Where does Highway Glare-Ice Come From?

Have you ever noticed that, even when there has been no snow for a long time, there almost always seems to be a thin layer of ice on the highways in the winter? You would think that unless it is replenished in some way, it should eventually be broken up and knocked aside, or maybe just sublimated away.

I posed this observation to various people at the State Department of Transportation and Public Facilities (both the Maintenance and Research branches), the U.S. Weather Service Forecast Office, the Cold Regions Research and Engineering Laboratory of the U.S. Army Corps of Engineers, the University of Alaska Departments of Civil and Mechanical Engineering, the Fairbanks International Airport, and several associates here at the Geophysical Institute.

In all, I have polled at least 20 individuals and come up with almost as many different opinions. I use the word opinions here, because there do not seem to be any black-and-white answers. Different causes seem to predominate, depending on the circumstances. Most of the responses could be listed as falling into one of three broad categories which sometimes overlap.

The first is that the thin layer of ice is simply the compressed remnants of an earlier layer of snow or freezing rain. Often, this is the obvious case, but it fails where the roads have been clear for a period of time before the ice appears and there has been no precipitation.

The second is that the ice is produced when moisture from automobile exhaust condenses and collects on the road surface. This is almost certainly responsible for part of the ice buildup at intersections and stoplights where cars remain motionless for long periods of time (and where you can actually see water dripping from tailpipes). However, this is not a realistic mechanism on the open road where vehicle speeds are such that exhaust gases quickly evaporate.

Third, there is the mechanism of water condensation or ice crystallization directly from the air onto the road. (This, by the way, was the reason most often given by the experts polled.) In weather conditions where the road surface is colder than the overlying air, particularly when the air is relatively humid, ice will often form even when the sky is perfectly clear.

This effect is particularly noticeable when the weather warms after a long cold spell. During the cold period, the ground is thoroughly chilled, and when the air temperatures warm, the ground does not keep pace so there is an abrupt temperature interface at the road surface. If the temperature of the air is near the dew point, water or ice crystals readily precipitate out onto the cold asphalt. (The dew point is that temperature at which the air becomes saturated and can no longer hold all of its moisture. Saturation can occur when either more water is added to the air, or the temperature of the air is lowered.)

A single respondent to the query suggested that water actually percolated upward from the ground through the asphalt. Most others felt that asphalt was too impermeable for this to be a major contributing factor.

I was surprised to find that there was such a diversity of opinions on the subject, and I'd be interested in hearing from any readers who have different ideas.