City Heat Spurs Summer Rain

Cities beget rain. Satellite measurements showing higher rainfall rates (in blue) downwind of major cities along Texas's Interstate 35.

Many city dwellers have experienced it time and again: the summer rainstorm that seems to come from nowhere. Now, using the world's first space-based “rain radar” instruments, researchers have found a strong link between heat generated by urban asphalt and summer rainfall around cities. Given projections that as much as 80% of the world's population will live in cities by 2025, it's vital to understand how cities alter rainfall, the researchers say.

Buildings, roads, and other manmade surfaces absorb sunlight, so cities tend to be 0.6 to 5.6 degrees Celsius warmer than surrounding suburbs and rural areas. That difference is especially pronounced in the summer. Researchers have used ground-based instruments to reveal more rain around a few cities such as St. Louis, Missouri, and Atlanta, Georgia.

To gain a wider perspective, a team led by meteorologist Marshall Shepherd at NASA's Goddard Space Flight Center in Greenbelt, Maryland, analyzed data collected by rainfall-measuring sensors aboard a satellite. The measurements showed that 30 to 60 kilometers downwind of five major cities, mean monthly rainfall was up to 51% greater than in upwind zones, and maximum downwind rainfall topped out at 116%. (The rainfall happens downwind because the city heat causes air flow changes that create clouds, which are carried by the wind.) The extra heat destabilizes the circulating air, says Shepherd, who thinks that the cityscape kicks in too: “We suspect the urban topography of variable building heights and surfaces forces air to converge and rise, producing clouds and rainfall.” The study appears in the July issue of the Journal of Applied Meteorology.

Past studies had to rely on scattered surface observations of rainfall, says meteorologist Robert Bornstein of San Jose State University in California, so the satellite analyses are a significant improvement. “Such studies are extremely valuable to urban planners in assessing the effects of proposed urbanization and in understanding the impacts of cities on precipitating storms,” he says.

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