Bigger Cities Causing Stronger Summer Storms, Experts Say

Richard A. Lovett
for National Geographic News
July 26, 2006
Scientists have long known that sun-baked cities form "heat islands"
that make hot days even hotter.

But new studies show that as cities grow, these heat islands increasingly affect the summer weather—not just by making cities hotter but by adding ever more power to summer thunderstorms.

In Houston, Texas, for example, another two decades of urbanization might be enough to double a small thunderstorm's intensity, increasing the risk of flooding.

J. Marshall Shepherd, a meteorologist at the University of Georgia in Athens, found that in 20 years Houston could see regularly see summer storms so large that they cover the entire city—and produce twice the amount of rain.

Shepherd reported his findings at a meeting of the American Geophysical Union (AGU) this spring in Baltimore, Maryland.

Growing cities are producing fiercer storms, Shepherd explains, because each new road and building provides more dark surfaces to soak up midday heat.

In the case of Houston, that heat causes stronger hot-air currents above the city, drawing moist air from the nearby Gulf of Mexico in a process that Shepherd describes as an "urban pump."

(See map of Texas.)

"Additionally, rough urban surfaces like tall buildings increase the piling of air, called convergence, into the pump," he said by email.

Houston Weather

Shepherd began his study by collecting data from a typical Houston thunderstorm.

He then used a computer model to project what would happen if the same storm hit Houston as urban-growth experts predict it will look in 2025.

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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