Commuter nightmare: Black ice and wheel track glazing

It happens every January.

The combination of cold temperatures, car exhaust and light fluffy snow turns roads into a high-speed icy crash test laboratory.

As expected, half an inch of snow at 5 degrees turned last night's metro commute into an instant nightmare. The physics of what happens to roadways in extreme cold produced the icy glaze. Two main culprits continue to provide challenging commutes in the days ahead — wheel-track glazing and black ice.

Wheel-track glazing

It happens fast.

This is why I was concerned in my MPR weather chats and blogs yesterday. Take temperatures at 5 degrees above zero and add half an inch of fluffy snow and you get an instant nightmare for commuters. Road salt and other chemicals just don't work very well with temperatures below 10 degrees.

As our tires crunch and compress the thin layer of ice crystals, compression heating and warm tires can cause melting, then cold air and pavement instantly freezes a thin layer of ice.

Here's a good explanation of "wheel-track glazing" from the Iowa Department of Transportation.

“Wheel-track glazing” is caused by warm tires trapping the ground level blowing snow. As more vehicles travel over the same wheel tracks a glaze of ice forms, which becomes very slippery. 

The combination of light, blowing snow and cold surface temperatures could result in icy roadways due to a phenomenon called wheel-track glazing. “Wheel-track glazing” is caused by warm tires trapping the ground-level light, blowing snow. As more vehicles travel over the same wheel tracks, a glaze of ice forms that becomes very slippery. The condition is very difficult for Iowa Department of Transportation crews to treat because the ice is continuously forming on heavily traveled roadways.

Black Ice

Black ice forms when water from our car's exhaust freezes on bitterly cold roadways. The thin icy coating is hard to see, a stealthy glare coating favoring intersections where cars sit and idle at stoplights.

Arctic high pressure overhead

Bitter high pressure drifts overhead today. Southerly winds push a warm front toward Minnesota tomorrow. The core of cold air slides east over the next 36 hours.

Temperatures moderate tomorrow

We struggle to reach zero today, then temperatures dip back to near -10 tonight in the metro with sub-zero teens to -20 up north.

Temperatures begin to moderate tomorrow afternoon. You'll notice the lack of bitter chill by late in the day. Much warmer air in the 30s (above zero) pools southwest and approaches southern Minnesota Thursday.

Thursday thaw, winter's coldest next week?

February may still deliver a colder punch, but the inbound Arctic slap from Saturday through Monday could be the coldest of winter. ECMWF guidance is cranking out three consecutive sub-zero days Saturday through Monday with lows approaching -15 at night. This may be a bit overdone, but not by much.

At least it's a weekend? Stay tuned.