Are water bodies effective for urban heat mitigation? Evidence from field studies of urban lakes in two humid subtropical cities.

Blue infrastructure is considered a nature-based solution to address urban heat problems. However, the potential nocturnal urban heat island (UHI) effect of water bodies has not been investigated and elaborated thoroughly. Besides, existing studies have primarily focused on surface temperature and are limited to individual cities during summer daytime. In this study, screen-height air temperature/humidity measurements at lake, residential, and rural sites in two humid subtropical cities of China (Nanjing, Guangzhou) were conducted continuously for one year. The UHI intensity (UHII), urban moisture island intensity (UMII, evaluated by humidity ratio difference), and heat index (HI) were analyzed across different weather conditions and time scales. Results demonstrated that (1) compared to residential sites, urban lakes can help mitigate urban heat excess, while this capability at night is not as significant as during daytime; compared to rural sites, urban lakes exhibit diurnal thermal behavior of daytime cooling and nighttime warming, with a weak urban cool island intensity (UCII) of 0.1–0.6 °C during the day but a significant UHII of 1.2–1.3 °C at night during warm months (May–September), which is comparable to residential sites with UHII of 1.1–1.3 °C; (2) urban moisture excess tends to occur during nighttime and cold seasons; (3) compared to rural sites, lakes contribute to a good relief on heat stress during daytime, but a significant negative effect at night due to the intensified warming and humidifying. Findings of this study can provide empirical evidence to help better understand the effectiveness of water bodies for urban heat mitigation. • Year-round air temperature/humidity were measured in two humid subtropical cities. • Urban lakes can help mitigate daytime urban heat, but not for nighttime UHI. • Urban moisture excess tends to occur during nighttime and cold seasons. • Increased humidity may offset heat stress relief achieved from evaporative cooling. • Weather conditions affect urban heat and moisture islands. [ABSTRACT FROM AUTHOR]