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April 26, 1999: 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.|
As the heat in a city builds, hot air rises. Colder air rushes
into the vacuum, creating winds. The warmer ascending air forms
clouds 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. |
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.
 n 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. 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. |
"This is the same basic instrument that Galileo has,"
Luvall says, referring to the unmanned NASA spacecraft that orbits
Jupiter. ATLAS' thermal data show that temperatures in parking lots
can reach 120 degrees Right: An ATLAS image of Atlanta, Georgia. 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. |
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.|
"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.
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." |
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.|
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