Cloud condensation nuclei or CCNs (also known as cloud seeds) are small particles typically 0.2 µm, or 1/100th the size of a cloud droplet on which water vapour condenses. Water requires a non-gaseous surface to make the transition from a vapour to a liquid; this process is called condensation. In the atmosphere, this surface presents itself as tiny solid or liquid particles called CCNs. When no CCNs are present, water vapour can be supercooled at about -13°C (8°F) for 5-6 hours before droplets spontaneously form (this is the basis of the cloud chamber for detecting subatomic particles). In above freezing temperatures the air would have to be supersaturated to around 400% before the droplets could form. The concept of cloud condensation nuclei is used in cloud seeding, that tries to encourage rainfall by seeding the air with condensation nuclei. It has further been suggested that creating such nuclei could be used for marine cloud brightening, a climate engineering technique.

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