New York painted some black roofs white, cutting surface temperatures by 42°F

White paint made a striking difference on New York rooftops during the city’s 2011 heat wave. NASA scientists found that roofs painted white reached temperatures 42°F, or 6°C, lower than the far more common black roofs.

The July heat wave pushed the thermometer to 91.4°F, or 33°C, while humidity made it feel like 111°F, or 44°C. Twenty-two people died, power outages hit the city and electricity consumption reached a record level.

The comparison took place on the roof of the Museum of Modern Art in Queens between June and August 2011. On a typical summer day, flat black asphalt roofs can reach 190°F, or 87°C, around 90 degrees above the air temperature.

“

The urban landscape of asphalt, metal and dark buildings absorbs more energy from sunlight than forests, fields or landscapes covered in snow and ice, which reflect more light.

Stuart Gaffin of Columbia University, the lead author of a NASA Goddard Institute paper on the subject, says widespread use of white roofs could lower urban temperatures, reduce energy consumption and cut the resulting greenhouse-gas emissions. Dark asphalt, metal and buildings absorb more solar energy than forests, fields, snow and ice, contributing to urban heat islands. In New York, the effect becomes more pronounced at night.

New York launched its official NYC CoolRoofs program in 2009 to whiten and cool roofs. The city says white roofs can reduce indoor building temperatures by up to 30%. The current program provides paid training and work experience installing reflective roofs, and aims to install one million square feet of white roofing each year. Urban planners are also considering green roofs, although white roofs are the less expensive option described in the article.

“

White roofs can reduce indoor building temperatures by up to 30%.