Black Ice

Question: Black Ice, why does it happen and what can I do?

Answer:

Black ice is dreaded by highway crews and drivers alike. A thin layer of ice becomes bonded tightly to the road surface. The road surface looks wet, and it is hard for drivers to tell that black ice is present until they hit the brakes. That makes it particularly hazardous.

Black ice can form in several different ways. The first two can happen in either day or night, when the sky is cloudy. The third way occurs only at night, when the sky is clear.

Sleet or freezing rain

The air temperature is at or a little above 32 degrees Fahrenheit. The pavement surface is slightly below 32 degrees Fahrenheit. There is no salt on the road. Sleet, or a light freezing rain begins. When it hits the pavement it freezes into a sheet of ice, very black in color, and extraordinarily slippery because it may be a bit wet on top. This is the most common way to get black ice, and it can occur over great lengths of a road.

Light snow dilutes the brine

The air and the pavement temperatures are well below freezing. There is some salt on the road, and the brine concentration is fairly dilute, typically less than 5 percent. An ice storm or a light snow begins. When the snowflakes fall on the salt brine they melt. The added water dilutes the brine, causing the freezing temperature of the brine to rise above the air temperature. (Keep in mind that salt brine freezes at temperatures below 32 degrees Fahrenheit.)

The brine freezes into black ice, occurring first in the lower valleys where the air may be a bit colder. This will cause black ice to occur in patches, over short lengths of a road.

Dropping temperature freezes the brine

The initial conditions are the same as either of the two situations described above. Then the storm ends and the clouds clear away. It is nighttime. Radiant cooling causes the air and pavement temperature to go down below the freezing temperature of the liquid brine on the road surface. The wet surface freezes, occurring first at higher elevations where the cooling effect is greater. This too will be patchy.

Anti-icing effective

Adopting anti-icing techniques will be effective in all three cases, helping to prevent black ice. You put down a small amount of salt or highly concentrated brine on the road surface before the icing conditions occur. This keeps the freezing temperature close to minus 6 degrees Fahrenheit, and the ice cannot bond to the pavement. Repeated light applications of brine or salt will be necessary during the storm.

The objective of anti-icing is to keep the road surface wet, and to not allow ice or compacted snow to bond to the road surface. In all three cases changing roadway conditions lead to the formation of black ice. You need good information about the current conditions on your roads in order to stay ahead of this problem, and you need to do some advanced planning for how you will deal with it.