[10.5.10.8] 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).*

AM = l_{m}/l_{z} \cong \sec Z, or 1/\cos Z, for Z \leq 62^{\circ}

 

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

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