By Staff
Well, we here in Michigan dodged the worst of the ice storm that hit the Heartland south of us. All we got (and still have as I write) is a charming coat of ice on the tree limbs. Everything at ground level held just enough heat to prevent ice from forming. Our conditions, though, were darned close to being treacherous. I'm speaking of the dreaded, invisible black ice that forms on paved highways.
We don't get black ice that often around here, but I'm especially conscious of it from living in the Columbia River Gorge of Washington State. Out there, we almost never saw snow, but black ice was a way of life.
Black ice forms on pavement when the temperature hangs right around the freezing point or even slightly above. Water droplets from a light, misty rain form on the pavement and flow and mix together well before slowly freezing. This forms a thin sheet of ice with few air bubbles in it. It's the air bubbles that make normal ice and frost white. With so little air in it, this ice is transparent and extremely smooth and slick. Black ice is not black; it's the dark pavement showing through the glass clear ice that gives it its name. You simply cannot see it as you're driving down the road. If it's really good black ice you can't even see it while walking along. ZIP-BANG… you're on your kieshter before you knew what happened.
Here in the east we most often see it forming on bridges where cold air both above and below the pavement cause freezing conditions while the rest of the roadway is ice free. Really watch those bridges when the road is wet and the temperature is near or below freezing.
Of course, us humans being what we are, we can't just call ice, ice. We have to break it down into types. The ice that formed on our tree limbs from this storm is called rime ice. Rime ice is similar to black ice in that it occurs when water forms on objects and then freezes. However, it has more air in it than black ice, so it's milky white and crystalline. Then there's the super crystalline ice called hoarfrost. Hoarfrost builds up in awesome, large, spiky white crystals. It's a product of warm, humid air freezing as its water particles condense on a surface (as opposed to forming water on a surface and then freezing). For this to happen the dew point must be warmer than the surface but colder than the air temperature and colder than the freezing point. Hoarfrost most often occurs on cold, clear autumn nights.
I think the coolest of ice anomalies (literally) is ice fog. I've never seen it, but it must be some spectacle. Ice fog only occurs at temperatures under 30 degrees below zero. This frigid air is so dense it can't absorb and hold much moisture. When moisture enters this air it floats around in microscopic ice particles looking like a cloud of cotton candy.
Ice fog only occurs naturally over comparatively warm, unfrozen water, a rare commodity at 30 below. A much better place to find ice fog is in far north cities such as Fairbanks, Alaska, considered the ice fog capitol of the world. Here, there are plentiful moisture sources such as water discharges from power plants, vehicle exhaust and human and animal sweat and breathing. Fairbanks and its people and dogs discharge some 6,000 tons of water into the air every day. When the temps drop below minus 30, as it often does, the ice fog show is on.
Ice fog was a major problem during construction of the Alaskan pipeline. As cold engines don't start at 40 below zero the thousands of trucks and other heavy equipment were left running around the clock, pumping thousands of tons of water into the air. At times the ice fog was so thick workers quipped that you could cut it with a shovel. You could barely breathe and visibility was virtually zero, bringing everything to a halt. Maybe I don't need to see ice fog after all. Carpe diem.