Rainmaker Technology’s bid to deploy cloud-seeding flares on small drones is being met by resistance from the airline pilots union, which has urged the Federal Aviation Administration to consider denying the startup’s request unless it meets stricter safety guidelines.

The FAA’s decision will signal how the regulator views weather modification by unmanned aerial systems going forward. Rainmaker’s bet on small drones hangs in the balance.

The Air Line Pilots Association (ALPA) told the FAA that Rainmaker’s petition “fails to demonstrate an equivalent level of safety” and poses “an extreme safety risk.”

Rainmaker is seeking an exemption from rules that bar small drones from carrying hazardous materials. The startup filed in July, and the FAA has yet to rule. Instead, it issued a follow-up request for information, pressing for specifics on operations and safety.

In its filing, Rainmaker proposed using two flare types, one “burn-in-place” and the other ejectable, on its Elijah quadcopter, to disperse particles that stimulate precipitation. Elijah has a maximum altitude of 15,000 feet MSL (measured from sea level), which sits inside controlled airspace where commercial airliners routinely fly. Drones need permission from Air Traffic Control to fly inside this bubble.

Rainmaker’s petition says it will operate in Class G (uncontrolled) airspace unless otherwise authorized. ALPA notes the filing doesn’t clearly state where flights would occur or what altitudes would be used. Rainmaker and ALPA did not reply to TechCrunch’s requests for comment. 

The union also objects to the flares themselves, citing concerns about foreign object debris and fire safety. ALPA points out that the petition does not include trajectory modeling of the ejectable casings or analysis on the environmental impacts of chemical agents.

However, Rainmaker says the flights will occur over rural areas and over properties owned by private landlords “with whom Rainmaker has developed close working relationships.”

Cloud-seeding already happens today, largely in the western U.S., with crewed airplanes flown in coordination with state agencies. Ski resorts commission the operations to help keep their runs white, and irrigation and water districts fly them to build snowpack in the winter to help feed their reservoirs during the spring melt.

The general practice of cloud seeding dates back to the 1950s. By spraying small particles into certain clouds, scientists found they could induce precipitation. Typically, cloud-seeding operations use silver iodide for the particles, mostly because they mimic the shape of ice crystals.

When a silver iodide particle bumps into droplets of water that are super-cooled, they cause the droplet to rapidly freeze because its water is already below the freezing point. Once the ice crystal forms, it can grow quickly if conditions are right, faster than a liquid water droplet would in similar circumstances. Plus, the rapid growth helps the crystals stick around longer than a water droplet, which might evaporate before it has a chance to fall as precipitation.

Rainmaker’s twist — doing this work with drones instead of pilots — could prove safer in the longer term. The company points out that the flight profiles are tightly bounded, overseen by a remote pilot and trained crews, over rural areas, with other safety checks in place.

What happens next hinges on whether the FAA thinks those mitigations are sufficient. However it’s decided, the agency’s response will likely set the tone for novel cloud-seeding approaches.