Winter inversions can really get you down

A heavy fog rolls over the Snake River and surrounding area near Ontario on Friday. Due to an air inversion, moisture trapped in the air builds up near the ground and with the added humidity, it leads to dense fog.

It’s wintertime, and the weather can be downright frightful. And here in the Western Treasure Valley, it has been, as we’ve already seen some early winter temperatures well below zero. Those extremely low temperatures may have been a bit unusual for early December, but they came in the midst of a temperature inversion, which is not so unusual for our winters. The Treasure Valley is somewhat infamous for its wintertime temperature inversions. The Blue, Owyhee and Boise mountains that surround the Treasure Valley tend to lead to the formation of some particularly severe inversions.

Normally, air gets colder with increasing altitude. That’s why it can feel so nice in the mountains in the middle of summer when the valley floors are baking hot. However, during a temperature inversion, cold air gets trapped underneath a layer of warmer air. So inversions create the somewhat bizarre and counterintuitive phenomenon in which valley floors can be much colder than the surrounding mountains.

Inversions can form in several ways; for us they normally form because of the typical high pressure systems that regularly set up in the winter over the Great Basin region of the western United States. Dense cold air sinks into the valleys, and the high pressure acts like a cap that pushed down on this layer of cold air and holds it in place. The calm winds that accompany a high pressure system keep the air from mixing as it normally would, and the cold air remains trapped below the warmer air above it.

Unfortunately, it’s not just cold air that gets trapped down at the valley floor. Because the air isn’t mixing during an inversion as it normally would, air pollutants become concentrated as they get trapped near the ground. Inversions not only trap the normal amounts of air pollution, they help create even more pollution. For example, with the colder temperatures, home heaters are working that much harder. Also, people are more likely to idle their cars to warm them when it is extremely cold, because let’s face it, no one is too thrilled about driving around in a freezing cold car. This buildup of smog can be especially severe in bigger cities such as Denver and Salt Lake, but the Boise area is seeing more and more incidences of poor air quality during winter time inversions.

It’s not just pollution that gets trapped in an inversion. Moisture in the air is also trapped and builds up near the ground. The added humidity leads to the dense fog and gray, sunless days that we can get in the winter. If there is any silver lining to the gray gloom of an inversion, it would have to be the spectacular frost formations that form across the landscape, on trees, bushes, and fences. If you look closely, you will see that the ice crystals that make up frost form in intricate, well-defined and detailed geometric patterns.

Given our topography and weather patterns, wintertime temperature inversions are a fact of life in the Treasure Valley. They can last from a few days to several weeks. The inversion that brought those cold temperatures in December lasted just a few days, but the one last January was a whopper that went on for over three weeks. However, the weather is always changing, and inversions break when a new weather front brings strong winds through the valley that restarts the process of mixing the air through the atmosphere, and leads to the return of blue skies and brilliant sunshine. So the next time we have an inversion, remember the old saying that this too shall pass.


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