Intensity Sea Level

ID is the intensity on a plane perpendicular to the sun's rays in units of kW/m2

AM is the air mass

1.353 kW/m2is the solar constant

0.7 arises from the fact that about 70% of the radiation incident on the atmosphere is transmitted to the Earth.

0.678 is an empirical fit to the observed data and takes into account the non-uniformities in the atmospheric layers