Definition from Wiktionary, the free dictionary
airmass (plural airmasses)
- (physics, meteorology) a widespread body of air, the properties of which can be identified as: (a) having been established while that air was situated over a particular region of the earth's surface (airmass source region) and (b) undergoing specific modifications while in transit away from the source region. An air mass is often defined as a widespread body of air that is approximately homogeneous in its horizontal extent, particularly with reference to temperature and moisture distribution; in addition, the vertical temperature and moisture variations are approximately the same over its horizontal extent.
Source: FM 3-6 Field Behavior of NBC Agents.