Particles, either liquid or solid, or ions upon which condensation of water vapour (or other substances) begins in the atmosphere. Condensation nuclei are usually very small hygroscopic aerosols (\(0.001\ \unicode[Times]{x3BC}\text{m}\) to \(0.1\ \unicode[Times]{x3BC}\text{m}\) in diameter), but these are not as abundant as the smaller particles. The number of CN which are active (initiate condensation) in a given air mass may be a function of the @R05267@. Usually CN are counted as the active nuclei at about 300% @R05267@, while cloud condensation nuclei (CCN) are counted as the number of active nuclei at @R05267@ less than or equal to 102%.