Motorists, pedestrians often fooled by its transparency

A5

Charles Perry
Weather

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When it comes to winter, there is no shortage of weather conditions that can make walking or driving treacherous.

There's snow, rain, blowing snow, drifting, ice storms, blizzards, freezing rain, ice pellets, sleet, slush and white-outs -- to name but a few.

Apart from the discomfort they create, they can also result in anything from a bone-breaking fall to a serious car accident resulting in injuries or even fatalities.

For certain, one of the most dangerous weather conditions this time of the year, mainly because it is so difficult to spot, is black ice. It has sent many a motorist skidding on what they thought was a dry road or pedestrian tumbling onto a hard surface which appeared to be ice free.

Actually, the term "black ice" is kind of a misnomer, said Jon Pacheco of State College, Pa., a meteorologist for AccuWeather.

The ice is not black in colour, but rather is transparent, said Pacheco. What you are seeing is not the ice, but the road surface beneath it.

This makes it extra dangerous because the driver or walker is not even aware it is ice until they or their car is on top of it, said the AccuWeather spokesman. He said that is because the road looks wet, not icy.

The reason that black ice is transparent, said the meteorologist, is because it is very thin. He said ice gets its colour from entrapped air, but because a layer of black ice is so thin, it is not going to be able to trap much air.

Pacheco said the thin layer is produced by deposits of drizzle, extremely cold rain droplets, mist or fog or sleet or rain from melted snow, merging to form a film before freezing into clear ice. He said this makes it very difficult to distinguish it from a clear road.

Black ice occurs often when it has just been raining and the temperature drops quickly below freezing, said the AccuWeather spokesman. Roadways are still wet from the rain and the quick freeze, he explained, will create the thin layer of ice.

Black ice is most common at night and very early in the morning when temperatures are at their lowest and traffic its lightest. It is thin enough that it often melts soon after the sunlight strikes it, but it can last much longer on a shaded stretch of road.

It can be especially hazardous when it develops on bridges and overpasses.

Black ice forms on these structures because the air can circulate both above and below the surface of the elevated roadway. This causes the temperature to drop more rapidly than it would on the surface of regular pavement.

n Charles Perry's Weather appears daily.