Optimizing retro-reflective surfaces to untrap radiation and cool cities

Extreme heat and its various impacts are a growing threat to cities and their residents, and it is increasingly clear that portfolios of solutions are needed to mitigate the resulting risks. Here we comprehensively evaluate and optimize the application of existing retro-reflective (RR) materials, which reflect incoming solar radiation back to the sky, on urban surfaces to cool them. Using detailed energy budget models, we show that RR walls and pavements decrease urban canyon surface temperatures by up to 20 °C and canyon air temperatures by up to 2.6 °C, outperforming highly reflective surfaces, with a notable improvement in pedestrian thermal comfort (up to 0.55 °C and 153 W m−2 reductions in human skin temperature and net radiative gain, respectively). We then develop optimized RR design guidelines for diverse climatic conditions, latitudes, seasons and urban geometries. On the basis of our analysis, we recommend RR pavements for open, low-rise areas and propose specific RR wall design strategies for compact, high-rise areas. Adapting to warming cities is increasingly crucial. This study determines the most effective retro-reflective surfaces, which reflect incoming solar radiation. The authors consider various latitudes, seasons, urban geometries, street orientations and wall directions to assess the broad applicability of this cooling strategy.