Cloud-seeding has a mixed reputation, both in Australia and overseas. Some see it as a reliable and cost-effective way to boost precipitation and water supplies. Others put cloud-seeding into the same basket as water-divining — something unworthy of attention by any proper scientist or water professional.
There are, however, strong scientific grounds for why some types of cloud seeding should work. These involve the use of compounds such as silver iodide or dry ice to induce ice-formation and precipitation. Such techniques can’t make clouds and rainfall appear out of nowhere. They work by catalysing a natural ‘chain-reaction’ that might or might not happen in a cloud (the weather is fickle like that!!), ultimately leading to snow or rain.
Cloud-seeding has a long research and operational history in Australia. Australian research teams have been testing cloud-seeding techniques and locations since 1947. Our national science organisation CSIRO has been involved in most or all of these research projects, working with various state governments and the hydro-electric power companies in Tasmania and NSW.
During 1947–1963 aerial cloud-seeding was tried in locations from southern Queensland to South Australia. Western Australia and Victoria began trials in the mid–late 1960s, but these, like the others before them, were inconclusive. Victoria, WA and Queensland ran further experiments between 1972 and 1982, but conditions were not suitable often enough and the experiments were abandoned as an inefficient use of resources. Trials in Tasmania between 1964 and 1994 were considered successful, and as a result cloud-seeding has been adopted there as an operational technique. Since 1998 Hydro Tasmania has seeded clouds from aircraft during May–October each year over mountainous parts of the state to boost hydro-electricity water supplies.
But, in spite of all the real-world applications and operational expenditure, the elusive goal for researchers and proponents has been in achieving ‘statistically significant’ results from cloud-seeding.
We all know that rain or snowfall varies enormously in time and location. This natural variability creates huge problems for practitioners trying to untangle the effects of cloud-seeding from the natural variability in precipitation. Also, dust and other background aerosols can stimulate precipitation and hence confound interpretation of what might otherwise appear to be positive results.
Now, for the first time possibly anywhere in the world, there is scientifically robust and defensible evidence that cloud-seeding has increased precipitation, in an area of the Snowy Mountains in Australia. Statistically significant increases of 9% to14% snowfall have been documented, with no negative environmental or ‘down-stream’ (rain shadow) effects detected.
The ‘Snowy Precipitation Enhancement Research Project’ (SPERP) trial has been conducted since 2004 by the government-owned Snowy Hydro Corporation. Independent review has been provided by the NSW Natural Resources Commission, which oversees the trial and ensures it does no environmental damage, including to sensitive alpine habitats (I was one of the advisors to that review process).
The trial uses ground-based burners on mountain tops to introduce silver iodide and a tracer into suitable clouds. The ‘suitable’ clouds are identified using modern Doppler radar identification techniques. Importantly, a ‘double-blind’ study design has been applied, with defined target areas and controls to allow for confounding effects. Snow fall is sampled carefully and chemical ratios in the snow are analysed to confirm the source (natural or seeded).
The October 2010 NRC review report on the SPERP trial can be found at www.nrc.nsw.gov.au/Workwedo/Cloudseeding.aspx.
The trial continues until 2014 when the final results will be announced. If I am still writing this blog then, I promise to provide an update on the results!