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Atlanta's Urban Heat Island

The ATLANTA project finds Atlanta's "urban heat island" increases pollution and changes the local weather.

The sprawling metropolis of Atlanta, Georgia is an island unto itself-an "urban heat island". A new study reports that with temperatures up to 10 degrees hotter than surrounding areas, the metropolitan area can create its own weather by making clouds, stirring up thunderstorms, and magnifying the area's already severe smog problem. The research is sponsored by NASA's Earth Observing System Program and was presented March 24, 1999 at the meeting of the Association of American Geographers in Honolulu, HI. All large urban areas are warmed by their own urban heat islands as a result of the removal of trees and the paving of land, according to Dale Quattrochi and Jeffrey Luvall of NASA's Marshall Space Flight Center, who lead the Atlanta Land-use Analysis: Temperature and Air-quality (ATLANTA) project. Dark, heat-absorbing materials for roofs and roads create the problem. During the day, dark materials absorb heat and hold it long after the sun sets, keeping cities hot hours longer than outlying rural areas. The added heat intensifies Atlanta's air quality problem. The city is plagued with serious ozone pollution. Smog levels are intensified by the urban heat island because with a 10-degree rise in temperature, the chemical reaction that creates ozone-the molecule responsible for smog-doubles, according to Luvall. Ozone is only produced in warm summer months. Ozone is a health hazard regulated by the Environmental Protection Agency (EPA).

The ATLANTA project began in 1996 to help solve the problems created and enhanced by urban heat islands. With funding through NASA's Earth Observing System, investigators from a variety of disciplines and institutions are looking at how land use changes since the 1970s have intensified the urban heat island effect. New results of these studies will be presented in a session at the AAG meeting.

To understand the distribution of increasing populations over the Atlanta metropolitan area, University of Georgia geographers Chor-Pang Lo and Xiaojun Yang use aerial photos and Landsat satellite data to study the area's growth since 1973. By interpreting these images, they can see where the vegetation is disappearing and being replaced by roads and suburbs. Lo and Yang will report that between 1973 and 1998, nearly 350,000 acres of forest area have been cleared for Atlanta's 13 metropolitan counties. Replacing the forests are mainly suburbs, according to Lo. Since 1973 the area of developed suburbs "low density residential area" has doubled to nearly 670,000 acres. Commercial development also doubled. The expanding population and loss of vegetated land leads to a larger urban heat island, according to Lo.

Robert Gillies, a Utah State University geographer, uses satellite data to map the heat coming off Atlanta's urban area. When land is covered by plants or soil containing water, heat absorbed during the day is quickly removed by evaporation and plant transpiration-the way that plants lose water through their leaves.

From an instrument aboard a National Oceanographic and Atmospheric Administration (NOAA) satellite that detects radiated heat from the earth, Gillies can map what parts of the city are hotter than others, based on which areas are losing heat more quickly. Gillies will report on the heat distribution around the city including the fact that in Atlanta's central business district there is an intense hot zone encompassing 17 square miles (45 square kilometers).

Robert Bornstein and Qing Lu Lin, meteorologists from San Jose State University use data from meteorological stations set up during the 1996 summer Olympics and discovered that the urban heat island in Atlanta creates thunderstorms south of the city. When the city heats up, low air pressure is created. Cold dense air rushes in from surrounding areas and causes the warm air to rise. The city creates its own wind, and hot air rushes upward, triggering convective thunderstorms, said Bornstein. Increasing thunderstorms could cause urban flooding, said Bornstein, especially because large areas of ground are paved and rainwater can't be absorbed into soil. One benefit of the added thunderstorms is that the precipitation cleans the atmosphere of pollutants and cools down the city. Colorado State University meteorologists Stanley Kidder and Jan Hafner are using Geostationary Environmental Satellite (GOES) and Landsat data to study how clouds interact with Atlanta's urban heat island. They will report on new research to understand how large urban areas effect cloud cover and how the clouds tend to decrease the amount of ozone production by blocking sunlight and cooling the ground surface.

ATLANTA researchers have several ideas to combat the urban heat island and its potentially harmful side effects. Luvall and Quattrochi offer a basic strategy for the city to help bring down the temperature. Light color materials could make roads, roofs and parking lots more reflective. And planting trees and vegetation will offer shade and cool the air more quickly when plants lose water to the atmosphere. Lowering the air temperature by just one degree could significantly lower the ozone levels, said Quattrochi.

Quattrochi and Luvall said that the ATLANTA team is now working with state and local government agencies to try and mitigate the effects of the urban heat island.

Atlanta Heat Island story provided couresty of Victoria Bruce, EOS Science News Office.