Air Mass (AM)
Solar radiation of approximately 6000K is approximately a blackbody radiation. The energy of the radiation that reaches the top of Earth atmosphere is around 1353kW/m2. As it passes through the atmosphere, sunlight is attenuated by scattering and absorption. The more atmosphere sunlight passes, the more energy is attenuated. The spectrum outside the atmosphere is defined as AM0. Air Mass (AM) describes the relation of the length sunlight travels and spectrum and radiation attunement. θ is the angel between path of zenith and incident sunlight. Air Mass is defined as 1/cos θ.
▲ Appendix I. Definitions of AM0 & AM1.5
AM0：The spectrum outside the atmosphere. WMO announced the irradiance of AM0 is 1366 W/m2 in 1981. 1366 W/m2 is also known as solar constant.
AM1：The irradiance when the incident sunlight perpendicularly travels through the atmosphere (1 atmosphere) to Earth surface.
AM1.5： The irradiance on Earth surface when incident sunlight travels from solar zenith angle of θ=48.2° and passes through 1.5 atmosphere thickness.
AM2：The irradiance on Earth surface when incident sunlight travels from solar zenith angle of θ=60.1° and passes through 2 atmosphere thickness.
Being absorbed by ozone, oxygen, water vapor, carbon dioxide…etc, the sunlight not only is reduced in irradiance but changed its spectral distribution.
▲Appendix II. AM0 & AM1 spectral distributions show how absorption in the atmosphere changes sunlight.