Temperature inversions are most common in valleys where cool mountain air sweeps down into the valley at night , below the warm ,polluted air surrounding the city . Los angeles commonly experiences temperature inversion that traps a smog layer with cool air above and cool air below. This inversion keeps the smog (and therefore the pollutants) close to the ground instead of allowing it to disperse into the atmosphere. Cities that experience such trapped smog may issue local air quality warnings so that people with asthma or other respiratory troubles know to stay indoors.