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.

italian workers migrated to northern provinces

answer (a) more rain in u s and less rain in australia and asia