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China's Pollution Leaving Mountains High and Dry, Study Finds


By Anne Minard
National Geographic News
March 8, 2007


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."

Worldwide Phenomenon

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.

"[The new study is] corroborating and buttressing what we've been
doing in California."

While some governments have taken steps to limit so-called
large-particle emissions, research by Rosenfeld, Woodley, and others
is showing that even small particles like aerosols can affect
weather both on local and global scales.
























 

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