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Last Updated: Monday, 12 March 2007, 15:13 GMT
China's pollution cuts local rain

Smog-filled street in China. Image: Getty
China's rapid industrialisation may be affecting its rainfall
Pollution from China's factories and vehicles is significantly reducing rainfall in hilly areas of the country.

Writing in the journal Science, researchers report that rainfall is reduced by about 50% in some cases.

Clouds form more easily when pollutant particles are abundant, but rain droplets and snowflakes are less likely to grow large enough to fall.

Last week, other researchers reported that Asian pollution is creating stronger storms in the Pacific Ocean.

In the latest work, a team of Israeli and Chinese scientists used more than 50 years' worth of records to show a clear correlation between air pollution and decreased precipitation over hills in central China.

By comparing visibility and amounts of rain and snow, they have been able to demonstrate an average reduction of 20% over this 50-year period.

High life

"The pollutants responsible for this are all the usual particles that our everyday life produces," said Daniel Rosenfeld from the Hebrew University of Jerusalem.

We can more safely say that this is what also happens elsewhere in the world
Daniel Rosenfeld
"These are things like emissions from cars, agricultural burning that emit smoke into the air, emissions from power plants and agricultural practices," he told the BBC's Science in Action programme.

In recent years it has become clear that rapid industrialisation in east and south Asia is producing large amounts of particulate pollution, which has become known as the "Asian brown cloud" or "Asian haze".

Scientists have suggested before that it might affect rainfall because of the influence of tiny aerosol particles on rain clouds.

When moist air is pushed over mountains, it cools down and raindrops are formed. But when pollutant particles are plentiful, the clouds retain their moisture instead of losing it as rain.

The layers of air pollution are a by-product of burning fossil fuels

Over lowland areas, the mechanism is rather different. Moist air is forced upwards by upwinds, and small droplets still have time to coalesce into proper rain drops or snowflakes; so increasing aerosol concentrations should not affect lowland rainfall.

Records kept at a meteorolgical station on Mt Hua in central China, 2,060m above sea level, gave researchers trends in rainfall and in visibility since 1954. Visibility was used as a measure of aerosol concentration.

Data on lowland rainfall came from weather stations on the plains nearby, at Xian and Huayin.

Rainfall over the hills was markedly less on hazy days, but not over the plains. Meanwhile, the overall visibility declined over the 50-year period, a reflection of increasing aerosol production.

Wider perspective

The researchers believe their conclusions may be applicable beyond central China.

"We could not find any alternative explanation why rain would preferentially be reduced over hills in much of the western USA or the Middle East," said Dr Rosenfeld.

"Now, when we find this direct linkage in China between air pollution and these decreasing amounts of mountain rainfall and snow, then we can more safely say that this is what also happens elsewhere in the world."

Findings from other parts of the world could prove very useful in honing scientific understanding of cloud behaviour, which is in turn vital for accurate predictions of how rainfall will change in future.

Global impact of Asia's pollution
06 Mar 07 |  Science/Nature
Pollution 'reducing rice harvest'
05 Dec 06 |  Science/Nature
Third of China 'hit by acid rain'
27 Aug 06 |  Asia-Pacific
Deadly Asian haze 'can be tamed'
12 Aug 02 |  Science/Nature


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