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. 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, researchers say. 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 a combination of data collected by Advanced Very High Resolution Radiometers onboard National Oceanographic and Atmospheric Administration satellites and the Visible and Infra Red Sensor and Precipitation Radar onboard NASA's Tropical Rainfall Measuring Mission satellite. 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. Rosenfeld notes that the most prominent pollution tracks are seen in Australia, the least polluted of continents that are inhabited.