Street albedo distribution's impacts on air temperature and outdoor thermal sensation during heat and cold waves

Climate change adaptation and mitigation require a re-evaluation of urbanisation practices. Uncomfortable summer microclimates discourage outdoor activities, driving people into air-conditioned spaces, the use of which raises outdoor thermal sensation (OTS) and air temperature (T air ), through expelled waste heat. To avoid this adverse feedback loop, thermally acceptable public spaces are crucial. Streets are the backbone of urban areas and optimising their albedo distribution is a leverage for reducing summer T air and OTS. High-albedo surfaces are beneficial for lowering T air by reflecting solar radiation, but the reflection might hit people nearby. In contrast, low-albedo surfaces absorb more solar radiation instead of reflecting it. Many cities feature dark-coloured roadways for vehicle traffic and brighter sidewalks for pedestrians. This distribution appears unsuitable for extreme summer conditions: dark roadways retain heat despite low foot traffic, while light-coloured sidewalks, due to their smaller dimensions, have less impact on T air and may increase pedestrian OTS through reflected radiation. Our work demonstrates via validated ENVI-met simulations that a reversed street albedo distribution, featuring light-coloured roadways paired with darker sidewalks, would improve current urban microclimates during heatwaves. This change lower T air and OTS daytime values and decrease the percentage of frequented spaces posing health risks, with limited rebound effects during cold waves.