Bacteria in the atmosphere, for instance, work as nucleation sites for water droplets. (judithcurry.com)
In contrast, if freezing is slow, the crystal growth will be slower with few nucleation sites resulting in larger ice crystals. (fao.org)
This state is reached because of a lack of nucleation sites — microscopic locations where the atoms can begin to arrange into a lattice. (sciencedaily.com)