http://www.cosmosmagazine.com/news/2514/major-study-proves-cloud-seeding-effective
SYDNEY: A 45-year Australian trial is the best evidence yet that could seeding - the practice of artificially inducing clouds to make rain - really works.
Since the mid-20th century scientists have attempted to produce rain by dispersing chemical substances into the clouds and stimulating precipitation. However, until now, there has been little concrete scientific evidence that cloud seeding is effective.
"This is the first time that an independent analysis of cloud seeding data over several decades has shown a statistically significant increase in rainfall," said Steven Siems, a meteorologist from Monash University in Melbourne and leader of the study.
Significant finding
The Monash team, in conjunction with renewable energy firm Hydro Tasmania, analysed monthly rainfall patterns over the hydroelectric catchment area between May and October from 1960 until 2005.
As they detailed in the Journal of Applied Meteorology and Climatology the analysis revealed higher levels of rain in the parts of the catchment where the rain making technique was used than in those where it was not.
"A number of independent statistical tests showed a consistent increase of at least five per cent in monthly rainfall over the catchment area," said Siems.
For the could seeding technique, the researchers select clouds using specialist weather radar technology that allows them to see all the tiny processes that take place within them.
Once clouds for seeding are chosen, minute particles of a silver compound are dusted into them by light aircraft to stimulate rain formation.
Super-cooled water
Anthony Morrison, a climatologist at Monash and co-author of the study, explained that these silver particles cause super-cooled water in the clouds to freeze. As these particular clouds are so high in the atmosphere that they are below freezing point, the frozen drops recruit water and get heavier causing them to fall from the clouds as rain.
However, the researchers caution that the result may be due to the unique clouds in this part of Tasmania and would be difficult to reproduce elsewhere.
"Clouds over the Southern Ocean are different to any other clouds", Siems told Cosmos Online. "They are really loaded with super cool liquid water." Just as important, he said, is the remoteness of the location: "the air in the Southern Ocean is exceptionally clean with virtually no pollution."
And the researchers are still at a loss to precisely explain how the technique was successful.
"They're really not comparable to clouds that have been seeded anywhere else in the world," said Morrison. "Further field measurements of cloud microphysics over the region are needed to provide a physical basis for these statistical results."
Despite the caveats, other experts are excited by the results.
"At long last there is scientific backup for the [cloud seeding] hypothesis that has been suggested over the years," commented Roger Stone, director of the Australian Centre for Sustainable Catchments at the University of Southern Queensland in Toowoomba.
However, while the study is a breakthrough, he noted that cloud seeding does not work in all locations and specific techniques have to be developed for each region.
"For example, in Queensland the conditions are highly different. It has to be the right time and exactly the right cloud for it to work," he said. "The key is to get a very good weather radar."
Let it snow
Paul Johnson, a spokesperson from Snowy Hydro, who are conducting similar experiments to artificial induce snowfall in Victoria's Snowy Mountains, said the results were promising. "It's another indicator that supports our preliminary data and backs up what the experts said in the beginning. That we would see an increase in snow."
Because of the unusual nature of the Tasmanian clouds, additional studies may be needed to determine if the cloud seeding really was the cause of the increase in rainfall.
"Unfortunately, very little cloud physics research has been associated with the cloud seeding experiment in Tasmania, so that we are at the full mercy of the statistics," commented Daniel Rosenfeld, a climatologist from Hebrew University of Jerusalem in Israel.
"Clouds do not distinguish between the impacts of aerosols based on our intentions when we disperse them," he said. "Therefore, understanding cloud seeding and impacts of air pollution is inseparable."
Pollution effects
Rosenfeld has previously worked on computer models of weather systems to understand the effects of cloud manipulation, and he admitted it is difficult to directly link cloud seeding with weather patterns.
"It has been much easier to detect impacts of air pollution, because we pollute the clouds at a much grander scale than we seed them intentionally," he said.
Siems said the new study may have wider implications. He hopes the research highlights the importance of weather radar technology and will pave the way for a better understanding of weather patterns.
"The more we understand precipitation and the better climate models we have, the closer we will be to understanding droughts," he said, a significant problem in Australia.
