Application of operative temperature to characterize daytime thermal comfort in urban areas during a heatwave: the case of two French cities

Given the impact of extreme temperatures on health, particularly in urban areas, the assessment of thermal comfort has become a central topic in the scientific community since the 20th century. Among the various indices developed for this purpose, this study aims to spatially analyze the thermal comfort of the urban areas of Grenoble and Toulouse (France) using the Operative temperature (Ot). The analysis covers the period from August 11 to August 26, 2023, corresponding to the longest and most intense heatwave recorded by Météo France during the summer of 2023 in the Auvergne-Rhône-Alpes and Occitanie regions.

The choice of Ot is based on its ability to define a variable for assessing daytime thermal comfort, which can subsequently be integrated into social vulnerability models initially developed for the nighttime phase. Although thermal comfort and social vulnerability share similarities, particularly in the construction of a composite index, some parameters, such as social predisposition factors, may overlap. Ot, by incorporating thermal exchanges between an individual and their environment through measurable climatic variables (air temperature, mean radiant temperature, wind speed), serves as a relevant indicator for an integrated approach to thermal comfort and social vulnerability.

The calculation of the variables necessary for composing the index was carried out using three models in urban climatology: SOLWEIG, URock, and TARGET. These models were forced by open-access geographical data (from IGN) and atmospheric data (ERA-5). The result is a composite index expressed as an equivalent temperature, with a spatial resolution of one meter. This methodology makes it possible to identify the most severely affected zones in the urban areas studied during the daytime heatwave. Finally, this analysis also aims to assess the relevance of the Ot index for thermal comfort studies in comparison to other commonly used indices (such as UTCI, SET*, PET, PMV, etc.).