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Urban Heat Alters Weather Patterns

As the heat builds during a blistering summer day in Atlanta, Georgia, you can almost hear the clouds overhead cry, "Let's get ready to rumble!"

Urban growth has transformed Atlanta's environment, creating a uniquely altered arena of weather. Because urban areas both generate and trap heat, a bubble or "urban heat island" forms around the city. The temperature in Atlanta is 5 to 8 degrees Fahrenheit higher than outlying areas, and this excess heat produces increased rainfall and thunderstorms.

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.

Mounds of lush color can be created in even hot, sunny conditions, by planting the new compact landscape roses. Flower Carpet-Pink by Tesselaar

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.

Traffic versus Trees

Heat builds in a city when forests are cut down to build roads. Not only do paved roads hold in heat, but cars compound the problem by generating smog and more heat.

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.

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