Estimation of Cloudiness and Solar Energy Availability in Georgia Using a Clear Sky Model

The underlying general equation in the model for computing irradiance at the surface of the earth takes the form: I = Io(cosZ)TRTgTwTa [1] for which Io = 1353 Wm-2{1 + 0.034 Cos[2?(n-1)/365]} [2] Io is the irradiance at the top of the atmosphere normal to the solar radiation beam, and n is the day number (Jan 1 = 1, Dec 31 = 365, etc.). TRTg = 1.021 – 0.084 [m(949p x 10-5 + 0.051)]1/2 [3] TR is the transmission coefficient after Rayleigh scattering, Tg the transmission coefficient for absorption by permanent gases, p the pressure in Kpa and m is the optical air mass at pressure of 101.3kpa given by m = 35(1224 Cos2(Z) + 1)-1/2 [4] The solar zenith angle, Z, is computed using a subroutine written by The Astronomical Almanac. It takes into account the refraction of atmosphere at large zenith angles. Tw = 1 – 0.077 (um)0.3 [5