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Earth Sense: Visibility becomes a challenge when driving in night fog
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Episodes of thick fog, like the one of last weekend, are almost always caused by air of different temperature and moisture content trying to mix. Last weekend’s whiteout was caused by a thick stream of warm, humid air entering southern Georgia from the Gulf Coast of Florida.

Reaching the higher altitudes of Hall County, the atmosphere cooled near the ground. When air cools, its relative humidity increases. At some point, the temperature drops to where the water vapor can’t remain in its gas form. Tiny droplets of water form and stay suspended in the air. They scatter the light in random directions, and visibility can be reduced to a few feet.

A similar effect occurs when one drives across a mountain area. Where a road rises to high elevation, it may start in clear conditions and then climb into a layer of low-hanging clouds. The result is a winding roadway with almost nothing visible ahead.

One night I had to travel from Andrews, N.C., back to Gainesville. U.S. 19 crossed Blairsville (1,880 feet) in clear night air, then rose steadily toward Blood Mountain. At Lake Trahlyta, I reached the “cloud ceiling,” which is the bottom of the clouds, not the top.

The highway kept climbing past 2,600 feet, and its hairpin curves ahead changed into a white blur. By the time U.S. 19 reached its Blood Mountain peak at 3,105 feet, the road surface was visible only to a distance of 5 or 6 feet. Not one other vehicle was in sight, let alone another motorcycle.

My single white beam did little to illuminate the direction in which I was traveling. Only the white line at the right edge of the road provided some guidance. Progressing at 15 mph, I followed that line until the cloudy fogginess thinned out again at 1,700 feet and the lights of Cleveland appeared in the distance.

When one drives in fog at night, the headlight beam causes a gazillion reflections on the water droplets in the air. Turning on the high beam makes it worse. If a vehicle has low-mounted fog lamps, they can help some because the reflections occur below the driver’s line of vision.

This is why truck drivers often have better visibility from their high perch than people in cars. Riding a motorcycle in these conditions turns out to definitely be a poor choice.

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