Air mass

Solar radiation is attenuated by the atmosphere before reaching the earth’s surface, to an extent dependent on the angle of the sun above the horizon. This dependence is measured by the ‘air mass’, defined as the ratio of the distance actually travelled by the radiation through the atmosphere to the distance vertically up through the atmosphere. ‘Air mass zero’ refers to radiation not attenuated at all by the atmosphere, i.e. to the full radiation from the sun in space.

 See also solar constant.