We all look forward and enjoy the sunshine during the warm summer months, but as many people know, much of a city’s infrastructure—roads, sidewalks, brick buildings, and dark, asphalt roofs—absorb and generate intense heat. These surfaces continue to hold and emit heat even at night, which creates what is known as the "Urban Heat Island Effect." In other words, a dome of hot air resides over a city, which can cause an increase in temperature of 10 degrees and increase levels of harmful ozone.

Stuart Gaffin, an associate research scientist with the Earth Institute at Columbia University, says, “The biggest temperature reductions could be achieved by addressing a city’s dark, impervious surfaces such as roads and roofs. His assessment of the city indicated that 64 percent of the city area consisted of such surfaces."

A staggering amount of impervious surface area consists of rooftops, most of them dark, heat-generating surfaces, typically tar, and sometimes overlain with gravel.

"The tar beach roof—ubiquitous in cities—is an oven in the summer, reaching 160 degrees Fahrenheit," Gaffin explains. "So cooling off this roof is like turning off an open oven."

--Beating the Heat in the Worlds Big Cities, by Michon Scott