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Removing seeds of doubt

18 February 2009

A new analysis of precipitation records from a long-term cloud seeding operation in Tasmania shows a promising increase in rainfall during periods of seeding.

A Monash University research team worked with Hydro Tasmania analysing the cloud seeding activity over the hydroelectric catchment area in central Tasmania for more than four decades -- from 1960 to 2005.

The analysis used monthly rainfall figures in the catchment area where the seeding took place and compared them with data from nearby control areas.

Associate Professor Steven Siems from the Faculty of Science said a number of independent statistical tests showed a consistent increase of at least five per cent in monthly rainfall over the catchment area.

"This is the first time an independent analysis of cloud seeding data over several decades has shown a statistically significant increase in rainfall," Associate Professor Siems said.

However the Monash team has also cautioned against becoming too excited by the results.

PhD candidate Anthony Morrison said clouds over the Southern Ocean and Tasmania are unique because they can contain vast amounts of supercooled liquid water and are unusually clean.

"Even though we suspect that cloud seeding has made a significant contribution, there could be other explanations for the increased rainfall," Mr Morrison said.

"Further field measurements of cloud microphysics over the region will provide a physical basis for these statistical results."

Cloud seeding is a form of weather modification where substances are dispersed into the air in an attempt to change the amount or type of precipitation that falls from clouds. The practice of cloud seeding remains a contentious issue in the scientific community with many attempts made since the beginning of cloud seeding programs in the 1940s to measure and explain rainfall levels.

Tasmania has long been a location for cloud seeding programs, following apparent success with the programs in the 1960s and 1970s when significant rainfall increases of more than 30 per cent were measured and reported by CSIRO scientists.

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