air mass (optical air mass)

[10.5.5.3]

NOTE: This definition was changed in RP-16-17 Addendum 2. Deletions have a strike-through. New text is bold.

The ratio of the path length of radiation through the atmosphere (lm) at any given angle, Z degrees, to the sea level path length toward the zenith (lz).*

The ratio of the path length of solar radiation through the atmosphere to a given point, relative to the path length from sea level to the zenith.

*Standard Solar Constant and Zero Air Mass Solar Spectral Irradiance Tables, ASTM E490-00a. West Conshohocken, PA:  ASTM International; 2014.

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