Cloud control

G Magazine

We can’t rely on the weather to be reliable, but are machines that can play God any more trustworthy? We investigate cloud seeding.


One of the Snowy Hydro cloud seeding units.

Credit: Snowy Hydro

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In early June, skiers and boarders turn their attention to the weather forecasts hoping for some good dumps of the fluffy white stuff over the resorts of the Snowy Mountains. But in recent years, a different set of weather-watchers get off-piste weeks earlier and these ones certainly don’t wear skis. At hidden-away spots on the same slopes, there are 23 pairs of cloud seeding units, powered by gas burners seeded with tiny amounts of silver iodide.

Carried up on the hot air, once the silver particles reach the super-cooled water in the clouds, these tiny particles can make the difference between the moisture evaporating, or gathering around the silver iodide and then falling as snow.

And while the operation might benefit the skiers, the motive for this experiment, which started in 2004 and will end in 2015, is to try and encourage more snow to fall in the catchments of the area’s hydro-electric company, Snowy Hydro.

In central Tasmania, a similar operation is carried out each year as planes release silver iodide to encourage the clouds to give up their moisture to help fill hydro-electric dams there.

On the face of it, the science is straightforward. All water in clouds need some sort of aerosol particle to gather around in order to form ice or droplets big enough to fall.

And while the concept of exerting an element of control over the weather might seem futuristic, scientists in Australia have been at the forefront of cloud seeding research since the mid-1940s.

“One of the interesting aspects of cloud seeding research was that sometimes it appeared to work extremely well and at others, it fell flat or even reduced rainfall,” says Roger Stone, Professor in Climatology and Water Resources at the University of Southern Queensland.

Success or failure for cloud-seeding can depend on myriad factors, such as the amount of naturally-occurring
and human-caused particles already in the clouds, the amount of moisture, the efficiency of radars, the right
types of clouds for the kind of particles used.

Australian examples in the Snowy Hydro and Tasmania suggest a reliable increase in rainfall by up to 14 per cent. A state government-sponsored experiment over southeast Queensland over two summers of 2008 and 2009 also suggested a measurable increase in rainfall.

But cloud seeding – also known as weather modification – is also deployed to try to suppress hailstorms, prevent fog or even to hold off rain.

Between 2003 and 2006, cloud seeding was used to prevent rain during special events in Moscow and St Petersburg. Anyone present at the sixth Moscow festival of beer in 2004, where no rain fell, may not have noticed the aircraft circling above.

Several types of cloud seeding has been deployed in recent years in dozens of countries including the US, Australia, Russia, China, South Africa and Mexico.

Professor Stone says there is also “promising” cloud seeding research being carried out in Thailand, where
the aim is to add rainfall into food-growing areas.

The World Meteorological Organisation’s conference on weather modification held in October 2011 concluded, “the impacts of operations in rainfall enhancement and hail suppression have still not been properly quantified and modification remains an area of active research”.

Meteorologist Steven Siems, an associate professor at Monash University who has researched cloud-seeding technologies and also worked as a consultant with Hydro Tasmania, says Australia is one of the few places in world where “robust” results have come from cloud-seeding activities.

“The type of seeding that we see as being effective only really happens over the mountains and not really on farmlands,” he says. “It’s had pretty poor results around the world but things in the Snowy and Tasmania look promising.”

Questions have been raised about the environmental impact of cloud seeding. Residents downwind, for example, question whether they are being robbed of rainfall.

In west Tasmania, cloud seeding was temporarily suspended for a season in 2008 when the regional council questioned whether increased rainfall was damaging the region’s economy.

Others ask if there is a risk that the chemicals deployed in the atmosphere could accumulate in flora, fauna or in water courses.

A review of the Snowy experiment in October 2010, tested soils, sediments, water and moss to find “no evidence” that silver iodide and a second chemical, indium trioxide (which is used as a tracer chemical), had accumulated.

The review also claimed there were no detectable impacts on snow habitats. Rainfall downwind of the experiment was unaffected, the review said.

Snowy Hydro had also collected drinking water samples in the area and found that concentrations of silver were “at background levels” of about one part per trillion - almost “100,000 times lower” than guidelines. Distilled water bought from a nearby supermarket had a silver concentration of 40 parts per trillion.

“There are critics concerned with the environmental impact but I think the fears are largely unfounded or unsupported,’’ adds Siems.

“A common concern is that you might be taking water from elsewhere but the evidence suggests that’s not a concern. These clouds fill and rain in the mountains and when they pass on they disappear, so there would be no rain anyway.

“It’s an emotional issue, but looking at the science in the Snowy and in Tasmania, we don’t see evidence of a downwind effect.”

As for the ethical issue of using technology to in some way control the weather, Professor Stone says we already affect nature simply by driving cars or adding fertiliser to soils.