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
a = 0.14
h is the location height above sea level in kilometers