Bright Rooftops Offer Big Relief for Urban Heat Islands
When we think about the hottest places on Earth, we tend to look to deserts. And, in fact, research shows that the highest land-surface temperatures recorded on the planet are almost always in places that are dry, rocky, bare and dark-colored. It turns out that such conditions appear in cities as well as deserts, which is why researchers are thinking about how to change the face of urban areas—or at least the rooftops.
The accompanying natural-color image from DigitalGlobe’s Worldview-2 satellite shows a section of Queens, N.Y., on Sept. 15, 2010. Viewed from space, the city is a patchwork of dark (often black) surfaces, along with white and other colored rooftops. The plot below the image, based on data from researchers at Columbia University and the NASA Goddard Institute for Space Studies, shows the surface or “skin” temperature of different-colored rooftops in early August 2010.
“Cities have been progressively darkening the landscape for hundreds of years,” says Stuart Gaffin, who studies the urban heat island effect, the propensity of cities to trap heat and grow considerably warmer than the surrounding natural environment. “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.’”
Gaffin and colleagues compared the surface temperature of black, white, and green roofs and found that black roofs can be up to 54 degrees hotter than a green or white roof. Installing a green, 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 the researchers found that even a simple step like painting black roofs white—increasing the albedo, or reflection of light—can reduce temperatures by as much as 43 degrees compared with typical black rooftops.