What is black ice?
Find out what you can do to avoid the dangers of black ice.It is wintertime and as you are traveling down the highway you speed up because the road ahead of you is clear. That is the problem. The temperature has dropped and earlier in the day it was snowing, but you believe that the road is clear of snow and any other obstacles. As you go over a certain spot of the road, you lose traction and your car starts to shift left, then right. You have just hit a patch of black ice. What is black ice?
What is Black Ice?
You did not see the black ice because it is ice that is transparent; it reflects the color of the asphalt underneath and lies upon a road. It can be a significant traffic hazard where it occurs because you are not expecting it.
How Black Ice Forms
The ice forms when you have deposits of extremely cold rain droplets, mist or fog. The chemical process occurs with a combination of freezing which slows down because of the latent heat given off in the road, followed by the quick process of going from a solid to a gas. This physical/chemical process allows the rain droplets to flow and merge together on the surface, forming a film before freezing into clear ice.
The consequence of this process is that there are few trapped air bubbles. It is transparent and very difficult to see compared to snow, frozen slush or other ice types formed on roadways.
In addition, it often has a matte appearance rather than the expected gloss; and often it is interleaved with wet pavement, which is identical in appearance. For this reason it is especially hazardous when driving or walking because it is both hard to see and extremely slick.
Where Black Ice Forms
Black ice can form anywhere, but bridges and overpasses can be especially vulnerable. It can form first there because air has the opportunity to circulate both above and below the surface of the elevated roadway. This will cause the temperature to drop faster than on regular pavement. One result is the warnings near bridges, "Careful: bridge ices."
Preparing for Black Ice
Knowing how black ice forms is one thing, but being on the lookout for it is another. You should look for signs of ice on places other than the roadway, such as on windshields or side view mirrors, on road signs, trees or fences along the highway. These places will have different exposure areas. If ice is forming on any of those things, it's possible that it may be on the road as well.
Also, if you have had varying temperatures during the day, say cold, then a warming spell, then another freezing spell, that could create the conditions for black ice.
A caveat: A four-wheel drive doesn't make you immune to black ice. The additional traction that you are supposed to get from four rotating tires that are all in push formation won't mean much. All it means is that the four wheels have no additional traction – four wheels can spin just as well as two wheels.