Farmers often complain about the encroaching urban development, but a study indicates they may also benefit from a longer growing season as a result.
Cities have long been known to be warmer than surrounding rural areas. It's an effect known as the urban heat island that occurs when buildings, roads and other surfaces absorb heat in the daytime and release it slowly at night.
Researchers from Boston University studying satellite data report that the warming extends as much as six miles from the cities, lengthening the growing season in those areas by as much as 15 days. Their findings were reported in a recent issue of Geophysical Research Letters.
The team, led by research professor Xiaoyang Zhang, looked at about 70 cities in the Eastern states.
They found the warming effect was strongest near the Washington-Philadelphia-New York corridor, with spring greening occurring as much as 8.7 days earlier than in more distant farming areas.
On average for cities in the Eastern United States, the report said the spring greenup occurred about seven days early near cities, and in the fall dormancy was delayed by about eight days.
The "footprint" of the warmed area extended to about 2.4 times the size of the actual urban areas involved, the report said.
The study covered Jan. 1 to Dec. 31, 2001, using data collected by NASA's Moderate Resolution Imaging Spectroradiometer.
That instrument, which scans the Earth's surface, can determine when vegetation turns green in spring and goes dormant in the fall. The team used nighttime light data collected by satellite to determine the size of the urban areas studied.
The research was funded by the National Aeronautics and Space Administration.