Air Pollution Can Prevent Rainfall

The new study, by Daniel Rosenfeld, of the Hebrew University of Jerusalem, presents satellite images and measurements of "pollution tracks" downstream from major urban areas and air pollution sources such as power plants, lead smelters, and oil refineries. The tracks consist of polluted clouds that have shut off virtually all precipitation because they contain abnormally small water droplets.

The droplets' small size is caused by pollution particles that act as "seeding" sites around which cloud moisture condenses. Approximately one million small droplets must collide and coalesce in order to make a precipitation-sized drop-that is, one large enough to fall below the cloud base and reach the ground before evaporating. In polluted clouds, there are too many small droplets and not enough larger ones. These small droplets float in the air with low probability of bumping into each other and merging into raindrops. The smaller droplets are also slower to freeze into ice crystals, resulting in less sleet and snowfall.

Because urban and industrial air pollution is a significant problem in many regions of the world, Rosenfeld's findings suggest that human activity may be affecting rainfall patterns on a global scale.

These data are the first direct evidence of how urban and industrial pollution affects rainfall levels, a question scientists have debated for several decades. In fact, some previous studies have concluded that air pollution might increase rainfall, but the debate has continued due to a lack of convincing data.

"In the past, scientists had to collect information by poking little holes in clouds from airplanes, or using statistics about rainfall patterns because you can't replicate rain-clouds in the lab. Now, new satellite instruments allow us to have a comprehensive look at the problem. For the first time, we can measure cloud precipitation and microstructure simultaneously over large areas," Rosenfeld said.

In his Science paper, Rosenfeld presents the first images of pollution tracks over land. The images, taken over regions in Turkey, Canada, and Australia, all contain known sources of industrial or urban air pollution. The tracks stream away from these pollution sources in long narrow plumes.

Rosenfeld took yet a closer look at the pollution tracks in Australia, where the plumes were particularly striking. Further measurements from a bevy of satellite instruments showed that precipitation of both raindrops and ice crystals-which was occurring in the unpolluted clouds-was practically shut off in the clouds within the pollution tracks. However, the total amount of moisture in the polluted clouds was sufficient to produce rain and snow.

Rosenfeld also notes that in other parts of the world air pollution is more widespread and not as easy to distinguish as it is against the relatively clean Australian atmosphere. Thus the well-defined tracks identified in the study "serve as a Rosetta stone for the potential impact of more widely distributed aerosol pollution on clouds," writes Owen Toon, of the University of Colorado at Boulder, in a related commentary article.