ice nucleus Crystalline, microscopic particle, either of ice or some other substance, which can induce the growth of ice crystals upon it from saturated air in cloud at temperatures of about −25°C or below. Deposition of water in this way—directly from vapour to form ice, without forming liquid in between—is called ‘sublimation’. During growth, splintering of the crystals (e.g. as a result of updrafts) causes large numbers of new nuclei to be formed. Compare FREEZING NUCLEI.

ice nucleus A crystalline, microscopic particle, of ice or some other substance, which can induce the growth of ice crystals upon it from saturated air in cloud at temperatures of about −25°C or below. Deposition of water directly from vapour to form ice is called ‘sublimation’. During growth, splintering of the crystals (e.g. as a result of updrafts) causes large numbers of new nuclei to be formed. Compare freezing nuclei.