Passive daytime radiative cooling surfaces can mitigate extreme heat from the urban heat island effect which occurs in over 450 cities worldwide, where it can be as much as 10–12 °C (18–22 °F) hotter in urban areas in comparison to surrounding rural areas. On an average hot summer day, the roofs of buildings can be 27–50 °C (49–90 °F) hotter than the surrounding air, warming air temperatures further through convection. Well-insulated dark rooftops are significantly hotter than all other urban surfaces, including asphalt pavements, further expanding air conditioning demand (which further accelerates global warming and urban heat island through the release of waste heat into the ambient air) and increasing risks of heat-related disease and fatal health effects.