By Phil Williams
Atlanta is an island unto itself--an “urban heat island”--that can have temperatures up to 10 degrees Fahrenheit higher than surrounding areas, creating its own weather and causing thunderstorms.
That’s the conclusion of a new NASA-sponsored study whose results were revealed this past month, at the annual meeting of the Association of American Geographers, by C.P. Lo and graduate student Xiaojun Yang of UGA’s geography department.
“We used geographic-information-system technology to see how land use has changed over the past two decades,” says Lo. “It’s a very useful technique to see how land cover has changed.”
All large urban areas are warmer than surrounding rural areas 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 led the Atlanta Land-use Analysis: Temperature and Air-quality (ATLANTA) project. Dark materials for roofs and roads absorb heat during the day and hold it long after the sun sets, keeping cities hot hours longer than outlying rural areas.
Atlanta is plagued with serious ozone pollution, and the added heat intensifies the air-quality problem. With a 10-degree rise in temperature, the chemical reaction that creates ozone--the molecule responsible for smog--doubles, according to Luvall. Ozone, which is only produced in warm summer months, is a health hazard regulated by the U.S. Environmental Protection Agency.
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 changes in land use since the 1970s have intensified the urban-heat-island effect.
“NASA had already done a study like this with Huntsville, Ala., and when it was finished I suggested that we do Atlanta,” says Lo. “It fit well into NASA’s project examining urban environments and global change. And that’s when we realized we needed many other experts if we were to understand the effects of development on Atlanta.”
In their part of the project, Lo and Yang used aerial photos and Landsat satellite data to study the growth in the Atlanta metropolitan area since 1973. By interpreting these images, they can see where the vegetation is disappearing, replaced by roads and suburbs.
Lo and Yang report that nearly 350,000 acres of forest area were cleared in Atlanta’s 13 metropolitan counties between 1973 and 1998. Most of the forested area was replaced by suburbs, according to Lo.
Since 1973 the area of developed suburbs--“low density residential areas”--has doubled to nearly 670,000 acres. Commercial development has also doubled. The expanding population and loss of vegetation has meant the growth of the urban heat island, Lo says.
Robert Gillies, a geographer at Utah State University, used 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 satellite that detects heat radiated from the earth, Gillies mapped which parts of the city are hotter, based on which areas are losing heat more quickly. Gillies reports that Atlanta’s central business district is an intense hot zone encompassing 17 square miles (45 square kilometers).
Robert Bornstein and Qing Lu Lin, meteorologists from San Jose State University, using data from meteorological stations set up during the 1996 summer Olympics, discovered that the urban heat island in Atlanta creates thunderstorms south of the city.
When the city heats up, air pressure falls. Colder, denser air rushes in from surrounding areas, causing the warm air to rise. The city essentially creates its own wind, and the upward-rushing hot air triggers convective thunderstorms, says Bornstein.
An increase in the number of thunderstorms can cause urban flooding, Bornstein says, because large areas are paved and rainwater can’t be absorbed into soil. On the other hand, the precipitation cleans the atmosphere of pollutants and cools the city.
Colorado State University meteorologists Stanley Kidder and Jan Hafner used Geostationary Environmental Satellite and Landsat data to study how clouds interact with Atlanta’s urban heat island.
Their research examined how large urban areas affect cloud cover and how the clouds tend to decrease ozone production by blocking sunlight and cooling the ground surface.
“The presence of forest has a large modification effect on local climate,” says Lo, “but we can’t really tell exactly where it begins or how much it changes local climate generally. What we can say is that there is a huge increase in urban heat, making Atlanta an island in this regard.”
The project team now hopes to extend the project to include modeling of land-use practices affecting the area.
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