condensation nuclei

Microscopic atmospheric particles, which attract water droplets, and which may then coalesce to form a raindrop. They may be specks of dust or clay, or particles formed from industrial processes (e.g. sulphur dioxide, sulphur trioxide), known to American meteorologists as combustion nuclei. Condensation nuclei are vital in cloud formation, since condensation only occurs spontaneously when relative humidity exceeds 400%. However, in accordance with Raoult's law, soluble condensation nuclei will lower saturation vapour pressure enough for condensation to occur. In the Bergeron-Findeisen theory, ice crystals act as condensation nuclei, initiating condensation in unsaturated air. See also cloud seeding, hygroscopic nuclei.