Small particles with a diameter of about 0.1 nanometres (nm) - 1 µm, on which oversaturated water vapour settles, are called nuclei of condensation. Because of their small size, nuclei of condensation no longer obey Stokes’ law of precipitation. They are regarded as bodies dissolved in air and the result is described as a colloidal or divisible system. Nuclei of condensation are not formed as a result of mechanical break up but of chemical or physical processes. Smoke and fog are such colloidal systems, where smoke is based upon the spread of solid particles in air, and fog on that of liquid ones. Nuclei of condensation, which are charged either positively or negatively, are described as ions. The number of nuclei varies greatly from 0 - 4 x 102 nuclei per m3.
