From urban heat island to thermal resilience: Lessons from low-cost microclimate monitoring in a small tropical island

Urban overheating is a pressing challenge for tropical island cities, where high humidity and intense solar radiation amplify heat stress. This study investigates the interplay between the urban heat island (UHI) effect and outdoor thermal comfort in Réunion Island, a small tropical island in the southwest Indian Ocean. An innovative LoRa network of validated low-cost sensors was deployed in two contrasting districts: Ruisseau, a compact high-density neighborhood, and Le Port, an open mid-rise district with minimal vegetation cover. The network simultaneously monitored air temperature, humidity, wind, and solar radiation, enabling the calculation of UHI intensity and the Universal Thermal Climate Index (UTCI). Results show that UHI magnitude alone does not adequately describe heat stress in tropical cities. In Ruisseau, higher UHI intensities coincided with neutral or moderate UTCI values due to effective shading from compact morphology. In contrast, Le Port recorded lower or comparable UHI but significantly higher UTCI values, often in the strong heat stress category, driven by solar exposure and weak nocturnal ventilation. Seasonal analysis further revealed extreme and persistent heat stress during the hot–humid season, with nighttime UTCI exceeding 30 °C in Le Port, leaving no recovery window for residents. These findings highlight the need to prioritize outdoor thermal comfort, rather than UHI reduction alone, in tropical island planning. Strategies should co-optimize shading and ventilation while embedding thermal comfort targets into building codes and zoning. The study demonstrates the potential of low-cost sensor networks to inform climate-responsive and resilient urban design in vulnerable island contexts. • Low-cost LoRa sensors mapped UHI and UTCI in two tropical island neighborhoods. • UHI peaks do not always coincide with highest outdoor thermal stress in humid tropics. • Le Port shows extreme heat stress in hot–humid season with no nocturnal recovery. • Shading and ventilation emerge as key levers for thermal resilience in tropical islands. • Findings stress the need to design for hottest, most humid months in urban planning.