Generally air masses form over large areas of the surface of the Earth that have similar characteristics. Because these regions are common sources of air masses, we refer to them as source regions.
A source region might be an ocean, a large forest, a desert, or open grasslands. Source regions must be large and have similar, or uniform, traits throughout.
In the Northern Hemisphere, cool air masses often form over the North Atlantic and Pacific Oceans, as well as over the Northern continents. Warm air masses generally form in the South Pacific and South Atlantic, and over the southern portion of continents. The opposite is true in the Southern Hemisphere.