U.S cities becoming 'heat islands'

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GREENBELT, Md., Dec. 13 (UPI) -- The size and development patterns of northeastern U.S. cities make them unusually warm, more than 10 degrees warmer than rural areas, researchers say.

NASA scientists presenting research Monday at an American Geophysical Union meeting, in San Francisco, say summer land surface temperatures of cities in the Northeast were an average of 13 degrees F to 16 degrees F warmer than surrounding rural areas during a three-year period, a NASA release said.

The phenomenon that pushes up temperatures of cities such as Boston, Philadelphia and Washington is called the urban heat island effect.

A study of 42 cities in the Northeast showed a city's development pattern can have a significant impact on the strength of a city's heat island, the researchers said. Densely-developed cities with compact urban cores are more apt to produce strong urban heat islands than more sprawling, less intensely-developed cities, they found.

Development produces heat islands by replacing vegetation, particularly forests, with pavement, buildings, and other infrastructure that limits plant transpiration, an evaporative process that helps cool plant leaves and results in cooler air temperatures, NASA scientist Robert Wolfe said.

Dark city infrastructure, such as black roofs and asphalt roads makes urban areas more apt to absorb and retain heat, adding to the heat generated by motor vehicles, factories and homes.

Of the 42 northeastern U.S. cities analyzed, Providence, R.I.; Washington; Philadelphia; Baltimore; Boston; and Pittsburgh had some of the strongest heat islands, the researchers said.