Fairfax Media Network

Weather News

Cloud seeding stimulates rainfall, but what is it and how does it work?

Kate Doyle, Friday March 2, 2018 - 08:06 EDT

Tasmanian Liberals have said that if re-elected they will put a stop to cloud seeding. Looking in from the outside the obvious question is, why?

Cloud seeding is the process of adding chemicals to clouds to increase rainfall.

Usually when it comes up, the debate is around if it actually works, but this time the stakes are higher than scientific curiosity, expense justification or environmental concerns.

In 2016, Hydro Tasmania conducted cloud seeding in the lead-up to deadly flooding in north-western Tasmania.

It was later found by Hydro Tasmania's independent report that the seeding "had no measurable impact on precipitation" — a conclusion further supported by an independent expert that caused community backlash.

After all, why were they doing it if it does not work?

How cloud seeding works

Professor Steven Siems of Monash University is one of Australia's leading cloud seeding experts and was involved with reviewing the 2016 event.

He said that the type of cloud seeding that had been found to be effective in Australia was "glaciogenic" cloud seeding.

The idea behind glaciogenic seeding is you take tiny little drops of super-cooled liquid water that are not growing efficiently enough to become rain drops and you convert them to ice by adding silver iodide.

This then helps the super-cooled water grow into snowflakes or raindrops ready to fall from the sky.

"Then we know that it will grow more quickly and those particles will grow to the size where they will precipitate appreciably, where you will get some meaningful precipitation from those clouds," Professor Siems said.



Glaciogenic seeding only works if the conditions are just right.

One of the places where it works is Tasmania, where Hydro Tasmania's cloud-seeding operation used light aircraft to deliver the silver iodide when the right conditions were forecast.

Another good spot for glaciogenic seeding is over the Snowy Mountains in New South Wales, where Australia's other big operation takes place.

The Snowy Hydro also uses silver iodide, but its delivery method is a little different.

James Pirozzi, manager of weather and water at Snowy Hydro, explained how.

"We've got a series of ground-based generators on the western side of the Great Dividing Range in the Kosciusko National Park, and we use the natural terrain features in the wind to give us uplift to get that material from the ground up into the clouds," Mr Pirozzi said.



"Those generators, the burners themselves [are] essentially a big propane flame.

They're almost like a barbecue on the back of the trailer."

Mr Pirozzi said the organisation did about 100 hours of burning a year, but it was highly variable and dependent on the conditions.

At the Snowy Hydro cloud seeding was only done when conditions were right to make snow and not rain.

Filling the dams was the top priority, but maintaining a good snowfield was seen as a welcome bonus of the program.

But does it really work?

Cloud seeding in Australia in 1947 when the CSIRO got up in a light plane and released crushed dry ice into promising-looking clouds.

Researchers have been trying to definitively decide if it works ever since.

A looking into cloud seeding in Tasmania found "the rainfall over the target was somewhere between 5 per cent and 13 per cent greater than over a nearby control region".

Another suggested an average 14 per cent increase in rainfall due to the Snowy Mountain cloud seeding trials. An independent review found the results to be positive but inconclusive.

Proving anything beyond doubt is hard, especially when dealing with the real world. Critics would say these studies were based on statistical analysis of rainfall data and did not include the comprehensive field work that could show cause and effect.



This criticism could soon be invalid. has been doing the field work needed to establish the cause and effect relationship.

But according to Professor Siems there is not a move to conduct those kinds of tests here in Australia.

"You're looking at getting aircraft and getting radars out there. It's not a cheap process."

Conditions different from normal

Hydro Tasmania's flight on June 5, 2016 lasted for an hour and 34 minutes over the Upper Derwent catchment, targeting Lake Echo, which was below preferred levels at the time.

found the cloud seeding flight did not contribute to subsequent flooding.

Professor Siems, who made a submission to the coronial inquest into the flooding, said, "Basically the cloud they had seeded already had tons of ice in it, or already was heavily glaciated, so adding more ice nuclei to it had really no impact whatsoever.

"If there's already ice in there, then putting in more ice nuclei isn't going to have any impact."



The flight was not effective because the conditions were very different from normal.

Professor Siems said normally Hydro Tasmania looked at the baseline air coming off the southern ocean where there was a lot of super cooled liquid water, and that is what it targeted.

On June 5, 2016, Tasmania was dealing with an east coast low coming down the coast that brought very different conditions.

There were no flood warnings in place for the Upper Derwent or Great Lake catchments at the time of the cloud seeding flight, and in a statement, Hydro Tasmania pointed out that the target seeding catchment was not the one that caused the Ouse River flood.

Hydro Tasmania's cloud seeding operation has been on hold since the incident.

Other issues of concern

The other issue that is commonly brought up is that by seeding, one area 'steals' the rain that would have fallen downwind.

Mr Pirozzi said in the Snowy Mountains, despite annual reports they were "yet to see any distinguishable trend in terms of changes in downwind precipitation".

He said conditions that were good for cloud seeding were often associated with dry conditions on the other side of the range, with or without seeding.



The other common concern is about the release of silver iodide.

Mr Pirozzi and his team at Snowy Hydro have been through extensive environmental testing that has shown the amount of silver iodide used is not harmful.

"It is literally like looking for a needle in the haystack. We released about 20 kilos in total of silver iodide a year on average, across an area of over 2,000 square kilometres.

"So, we're looking at parts per trillion of silver in the atmosphere."

While Hydro Tasmania's program could soon come to an end, there are no plans to stop Snowy Hydro's cloud-seeding operations.


- ABC

© ABC 2018

More breaking news

Sydney Morning Herald
ABC News
National Nine News
News Limited

Display Your Local Weather

Weather News

Sydney's Bondi beach engulfed in thick sea of fog

17:15 EDT

Heavy sea fog has engulfed Sydney's eastern suburbs and reduced visibility at the city's most popular beaches.

Flash flooding washes cars off roads in Queensland's south as the north cleans up

15:48 EDT

Flash flooding has cut roads on Queensland's Darling Downs after heavy rain overnight, while in the state's north locals are counting the cost of Cyclone Owen and its record-breaking rain.

Kids, don't let weather spoil your school holidays

13:37 EDT

The school holidays have just started, or are about to start, in each state and territory across Australia.

Help us build a better Weatherzone experience by testing new concepts and providing your feedback.
Sign up to be notified when new test activities become available.