Mar. 25, 1999 ATHENS, Ga. -- Atlanta, Georgia, 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 today in Honolulu, Hawaii, at the annual meeting of the Association of American Geographers.
"We used geographic information system technology to see how land use has changed over the past two decades," said Dr. C. P. Lo, a geographer from the University of Georgia. "It's a very useful technique to see how land cover has changed." Lo and graduate student Xiaojun Yang presented their data at the meeting today in Honolulu.
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.
"NASA had already done a study like this with Huntsville, Alabama, and when it was finished, I suggested that we do Atlanta," said 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."
To understand the distribution of increasing populations over the Atlanta metropolitan area, Lo and 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 report today 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 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.
"The presence of forest has a large modification effect on local climate," said 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 gain funding to extend the project to include the modeling of numerous events and land-use practices affecting the area.
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