Picture this: 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 actually 25 degrees! You see that the pavements are dry and all is well, so off to bed you go, feeling content that there will be no issues in the morning. Flash forward to 6 AM. You're casually getting ready for the day, sipping on a hot cup of coffee and admiring the heavy frost in the backyard. Suddenly, calls come streaming in about ice on the roads and parking lots! What?! How?! Was there possibly some rain overnight that wasn't forecasted? Running out the door towards your salt truck, you nearly break your neck on a frosty driveway. So What happened?
In our 35+ years in the snow and ice industry, we've seen this happen many times, and it's all because of one phenomenon: Pavement frost. While typical frost you hear about happens on grass, unfortunate plants, and begrudgingly...car windows, it can form on paved surfaces and sidewalks. "But, you're saying ice can form without any falling precipitation?" YES! And here's how it works
Typical Frost formation on a surface
Ordinarily, frost forms on cold surfaces that "radiate" heat easily. This allows those good radiators (like a metal railing or car top) to cool fairly rapidly, reaching the air's dewpoint temperature. To refresh on what the dewpoint is, it's a temperature that the air needs to reach in order to achieve 100% Relative Humidity. This allows moisture in the air to condense on surfaces and in the case of frost, freeze to surfaces like grass or elevated surfaces.
So, same situation for pavement frost to occur, right? Well yes, but it's a bit more difficult for the radiative process we described above to create it. It's typically hard to get pavements to cool so rapidly to form frost (due to the heat stored in them from the daytime). It's also hard to get a cold air mass to contain enough moisture to allow this to occur (most true cold air masses are typically drier than warmer and more humid air masses). In other words; pavement frost is quite rare, at least out here on the east coast. But it does happen from time to time, especially across the Midwest and parts of Canada, where cold/dry air can have sudden warmups with more moist airmasses. For us east coasters, here's how we can cheat the system, so to speak.
Suppose there's just been a long stretch of cold air, with highs in the 30s and lows well below freezing. Pavements will start cooling to those sub-freezing temperatures. This is especially true on those clear nights, which allow the surfaces to radiate and release all that built up heat from the daytime. Now, with the pavement temperatures well below freezing (let's say 20 - 25 degrees), we introduce a sudden increase or "advection" of higher dewpoints. We're pushing in a more moisture laiden airmass into our cold airmass. The dewpoints rise quickly and then suddenly, your pavement temperatures are colder than the dewpoint temperature.
Condensation of water vapor on a cold bottle
Much like a cold bottle of soda on a warm day, water will start to condense on the surface. In our case, this moisture will freeze on the surface as air and dewpoint temperatures are still below freezing. And then voila; pavement frost occurs. In many instances, pavement frost can happen suddenly and without warning. While we know all of the factors that go into the formation of pavement frost, it is one of the more tricky atmospheric conditions to predict ahead of time. When it does though, it causes slippery and dangerous travel, even with no precipitation falling from the clouds.
As we discussed, one important factor with pavement frost is, well...the pavement! And that all is based on just how cold the pavements are. Thankfully here at WeatherWorks, we have developed an in-house product to gives you insight into projected pavement temperatures. We call it: PavementRisk™: A 36-hour pavement temperature forecasting tool to provide insight into whether or not your pavements are cold enough for ice to be present. It's a powerful visual support aid to optimize your snow and ice removal operations.
For more information on how you can get your hands on Storm Alert and change the way you prepare for wintry events, visit weatherworksinc.com or email us at email@example.com. You can also reach our friendly staff Monday - Friday by calling 908-850-8600!