I’ve talked about snow that is undisturbed without a temperature gradient, and about disturbed snow without pressure. Today we’re going to take that a step farther and look at snow that is disturbed by pressure—specifically, by car or foot traffic—when temperatures are below freezing. Here in Alaska we call it white ice, and the less-traveled roads and almost all the parking lots are solid white ice this time of year.
Wind or an avalanche will break snow crystals, but it doesn’t in itself press the broken grains together. Walking or driving over dry snow, however, not only breaks the crystals, it also presses them together. If it is near freezing, the pressure may even cause slight melting. In any event the crystals are very firmly welded together. The result is a mass that has some air trapped—that’s why it looks white. But it may be only a little less dense than ice, and is only slightly softer. An icebreaker or a sharp-edged shovel will generally break it much more easily than it will break true ice, but white ice is definitely solid.
As a driving surface, white ice is something most Alaskan drivers learn to deal with. It is not impossibly slippery if it is not polished or near freezing temperature, though most of us drive on it with caution and learn to feather our brakes. Mine are anti-skid, but I’ve learned to brake softly enough that the anti-lock feature almost never engages—except at intersections. Those are often polished to the point that they are extremely slick, even though graveled.
Yes, graveled. We don’t use salt much because our temperatures are generally so low that even salt water freezes and salt simply will not melt ice. Salt’s used on sidewalks sometimes, but in cold weather each salt pellet simply melts its way down to the pavement without having much effect on the main ice mass—except to make it slicker.
The road I live on is gravel, and a coating of white ice actually improves it. But it does do some things you might not think about to paved roads.
First, it covers any marking painted on the road or parking lot—lane markings, turn arrows, lines that mark parking spaces.
Second, it can at times be thick enough that when part of a road is cleared in the spring, a considerable drop-off may result.
Third, and especially a problem when it is overcast and the light is flat, is whiteout conditions. It’s not as bad as in an airplane north of tree line when the pilot may not even be able to see which way is up, but telling what is road and what is not can be very difficult when the road is white ice and the verge is snow and both are exactly the same color. It’s hard to see even with directional light and shadows, but in flat light everything looks the same.
I managed to high-center my car a few weeks ago taking an exit between a four-lane highway and a major side road. The exit was pure white ice, and I couldn’t quite see what was road and what was the curbed triangle between the side road and the exit. I wound up on the triangle and had to be pulled off by a tow truck. Neither the trooper who stopped to see if I needed help nor the tow truck driver seemed to have the slightest problem understanding how I’d gotten there, or even consider my situation unusual. “Whiteout” was all the explanation I needed.