Neither rain, nor sleet, nor snow falls heavily in extremely polluted areas, suggests a new study. Scientists modeling cloud systems have debated whether air pollution tends to increase or decrease precipitation. New evidence from satellites now indicates that aerosols—pollution made up of fine, airborne particles—have a dampening effect on rainfall.
Previous satellite pictures showed that the particle-laden exhaust that oceangoing ships pump out makes visible trails in low clouds. The particles change the structure of clouds by breaking large water drops into droplets that reflect more light into space, creating a shiny trail. The polluted clouds in a ship’s track produce less rain than pristine clouds, but scientists didn’t know how massive pollution would affect precipitation, says Yoram J. Kaufman of NASA’s Goddard Space Flight Center in Greenbelt, Md.