To give a few more details, an inversion is where there's a layer of air where it gets warmer instead of colder as you go up. For example, last night at the SLC

To give a few more details, an inversion is where there's a layer of air where it gets warmer instead of colder as you go up. For example, last night at the SLC airport it was 4 degrees F whereas at the top of one of the 11,000 foot peaks near Snowbird it was 34 degrees F. When we're in a high pressure system (i.e. nice weather) and solar heating is limited due to low sun angle and snow on the ground, this situation is stable. It happens in many valleys in the western US. This becomes a problem in a few specific valleys where a lot of people live (and drive cars). Usually we get a few weeks of this every winter.