The Whole Truth: No, cloud seeding isn’t causing all this awful weather

This reporting is part of Stuff’s fact-checking project, The Whole Truth – Te Tikanga Katoa. You can read the rest of our fact-checks here.

Read this story in te reo Māori and English here. / Pānuitia tēnei i te reo Māori me te reo Pākehā ki konei.

What’s the issue?

Some have claimed on social media that Auckland’s historic rainfall event was the result of geoengineering.

The claims were seen on several Telegram groups and Facebook pages.

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The notion that the weather is artificially controlled by state actors has a long history in conspiracy theory movements.

It is often linked to theories about “chemtrails”, in which chemicals are supposedly sprayed in the atmosphere to make people sick or to control the weather, a claim that has been widely debunked.

What we found

The Auckland rainfall was caused by a subtropical low-pressure system, and could not have been synthetically generated with existing technologies.

There have, however, been attempts by scientists to influence the weather, usually in the context of alleviating drought.

The most common method is cloudseeding.

Cloudseeding is a technique that manipulates clouds to produce precipitation, which falls as either snow or rain. It involves spreading a natural compound such as silver iodide into clouds, either through rockets fired from ground level, or from planes.

The moisture in the cloud condenses around these chemicals and falls as rain or snow.

Cloud seeding has been developed over more than half a century, but remains a rudimentary technology with mixed success.

Research shows that cloud seeding in winter to generate snowfall has occasionally worked.

Doing so can increase the winter snowpack, which can have benefits for hydroelectric schemes when the snow melts in the spring. It can also decrease the size of hail stones.

There is less support for the benefits of summer cloud seeding. Multiple major studies have concluded that cloud seeding can have a modest impact in certain conditions, but cannot generate large amounts of precipitation.

One country that has embraced the technology is the United Arab Emirates (UAE), where annual rainfall can be less than 100mm per year. China and some parts of the US also use cloudseeding.

Although some studies show cloudseeding works, it is inherently difficult to know what proportion of rain can be attributed to the technology. Scientists in the UAE have said they do not know if the country’s ambitious cloudseeding programme is working.

Doubts about its effectiveness led Israel to abandon its decades-long cloudseeding programme.

Was the Auckland rainfall caused by cloudseeding then?

No. Cloudseeding is an expensive – and difficult-to-hide – process that is not regularly undertaken in New Zealand.

While seeding cold clouds to generate snowfall has modest success, there is little evidence that it can reliably generate rainfall in summer.

Even if it had been used in Auckland, the experience of other countries suggests it would have had a negligible impact on the record rainfall.

One analysis said that 247 cloudseeding operations in the UAE over the course of a year could theoretically have, at most, added up to 50mm of annual rainfall.

Other estimates suggest cloudseeding can increase winter snowfall by 5-15% over the course of a season.

In one 24-hour period, parts of Auckland experienced around 280mm of rainfall. At the airport, the previous rainfall record was broken by around 90mm, or 55%.

There is a clear explanation for where the rainfall came from; a subtropical low-pressure system, which was tracked days in advance forming over the Pacific Ocean. It was fuelled by La Niña conditions and a marine heatwave that made more moisture available and was blocked by a high-pressure system in the south.

It is also consistent with the impacts of a warming climate. Warmer air holds more water vapour, which can fall as rain.

In summary

Weather modification is possible in specific circumstances, mainly through cloud seeding.

Its primary usage is to increase snowfall in winter. There is little evidence the technology can generate a meaningful amount of rainfall in summer, particularly in one-off events.

It is not widely used in New Zealand, and even if it had been, its impact would have been marginal to non-existent in the Auckland rainfall event.