Monitoring and determinants analysis of canopy layer urban heat island in China based on both air temperature and apparent temperature

The canopy layer urban heat islands (CLUHIs) are closely linked to human well-being and have been extensively studied. Nevertheless, large-scale CLUHIs were insufficiently researched by sites observation, particularly from the perspective of apparent temperature. Therefore, the spatial-temporal differences of CLUHI intensities (CLUHIIs) were analyzed based on site-observed air temperature and apparent temperature. Then, the accuracies were explored for remotely sensed CLUHIIs, comparing with site-monitored CLUHIIs. Finally, fourteen associated factors were analyzed for CLUHIIs derived by site-measured air temperature and apparent temperature from four aspects, including human socio-economic activities, urban underlying surface conditions, meteorological factors and air pollution. The main findings were as follows. (1) 71.68 % of CLUHIIs were between −1.0 and 1.0 °C, while the maximum and minimum were 12.96 and − 6.47 °C, respectively. Under humid conditions, most differences exceeded 0.40 °C between CLUHIIs derived by air temperature and apparent temperature and even reached 4.95 °C. (2) The root mean square errors (RMSEs) for CLUHIIs derived by remote sensing didn't exceed 0.94 °C and 1.41 °C in Southern and Northern China, respectively. (3) Two types of CLUHIIs presented clear and regular relationships with fourteen factors, featuring varying positive or negative characteristics and intensities, involving three socioeconomic factors, two urban underlying surface conditions, six meteorological conditions, and three air pollutions. This study can provide theoretical basis and practical reference for the construction of climate-adaptive, livable, low-carbon and sustainable cities.