How to

14 February 2017

How Road Surface Temperature Affects Ice Build-up

During and after a particularly cold spell, you may notice one or two things that seem rather illogical, all things considered. For one, why, even days after the worst of the weather has passed and air temperatures are once again above freezing, do the roads keep their icy coating? On the other hand, bridges and overpasses will rapidly become frozen while other roadways remain clear. So, why do our roads behave in such a seemingly bizarre way?

The route of all of this is the ‘lag’ between air temperatures reaching zero or below and the road surface temperature following suit. Road surfaces connected to the ground, for example, will need several sub-zero days or to experience temperatures far below freezing in order for the road itself to freeze due to the heat held in the ground. Bridges and overpasses, subjected to freezing air on both sides, will quickly drop their own temperature to match, leading to faster freezing.

This effect is less pronounced if precipitation is falling as sleet or snow, as the melting process will absorb latent heat from the surface, reducing lag time between surface and air and thereby speeding up the freezing process.

To summarise, if the air temperature is below zero while surface temperatures are above, bridges and overpasses will freeze while roads connected to the ground will simply remain wet following precipitation.

Once surface temperatures drop to zero or below, ice will form on all road surfaces.

If the precipitation is heavy enough, and falls as sleet or snow, then it is possible for this to build up on surfaces above zero. However, as soon as snowfall ceases this will quickly melt, leading to the signature slush we all so loathe.

Sam Bonson

Sam is an aspiring novelist with a passion for fantasy and crime thrillers. He is currently working as a content writer, journalist & editor in an attempt to expand his horizons.