Air Mass
Air Mass in conjunction with photovoltaics
The spectrum of the solar radiation depends on the light's length of run. Certain spectra and radiant powers are attributed to a defined measure of length.
A diagonal incidence of light implies a decrease of radiant power and a change of the spectrum. Various spectra and radiated powers were defined for comparative measurements. AM 0 means the space-related spectrum extraterrestrial spectrum). The radiant power there adds up to 1367 W/m2 (solar constant). AM 1 signifies the spectrum of the rays of sun which hit the earth's surface vertically. To achieve AM 1, the sun has to be at the zenith and the rays have to take the shortest way to the earth's surface. AM 1.5 results in the zenith angle of approximately 48.2°. At this spectrum the global radiated power amounts to 1000 W/m². That is the reason why AM 1.5 was introduced as the standard value for measuring of solar modules.
In midwinter the zenith angle in Berlin adds up to 76° at noon and the outcome of this is AM 4.13. In midsummer and at the apex of the sun's motion the zenith angle amounts to circa 29° which equals AM 1.14.
User Schusch, from Wikipedia, GNU-licence for free documentation

