The valley and ridge landscape of North Georgia produces effects that can’t be found in the coastal plain. Stand at one of the overlooks on the Russell Scenic Byway. On a clear morning, near noon time, you’ll feel a cool breeze coming up from the valley.
At night, the slopes cool under a clear sky. This process makes the ground work like a huge radiator, sending its heat out into space. At the bottom of the valleys, in the town of Helen, for example, cool air accumulates as it gets heavy. The drier the air, the heavier it is. This may seem counter-intuitive. With terms like “heavy cloud cover” or “heavy rain”, it’s hard to imagine that this is the most light-weight air producing it. But the hydrogen atom, the first ingredient in water vapor, is the least heavy of them all. So during the night, the mountain sides above Helen, Blairsville or Blue Ridge are cooling. Air slides downward as if on a ski slope. This process is called the “mountain breeze”. As the air climbs upward again during daytime, one can feel a “valley breeze” high up above the town.
Roanoke, Virginia is surrounded by tall mountains on all sides, and I’ve always found the effect to be highly noticeable there. On some early mornings, the air dropping down into the town cools so strongly that its moisture condenses into fog. One can drive a stretch on I-81 in a milky haze, then cross the city and ascend to the Blue Ridge Parkway on its south side. During the climb, the fog suddenly goes away. Under clear blue sky, the Appalachian Mountains are in the bright sunshine. Down below, Roanoke is hidden beneath a thick white blanket until the sun rises high enough to reach the valley bottom. The effect is called an inversion, and it’s quite common in the Blue Ridge, Smoky Mountains, and the rest of the Appalachians. Cool air stays at the bottom and is hard to disperse, while warmer air above holds it there. It isn’t dangerous in North Georgia. But where a busy metropolitan area is located beneath tall slopes, it can be a smog trap. The city of Ulaanbaatar, Mongolia, with thousands of feet of mountains around it, has stubborn inversions that make it one of the world’s worst places for polluted air.
Rudi Kiefer, Ph.D., is a professor at Brenau University, teaching physical and health sciences on Brenau’s Georgia campuses and in China. His column appears Sundays and at gainesvilletimes.com.