for National Geographic News
Air pollution from the burning of fossil fuels in China is depriving nearby hills and mountains of rain and snow.
That's the finding of a new study led by Daniel Rosenfeld, a professor of atmospheric sciences at Jerusalem's Institute of Earth Sciences, in this week's issue of the journal Science.
To research the effects of pollution on high-altitude areas, Rosenfeld and his colleagues combined records of visibility, precipitation, and tiny pollution particles in the air—known as aerosols—on Mount Hua, near Xi'an in central China (see China map).
The results showed that the aerosols are causing clouds to withhold their moisture in hilly regions.
The findings explain the 10 to 25 percent drop in rainfall that has occurred at higher altitudes downwind of cities compared to lowland areas, the team said.
Aerosols are competing to attract the limited moisture in clouds, which reduces the size of water droplets, Rosenfeld explained. Smaller droplets in turn take more time to combine to form raindrops.
"It creates short-lived clouds," he said. "You don't have enough time for rain to fall before they get to the downwind side of the hills."
Scientists have long suspected a connection between pollution and decreased rainfall in many parts of the world.
But there no solid proof until Rosenfeld hit upon a scientific gold mine in China: records of visibility going back 50 years.
Using that data, his team has made a direct connection between aerosols and rainfall on a local scale that's been missing from observations in other parts of the world.
"It's an important story," said William Woodley, who has been documenting the same effect in the Sierra Nevada mountain range downwind of San Francisco, California, for the California Energy Commission.
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