What is an air mass and how does it form?

An air mass is a large body of air having little horizontal variation in temperature and moisture. Air masses form over large surfaces with uniform temperatures and humidity, called source regions. Low wind speeds let air remain stationary long enough to take on the features of the source region, such as heat or cold. When winds move air masses, they carry their weather conditions from the source region to a new region.

Further Reading:

• General Circulation Of The Atmosphere
• Heating And Cooling Of The Atmosphere