Best explanation for "how" air masses move across the United States:

Air Masses

An air mass is a large body of air with generally uniform temperature and humidity. The area over which an air mass originates is what provides it's characteristics. The longer the air mass stays over its source region, the more likely it will acquire the properties of the surface below. As such, air masses are associated with high pressure systems.

There are two broad overarching divisions of air masses based upon the moisture content. Continental air masses, designated by the lowercase letter 'c', originate over continents are therefore dry air masses. Maritime air masses, designated by the letter 'm', originate over the oceans and are therefore moist air masses.