In Mexico City, during the winter months, sub-tropical high-pressure systems can cause a warm layer of air to get trapped above the colder denser layer of air in the Valley of Mexico. This phenomenon can increase surface air pollutants and is called

a thermal inversion.                    

Explanation:

In climatology an inversion, is referred to as an aberration of temperature, is a variance from the natural altitude shift in an atmospheric area. This almost often relates to a spectral delay rate inversion. Outdoor temperature usually declines with a growth in altitude. Hotter air is kept over colder air while on an inversion; the usual weather profile is reversed with altitude.

An inversion causes near- air emissions, including smog. The inversion will also remove convection by serving as a 'bar' When, for any of the causes, this cap is removed, convection about any precipitation existing can then burst into destructive rainstorms.