Back in September, thermals were visible in the sky almost every day. Thick, puffy clouds built up in the afternoon heat. As the heated air climbed, it cooled and had to shed some of its water vapor. The tiny drops of liquid water made the clouds visible. Glider pilots love thermals because the air in them can push the plane upward faster than gravity is pulling it down. A good day with strong thermals can keep glider planes in the sky for hours.
In November, Canadian air dominates our region for days at a time. Once the cold front that brought it has moved on, conditions in North Georgia get very dry. This air is from the continent, not from an ocean. So it’s goodbye to balmy evenings. Dry continental air doesn’t hold heat well. As the sun drops to the horizon, the ground surface cools down rapidly.
The air that’s in touch with it is cooled as well as the ground radiates its heat out into space. This tends to produce a common evening situation called “temperature inversion.” In an inversion, the layer of air near the surface is cooler than the air above it. Normally, you’d find the warmest air on the ground, getting progressively cooler with altitude.
On an evening with clear sky, look into the distance. You’re likely to see a grey layer of air near the ground. That’s the inversion, which can be some hundreds of feet thick. Above it, the sky tends to be yellow, orange or red, indicating that there’s little moisture. The grey is caused by weak condensation in the cooler air.
An inversion is the opposite of a thermal. Instead of rising, air remains stubbornly on the ground. This isn’t glider plane heaven. There might be some weak uplift within the inversion layer. You’ll still see smoke rising from chimneys and outdoor fires. But it can’t go past the inversion boundary, which works like a lid on a pot. So the leaf fire that someone started in their back yard will end up spreading smoke across the neighborhood.
The good news about inversions is that they don’t tend to affect commercial jetliners significantly. Passengers won’t have the bouncy flights that come with summertime thermals.
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.