- The same heat-trapping sprawl of asphalt
and concrete that cooks Atlanta in the summer months may be creating thunderstorms
that vent their fury on the southern part of the city, climate scientists
say.
-
- In a presentation this week at the Association
of American Geographers in Honolulu, Hawaii, meteorologists Bob Bornstein
and Qinglu Lin of San Jose State University presented their findings on
how large cities can create thunderstorms.
-
- "People think of thunderstorms as
random phenomena," says Bornstein, "but they seem to have a preferred
location."
-
- Asphalt and other man-made surfaces store
energy from the sun and release it as heat, warming the air above the city.
This is the urban heat island effect, which plagues most major cities,
making them hotter than the surrounding countryside.
-
- The air rises as it heats up, leaving
behind a low-pressure zone that sucks in cooler winds from the surrounding
regions. Air masses rising and converging cause thunderstorms.
-
- Bornstein and Lin studied data from weather
stations set up in Atlanta during the 1996 Summer Olympics. Of the nine
days that they studied in detail, six had thunderstorms over the city,
but not the nearby countryside.
-
- The study is part of a NASA project known
as ATLANTA. The project analyzes satellite images for land use changes
over the last 25 years, then combines it with studies of day and nighttime
temperatures and air pollution over the Georgia city.
-
- Clouds that form when hot air rises help
reduce air pollution, says Colorado State University meteorologist Stan
Kidder, another member of the ATLANTA team.
-
- Ozone is formed by chemical reactions
fueled by light and heat; clouds shade and cool the city. Kidder looked
at cloudiness over Atlanta using weather satellites and found that the
city is cloudier than its environs -- and summer ozone levels are lowest
on the cloudiest days.
-
- "If you live in Atlanta," Kidder
says, "clouds are your friends."
-
- Climatologist Nancy Selover of the University
of Arizona notes that, though the thunderstorm data set is limited, the
findings make sense."I believe urban heat islands are enhancing the
environments in which thunderstorms form," she says.
|
