Pavement Frost, A Slippery Puzzle

January 17, 2020 // Article by: Mike Mihalik

It's a cold winter night, skies are relatively clear, but there's a bit of haze in the air. The haze is fairly curious as it almost looks like a warm, humid summer night, but it's not, it's 25 degrees! However, with a mainly clear sky overhead and dry pavements, you go to bed content that there will be no ice threats tonight. 6 AM rolls around and you casually get ready for the day, sipping on some coffee and admiring the heavy frost in the backyard. Suddenly, calls are streaming in about icing on your properties! What?! How?! Running out the door towards your salt truck you nearly break your neck on a frosty driveway. What happened?

Pavement frost was the culprit. While frost typically occurs on grass, unfortunate plants, and begrudgingly... car windows, it can form on paved surfaces and sidewalks. Here's how it works... Ordinarily, frost forms on colder surfaces that "radiate" heat easily. This allows good radiators (like metal railings and car tops) to cool rapidly, reaching the air's dewpoint, which allows moisture in the air to condense and in the case of frost...freeze to these surfaces.

For pavement frost to occur, a radiative process as described above can create it, but it's typically hard to get pavements to cool rapidly enough to form frost (due to the heat stored in them from the day). It's also hard to get a cold air mass to contain enough moisture to allow this to occur (as cold air masses are typically drier). In other words, pavement frost is quite rare. However, here's how we can cheat the system, so to speak. Suppose there's just been an arctic outbreak for several days, with highs in the 20s and lows in the single digits. Pavements will start cooling to those sub-freezing temperatures. Now, with pavements well below freezing (let's say 20 degrees), we introduce a sudden spike or "advection" of dewpoints, in other words, pushing in air that contains more moisture. Dewpoints rise quickly from 5 degrees to 25 degrees...suddenly, pavements are colder than the dewpoint...and much like a cold bottle of soda, water will condense on the surface, which freezes as air and dewpoint temperatures are still below freezing. Voila, pavement frost and slippery travel is created with no precipitation falling from clouds. Pretty crazy, huh?

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