The stagnant air sitting across Salt Lake County last weekend seemed to grow unusually dense to the untrained eye, even for an inversion.
Yet, according to a meteorologist who works in the National Weather Service’s Salt Lake City office, it wasn’t all pollution — the air quality didn’t significantly dip Saturday or Sunday compared to earlier last week.
The culprit, instead, was fog, making the air seem strangely thick. “That was a strong valley inversion that had quite a bit of moisture trapped in the valley,” said the meteorologist, Michael Wessler.
What happened?
Even though it was eerie, the misty air was “pretty typical” of a winter inversion — “especially for one where we don’t yet have the snow on the ground,” he said.
Snow can actually exacerbate poor air quality, he said, because it can cause “really strong, really shallow inversions that allow us to trap a lot of pollutants in a much shallower layer.”
Inversions are caused when cold air travels into valleys and cannot escape the layers of warmer air above. As that cold air stagnates, pollutants gather, intensify and linger — at least until a storm or low pressure system sweeps in to clear the valley.
But just like pollutants, moisture is also unable to escape.
“When you get that moisture and it cools down, basically you condense all the water out and you get that nice visible fog,” Wessler said. “Our one saving grace is that we don’t have snow on the ground yet in the valley.”
Had there been, the air could have reached more hazardous levels and become unhealthy for everyone, he said. Last weekend, though, did not bring those circumstances.
“Definitely not a pleasant amount of particulate matter in the air, but not as bad as visibly it would seem,” he said.
Could it happen again?
(Francisco Kjolseth | The Salt Lake Tribune) Inversion conditions in the Salt Lake valley trap pollution as air quality continues to deteriorate on Wednesday, Dec. 4, 2024.
The longer an inversion lasts, the more moisture it will build, leading to fog.
“Later in a longer inversion period, that’s when you’re more likely to see fog, or that really heavy cloud layer on top of the inversion,” he said.
The inversion last week started around Monday, Dec. 2, and remained through Sunday. All the while, pollution continued to gather, too, contributing to the decreased visibility.
Overnight Sunday brought “a really good pulse” to the Salt Lake Valley that helped clear out much of last week’s inversion buildup, though, leaving relatively good air for people to breathe.
Wessler expected more conditions this week that will fully flush out the valley — for now.
