Definitions for "Airmass"
Keywords:  kasten, secant, radiance, zenith, noon
The relative path length of the direct solar beam radiance through the atmosphere. When the sun is directly above a sea-level location the path length is defined as airmass 1 (AM 1.0). AM 1.0 is not synonymous with solar noon because the sun is usually not directly overhead at solar noon in most seasons and locations. When the angle of the sun from zenith (directly overhead) increases, the airmass increases approximately by the secant of the zenith angle. A better calculation (Kasten, F. and A. T. Young (1989). Revised optical air mass tables and approximation formula. Applied Optics 28 (22), 4735-4738)  follows: m = 1.0 / [ cos(Z) + 0.50572 * (96.07995 - Z)-1.6364] where Z is the solar zenith angle. The figure below illustrates the concept of airmass.
An extensive body of air having fairly uniform properties of temperature and moisture.