freezing nuclei Any nuclei, commonly ice crystals but sometimes the suitably shaped crystals of other substances, that when present in clouds at temperatures below 0°C will cause any supercooled droplets with which they collide to change to ice (in the form of a crystalline growth upon the nucleus). See also NUCLEUS. Compare ICE NUCLEUS.

freezing nuclei Any nuclei, commonly ice crystals but sometimes the suitably shaped crystals of other substances, which, when present in clouds at temperatures below 0°C, will cause any supercooled droplets with which they collide to change to ice (in the form of a crystalline growth upon the nucleus).