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 temperatureTa, mean radiant temperatureTmrt, 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 ofTa,Tmrt, and PET. With respect to both shading devices, theTareduction 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. AsTmrthas the role of a key variable for outdoor thermal comfort during Central European summer weather, all radiant flux densities relevant to the determination ofTmrtwere directly measured and analyzed in detail. The results show the crucial significance of the horizontal radiant flux densities forTmrtand consequently PET.

The applicability of the Universal Thermal Climate Index (UTCI) at the urban pedestrian level is analysed in relation to the approach for the vertical wind speed profile (VWSP) used in the UTCI basics. As the UTCI refers to the human-biometeorological reference height (z _h−b ) of 1.1 m above ground level (agl), all meteorological input variables for UTCI calculations should also be originated from z _h−b . Since the development of the UTCI considered meteorological conventions for stations routinely operated by national weather services, the wind speed (v) as one of four meteorological input variables is initially required at 10 m agl (v _{10 m} ). In order to calculate v at z _h−b from v _{10 m} , the logarithmic law for the VWSP is implemented in the UTCI basics, but only for neutral atmospheric stability and a roughness length (z ₀ ) for short-cut grassland (z ₀ = 0.01 m). This methodological approach cannot be changed in the UTCI basics so far. The UTCI developers also recommend the application of the same logarithmic law, if v values at lower heights have to be extrapolated to 10 m agl. However, the use of this VWSP approach disregards basic air flow findings and principles in the urban canopy layer (UCL). If the UTCI is nevertheless applied at the urban pedestrian level, inaccuracies in the UTCI values will occur. With reference to z ₀ = 0.80 m, which is more typical for the UCL, they can be up to 7 K, as shown in the example of three different climate zones.