in calm water, the first sea ice to form on the surface is a skim of separate crystals which initially are in the form of tiny discs, floating flat on the surface and of diameter less than . each disc has its c-axis vertical and grows outwards laterally. at a certain point such a disc shape becomes unstable, and the growing isolated crystals take on a hexagonal, stellar form, with long fragile arms stretching out over the surface. these crystals also have their c-axis vertical. the dendritic arms are very fragile, and soon break off, leaving a mixture of discs and arm fragments. with any kind of turbulence in the water, these fragments break up further into random-shaped small crystals which form a suspension of increasing density in the surface water, an ice type called frazil or grease ice. in quiet conditions the frazil crystals soon freeze together to form a continuous thin sheet of young ice; in its early stages, when it is still transparent – that is the ice called "nilas". once nilas has formed, a quite different growth process occurs, in which water freezes on to the bottom of the existing ice sheet, a process called "congelation" growth. this growth process yields first-year ice.