An air mass is a large body of air that exhibits uniform
temperature and moisture characteristics and helps to determine
the weather within an area. Warm air masses tend to form over
oceans in warm-temperature regions of the tropics and sub-
tropics, while cold air masses tend to form over land in cold-
weather regions nearer the poles. The cold air masses that form
at the poles move toward the equator, while the warm air masses
that form at the equator move toward the poles. These air masses
can undergo gradual, but significant, changes as they pass over
warmer or colder land surfaces encountering different types of
air. These boundary lines are called fronts. Cold fronts occur
when a cold air mass moves into warmer air. Since cold air is
heavier than warm air, the incoming colder air forms a wedge
that displaces the warm, and often moist, air at the surface. As
the warm moist air rises, towering clouds often form releasing
energy in the form of brief, intense rain storms with high winds
and lightning. On the