ANISOTROPIC SKY RADIANCE MODEL-BASED ON NARROW FIELD-OF-VIEW MEASUREMENTS OF SHORTWAVE RADIANCE

A model for the average anisotropic sky radiance (or intensity) as a function of the position of the sun, the diffuse fraction k, and the atmospheric clearness index k(t) are presented in this article. The complete range of sky conditions from clear to turbid to overcast is covered. Analysis of the observed data indicates that the model can be used to estimate instantaneous sky radiance values with a mean bias error of-11% and a root mean square error of 65% of the mean. The model is shown to account for 83% of the deterministic part of the variance of the instantaneous sky radiance measurements. The formulation for the sky radiance L(theta, phi) in the direction (theta, phi) is L(theta, phi) = G(d)[a0 + a1cos theta + a2exp(-a3.PSI)/pi(a0 + 2a1/3) + 2a2I(theta(s), a3)] where PSI is the scattering angle from the solar beam, a0, a1, a2, a3 are tabulated functions of k and k(t), and I(theta(s), a3) is a specific function of a3 and the solar zenith angle theta(s).