A construction crew paints a white roof in downtown Washington, D.C.
Click on image for full size
Courtesy of Maria Jose-Vinas, American Geophysical Union
White Roofs May Successfully Cool Cities
News story originally written on January 28, 2010
Cities are affected more by global warming than rural areas. Roads, dark roofs and other surfaces in cities hold onto heat from the Sun. This creates an urban heat island effect that can raise the temperature in a city as much as 5 degrees Fahrenheit (about 1-3 degrees Celsius).
Scientists say that painting roofs white can keep cities cool. White roofs would reflect some of that heat back into space and cool temperatures, much like wearing a white shirt on a sunny day can be cooler than wearing a dark shirt.
They used a computer model to look at how white rooftops affect what happens to energy from the Sun that comes to Earth. Some of that energy is absorbed by the roof, which warms things up. Some of it is reflected back out to space.
Their model shows that, if every roof were painted white, cities would be almost one degree Fahrenheit cooler on average. Some cities would have more cooling than others. New York City would be almost 2 degrees Fahrenheit cooler on summer afternoons.
But it’s hard to keep a roof looking white. Over time white paint may look darker because of dust and decay. Some parts of roofs, such as vents, can’t be painted white. So there might not be as much cooling as the model predicts.
Last modified February 19, 2010 by Lisa Gardiner.
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