In 1998, Rosenfeld detected similar "pollution tracks" over land areas of Earth for the first time. "We had known that aerosol particulates emitted by ships can trigger changes in clouds and even their formation over the ocean," said Toon. "What Rosenfeld has done in his latest paper is to use satellite data to demonstrate a clear, widespread influence of aerosol pollution on continental precipitation. "

Rosenfeld used data taken by Advanced Very High Resolution Radiometers onboard several U.S. weather satellites and NASA's new TRMM satellite for his Science study. The images showed pollution tracks caused by urban and industrial activity in areas of Turkey, Australia and Canada, as well as data indicating clouds making up the pollution tracks were prohibiting rain and snow from falling downwind from the sites.

Since each cloud droplet must form on a pre-existing particle, additional aerosols in clouds like sulfates or sulfuric acid increase the number of water droplets in clouds, said Toon. Because temperatures and atmospheric motions driving cloud formation control the mass of water condensing in the clouds, the droplets formed on aerosol particulates tend to be smaller in size.

Clouds harboring smaller droplets have larger surface areas, making them more reflective and sending more sunlight back to space, said Toon. Because of their diminutive size, the chances of the droplets coagulating into raindrops large enough to fall as precipitation are greatly diminished.

A typical microscopic cloud droplet can travel little more than an inch through dry air before evaporating. "About 1 million cloud droplets must collide and coalesce in order to form a precipitation-sized drop," said Toon, who noted that a typical raindrop -
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Contact: Owen B. Toon
brian.toon@lasp.colorado.edu
303-492-1534
University of Colorado at Boulder
8-Mar-2000