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Earth Sense: Black ice a major danger in winter storms
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“They said we’d get 5 to 6 inches of snow. Now look at this.” The cashier at the Oakwood fast food restaurant was disappointed with the small amount we got in Hall County last weekend.

After a grueling drive to Hartsfield-Jackson Atlanta International Airport and back, I wasn’t. Black ice had made the roadway treacherous, and traffic on Interstate 85 had slowed to a dead crawl, even at 5:30 a.m.

North Georgia weather forecasts are generally more reliable during winter than in the summer. This applies mainly to fronts bringing the rain that’s so typical for the cold season here.

Those big, wet weather systems are easier to track than the small local rainstorms we see on hot days. But predicting whether we’ll have snow, freezing rain or sleet, and the respective amounts, remains a highly complex task.

We have to rely on computer models that calculate the greatest probability of what will happen. As always in statistics, there is that “degree of freedom” that can’t be accounted for. It’s the fish that got away, in other words. Weather can never be predicted with absolute certainty.

Freezing rain is the most damaging type of winter precipitation. It occurs when the air near the ground is generally above 32 degrees, but the surface is frozen. That’s when a layer of ice forms on roadways, buildings, power lines and trees. It’s been known to reach a thickness of several inches. Tree branches and entire trees break and block the roads, and power lines snap, leaving thousands in the dark.

Snow is much more benign. It accumulates when both the air and the ground are below freezing. However, on heavily traveled highways, heat and pressure from car tires melt it. That could be nice, but if temperatures keep dropping, the melted water can refreeze.

Because of its rough surface, it doesn’t reflect brightly. The tar underneath makes it appear dark, and that’s why it’s called “black ice.” Motorists often get taken by surprise on an extremely slippery surface that looked like harmless asphalt.

Sleet is partly frozen rain. If it falls on cold ground, it can also produce an icy layer and make roadways slippery. It’s very difficult to predict where that temperature boundary is going to occur and whether we’ll see snow or sleet. But the biggest danger to be alert for is black ice.

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