[William Terrell]

There are cities that announce the Air Quality Index on the evening weather report, with special alerts from time to time.

[paulat]

There are cities that announce the Air Quality Index on the evening weather report, with special alerts from time to time.

Mostly cities around mountains...Los Angeles, Seattle, Denver....

The geography traps low temp air under high temp air, creating a temperature inversion. It is a visible effect...we can't see the mountains when it happens.