It has long been observed that metropolitan areas are significantly warmer than surrounding rural areas as a result of human activities. These areas are known as urban heat islands.
Urban heat islands make life more difficult in cities by decreasing air quality, decreasing water quality and stressing ecosystems. They impact human health by worsening the effects of heat waves.
There are a number of causes for urban heat islands. A significant one is the difference in albedo – the ability to reflect sunlight or radiation – between human-built surfaces and those that occur in nature. The surfaces of our cities store more heat than is retained by vegetation and soil.
A recent study at Yale University has determined that the dominant phenomenon leading to urban heat islands is the impaired convective efficiency of urban environments. Convection is the primary process by which heat diffuses out of a region on the surface of the earth. The smooth surfaces of buildings and other human-made features in cities are far less conducive to heat diffusion than the densely vegetated rural areas that surround them. In cities with high humidity, this effect can add five Fahrenheit degrees to the average daytime temperature.
There isn’t much we can do about the convection efficiency of our cities. One thing we can do is to try to aggressively manage surface albedo by doing things like changing the color of parking lots, roads and roofs. Making these surfaces light colored could reduce the amount of heat absorbed during the day and the amount that needs to be released at night.
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Urban heat — not a myth, and worst where it’s wet
Photo, posted August 10, 2014, courtesy of Hagen Strobach via Flickr.
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Earth Wise is a production of WAMC Northeast Public Radio.