Recognising vehicles as thermally active surfaces in urban microclimates based on evidence from Lisbon

Abstract Urban climate studies have extensively examined buildings and pavements, yet vehicles as thermally active elements within street canyons, remain largely unaccounted for in surface energy assessments. This study addresses this gap by empirically analysing the radiative and thermal behaviour of parked vehicles in Lisbon during summer clear-sky conditions. Measurements of surface temperature and radiative fluxes were collected across vehicle colours using mobile transects and fixed-point sensors, applying an urban surface energy balance framework to vehicle roofs. Results show pronounced colour-driven thermal contrasts: black vehicle roofs reached median temperatures of ~ 71 °C, approximately 20 °C higher than white vehicles under peak solar exposure. These differences translated into distinct radiative responses, with darker surfaces exhibiting consistently higher net radiation gains, particularly in narrow street canyons with limited sky view. The findings demonstrate that vehicles function as radiatively active surfaces capable of modifying near-surface thermal conditions at pedestrian height, a process not currently accounted for in urban climatology studies. By recognising vehicles as temporary yet thermally relevant urban surfaces, this exploratory field investigation contributes to high-resolution understanding of urban heat exposure and microclimate variability in densely built environments. While not intended to generalise across all urban settings, the study provides an empirical benchmark for evaluating vehicle radiative behaviour in real street canyons and informs future modelling and multi-site research.