Freezing Fog Can Be An Icy Problem

November 3, 2020 // Article by: Mike Mihalik

As winter slowly approaches, more and more thoughts go into ice formation and treatment, especially if you are a snow and ice removal contractor or road department. Most times road crews deal with ice from freezing rain, a freeze up after a storm, or melt water refreeze around existing snow piles. Although fog is driving hazard from its ability to significantly reduce visibility, it doesn't seem like it would be a concern for icing. However in the winter, freezing fog can occur with temperatures at or below freezing, leading to icy conditions on pavements. Let's investigate more...

There are three main ingredients you need for fog formation: moisture at the surface, calm winds, and a clear sky. Fog can also form near a stationary front and is more favored after rainfall. During the winter, freezing fog is typically confined to valleys and fields due to radiational cooling. However, it can be more widespread especially when rainfall ends late in the day, skies clear, winds calm, and temperatures fall quickly to near or below freezing at night. To top things off, this situation also leads to a temperature inversion (temps increase with height, instead of cool) which prolongs the duration of fog. In these situations, the fog keeps things wet and black ice an easily form as temperatures fall below freeezing.

Pavement frost example from eastern PA in February of 2020.

There are times when freezing fog may be around, but it's more of a pavement frost situation. Believe it or not, pavements can frost up just like the grass in the winter, but it takes more than a clear, cold night. Typically, pavement frost needs roadways and the air temperature to be at or below freezing and moisture needs to be "advected" or pushed into the region quickly. As the moisture increases, dewpoints rise...and when the dewpoint equals the air temperature... condensation occurs. With the temperature of the roadway at or below freezing during this process, pavements can frost up creating slippery driving conditions. These pavement frost situations are most favored on bridges, shaded areas, and near water sources, but are rather rare. Read more about pavement frost in our blog Pavement Frost, A Slippery Puzzle.