Quite soon, cold nights will be a thing of the past for a while. That’s true for temperature inversions as well. On a cloud-free evening, it’s easy to spot an inversion building up.
The sky turns yellow and red. But near the horizon, it lacks color. Just before dark, you see a thick gray layer at a distance, settling on the landscape. Inversions form when the ground cools faster than the atmosphere above it.
Closest to the bottom, the air that’s in ground contact will also get cold. Higher up, maybe at 1,000 feet, it’s a few degrees warmer. Cold air cannot rise above warmer air, so the inversion layer rests on the surface like a heavy quilt.
Just north of Gainesville, the Chattahoochee River flows in a valley just 2 miles wide, at an elevation close to 1,100 feet. On both sides of Ga. 365, the hills rise quickly to 1,200 feet and more. Driving southwest from Cornelia, one can see inversions developing against the light of the evening sun. The margins of the Chattahoochee valley serve as boundaries.
Inversions aren’t bad by themselves. They only mark temperature differences. But anything that’s in the air near the ground will stay there. Dense fog is common along Ga. 365.
Smoke from fireplaces, outdoor burnings and wildfires doesn’t get removed by the wind. Instead, it spreads all over the area affected by the inversion.
The rough smell is caused by the tiny particles that constitute the smoke. This time of the year, pollen from trees, shrubs and grass is an additional problem. A strong temperature inversion will hold those tiny particles in the air near the ground, with much higher concentrations than we see at other times. Some people call it “the brown cloud” when high pollen count combines with the stagnant air of a temperature inversion.
Because this happens when the ground is cold, inversions tend to start at sunset and last through the night. They get strongest just before sunrise, when temperatures are lowest. If the air is loaded with particles, opening windows for “fresh air” probably won’t help.
In large cities, the smog reports are updated in the morning because that’s when inversions cause the most problems. When the night is forecast to be cool, people sensitive to pollen and air pollution should best close the windows before going to bed.