A start-up named Rainmaker recently conducted a cloud seeding test in Alaska. The company used drones to release silver iodide into an existing storm.
Over a three-hour period, seven drone flights released the particles into the clouds above. Roughly 30 minutes later, seven new patches of snow appeared on radar in the same locations.
Rainmaker estimates the operation produced roughly 19 million gallons of extra precipitation.
The company acknowledges a wide range of uncertainty in that figure, between 13 and 29 million gallons.
How cloud seeding is supposed to work
Cloud seeding works by releasing particles, such as silver iodide, into clouds to give small water droplets a surface to cling to.
As those droplets gather more moisture and grow heavier, they eventually fall as rain or snow.
Water districts across the American West have practised the technique since the 1950s, mainly to increase mountain snowpack that later melts into drinking water supplies.
Worsening droughts and shrinking freshwater supplies have given the practice fresh urgency in recent years.
States are increasingly investing millions of dollars annually into weather modification programmes.
Utah alone currently operates 187 ground-based generators for this purpose.
Why proving it works is so difficult
Proving cloud seeding's exact efficacy remains genuinely difficult.
Clouds are chaotic systems, and there is no "unseeded" version of the same storm available for direct scientific comparison.
That makes it hard to say with certainty how much of any given snowfall came from the seeding itself, rather than from the storm doing what it would have done anyway.
Controversy
Rainmaker has faced significant public backlash, including death threats directed at its founder.
The backlash followed accusations that the company had caused deadly floods in Texas last year.
Those accusations were false.
The company's seeding mission had taken place 150 miles away, two days before the floods occurred.
A single seeding mission typically adds only a fraction of an inch of precipitation, compared with the roughly 20 inches of rain that fell during the actual Texas flooding event.
Patchwork of responses
Three states have responded to public concern by implementing outright bans on weather modification.
Others, including Utah, continue to expand their own seeding programmes in the opposite direction.
What isn't yet known
Current levels of silver iodide used in seeding appear safe based on existing research.
The long-term effects of widespread, large-scale use, however, have not been fully studied.
Should this happen at all?
The emergence of private companies capable of altering rainfall patterns raises significant ethical and governance questions.
Chief among them is who should actually decide when the potential benefits of weather modification are judged to outweigh the risks, a question that current regulation has not yet caught up with.