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.