Supercooled liquid droplets drift in a rising column of air. On their own they are far too light to fall — their terminal velocity never exceeds the updraft, so they stay suspended as cloud.
Silver-iodide aerosol released from the "seed reagent" acts as an ice-nucleus: any droplet that touches an aerosol particle has a chance to freeze, weighted by how close the cloud temperature is to AgI's effective range (roughly -6°C to -20°C, peaking near -12°C).
- Bergeron-Findeisen growth: once frozen, a crystal's mass grows at a rate proportional to the supersaturation, because the saturation vapor pressure over ice is lower than over liquid water — vapor deposits onto ice at the expense of the surrounding droplets.
- Fallout condition: terminal velocity scales with mass0.6. A crystal only precipitates once its fall speed exceeds the updraft holding it aloft.
Turn seeding rate to zero and watch precipitation stop almost entirely — that is the effect real cloud-seeding operations are trying to trigger on demand.