air mass

noun

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
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
optical path length through Earth’s atmosphere for light from a celestial source

Pronunciation: /ˈɛː mas/ / /ɛəɹ mæs/ / /ɛːɹ mæs/

Etymology: From air + mass. Perhaps a calque of German Luftmasse.

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.
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[…]”