"The term "heat island" refers to urban air and surface temperatures that are higher than nearby rural areas. Many cities and suburbs have air temperatures that are 2 to 10°F (1 to 6°C) warmer than the surrounding natural land cover. Figure 1 shows a city's heat island profile. It demonstrates how urban temperatures are typically lower at the urban-rural border than in dense downtown areas. The graphic also show how parks, open land, and bodies of water can create cooler areas. Elevated temperatures can impact communities in a number of ways. Elevated temperatures can impact communities by increasing peak energy demand, air conditioning costs, air pollution levels, and heat-related illness and mortality.
The remotely sensed image of Sacramento, CA in Figure 2 illustrates the heat island phenomenon. In the aerial photo (left), the white areas, mostly rooftops, are about 140 degrees Fahrenheit (60 degrees Celsius) and the dark areas, primarily vegetative areas or water, are approximately 85-96 degree Fahrenheit (29-36 degrees Celsius).
The hottest spots are the buildings, seen as white rectangles of various sizes. In the thermal image (right), Sacramento's rail yard is the orange area east of the Sacramento River, which flows from top to bottom. Red and yellow areas indicate hot spots and generally correspond with urban development, while blue and green areas are cool and generally correspond to the natural environment.
Cities in cold climates may actually benefit from the wintertime warming effect of heat islands. Warmer temperatures can reduce heating energy needs and may help melt ice and snow on roads. In the summertime, however, the same city may experience the negative effects of heat islands.
http://www.eoearth.org/article/Heat_island
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