Condensation nucleus

From Glossary of Meteorology

condensation nucleus

An aerosol particle forming a center for condensation under extremely high supersaturations (up to 400% for water, but below that required to activate small ions).

High supersaturations are produced in a condensation nuclei counter (often using a condensate other than water) to measure the concentration of such particles. Supersaturations greater than a few percent are rarely produced in clouds in the atmosphere but may exist in aircraft exhaust and over hot springs. Nuclei only active at such high suersaturations may coagulate to form larger particles that become condensation nuclei.
Compare cloud condensation nuclei.