Microscopic particle (e.g. of sulphur dioxide, salt, dust, or smoke) in the free air, on which water vapour may condense to form droplets. Aerosols that are soluble in water (e.g. salt or sulphuric acid) can induce condensation in unsaturated air, e.g. salt nuclei can induce it at a relative humidity of less than 80%. The size of nuclei may be from 0.001 μ‎m to more than 10 μ‎m (i.e. ‘giant’ nuclei such as particles of sea-salt). See also aitken nuclei; nucleus.