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air mass or air mass ratio
Equal to the cosine of the zenith angle or that angle from directly overhead to a line intersecting the sun.

The air mass is an indication of the length of the path solar radiation travels through the atmosphere. An air mass of 1.0 means the sun is directly overhead and the radiation travels through one atmosphere (thickness).

This entry is located in the following unit: Photovoltaic Conversion Efficiency Terms + (page 1)