air mass

noun2

1. •
   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.
2. •
   The amount of air through which light from a celestial object passes, normalized to 1 for an object at zenith.

   • Unfortunately, the extinction is very sensitive to the airmass[…] — 2007