To form rain, each cloud droplet must form on a pre-existing particle, and approximately 1 million small droplets must coalesce to form an ample raindrop. Pollutants such as sulfates and sulfuric acid increase the number of water droplets in clouds. These small droplets float in the air with a low probability of bumping into each other and merging into raindrops, Rosenfeld reports. Aerosol pollutants also reflect sunlight back to space, rendering a cooling effect on Earth that may cancel out the warming effect of greenhouse gases, said Owen Toon, an atmospheric scientist at the University of Colorado at Boulder, in an accompanying article. The satellite imagery is the first direct evidence of the effect of urban and industrial pollution on rainfall levels, a source of scientific debate for several decades.
Previous studies suggest pollution increases rainfall.
This is true of very large aerosol pollutants such as those emitted by paper mills, which induce the formation of very large water droplets that collect smaller droplets as they fall. The scientist based his study on data collected by Advanced Very High Resolution Radiometers onboard NASA's Tropical Rainfall Measuring Mission satellite. The spacecraft was launched in 1997 to study rainfall between 35 degrees north latitude and 35 degrees south latitude. Images were recorded over regions of Turkey, Canada and Australia that contain known sources of industrial or urban air pollution. The tracks stream away from these pollution sources in long, narrow plumes.