Aerosol particles floating in the atmosphere absorb or scatter solar radiation, and prevent it from getting to the ground. This cools the Earth's surface and reduces daytime vertical convection that mixes the slower winds found near the ground with the faster winds at higher altitudes. The overall effect is a reduction in the speed of near-surface winds, which study has calculated to be up to 8 percent slower in California. In addition, the accumulation of aerosol particles in the atmosphere makes clouds last longer without releasing rain. Pollution in the atmosphere provides more nuclei for water to deposit on creating smaller droplets which in turn takes longer to coalesce and form raindrops. In some cases rain may not ever happen when the clouds stay on longer and move to drier air zones and evaporating faster.

Read more: http://earthobservatory.nasa.gov/Newsroom/MediaAlerts/2007/2007012224189.html