The Source of Air Masses

Air masses develop above parts of the Earth that have similar characteristics, such as the ocean, or a large forest. In order for an air mass to take form, the air must remain above that location for at least a couple of days, before moving on. In order for the air to be stable enough to remain over a large area for a sufficient amount of time, generally there needs to be a high pressure in the area. Without this high pressure, the air will simply be too unstable, and will never acquire the uniform characteristics so important in an air mass.