The winds that blow near the surface of the Earth have two beneficial effects: They provide a renewable source of clean energy and they evaporate water, helping rain clouds to build up. But aerosolized particles created from vehicle exhaust and other contaminants can accumulate in the atmosphere and reduce the speed of winds closer to the Earth's surface, which results in less wind power available for wind-turbine electricity and also in reduced precipitation, according to a study by Stanford and NASA researchers.