Mountains are fountains. Humid air crashes into upthrusted rock and releases its water in the form of rain, snow or ice. But the tiny particles created when fuel is burned—aerosols—can interfere with this process by providing even more impurities in the air on which water can condense. The many more resulting smaller droplets collide less often, thus forming fewer raindrops and, ultimately, less rainfall. Or so the theory goes. And now, records stretching back 50 years for a mountaintop in China strongly support this idea.

Atmospheric scientist Daniel Rosenfeld of The Hebrew University of Jerusalem, Jin Dai and colleagues from the Meteorological Institute of Shaanxi Province in China and Zhanyu Yao of the Chinese Academy of Meteorological Sciences studied the records of precipitation and visibility at Mount Hua in central China. The site, one of five sacred mountains in China (familiar to many as a backdrop in martial arts films), has had a meteorological observatory on its peak since 1954.

The observatory's records show that visibility on the mountain on average has declined from roughly 30 kilometers to just over 10 kilometers from its two-kilometer-high summit. They also show that rainfall has dropped by as much as 17 percent compared with the precipitation in Huayin, a town at the foot of the range, and in Xi'an, the closest big city. "The average precipitation over Mount Hua is almost double that of Huayin in relatively good visibility of around 20 kilometers," the researchers write in a study published in the current Science. "The amount of precipitation approaches that of Huayin at visibilities of eight kilometers and below."

The sacred mountain is not the only place affected—an observatory in the U.S. Rocky Mountains detected a 50 percent reduction in snow as a result of aerosol pollution—but it is the only one with longstanding records to prove it. It also explains the widely observed trend—from Canada to South Africa to Israel—of a decrease in highland precipitation compared with that in adjacent lowlands. Pollution is not just obscuring the majestic view, it is also choking the mountain streams.