On 23 August, Cooper Freeman had no idea clouds above his head were being filled with a silver compound in order to cause rainfall.

Neither did thousands of other residents in Homer, Alaska, who later discovered San Francisco start-up Rainmaker conducted an experiment in that region using cloud-seeding technology.

The company claims it produced 19 million US gallons of water in the sky in three hours.

This approach to weather modification, now more than 80-years-old, involves using planes and drones to disperse compounds such as silver iodide into clouds to form ice crystals. Those crystals eventually become large enough to fall as rain or snow, depending on the temperatures below.

Essentially, cloud seeding speeds up the natural process of water vapor condensing inside of clouds. Gravity then pulls those crystals to the ground. The technology, though, needs ideal climate environments, such as mountainous or cold regions such as Alaska.

Freeman, the Alaska director of the Center for Biological Diversity, isn’t just concerned Homer residents weren’t widely alerted to these experiments. He’s sceptical about what was shot into the clouds, and questions how safe it was for both residents and their habitat.

“It doesn’t appear that there was any downstream monitoring to verify that this silver iodide didn’t impact the environment,” he says.

Freeman adds, “Having a company come in to do this seeding isn’t going to solve our water woes, and it feels like a distraction from solving the urgent issues related to water conservation.”

Concerns are mounting about the challenges surrounding water security. United Nations Secretary-General António Guterres said in July “our world is using freshwater faster than it can be replenished, external“, which echoes reports noting how the past five years have led to the driest period for global rivers, external in more than three decades.

Cloud-seeding technology may be a drop in the bucket, as it doesn’t create rainstorms. For example, Rainmaker’s 19 million gallons equates to around 0.01 inch of rain across 100 square miles.

Nevertheless, more than 50 countries, external are developing programmes to modify the weather and a surge of start-ups are developing systems to create rain.

As well developed and promising as this innovative field has become, these businesses continue to face thorny questions on safety and efficacy, leading to novel approaches with both finding the right material to spray into clouds and how they do it.

Augustus Doricko, the founder and CEO of Rainmaker, is quick to answer any questions on the consequences of how his company uses the silver iodide compound. It’s clear he’s been asked about chemical safety many times.

He says Rainmaker releases this form of silver at 20 grams per flight, distributed across thousands of square kilometres. “The resultant increase in concentration on the ground is in the parts per quadrillion, way below the threshold laid out by the FDA [Food and Drug Administration] on what is allowable.”

Rainmaker’s flights use drones to soar into clouds because without clouds, “we can’t do anything,” he says, pointing to one of the key limits of cloud-seeding processes. Cloud-seeding sessions have to wait for clouds to form to try to coax them into precipitation.

Their drones are outfitted with silver iodide flairs, and when lit the burning atomizes the water particles into a small enough size so they can stay suspended in the freezing cloud for hours, eventually turning into crystals and later rain.

Rainmaker operates in states such as Idaho, Utah, Colorado and areas of the Middle East such as Jordan. Doricko says, “Jordan residents often only have water for hours per week from their utility because of how scarce water is, and it’s a privilege to try to help this storied region.”

Cloud seeding’s roots began in 1946 when researchers from the General Electric Research Laboratory used dry ice as the first seeding agent. A year later, silver iodide was found to be more effective than dry ice, which quickly turns into useless gas.

While seeding agents caused more rainfall, they didn’t move the needle much. In fact, sobering statistics point to how patchy this technology can be in what it pours on the ground.

“With the right conditions, cloud seeding squeeze out about an extra five to 10% of precipitation,” says Jon Meyer, an assistant state climatologist with the Utah Climate Center.

What was encouraging, though, for the cloud-seeding sector was a landmark 2017 study that found how silver iodide worked as expected, one of the first reports to observe cloud seeding in action using radar and precipitation gauges.

In the past several years, Meyer has been encouraged to see more companies such as Rainmaker bring innovation to an old industry. “Drones [such as Rainmaker’s products] present this new avenue,” says Meyer, “and they offer a more targeted nature to these clouds we hope will be responsive.”

What is sprayed into clouds may also be undergoing a major shift. Recast Systems in San Francisco is experimenting with a type of protein made from amino acids found in the soil, says CEO Olivia Li.

While still undergoing tests at Texas A&M University, this new seeding agent moves Recast away from silver iodide as the go-to compound.

“It’s safe, biodegradable and nucleates ice at a higher efficiency than silver iodide,” says Li, whose company currently uses silver iodide to seed clouds in several US states.

Meyer is optimistic this technology’s trajectory will only continue to rise. “We’re going to see more demand for these services as so many regions are experiencing water demand,” he says, “and they’re going to be looking for ways to help bridge that gap.”