Abstract
The difference in estimation of the diffuse site factor due to different models of the spatial distribution of sky luminance was examined. The examined models were the uniform overcast sky (UOC) and standard overcast sky (SOC). The difference between the diffuse site factors for UOC (ΨUOC) and for SOC (ΨSOC) was 1.5% in average and ranged from -0.4% to 5.2%. A strongly positive correlation was observed between ΨUOC and ΨSOC: ΨSOC=1.109ΨUOC (n = 270, r = 0.999, p < 0.001). This shows that ΨSOC tends to be larger than ΨUOC and the difference increases with the increase in ΨUOC or ΨSOC. The difference is determined by three factors: 1) the decrease in gap fraction (gap ratio) with zenith angle; 2) the incident angle of cosine law; 3) the difference in spatial distribution of sky luminance.
