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.« Back to Definitions Index