The original thunderstorm produced a single small cell of rainfall.
But in Shepherd's Houston of the future, it was much larger, spreading across the entire urban area.
In addition, it was more intense, producing twice as much rainfall.
The finding has considerable significance, from commuters facing afternoon downpours to increased water pollution caused by extra storm runoff.
More important, more rainfall would mean increased flooding unless city planners take more storms into account as cities continue to sprawl.
And this is not solely a problem for coastal cities like Houston, scientists say.
Phoenix, Indianapolis, Atlanta
In a paper published last month in the Journal of Arid Environments, Shepherd finds that the heat-island effect also affects desert cities like Phoenix, Arizona.
(See map of Arizona.)
Using satellite data and 108 years of rainfall statistics, Shepherd found that since 1950, Phoenix's urban sprawl has increased nearby summer thunderstorm precipitation by 12 to 14 percent.
He adds, however, that in Phoenix the heat-island effect isn't the only cause of these changes.
Lawn watering and air pollution probably also play a role by adding humidity to the air.
Meanwhile, similar effects have been detected in the U.S. Midwest.
Patrick Pyle of North Carolina State University in Raleigh has found that thunderstorm rainfall has increased significantly around Indianapolis, Indiana.
(See map of Indiana.)
At the AGU's Baltimore meeting, Pyle reported that in his study of an apparently typical storm, the city's heat island appears to deflect thunderstorms away from the urban core, concentrating their power in nearby suburbs and farmlands.
Dale Quattrochi of NASA's Marshall Space Flight Center in Huntsville, Alabama, has done similar studies of Atlanta, Georgia.
(See map of Georgia.)
"We have a digital movie that shows how much land-cover change has occurred over Atlanta between about 1973 and 1999," Quattrochi said in an email.
Due to those changes in urban growth, he said, "thunderstorms [are] building up downwind of Atlanta due to the urban heat-island effect pumping hot air into the lower atmosphere."
Because this effect is likely to intensify as cities grow, Quattrochi advocates using simple measures to counteract the heat-island effect.
Such measures could include planting more trees and using light-colored materials for roofing homes and commercial buildings.
To date, however, nobody has modeled the degree to which these measures might minimize thunderstorm formation.
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