Figure 3-1: The atmosphere acts as a temporally and spatially variable filter on solar radiation. The path length (air mass) of direct-beam radiation increases with increasing solar zenith angle; scattering and absorption increase with longer path lengths. Global radiation on a horizontal plane (Gh) is equal to the direct-beam radiation (from the solar disk) normal to the surface (Dn), multiplied by the cosine of the solar zenith angle (z), plus scattered radiation from the sky (S). For a tilted (rather than horizontal) plane, Dn is multiplied by the cosine of the incidence angle. The surface also receives radiation reflected from the ground to the surface (R).

These relationships are:

  Direct normal		Dn
  Global horizontal	Gh = Dn x Cos(z) + S
  Global normal		Gn = Dn + S + R
  Global tilt		Gt = Dn x Cos() + S + R


http://www.nrel.gov/rredc