Defining the Urban Heat Stress Island: A novel characterization of human discomfort for urban environments

Urban areas are becoming increasingly affected by the impacts of climate change, particularly through intensifying heat extremes that constitute a threat to public health and economic productivity. A key driver of urban heat stress is the Urban Heat Island (UHI) effect, a phenomenon in which nighttime cooling is reduced due to heat accumulation within the city structure.The UHI is usually defined as the temperature contrast between urban and rural areas; however, thermal discomfort is not solely determined by temperature: humidity, wind and radiation strongly modulate heat perception. Therefore, a purely temperature-based definition may underestimate its actual impact on the urban population. This study proposes a human-centered characterization of the UHI effect, introducing the concept of Urban Heat Stress Island (UHSI), by integrating several heat stress indices and assessing their complementarity.We analyze observations from ten meteorological stations in Paris and its surroundings for 1980-2017, within the CORDEX URB-RCC Flagship Pilot Study (Langendijk et al. 2024). The dataset includes subdaily records of air temperature, relative humidity, wind speed and radiation, from which widely used multivariable heat stress indices are computed. The UHSI effect is consequently defined as the urban-rural degree difference for each index, calculated at 3-hourly resolution and restricted to summer nights (from 21:00 to 06:00), when the phenomenon is strongest. Daily mean and maximum differences are considered to capture both average and extreme contrasts.We examine the UHI (for air temperature) and UHSI for each index independently, assessing complementarity and redundancy through correlation analysis and Kolmogorov-Smirnov distance, which quantify temporal co-variability and distributional similarity, respectively. We also evaluate the sensitivity of UHSI to key drivers, specifically temperature and humidity.Results show that the highest UHSI contrasts systematically occur for relatively cool rural nights, typically below the 20th percentile of rural temperature. Two highly redundant subgroups of heat stress indices emerge: one formed by air temperature and the Heat Index, and another dominated by humidity-sensitive indices such as the simplified WBGT, humidex and WBGT in the shade. In contrast, UTCI and Effective Temperature exhibit consistent independence from the rest, highlighting the added value of a multivariable UHSI approach over a solely temperature-based UHI definition by capturing complementary dimensions of urban thermal stress. Hence, the UHSI reframes the traditional UHI definitions offering a novel framework to quantify urban thermal risk beyond temperature, with implications for urban climate adaptation and public health.This work is part of Grant PID2023-149997OA-I00 (PROTECT) funded by MICIU/AEI/10.13039/501100011033 and by ERDF/EU. C.R.R. acknowledges support from Grant PREP2023-001919 funded by MICIU/AEI/10.13039/501100011033 and by ESF+. Langendijk, G. S., et al. (2024). Towards better understanding the urban environment and its interactions with regional climate change—The WCRP CORDEX Flagship Pilot Study URB-RCC. Urban Climate, 58, 102165. https://doi.org/10.1016/j.uclim.2024.102165