We often get atmospheric inversions where the air at higher altitudes is warmer than the air at lower. It creates a clearly defined line holding all exhaust etc below. With unleaded gas and scrubbers, etc, we rarely have bad brown days like we used to, even with the explosion in population but still get the inversions (mostly in the winter).
My dad lived on Green Mountain in the 70s. From his backyard deck the view of smog was amazing, it actually had a copper color.