This finding was presented at the annual meeting of the Association of American Geographers in Honolulu, Hawaii on March 24 by meteorologists Robert Bornstein and Qing Lu Lin from San Jose State University in California. Dale Quattrochi and Jeffrey Luvall of NASA's Global Hydrology Center lead this NASA-sponsored study .

The Atlanta Land-use Analysis: Temperature and Air-quality (ATLANTA) project began in 1996 in order to study the impact of urban heat islands on the environment.

As the heat in a city builds, hot air rises. Colder air rushes into the vacuum, creating winds. The warmer ascending air forms clouds that drop water as they continue to rise. Bornstein and Lin found that Atlanta's urban heat island causes convective clouds to form over the city. "Convective clouds typically produce rains that are intense and localized," says Bornstein. "These types of clouds should also produce thunder and lightning."

Bornstein and Lin used data collected by the National Weather Service and the Georgia Automated Environmental Network. They also used data from the Geostationary-orbiting Operational Environmental Satellite (GOES), which monitored Atlanta's weather conditions during the 1996 Summer Olympics.

Southern cities in the United States usually experience summer afternoon thunderstorms. Atlanta's storms, however, are out of the ordinary. The Project ATLANTA team found that storms around Atlanta were generated during heat island periods. Rather than only experiencing afternoon storms, Atlanta would also experience pre-dawn or early morning rain showers that would continue until noon.

"There's no doubt when you look at the patterns of precipitation development that the showers were forced, or created by the 'sucking in' of cooler air into the urban heat island over Atlanta," says Quattrochi.

Heat islands are created through the process of urbanization. As a city grows, trees are cut down to make room for commercial development, roads, and suburban growth. Forest growth normally reduces the amount of heat and smog generated by populated areas.

Plants and water-retaining soils absorb heat during the day, and then carry the heat away through evaporation. In Atlanta, commercial and suburban development dramatically increased between 1973 and 1992, and nearly 380,000 acres of forest were cleared to accommodate that growth.

The materials used to build over these forests compound the urban overheating problem. Asphalt roads, tar roofs, and other dark, heat-absorbing materials hold in heat long after the sun sets, keeping the cities hotter for longer periods of time. Atlanta experiences early morning rain showers because urban heat islands retain their temperature long after nightfall.

This rise in temperature also increases the amount of air pollution. Not only is heat and pollution produced from automobiles and commercial facilities, but Atlanta's 5 to 8-degree rise in temperature contributes to an increase in ozone, a particularly destructive type of smog.

Ozone interferes with photosynthesis, the process by which plants make food, and it damages the lungs of humans and animals, sometimes causing permanent lung damage. As heat levels rise, the city environment becomes increasingly hazardous.

Thunderstorms may be nature's way of keeping its cool. The storms also help clean the air because the fresh rainfall acts like a scrub-brush on air pollution. On the downside, thunderstorms can cause flooding in urban areas because paved ground doesn't allow water to soak into the soil.

Project ATLANTA often uses images from ATLAS, the Airborne Thermal and Land Applications Sensor, which is onboard a Lear 23 jet. These images help scientists clearly see urban growth trends. For instance, ATLAS images indicate that interstate and other major highways generate urban sprawl. The images also show temperature variations, color coding hot and cold areas for easy recognition.

"This is the same basic instrument that Galileo has," Luvall says, referring to the unmanned NASA spacecraft that orbits Jupiter.

ATLAS 'sees' in 15 colors, as well as in three visible light, six near and middle-infrared, and six thermal infrared channels. The ATLAS data collected over Atlanta has a resolution of 10 meters (33 feet), which means objects the size of an average house are easily recognizable.

ATLAS' thermal data show that temperatures in parking lots can reach 120 degrees Fahrenheit during the day, while the tree islands in the same parking lot reach only 89 degrees. And the parking lot retains that heat much longer than do the tree islands or wooded areas.

The study has helped bring attention to the problems of urban sprawl in Atlanta. Because Georgia Governor Roy Barnes has shown interest in the work, the scientists hope he will use the data to initiate legislative action to support the use of tree planting and installation of highly reflective rooftops to help cool Atlanta and other major Georgia cities.

"Planting trees to shade the city and installing highly reflective roofing materials are seen as measures that are politically palatable as opposed to passing more restrictive air quality legislation," says Quattrochi.

Already, the Georgia government has eased insulation requirements for buildings constructed with highly reflective roofs. The scientist's study has also been embraced by high-reflecting roofing material manufacturers, who use the data to illustrate how such materials can keep a building cool while keeping energy costs down.

Although Quattrochi and Luvall have collected data for EPA studies in many other cities, Project ATLANTA is unique. By looking at changes in urban growth over the past 25 years, the scientists have been able to monitor changes both in the area's environment and meteorology.

Quattrochi says he hopes to duplicate Project ATLANTA in Houston beginning next summer. Because Houston's location, vegetation and weather conditions are different from Atlanta's, the scientists are interested to see whether Houston experiences similar urban heat island effects.

"There are a lot of complicating factors to consider," says Quattrochi. "But my suspicion is if the city is large enough, it probably has some impact on local weather."

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