In order to create more comfortable and healthy outdoor living spaces, it is important that the effects of solar shading by a tree on the thermal environment, light environment and ultraviolet environment are simulated in the planning stage. Particularly, a deciduous tree prevents solar radiation in summer by lush leaves, while transmitting solar radiation because leaves are shed in winter. The present study discussed solar shading by numerical analysis of the relationship between solar shading ratio and tree geometry indexes using CG tree models. Four tree species were chosen from various deciduous trees because they were popular shade trees in Japanese urban space. 50 trees of each species were created by CG software. Indexes of tree geometry such as leaf area index and branch area index were calculated using these CG models. The solar shading ratio of tree was computed by the ray-tracing method on the summer solstice and winter solstice in Tokyo. The relationship between these indexes and the solar shading ratio of tree was investigated. The results showed that solar shading ratio of tree in summer depended on solar attitude in most tree species. In winter, solar shading ratio of tree was constant any solar attitude. It is clear from this analysis that calculation of the solar shading ratio requires combined indexes that consider solar altitude and tree geometry, such as the plant hit count per virtual rays.
