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.
