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Atlanta' Changing Weather Patterns - NASA |
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Written by Webmaster
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Saturday, 06 December 2008 |
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By PiBa
 Welcome to the Thunder Dome
Atlanta's Urban Heat Alters Weather Patterns
As the heat builds during a blazing summer day in Atlanta, Georgia, you can almost hear the clouds overhead cry, "Let's get ready to rumble!"
Urban and city growth has changed Atlanta's environment, creating a uniquely altered world of weather. Because urban areas both generate and trap heat, a bubble or "urban heat island" is created around the city. The temperature in Atlanta is 5 to 8 degrees Fahrenheit higher than surrounding areas, and this higher heat produces increased rainfall and thunderstorms.
This finding was shown at the annual meeting of the Association of American Geographers in Honolulu, Hawaii on March 25 by meteorologists Robert Bornstein and Qing Lu Lin from San Jose State University in California.
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, increasing
rainfall.
"Convective clouds typically produce rains that are intense and localized," says Bornstein. "These types of clouds should also create more thunder and lightning."
Bornstein and Lin used large data sets collected by the National Weather Service and the Georgia Automated Environmental Network. They also gathered data from the Geostationary-orbiting Operational Environmental Satellite (GOES), which monitored Atlanta's weather conditions during the 1996 Summer Olympics held in Atlanta.
Southern cities in the U.S.A. typically experience summer afternoon thunderstorms. Atlanta's storms, however, are more out of the ordinary. The Project ATLANTA team found that storms around Atlanta were created during heat island periods. More 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 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 produced through the process of urbanization. As a city grows, trees are cut down to make room for development and roads. 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 has dramatically grown over the last nine years, and nearly 380,000 acres of forest were cleared to make way for this growth.
The materials used to build over these forests aggravate the urban overheating problem. Asphalt roads, tar roofs, and other dark, heat-absorbing materials hold in heat long after the sun sets. Thus the cities stay hotter for longer periods of time. Atlanta experiences early morning rain showers because urban heat islands retain their temperature long after sunset.
This rise in temperature also increases 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, 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 stabilizing this by helping 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 on board a Lear 23 jet. These images help scientists clearly see urban growth trends. For instance, ATLAS images show that interstate and other major highways generate urban sprawl. The images also show temperature variations, color coding hot and cold areas for easy analysis.
ATLAS' thermal data show that temperatures in parking lots may reach 120F during the day, while the tree islands in the same parking lot reach only 89F. The parking area retains that heat much longer than the tree islands or wooded areas.
The study has helped bring attention to the problems of urban sprawl in Atlanta. Scientists hope to get the Governor of Georgia to use the data to initiate legislative action to push for the use of tree planting and installation of highly reflective rooftops to help cool Atlanta.
"Planting trees to shade the city and installing highly reflective roofing materials are seen as ways that are politically palatable as opposed to passing more restrictive air quality legislation," says Quattrochi.
At this time the Georgia government has already reduced insulation requirements for buildings constructed with highly reflective roofs. The scientist's study has also been endorsed by high-reflecting roofing material manufacturers, who use the data to illustrate how such materials can keep buildings cool while lowering energy costs.
Although Quattrochi and Luvall have collected data for EPA studies in many other cities, Project ATLANTA is unique. By looking at variations in urban growth over the past 25 years, the scientists have been able to monitor changes in environment and in meteorology. Quattrochi says he wants 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 compare Houston for similar urban heat island effects.
NASA study info and other Atlanta weather related info are available from
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