In black and white noir films from the ?40s, every day seems gray and rainy. It turns out cities make their own weather and yes, it does rain more often in major cities.
NASA researchers used their Tropical Rainfall Measuring Mission (TRMM) to study "urban heat-islands" and see if they create more summer rain than normal over major cities. The cities they studied included Atlanta, Dallas, San Antonio and Nashville.
Dr. J. Marshall Shepherd of NASA's Goddard Space Flight Center found that urban areas with high concentrations of buildings, roads and other artificial surfaces retain heat and create urban heat-islands that lead to rising warm air that produces clouds and changes the weather in the immediate area. "Cities tend to be one to 10 degrees Fahrenheit warmer than surrounding suburbs and rural areas and the added heat can destabilize and change the way air circulates around cities," says Shepherd.
"A recent United Nations study estimates that 80 percent of the world's population will live in cities by 2025, so a better understanding of the impact of urban land use change on Earth's water cycle system is vital," he says.
Shepherd found that average monthly rainfall rates within 18 to 36 miles downwind of the cities were, on average, about 28 percent greater than in the upwind region. In some cities, the downwind areas had as much as 51% more rain. That?s hard news to hear these days, when farms and ranches are suffering from drought. What little rain there is may be falling over urban areas.
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