The physics of black ice and “wheel-track glazing”

It's a stealthy hazard for Minnesota winter drivers.

The dangerous combination of extreme cold, car exhaust, and a thin layer of blowing snow turns roads into a high-speed icy crash test laboratory. Friday's icy crash swarms were probably the result of black ice and "wheel-track glazing."

The Minnesota State Patrol reports 165 crashes as of this post Friday.

Black ice

It's an irony of winter in Minnesota. Roads are often more perilous in sub-zero cold than when it's 25 degrees and snowing heavily. The reason is physics of snow and ice. A few inches of snow actually offers much more traction than a thin layer of ice.

Black ice is particularly stealthy. Water vapor in your car's exhaust can freeze on contact on sub-zero pavement.  The thin icy layer is difficult to see, and nearly impossible to stop on.

Bridges and overpasses are special breeding grounds for black ice. Because frigid air circulates both above and below the surface of the elevated and exposed roadway, bridge pavement temperatures fall more rapidly.

Wheel-track glazing

A thin layer of blowing snow across roadways may have also contributed to Friday's crashes. Wheel-track glazing occurs when ice crystals within the thin layer of snow is crushed beneath your warm tires. The result is rapid melting, then instant freezing.

Here's more on 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.

Keep in mind that even though the snow is over, icy roads are still a daily issue with sub-zero temperatures. Road chemicals just don't work so well when it's this cold. Drive accordingly.

Arctic weekend

Our latest feed of Arctic air continues over Minnesota into early next week. Temperatures begin to moderate by next Thursday.

Sunday Clipper

A Clipper spreads snow southeast across Minnesota Sunday. NOAA's GFS capatures the essence of the system, which could produce widespread 1" to 3" snowfall totals. Slick roads may be an issue across Minnesota again Sunday. The snow likely begins in the Twin Cities by mid-afternoon. Roads could be challenging after the Vikings playoff game Sunday evening.

Milder late next week

The upper air pattern goes zonal again late next week. Winds off the North Pacific feed milder air into the Upper Midwest.

Temperatures respond into the 20s and 30s again. The second half of January likely features alternating cold and mild spells.

Stay safe out there Minnesota.