relative optical air mass

m

quotient of the optical thickness of the atmosphere at angle ε to the vertical, δ (ε), and the vertical optical thickness, δ (0°), of the atmosphere

Note 1 to entry: See also "optical thickness".

Note 2 to entry: Several equations are available to express this concept. Usually the Kasten–Young formula is used:

where γ_s (= 90° − ε) is solar altitude (complementary angle of zenith angle, ε) in degrees.

Note 3 to entry: When the curvature of the atmosphere and atmospheric refraction are neglected:
.

Note 4 to entry: The relative optical air mass has unit one.

Note 5 to entry: This entry was numbered 845-09-88 in IEC 60050-845:1987.

Note 6 to entry: This entry was numbered 17-1082 in CIE S 017:2011.