How Black Ice Works

Black ice is actually almost perfectly clear. It has no trapped air bubbles and no swirls (aka occlusions, if you want to use the proper term for bubbles and swirls trapped inside ice). White ice is, of course, white and translucent (some light gets through) or opaque (no light gets through) because of occlusions. Black ice has no such imperfections, so it's perfectly clear. It's only black because you can see the pavement surface underneath. Fun fact: before there were paved roads, motorists were never concerned about black ice! There was certainly ice without imperfections, but it would have been the color of dirt or whatever surface was underneath it. It seems the term "dirt ice" or "macadam ice" never really caught on, though.

In order to achieve this level of clear perfection, a couple of conditions have to be met. First, there has to be some water, and this water has to fall gently. Fog, misty rain, and even exhaust condensation from vehicles all fit the bill. If it were pounding, splashing rain, bubbles and swirls would form, and you'd be back to plain-old white ice that you can see. Slowly melting snow seeping back onto the pavement can also lead to black ice as the water refreezes on the roadway. Second, it cannot be windy. That would introduce bubbles and swirls, and we know what that means. White ice, again. Black ice can also form on a sunny but below-freezing day, when the sun warms up the road surface and any slushy stuff melts, despite the cold air temperature. Then, overnight, the melted snow and slush refreezes into a slick of black ice.

Because of the way the water gently lays itself down on the pavement, it freezes in a thin layer, which adds to the invisibility of black ice. It might just look like a wet patch of road — until you drive over it. Black ice is often so thin that it can form even when the air temperature is above freezing. If the surface of the road is at 32 degrees Fahrenheit (0 degrees Celsius) or lower, that may be enough to cause black ice to form, even if the thermometer outside your window says its above freezing. This is especially true in the morning hours, when the cold winter sun hasn't had time to warm up the pavement yet. It's also the time of day when you're still half asleep and trying to hustle to get to work on time, which doesn't help the safety factor.

There are places that black ice is more likely to form — Wisconsin, for example, as opposed to southern Florida. You probably figured that out on your own, though.

You've also surely seen the street signs that warn you of bridges freezing before roads. It's true, and the frozen water is likely to be black ice. Cold air and wind can circulate above, below, and all around a bridge or highway overpass, and there's no insulating ground beneath a bridge to help keep the temperature constant. Any little bit of water that's settled on the deck of the bridge can easily freeze into smooth, bubble-free black ice.

Black ice also likes to hang out where the sun don't shine. For people in crowded cities, that means tunnels and underpasses. Last night's commutersprobably left a fine sheen of moisture from their tailpipes, which can easily freeze into black ice for your morning commute. Thanks, guys! For rural drivers, black ice is more likely to be under trees with branches that hang over the roadway. And everybody with a shaded driveway should beware of black ice before they even leave home.

Rural drivers also need to be aware that black ice can linger on the back roads. Cars and trucks on crowded streets, with their warm engines and tire friction, warm up and dry off roads pretty quickly. But if there haven't been many (or any) cars on a stretch of road, there might not be enough warmth to melt the ice.

Also note that official temperatures — the ones that scroll across your television screen in the morning — are taken at about five feet above the ground. So if it's, say, 37 degrees Fahrenheit (2.8 degrees Celsius) according to the meteorologist, the road surface may be at freezing. Beware the black ice!

Originally Published: Jan 27, 2015