Urban heat islands characterized by six thermal indicators

The urban heat island (UHI) effect alters local thermal environment and affects thermal comfort of urban dwellers. However, the similarities and differences of the UHI when characterized by different thermal indicators (i.e., different types of temperatures and thermal comfort indices) remain poorly understood. Taking 30 major Chinese cities as an example, here we compared the spatiotemporal patterns and drivers of the UHI intensity (UHII) represented by six different thermal indicators, i.e., Ia (UHII represented by surface air temperature), Is (land surface temperature), Id (dew-point temperature), IHD (Humidex), Imrt (mean radiant temperature), and IUTCI (Universal Thermal Climate Index). These thermal indicators encompass temperature factors that capture changes in the natural environment and thermal comfort indices that reflect the human perception of changes in ambient heat and humidity. The results show that the ranking of the annual mean UHII is similar between the day and night (i.e., Is > IUTCI > Imrt > Ia > IHD > Id). From a seasonal perspective, the daytime Is and Imrt exhibit the highest variability among all indicators, while the IUTCI demonstrates the largest difference among climate zones. Regarding the UHI drivers, most of the UHI indicators (except Id) are negatively correlated with the urban-rural difference in vegetation index during the day. At night, the UHI indicators are negatively related to the climatic variables and the urban-rural difference in albedo. Our results by using the IUTCI to assess outdoor thermal comfort across local climate zones (LCZs) show that the inter-LCZ standard deviation of the IUTCI is 1.94 °C during the day and 2.16 °C at night. We believe that these results deepen our understanding of the UHII when characterized by different thermal indicators, especially from the human perception perspective.