Right now the Salt Lake (Utah) valley is in the middle of a temperature inversion. I’m no meteorologist, but I’ll try to explain.
A temperature inversion is an atmospheric condition where the temperature increases with height (or elevation). The warm air above traps the colder air below. Along with the colder air, the pollutants are not able to escape. This creates the situation shown in this photo I captured today of the Salt Lake valley. This valley is highly susceptible to inversions due to being surrounded my mountains (duh… isn’t that the definition of a valley?).
If you’re really bored and would like to read more about inversions, follow this link to Wikipedia.