HABY
Icing of the road is very dangerous to travel. Even a small amount of ice on the road can lead to accidents. The
temperature and the amount of precipitation determine how much ice will be on the roads. You have probably heard
that ice freezes on the bridges and overpasses first. Why is this? Ice will freeze first on surfaces that drop to
freezing or below. Bridges and overpasses are cooled on TWO sides while road surfaces connected to the ground are
only cooled on ONE side. When temperatures drop below freezing, there is a lag time in the soil dropping to
freezing. A road connected to the ground will stay above freezing even after the air temperature drops to freezing
(especially if temperatures had been above freezing the previous few days). Either temperatures have to fall well
below freezing or the air temperature needs to be below freezing for a significant amount of time before a road
surface connected to the ground will freeze. A bridge or overpass has air on BOTH sides of it. There is no
relatively warm surface to keep the bridge or overpass above freezing when the air temperature drops below freezing.
The bridge or overpass losses heat very quickly to the air as the air cools. The lag time between the temperature
of the bridge or overpass and the surrounding air is very short. Once the air temperature drops below freezing,
the bridge drops below freezing. If precipitation is occurring or there is standing water on the bridge or overpass,
it will freeze quickly once the air temperature drops below freezing. A road connected to the surface may stay
above freezing even after temperatures drop into the 20's. This is especially true for roads in the southern U.S.
Temperatures may be in the 50's or higher for several days before colder weather moves in. It may take several hours for
the soil temperatures to cool below freezing once the air temperature drops below freezing because of all the
heat the earth's surface has stored during the warm period.
When forecasting it is important to have an idea
of how much frozen precipitation will accumulate on the roads. If the surface is below freezing, frozen
precipitation will accumulate on all roads. If the temperature is below freezing and the surface is above freezing,
it is bridges and overpasses that will ice over while surface roads will remain wet. Once the surface reaches
freezing, ice will accumulate on all road surfaces. If precipitation is falling as sleet or snow, road surfaces
will begin to cool since the melting process absorbs
latent heat. If the wintry precipitation is heavy enough,
it CAN accumulate on road surfaces that are above freezing. Once the precipitation stops, the snow or sleet
quickly turns to slush and melts. Freezing rain or drizzle will only freeze on surfaces at or below freezing.
Any amounts of ice, whether just on the bridges or overpasses or on all roads is dangerous. Some motorists make
the mistake of thinking the bridges and overpasses will be safe to drive on because the rest of the surface
roads are not frozen.