In-situ observations for analyzing urban overheating in tropical island contexts: A case study from Reunion Island

Climate change poses escalating risks to urban populations, particularly in tropical regions where vulnerability is projected to rise, as highlighted in the IPCC's Sixth Assessment Report. Among these risks, the Urban Heat Island (UHI) effect—marked by elevated temperatures in urban areas relative to their rural surroundings—is amplified by  urban densification. However, research on UHIs and their impacts remains limited in tropical settings, especially on small islands where unique climatic and geographic conditions exacerbate the issue.

This study delivers the first detailed analysis of urban overheating impacts at the district scale on Reunion Island. Using a network of low-tech environmental sensors, we conducted high-resolution in situ measurements of key atmospheric variables, including air temperature, relative humidity, wind speed, and solar radiation. These microscale observations allowed for a precise assessment of the spatial variability of heat within the urban fabric and enabled the calculation of thermal comfort indices to fully capture the effects of urban overheating.

Preliminary results show that despite its modest population of approximately 850,000, Reunion Island exhibits a significant UHI effect, with temperature differentials exceeding 2°C between urban and rural areas. Furthermore, shading plays a critical role in mitigating heat stress, as evidenced by differences in the Universal Thermal Climate Index of up to 5°C between shaded and unshaded zones within the same neighborhood. These findings highlight the importance of incorporating shading, vegetation, and optimized urban morphology to enhance outdoor thermal comfort in tropical climates.

By demonstrating the value of in situ environmental observations in understanding urban overheating challenges, this study provides actionable insights for policymakers and urban planners. It underscores the importance of targeted mitigation strategies, such as increasing vegetation coverage and promoting natural ventilation, to reduce the impacts of UHIs in tropical island urban environments.