Heat on the Walked Path: Thermal Exposure, Urban Form and Vulnerability Within the Fifteen-Minute-City Paradigm in Metropolitan Barcelona

The fifteen-minute city is commonly based on strategies that aim to keep daily destinations within walking distance. However, thermal conditions are rarely considered, and these strategies fail to deliver in dense, vulnerable neighbourhoods during heatwaves. This study examines whether pedestrian proximity is thermally habitable at a walking scale. We measured air, shaded-surface, and sunlit-surface temperatures, as well as relative humidity, at different times of the day along two everyday itineraries in Santa Coloma de Gramenet. The study area stands as one of metropolitan Barcelona’s most vulnerable municipalities, and the field campaign took place during the July 2026 heatwave. Each reading was expressed as a differential from a simultaneous reference at the origin, and points were classified through a Thermal Load Index. We then compared the registered data with plan-view and satellite images, yielding four main findings. First, thermal contrast lives in surfaces, not air: sunlit-surface temperature varies two to three times more than air temperature (as expected), so air-based metrics under-represent lived exposure. Second, extremity concentrates in a few discrete micro-sites rather than a smooth gradient. Third, and most consequentially, plan-view greenness (NDVI) is largely decoupled from pedestrian thermal exposure; cool shelters instead arise from urban morphology—enclosure, orientation, and ventilation—often without vegetation. Fourth, exposure reverses between morning and evening. We read the study area as a situated urban microclimate, reframing green-centred heat adaptation in terms of walkable urban form.