White Roofs Cool Cities More Than Trees

Cities are hot spots. Their paved surfaces and dark rooftops absorb energy from the sun, which creates localized areas of high temperatures. Expanded out from the building scale to the city scale, these hot roofs and blacktops collectively create a blanket of retained warmth in a city, raising temperatures an annual average of about 2 to 5 degrees Fahrenheit. It’s a phenomenon known as the urban heat island effect, and it can have huge impacts on energy use and even health in the warm months. It’s like wearing a city-sized sweater on a hot day in July.

As cities become focused on reducing energy use and costs, this inherent increase in temperature is seen as an important, if largely amorphous, target. Combating the urban heat island effect has become, in a catch-phrasey sort of way, an official imperative in cities across the county and all over the world. There are various approaches to fighting the problem, but the primary methods are to plant more trees and make roofs reflect rather than absorb heat.