I had a car for a while that had a thermometer on the inside which told me what the temperature was on the outside. It was a very cool feature, and one of this particular vehicle’s few redeeming qualities, but that’s another story. Anyway, this car and I headed down the mountain one wintery day. It had been precipitating, the temperature was marginal—right around thirty-two degrees—and the road was a little slick. No worries, I thought. By the time I get to the bottom (I started out at over 2,200 feet), the temperature will be well above freezing and I won’t have an icy road to contend with.
Not so. My trusty interior thermometer told me the outside temperature was dropping, along with my elevation. The weather gods were having fun at my expense; I was caught in a temperature inversion.
Normally air temperature decreases about 3.5 degrees for every 1,000 feet of increase in altitude. With an inversion, the opposite happens. Technically speaking, an inversion is the condition during which the temperature of the atmosphere increases with altitude in contrast to the normal decrease with altitude. The inversion—a thin atmospheric layer—acts like a lid or a ceiling, keeping the “convective overturning” of the atmosphere from penetrating through the lid. In other words, the air gets kind of stuck. Why is that?
Sometimes a warmer, less dense air mass moves over a cooler, more dense mass, and the lid or ceiling effect comes into play. Geography can also be a factor, which was the case with my inversion experience, as the mountains are good at keeping the air in the narrow runs and valleys contained. Thanks to solar radiation, the atmosphere near the surface of the earth (the troposphere) can be warmer than the air above it. But, overnight radiative (spreading out from a central point) cooling of surface air can result in nocturnal temperature inversions, too. Those often dissipate after sunrise when the air near the ground is warmed, but in the areas where the base of the mountains hug the roads and the sun’s heat doesn’t get to the ground before mid-day, if at all, that cooler air stays put.
“Air is a pretty good insulator, but the ground is not,” says WETM weather forecaster Chip Maxham. “The temperature of air is slow to respond to sunshine, or the lack of it, but the temperature of the ground changes more quickly with more or less sun.”
The altitude of the inversion boundary is determined by the weight of the air above it; that boundary could conceivably be high enough to have little or no effect on us. Other times, when conditions are right, the earth’s surface cools the moist air above it (remember it is more dense than the warmer air layer) and we get fog. Temperature inversions are also good at trapping smog.