When a chilly wind is blowing from the northwest during daytime, it’s easy to understand why it feels so cold. But the reasons for temperature plunges at night are more complex.
The coldest nights tend to be the ones when the sky is crystal clear. The Earth acts like a giant radiator all the time, sending its heat back into space through a very long-wave type of infrared radiation.
While there’s a cloud cover, much of the heat is radiated back down. Meteorologists call this a “thermal blanketing effect.” Without clouds, though, there’s nothing to stop the heat from escaping.
The ground cools quickly, absorbing more heat from whatever may be in the air. Because the atmosphere is heated as well as cooled by the ground surface, temperatures can drop very rapidly below freezing on a clear night.
The coldest layers of air are then not found at a few hundred feet of altitude, but right where people live. A cold blanket of this type accumulates particularly in valleys, where the heavy air settles down. When temperatures are lower at ground level than above, we have what’s called an inversion.
Inversions aren’t fun. Not only is the coldest air trapped at the bottom, it also holds down pollutants that might otherwise be carried away. Smoke from fireplaces, automotive exhaust and industrial odors stay around and can cause situations ranging from unpleasant smells to full-blown smog.
Gainesville lacks the population density for severe smog episodes, even though there are hills on two sides that limit air circulation. It gets bad where tall mountains trap air in a city on several sides.
Cold air from the Pacific flows in from the west in Los Angeles, Calif., but can’t easily make it across the San Gabriel, San Bernardino and San Jacinto mountains that form a three-quarter ring around the city.
In Tucson, Ariz., the ring is complete. Nearly every part of that city is bordered by the Tortilita, Santa Catalina, Rincon, Santa Rita and Tucson mountains. Winter inversions there are known by the population as the “brown cloud” of Tucson.
Even without mountains, nighttime inversions can produce problems. Atlanta traffic is particularly hazardous at times when our Southern humid air cools, and its moisture condenses near ground level. The result is fog, which unfortunately tends to be the most dense early in the morning, at prime commuting hours.