Urban heat islands are created when solar energy is absorbed by non-reflective, impervious, and often rather dark surfaces, such as asphalt, stone, metal, and concrete, which are ubiquitous in cities. Exacerbating this solar energy absorption effect are abundant amounts of heat released from vehicles, factories and air conditioners, for example, as well as pollutants trapped in the lower troposphere that slow down the cooling of rising air.
In New York City, where this study was conducted, the “summertime nocturnal heat island averages 7.2ºF (4ºC). This means that during the summer months the daily minimum temperature in the city is on average 7.2ºF (4ºC) warmer than surrounding suburban and rural areas” [Rosenzwieg 2006: 1]. The authors tested the cooling effects of tree plantings, living rooftops and high albedo (light colored) surfaces, and found that curbside tree plantings were the most effective form of cooling per unit area, followed by living rooftops. High albedo (light/white) surfaces were the least effective at cooling per unit area, but were the most effective overall “because 64% of New York City’s surface area could be redeveloped from dark, impervious surfaces to lighter high-albedo surfaces” [Rosenzwieg 2006: 3], whereas only 17% of the city’s surface could be planted with new street trees.
 Maps (above) retrieved from NASA: https://earthobservatory.nasa.gov/features/GreenRoof/greenroof2.php.