${\displaystyle m=\frac{\delta (\epsilon )}{\delta (0°)}}$

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:

${\displaystyle m=\frac{1}{\left[\text{sin }{\gamma }_{\text{s}} + \mathrm{0,}\text{5}0\text{5 72}{({\gamma }_{\text{s}} + \text{6}\mathrm{,0}\text{79 95})}^{-\text{1},\text{636}\text{4}}\right]}}$

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

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

${\displaystyle m=\frac{1}{\cos \epsilon }}$.

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.