Air pollution stops rain

300km-long pollution tracks in South Australia: Power plant (5) and lead smelter (6)

By BBC News Online's Damian Carrington

Urban and industrial air pollution can stop rain and snow falling, satellite data have revealed.

Daniel Rosenfeld, of the Hebrew University of Jerusalem, studied satellite images of smog streaming out of power plants, lead smelters, and oil refineries.

100km-long pollution track from Hudson Bay Mining and Smelting compound, Manitoba

He showed that the pollution particles led to abnormally small droplets that were far too tiny to fall to Earth - the first direct evidence of how pollution affects rainfall.

Floods and fires

Professor Rosenfeld told BBC News Online that the new work suggested that the smoke from forest fires could prevent quenching rain from falling.

"Large amounts of smoke such as from burning vegetation might reduce the precipitation from clouds in some conditions," he said.

However, he thought it unlikely that clouds could be deliberately treated to prevent heavy rain from causing floods: "The clouds that are susceptible to the pollution are short living, but I would think that the most extensive floods are caused by long living cloud systems."

Millions of droplets

The reason why pollution stops rain and snow is that the particles allow the cloud moisture to condense into much smaller droplets than usual.

It takes about one million normal-sized droplets to collide and coalesce to produce a rain drop large enough to fall to Earth. With the smaller droplets, the chances of collision are much lower and drops do not form.

The smaller droplets are also slower to freeze into ice crystals, resulting in less sleet and snow.

Urban and industrial air pollution is a significant problem in many regions of the world, so Professor Rosenfeld's findings suggest that human activity may be affecting rainfall patterns on a global scale.

Poking holes in clouds

"In the past, scientists had to collect information by poking little holes in clouds from airplanes because you can't replicate rain-clouds in the lab," he said. "But now, new satellite instruments allow us to measure cloud precipitation and microstructure simultaneously over large areas.

"Water absorbs in parts of the infrared solar spectrum, so larger droplets absorb more IR solar radiation and reflect less back to space. Therefore, exact calculations based on the measured radiation by the satellite sensors provide information about the cloud particle size."

Professor Rosenfeld's work is published in the journal Science and focuses on particularly striking plumes of pollution in Turkey, Canada, and Australia.

300km-long pollution tracks in South Australia: Adelaide port (7) and oil refineries (8)

However, he also notes that in other parts of the world air pollution is more widespread and not as easy to distinguish.

Therefore, according to Owen Toon, of the University of Colorado at Boulder, the well-defined tracks identified in the study "serve as a Rosetta stone for the potential impact of more widely-distributed aerosol pollution on clouds".

Another impact of the new work is on schemes to provoke rain by seeding clouds from airplanes. These use large particles to attract water.

"With the new space technology, we can identify the clouds which could possibly benefit from cloud seeding and not just seed blindly as has been the case until now," said Professor Rosenfeld.

"Furthermore, we might be able to see the seeding effects on the clouds. In fact, observing clouds that were seeded with silver iodide, aimed at producing more ice, I was able to see the seeding signatures on the cloud droplets turning into ice."

All images copyright Science