Urbanization Amplifies Nighttime Heat Stress on Warmer Days Over the US

The impact of heat on human health is well-recognized, with excess heat stress in urban areas (urban heat stress intensity, UHSI) adversely affecting rapidly growing urban populations. However, the physical associations of UHSI with urban heat island (UHI), urban-induced change in moisture (UQI) and background temperature are not well understood. Multi-year convection-permitting simulations over the US show that UHI effect peaks during nighttime (2–5°C) but maximum UQI occurs in daytime (0.01–2 g kg−1), resulting in competing effects on UHSI. UHI dynamics dominate the diurnal variations in UHSI with intensified urban-induced human discomfort during nighttime (3–5 hr day−1). UHSI is very sensitive to the background temperature, especially over the southeastern US, with distinct nightime UHSI amplification of ∼0.5 hr day−1 degree−1 rise in the background temperature. Spatial variability of UHSI is also dominated by the UHI with possible constrains from background moisture availability.