What is the difference between polluted air and clean air? Most will agree that the former has a bad odour and causes respiratory problems.

Now scientists have found another answer: polluted air causes less rainfall in mountainous areas.

After studying a set of meteorological measurements, they found pollution particles in the air atop Huashan Mountain in Northwest China's Shaanxi Province had cut rainfall in that region by almost 50 percent on hazy days.

It is the first time scientists have observed direct evidence of the relationship between increasing pollution and decreasing precipitation, according to Israeli scientist Daniel Rosenfeld, the lead author of a research report published in yesterday's issue of US-journal Science.

He said the research was particularly important to China since "mountain precipitation is one of the main water sources of Northeast and Northwest China".

"Some big rivers, such as the Yellow River, receive much of their water supply from mountain precipitation," he said.

A particular kind of precipitation called orographic, occurs when moist air is deflected upwards by a topographic feature such as a mountain, which cools the air and causes cloud droplets and then raindrops to form.

"Polluted air carries more particles which divide cloud droplets into smaller ones. The smaller cloud droplets are slower to combine into rain drops," Rosenfeld said.

He and his fellow Chinese researchers analyzed records of Huashan Mountain dating back 50 years, and observed a trend of reduced precipitation and increased aerosols.

Aerosols are tiny particles suspended in the air. Some occur naturally from volcanoes and dust storms, and others from the burning of fossil fuels.

Although the researchers could not provide specific figures about the increase of aerosols in the area, statistics collected atop Huashan Mountain, 2,100 kilometers above sea level, show that the average visibility in 1950 was 30 kilometers, twice that of today.

This provides a measure of the increase in aerosols that reach into the clouds, Rosenfeld said.

In the same period, the amount of rain and snow atop the mountain had decreased by 20 percent, he said.

"The two trends can be related to each other, as more pollution leads to lower visibility and less precipitation," he said.

Rosenfeld said that this research could also shed light on drought conditions in western US mountain ranges that are downwind from polluted urban areas.

The New York Times on March 3, quoted a newly released US Climate Action Report as saying drought would become a persistent threat to the United States.

"Warmer temperatures along with increasing concentrations of greenhouse gases are expected to exacerbate present drought risks in the United States by increasing the rate of evaporation," the newspaper said.

But Rosenfeld said the scientists could not observe a direct long-term linkage between pollution and droughts in US due to a lack of records on aerosols.

Yao Zhanyu of the Chinese Academy of Meteorological Sciences in Beijing, who also took part in the research, said there has always been debate in the scientific community about whether an increase in aerosols will lead to a decrease in rainfall. "Our study has provided strong support for a confirmative answer to that question," he said.

(China Daily March 9, 2007)