Microscale thermal variations in a walkable urban area during hot days: Analysis through mobile measurements and Day PET Signatures

This article analyzes the thermal variations of a small urban area recently transformed in the city center of Nantes, France, which has been designed to promote walkability and make it comfortable in summer. A measurement method based on mobile climate metrology, which offers high spatial resolution, was developed. Four measurements were conducted per day over five days between June and September 2023. The data was analyzed using the concept of the Day PET Signature (DPS), which reflects the dynamics of a point in space over the course of the day through variations in the Physiological Equivalent Temperature (PET) index. Four clusters of DPS were identified in the dataset, enabling the accurate assessment of the thermal diversity existing in the study area at different times of the day in summer. The results demonstrate that the observed differences between clusters are solely attributable to the radiative conditions at the distinct positions analyzed. The concept of a Radiative Portrait is introduced in order to characterize the thermal diversity of the study area. In contrast to the TMRT synthetic index, the Radiative Portrait provides a detailed breakdown of the fluxes, revealing the intensity of short- and long-wave radiation in top, bottom and lateral orientations. The combination of the Radiative Portrait and the PET class qualification provides a comprehensive understanding of the local climatescapes in summer.