ia, Canada and Turkey from cities and factories. He used images provided by a satellite orbiting Earth. The satellite has instruments that can measure clouds and rainfall.
Raindrops form in clouds with little or no pollution when water in the air collects around dust particles. The study notes that about one-million small droplets are required to form a drop of water large enough to fall to earth.Mr. Rosenfeld observed the movement of the air pollution as it floated away from cities, power production centers and oil treatment centers. He found that heavily polluted clouds do not produce as much rain as cleaner clouds. He says this is because polluted clouds are made of only very small water droplets.
Mr. Rosenfeld says polluted clouds have too many dust particles. The large number of particles results in the formation of too many water droplets that are too small.
They are not big enough to form raindrops. The smaller droplets also are slower to freeze and do not easily form ice particles for snow.Last year, Mr. Rosenfeld found that burning trees and other plants also reduces rainfall amounts. This can be a problem in areas where forests are cleared for development.
Science magazine published a commentary on the findings. Owen Toon of the University of Colorado at Boulder said the study is the first direct evidence that pollution from cities and factories affects rain. He says more studies are needed to show if the effect found over Australia, Canada and Turkey is found in other areas.
This SCIENCE IN THE NEWS program was written George Grow. This is Bob Doughty.And this is Warren Scheer. Join us again next week for more news about science in Special English on the Voice of America.