Rainmakers: The drones used to seed clouds

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.