Modification of Human-Biometeorologically Significant Radiant Flux Densities by Shading as Local Method to Mitigate Heat Stress in Summer within Urban Street Canyons

Increasing heat will be a significant problem for Central European cities in the future. Shading devices are discussed as a method to mitigate heat stress on citizens. To analyze the physical processes, which are characteristic of shading in terms of urban human-biometeorology, experimental investigations on the thermal effects of shading by a building and shading by tree canopies were conducted in Freiburg (Southwest Germany) during typical Central European summer weather. Urban human-biometeorology stands for the variables air temperature T-a, mean radiant temperature T-mrt, and physiologically equivalent temperature PET, that is the human-biometeorological concept to assess the thermal environment which was applied. The measuring setup consists of specific human-biometeorological stations, which enable the direct or indirect determination of T-a, T-mrt, and PET. With respect to both shading devices, the T-a reduction did not exceed 2 C, while PET as a measure for human heat stress was lowered by two thermal sensation steps according to the ASHRAE scale. As T-mrt has the role of a key variable for outdoor thermal comfort during Central European summer weather, all radiant flux densities relevant to the determination of T-mrt were directly measured and analyzed in detail. The results show the crucial significance of the horizontal radiant flux densities for T-mrt and consequently PET