Observational studies of mean radiant temperature across different outdoor spaces under shaded conditions in densely built environment

Abstract

High-density urban environment affects urban microclimate, and thermal comfort of outdoor spaces. Shading by urban structure is the primary measure to reduce daytime mean radiant temperature (MRT) by blocking direct solar radiation. However, it is believed that overly-restricted Sky View Factor (SVF) limits the radiative cooling by the sky of lower effective temperature. Therefore, this study aimed at examining the relationship between SVF and MRT in built environment. To investigate the dependence of MRT on SVF via radiant fluxes, this study performed field measurement of six directional long-wave, short-wave fluxes and MRT across a number of outdoor spaces under shaded in densely built environment. Regression analysis was employed in examining desired relationships. Across different open spaces, MRT would increase by 1.6 K per 10 W/m2 increase of either long-wave or short-wave radiant fluxes. Global solar radiation and air temperature are the most influential meteorological parameters affecting radiant fluxes components in MRT. Besides, this study suggests the use of long-wave mean radiant temperature (LMRT) representing effective surface temperature of surrounding objects. LMRT generally follows the temporal pattern of reference air temperature. In a case study, if SVF is decreased by 0.1, the weighted sum of long-wave fluxes would be increased by 10 W/m2. This would bring an increase of 1.6 K in MRT. But, SVF has weak explanatory power to variations in short-wave fluxes when under shaded. Larger SVF is preferred for cooling open space in hot and humid region if direct sunlight is already blocked by urban morphology.