Satellites confirm cities cause more rain on some suburbs

WASHINGTON -- People moving to the suburbs -- or planning to farm near a city -- may find it's more important than they thought to know which way the wind blows.

That's because it rains more for several miles downwind from large cities, sometimes a lot more.

NASA researchers, using satellite measurements, have confirmed earlier reports based on rain gauges, that indicated these city-related wet spots existed.

Now, the satellite data can refine the size and areas of these rain zones, and researchers will be able to study the areas around more cities without having to set up a large array of ground-based gauges.

A team led by J. Marshall Shepherd at the space agency's Goddard Space Flight Center in Greenbelt, Md., studied rainfall patterns around Atlanta, Dallas, San Antonio and Nashville, Tenn. Previous ground-based studies have found similar patterns downwind of St. Louis, Chicago and other cities. Shepherd's study found that downwind of the studied cities, rainfall averaged 28 percent more than the area upwind of the city, with some areas showing increases of as much as 51 percent.

Cities' heat makes rain

Their findings are reported in the July issue of the Journal of Applied Meteorology, published by the American Meteorological Society.

It's well known that cities tend to be hotter than the surrounding countryside, thanks to the warmth absorbed by pavement and large buildings.

And it turns out that's also what makes the extra rain.

When the rising warm air over cities cools, water begins to condense, clouds form and drift downwind in the normal air flow, Shepherd said. That results in the areas of higher rainfall.