Re: At what Temperature does water freeze on a road?

I assume you want to know if radiation, evaporation, and wind, etc., can contribute to the cooling of the water layer and/or the road surface, and that we are assuming there are no chemicals (such as salt) which could depress the freezing point of the water. Given those assumptions, there are some ways that ice can form on a road surface even if the measured air temperature is above 32 degrees F.

You can find some previous answers in our archives (use our search engine) which discuss frost, and that frost can form when the measured are temperature is substantially higher than 32 degrees F (as high as 38 or 39 degrees F) due to the boundary layer's cooling due to the (mostly infrared) radiation of the road surface toward the clear sky above. (The boundary layer is a very thin insulating layer of air just above the surface. If the air is very still [very little wind] the boundary layer can insulate the surface enough so that it can cool below the air temperature above the boundary layer. When there is wind, however, there is not much of a boundary layer.)

Similarly, if there is almost no wind and the sky is very clear, the road surface can cool below 32 degrees F even if the measured air temperature (above the road) is as warm as 38 or 39 degrees F. However, the ground below the road will usually be warmer, and so the final answer depends on lots of factors. A bridge (or overpass) whose roadbed is exposed to the air will usually be closer to the temperature of the air, and so the radiative cooling will be less important.

Also, evaporation can cool the water layer, but usually when the road surface is wet the relative humidity is very high and so evaporation is not very substantial.

The answer, as you can tell, depends on lots of factors.

John Link, MadSci Physicist