Small particle of an atmospheric impurity (e.g. salt, dust, or smoke), which provides surfaces for the condensation of water. Condensation nuclei vary in size from about 0.1 μ‎m to more than 3 μ‎m. Some nuclei, e.g. salt and acid particles, can encourage condensation at a relative humidity well below 100%. See also aitken nucleus; hygroscopic nuclei.