Optical air mass
From Glossary of Meteorology
optical air mass
(Originally called air mass.) A measure of the length of the path through the atmosphere to sea level traversed by light rays from a celestial body, expressed as a multiple of the pathlength for a light source at the zenith.
It is approximately equal to the secant of the zenith distance of the given celestial body for zenith distances up to about 70°. Bemporad's formula must be used for more accurate determination. To get a representative value at high elevation, the above values must be multiplied by the ratio of the actual atmospheric pressure to the sea level pressure.
List, R. J., Ed. 1951. Smithsonian Meteorological Tables. 6th rev. ed., . p. 422.