White materials are highly reflective of the sun's radiant heat and may have a high thermal emittance value (a measure of how much absorbed radiant energy is given off into the atmosphere). As much as 40% of a city's waterproof surfaces are composed of roof area that is often black. That means more cooling energy is needed for occupied spaces in urban areas because on a 90° degrees day a white roof can be up to 50° degrees cooler than a black roof. That results in a lower rate of air pollution from the reduced production of ozone.
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