Pollution Found to Inhibit Rainfall in China

Pollution in central China has caused a steady decrease of rain in the mountains, according to new research. As an example, scientists say small particles from air pollution in the city of Xi'an are causing clouds to hold on to their moisture rather than dropping it on nearby Mt. Hua. On the haziest days, rainfall can be reduced by half.

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Particles from coal fires and vehicle tailpipes do more than make the air unpleasant to breathe. They're also changing rainfall patterns in some places. NPR's Jon Hamilton reports that the latest evidence for this comes from a sacred mountain in central China.

JOHN HAMILTON: On a clear day in the city of Xi'an, you could probably see Mt. Hua. The mountain is only about 70 miles away and it's so tall people once believed it was a pillar holding up the sky. But these days in Xi'an, it's often hard to see across town. Daniel Rosenfeld says that's because there's a thick haze of pollution.

Mr. DANIEL ROSENFELD (Hebrew University of Jerusalem): It is so bad that you can't actually discern whether it is cloudy or not. And when the sun is shining, you can see it as if it was the moon.

HAMILTON: Rosenfeld is an expert on clouds at the Hebrew University of Jerusalem. He spent some time in Xian recently, studying whether pollution from the city is affecting weather in the mountains. To find out, Rosenfeld and a team of Chinese researchers analyzed daily records of rainfall and visibility. These records have been kept at an observatory on Mt. Hua since 1954. Rosenfeld said the numbers showed a clear pattern.

Prof. ROSENFELD: The last 50 years, the average amount of rainfall and snow on the mountain has been decreasing by 20 percent.

HAMILTON: And on the days with the worst visibility, rainfall was only about half what it was when the air was relatively clear. Rosenfeld says these findings published in the journal Science suggest that tiny particles in the air affect the clouds that form over mountains. He says here's what normally happens when moist air reaches the foothills.

Prof. ROSENFELD: It is forced to go up cool. And then when it cools, the moisture condenses into raindrops and snowflakes, and that's how mountains get typically much more rain and snow than the nearby lowland.

HAMILTON: Scientists call this orographic precipitation. But Rosenfeld says when the air is polluted, tiny particles prevent raindrops or snowflakes from forming. The moist air simply passes over the mountain and becomes rain somewhere else. That's a problem, and not just in China. Bill Cotton is a professor of atmospheric science at Colorado State University. He says some big cities in the U.S. depend on orographic precipitation for their water.

Professor BILL COTTON (Atmospheric Science, Colorado State University): The western U.S. in particular, in terms it run off to, you know, rivers like the Colorado River basin and along the California and to its river systems is dominated by winter orographic precipitation.

HAMILTON: And study show that rainfall patterns have changed in the Rockies and the Sierras. But Cotton says it's very hard to prove that air pollution is the direct cause of a decrease in rain in any one place.

Prof. COTTON: It could be related to land use changes in that area - that is, as metropolitan areas grow and you have a lot more concrete surfaces, that produces things like urban heat islands.

HAMILTON: Hot spots that raise air temperatures and can change the wind patterns that bring rain to nearby mountains. Cotton agrees that particles can affect clouds and rain. But he says this can happen in a number of ways. For example, away from the mountains, particles may cause more rain, not less. Another study published this week provides an illustration. Scientists writing in the proceedings of the National Academy of Sciences say there's evidence that airborne particles from Asia are causing more intense storms in the Pacific Ocean.

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