Fall is a great time to see different sky colors. Most spectacular are the evenings following a dry afternoon. Our eyes can see three colors: red, green and blue. The red wavelengths are the longest, and the laziest in a way. They don’t scatter in different directions easily, like the blue and green ones do. As the sun drops toward the horizon, the air layer that its rays penetrate becomes thicker. When the evening (or morning) sunlight travels through a thick, dry atmosphere, the green and blue portions of it can scatter away, leaving mostly red for us to see.
A red sunset in Georgia usually means clear weather for the next day because most of our weather comes from the west. Sunset occurs in a similar direction (southwest actually), so we’re looking at dry air coming our way.
Another color we spot frequently on dry fall days isn’t a color at all. When you look far into the distance, the horizon seems capped by a grey layer. Sometimes there’s a sharp upper limit before the red color starts. This is called a low-level temperature inversion. A layer of cool air is settling on the ground. The air above it is warmer. It happens because the earth’s surface disposes of heat more quickly than the atmosphere.
Hall County’s forests, fields and towns cool rapidly after sunset, radiating their heat into space through the dry air. With no clouds in the way, there’s nothing that would slow or stop this process. Cold air now settles near the surface. The cooling also increases its relative humidity, and the inversion layer can get quite hazy.
Heavy, cold air cannot rise beyond the top of the inversion, so it’s confined to a few dozen to a few hundred feet above ground level. Smoke and other pollutants can’t disperse upward. That’s why you can smell smoke from the neighbor’s grill or fireplace more intensely during a nighttime inversion.
Deep blue sky is a phenomenon of fall and winter in North Georgia also, but during daytime. Greater humidity during spring and summer tends to scatter the short, jittery blue wavelengths so strongly that a lot of them get lost.
But now, at high sun around noontime, they arrive with great intensity. Until the next frontal system brings clouds, we get to enjoy picture-perfect blue sky color.
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.