Cloud seeding, explained

Precipitation begins when water vapor condenses or freezes onto a particle. In many clouds, especially cold winter clouds over mountain terrain, there is abundant supercooled liquid water — water that is below freezing but has not yet turned to ice, because nothing suitable is present for it to freeze onto. That water often passes over a basin and evaporates without ever falling.

Cloud seeding introduces ice nuclei into that environment. The supercooled droplets freeze onto the added particles, grow heavy enough to fall, and reach the ground as snow or rain. The intended effect is an increase in precipitation efficiency from a storm that was going to pass through anyway.

The honest framing matters: seeding is a marginal-gain tool. It can only augment existing systems, the response varies with conditions, and attributing a specific snowfall event to seeding requires careful measurement rather than assumption.

Suitable conditions generally require several factors at once:

• Moisture in the cloud. Seeding a dry sky does nothing. There must be supercooled liquid water present.
• The right temperature band. Glaciogenic seeding depends on cloud temperatures cold enough for the seeding agent to nucleate ice, but not so cold that the cloud has already glaciated on its own.
• Lift and transport. Wind and terrain need to carry the material into the target region of the cloud and hold it there long enough for crystals to grow.
• A defined target area. Precipitation falls downwind of where seeding occurs, so targeting has to account for storm motion.

Outside those windows, an operation should not fly. A disciplined program is defined as much by the storms it declines as the ones it seeds.

For decades, cloud seeding was mostly evaluated by comparing gauge totals in seeded and unseeded areas, and those statistical studies often gave inconclusive results. More recent field research has tracked the process directly. The SNOWIE project in Idaho used radar and aircraft to follow seeding-generated snowfall from release to the ground in individual storms (Friedrich et al., PNAS, 2020). The authors note that it covered only three cases.

The World Meteorological Organization neither promotes nor discourages weather modification. It warns that claims of large or dramatic effects lack a sound scientific basis (WMO Statement on Weather Modification). Utah has run a state-overseen program since the 1950s and describes how it evaluates effectiveness and environmental effects (Utah Division of Water Resources).

The practical reading: seeding can produce snowfall from suitable clouds. How much it adds over a season in a particular basin is uncertain and has to be estimated through careful evaluation. Alto Cirra does not publish efficacy figures for its own operations.

Silver iodide is the most widely used glaciogenic agent because its crystal structure closely resembles ice, making it an effective ice nucleus at relatively warm subfreezing temperatures. It is dispersed in very small quantities per operation.

Hygroscopic materials such as salt particles are used in warm-cloud seeding, where the goal is to encourage droplet coalescence rather than ice formation.

Dry ice and liquid propane work by rapid cooling, producing ice crystals directly from supercooled water without a chemical nucleant.

• Ground-based generators. Fixed or portable units burn a silver iodide solution and rely on terrain-driven updrafts to lift the plume into the cloud. Low cost and able to run for long periods, but dependent on favorable vertical transport and blind to where the material actually ends up.
• Manned aircraft. Crewed planes place material directly into the cloud using flares or dispensers. Accurate, but expensive per hour, limited by crew duty and icing risk, and often grounded in exactly the conditions worth seeding.
• Uncrewed aircraft (UAS). Drones place material in-cloud without putting a crew in the weather, at a far lower cost per flight hour, and can carry sensors that record the conditions at the moment of release.

• Water supply augmentation. Winter seeding over mountain watersheds aims to build snowpack that releases into reservoirs during spring runoff.
• Ski resorts. Additional natural snowfall reduces reliance on energy- and water-intensive snowmaking.
• Agriculture and hydropower. Both depend on predictable seasonal water volume.
• Ecosystem and lake levels. Watershed inflow affects terminal lakes and the habitats tied to them.

Alto Cirra is developing cloud seeding as an autonomous aviation operation. The approach pairs uncrewed aircraft with atmospheric analysis so that seeding material is released at a known place, at a known time, under measured conditions, and so each mission is designed to produce a record rather than an assumption.

Read more about drone-based cloud seeding, our work in Utah and the Wasatch, or the cloud seeding FAQ.