But during the daytime, convection is the dominant factor, researchers found — particularly in “wetter” cities of the southeastern U.S. In those places, the smooth surfaces of buildings and other human-made features are far less conducive to heat diffusion than the densely vegetated natural areas that surround them, researchers say. Overall, in wetter climates urbanization reduces convection efficiency by 58 percent.



“The ‘rougher’ surfaces of the vegetation triggers turbulence, and turbulence removes heat from the surface to the atmosphere,” Zhao said. “But where there is a smoother surface, there is less convection and the heat will be trapped in the surface.”



Convection plays a key role in drier cities, too — albeit with far different consequences. In those settings — including in urban areas of the southwestern U.S. where surrounding vegetation is typically shorter and scrubbier — the rural areas are less effective at dissipating heat. As a result, the urban landscapes are actually 20 percent more efficient in removing heat than their rural surroundings, triggering a 1.5-degree C cooling within the cities.



“In those urban areas we sometimes see urban heat sinks instead of the typical heat island,” Lee said. “It’s a paradoxical phenomenon.”



According to the study, managing convection efficiency or heat storage of urban land is not a viable option since it would require fundamental changes to the morphology of cities, such as altering the height of buildings. A more viable option, researchers suggest, is more aggressive management of surface albedo — such as changing the color of parking lots, roads, and roofs. Doing so would reduce the amount of heat absorbed during the daytime hours and the amount of energy that has to be released at night.



Other authors of the paper, “Strong contributions of local background climate to urban heat island,” are Ronald B. Smith of the Yale Department of Geology and Geophysics, and Keith Oleson of the U.S. National Center for Atmospheric Research.



The model computation was supported by the Yale University Faculty of Arts and Sciences High Performance Computing Center.