1. A widespread body of air, the properties of which can be identified as 1) having been established while that air was situated over a particular region of the earth's surface (airmass source region), and 2) 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. The stagnation or long-continued motion of air over a source region permits the vertical temperature and moisture distribution of the air to reach relative equilibrium with the underlying surface. See airmass classification. 2. In radiation, the ratio of the actual path length taken by the direct solar beam to the analogous path when the sun is overhead from the top of the atmosphere to the surface. Extrapolation of surface measurements to zero air mass was the original method for estimating the value of solar irradiance at the top of the atmosphere. 3. See optical air mass.