Developing a method for calculating the mean outdoor radiant temperature for thermal stress in Guangzhou

Cities around the world are exposed to increasing heat stress as a result of the heat island effect exacerbated by extreme weather events, population growth and increased urbanization. To promote outdoor activities and protect human health, there is an urgent need for a tool to assess the outdoor thermal environment. While the air temperature (Ta) and relative humidity (RH) are usually available from weather stations, but the mean radiant temperature (MRT) cannot. The MRT is a key variable in determining outdoor thermal comfort. Therefore, it is necessary to develop a simple and accurate method for calculating MRT, in order to compute the corresponding thermal indices, which can be used to assess the thermal environment more accurately. This study was conducted at a university in Guangzhou, China. Four different ground-cover surfaces (concrete, asphalt, lawn, and granite) were selected for study. The Ta, black-globe temperature (Tg), long-wave (L), and short-wave (K) radiation in six directions, and wind speeds (Va) were measured under unshaded conditions. The MRT obtained by the six-direction integral method was used as the base value and regression analyses were performed with Ta, K down arrow (downward short-wave), and Va. In addition, the six indices-significance (P), linear regression coefficient (R2), consistency index (d), root mean square error (RMSE), mean bias error (MBE), and mean absolute error (MAE)-were used for quantitative analysis to explore the feasibility of this method. This study compared five calculation models on MRT: MRTSDIM (six-direction integral method), MRTBGTM-Va-0.1Hz (black-globe thermometer method, 0.1 Hz wind speed measurement frequency), MRTBGTM-Va-10Hz (black-globe thermometer method, 10 Hz wind speed measurement frequency), MRTBGTM-P (polynomial regression based on black-globe thermometer method), and MRTSDIM-P (polynomial regression based on six-direction integral method). The MRTSDIM was used as a baseline value, and the values obtained by other four methods were discussed and validated separately. The results show that the calculation model about MRTSDIM-P has a relatively minimal error with the values <= 5 degrees C, which satisfies the accuracy requirements for thermal stress. Finally, the data from each area weather station were substituted into the model and compared with the actual measurement days, and the monthly mean and maximum universal thermal climate index (UTCI) were calculated for the entire Guangzhou daytime in 2022. The results obtained from the weather station data calculations fit well with the measured data. Consequently, this study provides a simple method of calculation for outdoor mean radiant temperature. Further, based on the calculated thermal indices, recommendations can be provided for outdoor work and activities.