A comparison of two models for the angular distribution of diffuse sky radiance for overcast skies

A comparison is presented of two models for the angular distribution of diffuse sky radiance under overcast skies: the generalized Standard Overcast Sky (SOC) model and the Three-Component Continuous Distribution (TCCD) model. The defining equations for the models are seen to be different, but are shown to be similar in form for the case of overcast skies. It is concluded that for overcast skies 1) the SOC and TCCD equations are capable of predicting the diffuse sky radiance equally well, 2) calibrations of the parameters of one model can be used to calibrate parameters of the other, and 3) parameters can be estimated more easily for the TCCD equation than for the SOC equation. These results substantially raise the confidence in the validity of both models for overcast skies, and support the choice of the most convenient for any particular application.