relative optical air mass [m]

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


Unit: 1

NOTE 1 See also "optical thickness of the atmosphere" and NOTE 1 to "spectral optical depth".

NOTE 2 Several equations are available, usually the Kasten-Young formula is used:


where γs is solar altitude in °.

NOTE 3 When the curvature of the atmosphere and atmospheric refraction are neglected:


NOTE 4 Further information on the use of this term in the field of terrestrial utilisation of solar energy is given in IEC 60904-3:2008.

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