High and Dry: Pollution may stifle mountain precipitation

Trends seen in meteorological data gathered on a Chinese mountaintop suggest that air pollution reduces the amount of precipitation that falls at high-altitude sites.

NO UMBRELLAS TODAY. Hazy conditions atop China’s Mount Hua may stifle precipitation there as much as 30 percent, a new study of long-term weather data suggests. Science

When winds force moisture-laden air masses up a mountainside, the air cools, its water vapor condenses, and precipitation often results. Therefore, many mountainous locales receive more rainfall and snow than do sites on the plains just upwind, says Daniel Rosenfeld, an atmospheric scientist at the Hebrew University of Jerusalem.

This moisture-squeezing effect seems to be faltering in some areas, Rosenfeld notes. Many mountainous areas in the western United States, especially those downwind of major urban areas, have experienced as much as a 25 percent drop in precipitation in recent years.

Some scientists suspect that air pollution wafting from those industrialized regions may be triggering the downturn in rainfall. However, because of a lack of long-term atmospheric data in affected areas, “there’s been no smoking gun,” says Rosenfeld.

Now, he and his colleagues have analyzed weather data gathered atop Mount Hua, a 2,060-meter-tall peak about 120 kilometers east of Xi’an, China. Meteorologists there have made detailed measurements, including rainfall, atmospheric visibility, and humidity, since 1954, he notes.

First, Rosenfeld and his team adjusted the data by removing the contribution of relative humidity to haze, thereby isolating the role of aerosols—particles suspended in the air—in reducing visibility. Then, they calculated the ratio of rainfall measured on Mount Hua to that collected at Huayin, the closest city on the plains nearby.

On days when the adjusted visibility was more than 20 km, Mount Hua received, on average, about 65 percent more precipitation than did Huayin. When aerosols reduced visibility to below 8 km, rainfall on Mount Hua measured only 20 percent more than that at Huayin. The researchers describe the findings in the March 9 Science.

Although the team’s findings bolster the notion that pollution stifles precipitation, they’re not conclusive, says Randolph D. Borys, an atmospheric scientist with the Desert Research Institute’s Storm Peak Laboratory in Steamboat Springs, Colo. For instance, the visibility data don’t distinguish between industrial pollutants and natural aerosols, which may have dissimilar influences on the formation of raindrops.

Wei-Kuo Tao, a meteorologist at NASA’s Goddard Space Flight Center in Greenbelt, Md., calls the new report the first to look at the connection between atmospheric visibility and precipitation. Nevertheless, he, too, points out a potentially confounding variable. While an abundance of tiny particles tends to produce small droplets that don’t coalesce, thereby stifling precipitation, a smaller number of large particles—which may also block long-distance visibility—may have no effect whatsoever on precipitation.

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