Cloud condensation nuclei

Cloud condensation nuclei (CCNs), also known as cloud seeds, are small particles typically 0.2 µm, or 1/100 the size of a cloud droplet on which water vapor condenses.

Water requires a non-gaseous surface to make the transition from a vapor to a liquid; this process is called condensation.

In the atmosphere of , this surface presents itself as tiny solid or liquid particles called CCNs.

When no CCNs are present, vapor can be supercooled at about −13 °C (9 °F) for 5–6 hours before droplets spontaneously form.

In above-freezing temperatures, the air would have to be supersaturated to around 400% before the droplets could form.