Analysis of data from a cloud seeding flight undertaken on 5 June 2016 shows the operation had no measurable impact on rainfall on that day.

The June floods had a devastating impact. Hydro Tasmania extends its sympathies to all who have been affected. Hydro Tasmania understands community concern about the possibility that the 5 June flight may have contributed to the flood event.

A preliminary report on the flight was provided to the Tasmanian Government on 4 July and significant analysis has been undertaken since that time, resulting in a final report being delivered to government today.

Key points:

• Hydro Tasmania brought forward the start of its cloud seeding season this year as part of efforts to rebuild storages after an unprecedented dry Spring and Summer.
• The cloud seeding operation on 5 June took account of current flood warnings and the Bureau of Meteorology’s forecasts.
• The operation was undertaken with the intent of enhancing rainfall into hydro storages in the Upper Derwent catchment (including Lake Echo). Had the seeding flight been successful it was possible there would also have been an effect in the Great Lake catchment, Arthurs Lake and Woods Lake.
• There were no flood warnings in place in the Upper Derwent or Great Lake catchments at the time of the flight.
• Cloud seeding began at 10:57am, in seeding conditions that were described on the Flight Log as “marginal”, and continued for 1 hour and 34 minutes to 12:31pm.
• Post-flight analysis of data has demonstrated that the cloud seeding operation had no measurable effect on rainfall on 5 June because the cloud that was seeded already contained significant ice and was already precipitating freely.

The 5 June flight was undertaken both as part of Hydro Tasmania’s usual practice of seeking to enhance rainfall and also having regard to the need for storage recovery following drought and the then current Basslink outage.

“Cloud seeding works by using silver iodide to introduce ice nuclei to clouds with high levels of super-cooled liquid water and low or no ice content,” said Hydro Tasmania CEO, Steve Davy.

“The process aims to convert the super-cooled liquid water droplets to ice, which will then fall as precipitation – rainfall or ice crystals. If a cloud has high levels of ice content and is already readily precipitating, introducing further ice nuclei will not enhance this process.”
 
Data collected by the aircraft’s instruments and data obtained from the Bureau of Meteorology, and analysed post-flight, show the cloud that was seeded on 5 June 2016 already contained significant ice and was already precipitating freely, meaning that any seeding effort to initiate precipitation was redundant.

“We appreciate there has been a high level of community concern that we undertook a cloud seeding flight on that date,” said Mr Davy.

“We will now review the cloud seeding program to make improvements in our processes, including in relation to seeding when there is a risk of floods, so that future decisions about cloud seeding are more in line with community expectations.

“Hydro Tasmania’s cloud seeding program remains on hold and will not resume until a full internal review of the program has been completed and any appropriate improvements have been implemented.

“We will also be engaging with stakeholders prior to any resumption of the program.”

Mr Davy said it is not expected that cloud seeding will be undertaken again this season.

“Given the unfortunate loss of life during the floods, we anticipate that there may be formal inquiries in the future in relation to the floods, including the cloud seeding flight, and we look forward to co-operating with any such processes and explaining our conclusions further at that point.”

The report can be viewed here.

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