Abstract
Circumsolar radiation is the diffuse solar radiation from the ring shaped area of the sky around the sun. When global solar radiation is separated into direct and diffuse component, it is usually included in the direct solar radiation. However, the amount and the angular distribution of the component have not been clarified. In this study, global solar radiation I_<TH>, direct solar radiation I_<DV>(θ_0), and diffuse solar radiation I_<dH>(θ) occluding the direct and diffuse solar radiation up to the angular distance θ from the center of the sun, were measured at a ground based station for various seasons, weather conditions, and solar altitudes h. for three θ ( = 8°,16°,32°). The instruments used were Eppley pyranometer, EKO pyrheliometer with opening half angle θ_0 (=4°), and the combined device of Eppley pyranometer and a sun shading disc attached to an automatic sun tracker. From these observations, circumsolar component S_<DH>(θ) on a horizontal plane was calculated by the following formula : S_<DH>(θ) = I_<TH> - I<dH>(θ) - I_<DV>(θ_0)sin(h). The estimation formulas for I_<dH>(θ_0) and S_<DH>(θ) as functions of θ, h, and atmospheric permiability P,as well as a relationship between I_<dH>(θ_0) and S_<DH>(θ) were developed.
