condensation nucleus[‚kän·dən′sā·shən ′nü·klē·əs]
a very small neutral or charged particle on which water vapor condenses. Without condensation nuclei, the condensation of water vapor and the formation of clouds in the atmosphere would not be possible.
Serving as condensation nuclei are hygroscopic particles that contain chlorides, sulfites, sulfides, nitrates, and nitrites. The particles range in size from 10–7 to 10–5 cm, and they are found in concentrations averaging 103 per cu cm over oceans, 104 per cu cm over rural landscapes, and approximately 1.5 × 105 per cu cm over cities; the concentration of condensation nuclei usually decreases with increasing elevation.