The underestimated impact of parked cars in urban warming.

The urban heat island (UHI), which describes the warmer temperature over urban landscape, is the most studied climate effect of cities. Most studies focus on the surface and canopy layers, particularly in common urban configurations such as street canyons. The causes of the UHI include aspects of physical form, fabric and of functions and, while urban forms are treated as fixed (over short time periods), functions are considered dynamic. In this context, the thermal and radiative properties of street facets like roads are critical urban canopy parameters (UCPs) that are used to understand heat storage and surface-air exchanges. However, the role of vehicles, especially parked ones, in modifying these surface properties and associated UCPs has been largely overlooked. This short contribution examines the impact of parked and mobile vehicles in cities using data from Lisbon, Portugal. Our findings highlight that parked vehicles significantly alter surface thermal properties in densely built areas, where road coverage is extensive and UHI intensity is greatest. These insights underscore the need to consider parked vehicles in urban heat island studies and the potential for spatially targeted mitigation strategies, such as restricting parking in identified hotspots, constructing shading structures, and promoting light, over dark, coloured vehicles.