Urban Heat Is Not A Myth - But It's Not Due To Lack Of Trees
    By News Staff | July 9th 2014 03:25 PM | Print | E-mail | Track Comments

    New York City residents think everything is about New York City. A NYC storm automatically becomes a Super Storm, the population between the Hudson River and the San Francisco Bay bridge are assumed to be mutant church Republican zombies, they even think it's hotter in the city than everywhere else.

    On that last part, they may be right. There has long been a belief in the "urban heat island" (UHI) effect, which makes the world's cities warmer than the surrounding countryside. In an analysis of 65 cities across North America, researchers found that variation in how efficiently urban areas release heat back into the lower atmosphere — convection — is the dominant factor in the daytime heat island effect. 

    Which means that people who believed in urban heat islands all along had the right answer for the wrong reasons - it isn't due to diminished evaporative cooling related to loss of vegetation.

    The effects of this impaired "convective efficiency" are even worse in wet climates, the researchers say. Cities such as Atlanta, Georgia, and Nashville, Tennessee, this factor alone contributes a 3-degree C rise in average daytime temperatures, according to the study.

    In addition to the changes in convection efficiency and evaporative cooling, effects include the tendency of buildings, pavement, and other structures to store more heat than vegetation and soil; heat generated by human-built industrial systems; and changes to the albedo of the Earth's surface - the proportion of sunlight or radiation reflected by the surface of the planet. Light-colored parking lots, for instance, reflect more sunlight back into space than darker surfaces.

    Using satellite data of land surface temperatures and vegetation cover from cities in the United States and Canada, researchers calculated the mean temperature differentials between urban centers and their rural surroundings during both daytime and nighttime hours. They also used climate modeling to produce a more complex range of variables — from air density to aerodynamic resistance — which were then used to quantify the roles of each of the primary drivers of UHI (radiation, convection, evaporation, heat storage, and human-generated heat).

    Their results reaffirmed the consensus view that, regardless of the local climate, the release of heat stored in human-built structures is the dominant contributor to UHI during the nighttime.

    But during the daytime, researchers found, convection is the dominant factor — particularly in "wetter" cities of the southeastern United States. In those places, the smooth surfaces of buildings and other human-made features are far less conducive to heat diffusion than the densely vegetated areas that surround them. 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," said Lei Zhao, a doctoral student at Yale and lead author of the study. "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 U.S. Southwest 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.