Salt Lake City exists in a big bowl surrounded by a bunch of mountains. When the seasons change, the changing temperature traps some air in the bowl. I don’t know the specifics behind it, but what you end up with is thick nasty smog that hangs over the entire city in a gross haze for days to weeks at a time.

I’m not native to Utah but I spent a little over a year there for school, and I got to experience the inversion. Looking out from the balcony on my apartment in broad daylight, I was unable to see the street. It just faded out into nasty brown fog like Silent Hill but grosser. People were wearing masks if they needed to go outside.