Most of us who live in cities are aware (very aware) of the urban heat island effect. We know it’s hotter in cities than in the surrounding countryside, especially at night. The buildings in cities block heat from being radiated effectively back to space. Plus the materials commonly used in cities for pavements and roofs – such as concrete and asphalt – absorb heat far differently from grass and trees, which among other things help cool with their moisture and shade. Studies have shown the benefit of painting roofs white in cities to help lessen the urban heat island effect. On April 20, 2012, the NASA Earth Observatory website ran a picture and a graph that demonstrate how white or “green” rooftops can help keep cities cooler.
Stuart Gaffin of Columbia University studies the urban heat island effect and was part of a research team that has been measuring what they call land-skin temperatures in greater New York City, including at the Con Edison (power company) buildings shown above, using portable infrared radiometers. In a recent study, Gaffin and colleagues compared the surface temperature of black, white and “green” (vegetated) roofs and found that black roofs can be up to 30°C (54°F) hotter than a green or white roof. These scientists say:
… installing a plant-covered roof is the ultimate technique to combat urban heat because it adds a combination of slight shading and a lot of cooling moisture. But … even a simple step like painting black roofs white—increasing the albedo, or reflection of light—can reduce temperatures dramatically.
These scientists say they have observed temperatures as high as 77 to 82°C (170 to 180°F) on the black rooftops of the city in mid-summer. In winter, they say, rooftop temperatures can be tens of degrees warmer than local air temperatures. Gaffin added:
Cities have been progressively darkening the landscape for hundreds of years. City roofs are traditionally black because asphalt and tar are waterproof, tough, ductile and were easiest to apply to complex rooftop geometries. But from a climate and urban heat island standpoint, it makes a lot of sense to install bright, white roofs. That’s why we say, ‘Bright is the new black.’
The natural-color image of Queens, above, is from DigitalGlobe’s Worldview-2 satellite.
Bottom line: Scientists from Columbia University in New York City and from NASA demonstrate that “green” (vegetated) or white roofs can help keep cities cooler.