At WeatherWorks, our meteorologists have helped our clients through each and every wintry event; from arctic outbreaks to severe blizzards. Along the way we’ve spoken to professionals in the snow & ice industry, and there’s one common theme...nobody is a fan of ice! In fact, when the topic of freezing rain is brought up, most give a long sigh…then typically followed with “I hate ice storms.” Even though the word “ice” brings stress and anxiety, let’s learn more about how it forms, the types of ice, and some rare circumstances that prove that ice is quite slick (pun intended).
First off, what processes form ice in winter? Well this question starts fairly basic, if air temperatures are at or below 32 degrees, you can have ice…simple enough. However, right off the bat, winter storms complicate things and produce ice three different ways. One is in the form of snow, when temperatures from the cloud to the ground are at or below 32 degrees. The second is sleet, which is when snow falls into a warm layer and melts into raindrops, then freezes into balls of ice as they pass through a thick sub-freezing later closer to the surface. Sleet looks like a bunch of BBs, but instead of shooting your eye out, it just becomes a dense, hard to push accumulation for plow drivers. Last, if the warm layer is thick and the cold layer is very shallow at the surface, the raindrops don’t have enough time to freeze into sleet. Instead, the drops freeze on contact with any at or below freezing surface. This is freezing rain, which forms a rather clear glaze of ice on everything from trees and power lines to roads and sidewalks.
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Left: A good example of sleet, which is basicially frozen raindrops. Right: Freezing rain, which is rain that freezes on contact and coats everything in a glaze of ice.
Ice can also form after winter storms, which typically involves melting of residual snowpack or piles, then the subsequent runoff freezes into ice once the sun sets and temperatures drop below freezing or it encounters a surface that is below freezing. Ice can also form if old snow isn’t removed properly and gets compacted by foot traffic or vehicles. This type of ice is generally harder to remove as deicer is needed to soften it up first before it can be scraped away. Finally, even plain rain storms can cause issues at times, when temperatures plummet below freezing shortly after rainfall ends. This situation is most hazardous when skies clear at night and winds are calm, allowing wet areas to freeze into black ice. Oh, and there’s also pavement frost, but that’s another puzzle you can learn more about here.
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Left: An example of black ice, that is transparent making pavements just look wet (it's not!). Right: Opaque ice appears whiter and typically has impurities or air trapped in it.
Now that we’ve covered how ice forms, what’s the difference between black ice and opaque ice? Basically, it comes down to purity. Black ice is transparent and free of a lot of impurities. When it forms on parking lots and roadways, the icy surfaces just look like wet asphalt, leading to the name. Since it’s clear, black ice is more dangerous as it’s hard to tell whether surfaces are wet or icy, leading to misjudgments and accidents. Black ice typically forms near snowmelt runoff, in refreeze situations after rain storms, and during freezing rain events. Ice that appears whiter or opaque basically has more impurities and many times has air trapped inside it. This type of ice is more prevalent when old snow is hard packed on roadways and sidewalks.
Notice the old snow in the travel lanes of this parking lot. It has been compacted and compressed by traffic and bonded to pavement. This will be difficult to remove and will require deicers first before scraping.
Pop Quiz Time: Can ice form when temperatures are above freezing? Answer: YES!
Although it seems improbable, here’s how it works. Air temperatures are usually measured 4 - 6 feet above the surface, since that’s the height of most people. However, on a clear night with light winds, heat radiates quickly from the surface and allows surface temperatures to cool faster than the air. So while it’s 35 degrees on the thermometer 4 - 6ft off the ground, temperatures at the immediate surface can be 32 or less, allowing lingering wet areas to freeze. Plus, pavement temperatures are critical as well, with typically shaded areas and the northside of buildings running cooler due to the lack of sunlight. These areas also freeze first. Not only that but if a walkway is surrounded by snowbanks, the snow will act like a freezer, keeping the surfaces in between colder than the air temperature with runoff freezing in these spots. As a snow & ice professional, it’s important to recognize and keep a close eye on these colder areas.
Here, notice the snowbanks surrounding the sidewalk. These act as a freezer and allow any snowmelt water to refreeze as it flows onto the sidewalk.
Last test: Can ice melt when temperatures are below freezing? Again, the answer is YES!
This typically occurs during the daytime with plenty of sun. The sun’s rays don’t get absorbed by the air much, allowing the air to remain below freezing. However, the energy from this is more readily absorbed by snow/ice on the ground or a blacktop surface underneath clear ice. Because these surfaces are absorbing more energy, they warm up, allowing the snow and ice to melt. Beware though; if meltwater flows into a colder, shaded area, as it can freeze once again!
Who knew icing in the wintertime could be so intricate! By utilizing all of this information in your snow & ice removal strategies, you can stay one step ahead of the weather, ultimately keeping your clients safer. WeatherWorks meteorologists are always readily available with our 24/7 consultation from Storm Alert. We’re happy to give guidance on icing situations, making your decisions easier. Good luck this winter!
