tiny suspended particle, either solid or liquid, upon which water vapour condensation begins in the atmosphere. Its diameter may range from a few microns to a few tenths of a micron (one micron equals 10^-4 centimetre). There are much smaller nuclei in the atmosphere, called Aitken nuclei, but they ordinarily play no role in cloud formation because they do not induce condensation unless the air is highly supersaturated with water vapour. Nuclei that have diameters of several microns and are composed of a hygroscopic, or moisture-attracting, substance (e.g., sea salt) are called giant condensation nuclei.

Most condensation nuclei are produced by natural and man-made fires over land and by wave action over the oceans. When mixed with the more hygroscopic material, dust and soil particles blown into the atmosphere also are sources of nuclei. See also nucleation.