Team probes Atlanta heat island

In this false color image of Atlanta, trees and other vegetation appear red and buildings, streets and other urban land covers appear white or blue-green to almost black in color. The lighter the color the hotter the surface

March 24, 1999
Web posted at: 1:45 PM EST

A hot time in the old town tonight is quite possible in Atlanta, Ga., where the temperature is often 10 degrees Fahrenheit warmer than the surrounding area because of the urban heat island effect.

A National Aeronautics and Space Administration-sponsored study in Atlanta has found that the swapping of trees and other vegetation for heat-absorbing asphalt, concrete and rooftops has created an urban heat island that can generate its own winds and thunderstorms, warm the city well into the night and dramatically increase the production of harmful ground-level ozone.

The study, called the Atlanta Land-use Analysis: Temperature and Air-quality Project, is led by Dale Quattrochi and Jeffrey Luvall of NASA's Marshall Space Flight Center. It was started in 1996 to address and solve problems associated with urban heat islands.

The project team includes researchers from the University of Georgia, Utah State University, San Jose State University and Colorado State University, all using different techniques to study the influences of an increase in the area of heat absorbing surfaces on the city of Atlanta. Project funding is provided through NASA's Earth Observing System.

"NASA had already done a study like this with Huntsville, Ala., and when it was finished, I suggested that we do Atlanta," said C. P. Lo, a geographer from the University of Georgia. "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."

"We used geographic information system technology to see how land use has changed over the past two decades," said Lo of his portion of the project. "It's a very useful technique to see how land cover has changed."

The added heat, according to Luvall, contributes to Atlanta's air quality problem. The city already has a serious ozone problem, he said, and the 10-degree rise in temperature doubles the amount of ozone that is produced.

Ground-level ozone is formed when nitrogen oxides and organic gases react chemically in the presence of sunlight. Nitrogen oxides are produced by motor vehicles and fuel-burning engines. Reactive organic gases are produced by motor vehicles, solvents, consumer products and the petroleum industry. Ground-level ozone can damage human lung tissue, manufactured materials and crops.

Using aerial photography and satellite imagery, Lo and Yang discovered that about 350,000 acres of forest have been cleared in Atlanta's 13 metropolitan counties between 1973 and 1998. During that time, the area of developed suburbs has doubled to nearly 670,000 acres. Commercial development has also doubled they found, leading to a larger urban heat island.

The heat coming off of Atlanta's urban area is being measured by Robert Gillies, a Utah State University geographer, using an instrument aboard a NOAA satellite. He is producing maps of the city that show which areas are hotter than others based on which areas are losing heat more quickly. He's found that there is a 17 square-mile hot zone in Atlanta's central business district.

Robert Bornstein and Qing Lu Lin, meteorologists from San Jose State University, have used data from meteorological stations set up during the 1996 Summer Olympics to show how the urban heat island in Atlanta creates thunderstorms south of the city. When the city heats up, according to Bornstein, 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, he said.

These thunderstorms are both a threat and benefit. They are a threat because of the flood risk they pose in combination with the city's paved surfaces, which do not absorb water. They are a benefit because the precipitation cleanses the air of pollutants and cools the city.

Colorado State researchers are studying how the urban heat island interacts with clouds and influences cloud cover. They are using NASA's Geostationary Environmental Satellite and Landsat data in their studies. Cloud cover is important in the equation because it blocks ozone-forming sunlight and cools surfaces on the ground.

"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 next step for the project team is to gain funding to extend their studies to include the modeling of numerous events and land-use practices affecting the area. Members of the team presented their data at the annual meeting of the Association of American Geographers in Honolulu, Hawaii, this week.

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