Cities Make Rain

Evidence gathered by satellites in Earth orbit shows that cities can alter the rainfall patterns around themselves. Rainfall and urban heat islands. Surfaces like asphalt, concrete and roofing shingles heat up more than treetops, grassy fields or soil. That’s why cities are some 5 to 10 degrees Fahrenheit warmer than the surrounding countryside.

And if you’re downwind of a very large city like Dallas or Los Angeles, you might be in a place that’s rainer than normal, too. Marshall Shepherd is a meteorologist at the Goddard Space Flight Center in Greenbelt, Maryland. He and his colleagues analyzed data from the Tropical Rainfall Measuring Mission — a satellite equipped with radar that measures rain. They looked at rainfall rates — how hard it’s raining over a given time — over several large U.S. cities.

Marshall Shepherd: What we found is that on average in the downwind region — anywhere from 30 to 60 kilometers downwind of the major cities in the study — the rainfall rates were on average 28 percent larger than the rainfall rates in a defined upwind region . . .

Shepherd says the reason is that hot air rising from urban surfaces helps form rain clouds. As these clouds are carried downwind, more rain falls. More difficult to determine is whether cities have a broader effect on worldwide weather and climate — like the broad effect caused by an El Nino or deforestation of the Amazon.

What might be done to keep cities cooler? According to Marshall Shepherd, “There are proposals to design cities with more reflective surfaces, or surfaces that don’t absorb as much of the sun’s energy, more whites, or reflective type or mirrored surfaces. There are efforts to plant more green trees and plants in cities. These are certainly things that can be done and have shown some promise at mitigating the urban ‘heat island.'”

In addition to studying U.S. cities, these researchers are also using satellite data to analyze the effect cities in other countries may have on rainfall patterns around them. In general, they are paying special attention to mega-cities like Los Angeles, Dallas and Rio De Janeiro.

Shepherd noted that increased rainfall rates downwind of cities might have limited local benefits. For instance cropland might be helped by increased rainfall. But there are also local dangers. Larger, hotter cities clearly could lead to a rise in urban heat-related deaths.

And there also might be dramatic impacts on the global water cycle that are not understood. In general, scientists and urban planners agree it may be best to try to curtail urban heating, rather than let it continue unchecked and wait to see what happens later.

What you have in your mind?