Local variation of outdoor thermal comfort in different urban green spaces in Guangzhou, a subtropical city in South China

To understand the influence of different urban green spaces on outdoor thermal comfort, researchers have focused on developing a thermal comfort range and estimating neutral and preferred temperatures through questionnaire surveys and microclimatic measurements. The simultaneity of investigation among different sites is often neglected, which decreases the accuracy and reliability of the outcomes. To fill this gap and to better address the local variation of outdoor thermal comfort in different urban green spaces, both surveys and measurements were performed simultaneously at three sites, “central grassland,” “pond-side garden” and “grove” in a subtropical urban local area in warm and cool periods. Remarkable differences in microclimatic conditions, thermal sensations, thermal comfort, and both neutral temperatures and preferred temperatures were observed among the three sites and in the two periods. Preferred temperatures were lower than neutral temperatures among different sites in each period or both periods combined, indicating the instinctive preference of people from relatively hot regions for a cooler thermal state. The grove yielded the best cooling capability but was perceived as the hottest and most uncomfortable site in the warm period. The central grassland with the highest air temperature, conversely, was perceived as the most comfortable site. The subjective perceptions were also reflected by lower neutral and preferred temperatures in the grove than those in the central grassland in the warm period. In particular, for preferred temperatures, up to 8.8 °C operative temperature (Top) and 4.3 °C physiological equivalent temperature (PET) differences between these two sites were observed. Hence, a high density of trees does not necessarily result in better outdoor thermal comfort. Future urban planning and management, therefore, should emphasize the types and structures of different urban green spaces. To further comprehensively explore the roles of different urban green spaces in outdoor thermal comfort, high synchronicity among sites during investigation is important.