SUPERSATURATION AND TIME DEPENDENCE OF ICE NUCLEATION FROM THE VAPOR ON SINGLE CRYSTAL SUBSTRATES.

Abstract

Nucleation of individual ice crystals on large (3. 0 mm) cleaved crystals of solution-grown silver iodide and covellite is investigated by microscopy. Ice crystals do not usually appear immediately when a supersaturation is imposed. Nucleation, defined as the appearance of crystals of 1 mu m radius, is delayed between zero and 70 s near water saturation and between 20 and 400 s at a few percent ice supersaturation, the longer times occurring at higher temperature. This time decreases only marginally when the crystal is exposed to a period of higher supersaturation which ends a few seconds prior to the time crystals would appear at this higher value. The number of crystals per unit area increases with ice supersaturation at a given temperature. The absolute crystal concentration varies over the substrate surface. Results are applied to crystal nucleation in the atmosphere and the characterization of ice nuclei in laboratory instruments.