for National Geographic News
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 weathernot 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 cityand 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.
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.
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