Planes disperse chemicals such as silver iodide, which under the right conditions can bond with water vapour and form ice crystals. The ice crystals then fall as snow or, if it’s warm, rain. Jim Brandenburg/Minden Pictures

If only a downpour will vanquish the massive wildfire that has laid waste to parts of Fort McMurray, why not make it rain?

For more than half a century, farmers, kings and generals alike have sought to conscript the skies as a weapon, turning clouds into sprinklers.

Now, with wildfires burning until enough water falls to douse them, questions have arisen over whether the heavens could become a fire hydrant as well. After all, China's offices of weather modification employ tens of thousands of people in a massively ambitious project that, in 10 years time, spilled 490 billion tons of precipitation, enough to fill the vast Three Gorges Dam 12 times over – and, in the process, put out dozens of forest fires.

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In Thailand, too, the Bureau of Royal Rainmaking boasts that its patented techniques have helped quench burning forests. And the world's largest cloud-seeding company has aircraft that it could send north at a moment's notice from Fargo, N.D., less than 1,500 kilometres from Fort McMurray.

Would it not be worth trying to provoke a deluge over northern Alberta?

As one user on Internet forum reddit.com recently put it: "Can we do that thing that China does and shoot artillery shells into the sky that create rain?"

The answer, in simple terms is: probably not.

That, at least, is what scientists say today.

A Chinese worker fires rockets for cloud seeding in an attempt to make rain in Huangpi, central China’s Hubei province on May 10, 2011. STR/AFP/Getty Images

But the question of whether the clouds can be made to drown out fires is a long-standing and controversial one – and those with the most modern experience in rainmaking say it may be worth a try in Fort McMurray.

"Under the right conditions, it will work," said Patrick Sweeney, president of Weather Modification Inc., a Fargo-based company that dispatches 37 aircraft around the world to inject chemicals into clouds.

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"If we got a call we could go tomorrow morning. We have equipment ready to go," Mr. Sweeney said.

He is among the most practised people on Earth in the science of rainmaking, which typically involves shooting seeding agents – silver iodide or salt, depending on conditions – into a cloud, in hopes drops will then coalesce and rain will fall.

It is the modern face of a decades-long effort to modify the weather for all kinds of uses. At least 40 countries have experimented with the technology, whose effectiveness nonetheless remains the subject of scientific dispute.

Most research has sought new solutions for drought. But some has also looked at ways to attack burning forests.

In the mid-1950s, U.S. forestry researchers working on "Project Skyfire," tried to use weather modification to prevent forest fires – and found that clouds seeded by aircraft produced a third fewer lightning strikes.

In the late 1960s and early 1970s, Soviet scholars took to the skies to see if they could snuff out fires, by churning up downpours. They claimed success, writing in academic papers that they had suppressed 2,000 square kilometres of flame – and, they said, it cost 560 times less to unleash a ton of water by rainmaking compared to using water bombers.

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Thailand turned to rainmaking to fight wildfires in 1998, when 1.1 million hectares of forest burned, according to the website of the Bureau of Royal Rainmaking, which has one of the most active cloud-seeding programs on Earth. The idea is being actively considered in India, too, where a senior government official, Additional Chief Secretary S Ramaswamy, said just last week: "This technique can help in dousing of fires."

Aircraft technician Steve Bauer of Weather Modification Inc., inspects wing mounted burn-in silver iodide flare racks on a Piper Cheyenne II aircraft before the beginning of cloud-seeding operations at Jakkur Airfield, Bangalore, 17 August 2003. Karnataka’s government has contracted the US based company to perform cloud-seeding, which involves the aircraft being piloted into clouds and releasing silver iodide in order to attempt to bring rain to the severe drought hit regions of Karnataka state. INDRANIL MUKHERJEE/AFP/Getty Images

In China, meanwhile, rainmaking has become a common tool, used to fight dozens of forest fires in recent years, news reports show.

"The cannons sound like spring thunder, and the rain came down right after being requested. No need to be afraid of the forest fire," said an official in Hubei province in 2014, according to a Chinese state media report. Local workers had used artillery to shoot cloud-seeding rockets into the clouds over a fire that threatened a local electrical transmission line. A half-hour later, according to the report, rain soaked the flames.

But the science has been far from conclusive. The United Nations, for example, found that novel techniques – including cloud seeding– used to fight immense Indonesian forest fires in 1997 "had minimal effect."

In Australia, which faces regular threats from fast-spreading bush fires, cloud seeding is not used in fire-suppression. Even with "some slight precipitation on a 40-degree day, you can still potentially see quite large fires. I don't think it's going to be the silver bullet," said Brendan Doyle, spokesman for New South Wales Rural Fire Service.

Others have attacked the very underpinnings of the science: A sweeping report on rainmaking by the National Research Council in the U.S. concluded in 2003 that "there still is no convincing scientific proof of the efficacy of intentional weather modification efforts." In Israel, scientists who surveyed 50 years of data concluded: "seeding had little or no effect on total precipitation."

Alberta has historically shown more faith in its ability to edit cloud behaviour. Farmers eager to prevent violent hail organized the Alberta Weather Modification Co-operative in the 1950s; 20 years later, the government formed its own Alberta Weather Modification Board. In the 1980s, the Alberta Research Council even used an armoured plane to fly into hail clouds. Computer modelling on one storm found that seeding achieved a 9 per cent reduction in hail, and the idea has stuck. For nearly two decades, insurers have paid to seed clouds around Calgary, and claim a 50 per cent reduction in claims as a result.

Rainmaking may also have a role to play in a warmer world experiencing more weather extremes. Scientists in Australia, for example, suggest pulling more water from the clouds when it rains, to save for later.

"It can enhance the dam levels, so that communities have some water at least when they do go into drought," said Roger Stone, director of the International Centre for Applied Climate Sciences at the University of Southern Queensland.

"When you get into droughts and have massive forest fires and brush fires, it's the wrong time."

Smoke and flames from the wildfires erupt behind a car on the highway near Fort McMurray, May 7, 2016. Mark Blinch/REUTERS

It's not clear, either, whether cloud seeding will work when trees have already become torches, spewing smoke into the air.

"The smoke particles themselves won't seed the cloud in a positive way. In fact they may do the opposite by providing too many centres for condensation, so forming many small cloud droplets without much chance for any of them to grow large enough to fall," said Terry Deshler, a professor in the department of atmospheric science at the University of Wyoming.

His research has shown that cloud seeding can bump precipitation by 5 to 10 per cent when used in winter to help build snowpacks, which can then provide a better spring runoff.

But for Fort McMurray, he said, it's not clear anything but time will bring the moisture needed.

"To quell this will really require the weather to change. Unfortunately that doesn't appear to be likely in the near future," he said. "There appears to be persistent high pressure blocking moisture from the Pacific."