The formation of sea ice influences the global climate and the circulation of deep ocean waters. It occurs primarily along the sea ice pack margin or continental coastlines and in openings within the ice pack. Winds increase the rates of ice formation by opening gaps in the ice cover and maintaining open water conditions through sea ice drift and deformation. The seasonal cycle of sea ice formation is controlled by the amount of incoming solar radiation at higher latitudes. Before ice crystals can begin forming, seawater must attain a subzero temperature.
Freshwater freezes at a higher temperature than seawater.
The surface of the ocean is chilled by the polar air, but it is not just the air-sea contact that must be chilled to the freezing point. As surface ocean water cools and increases in density, it sinks and mixes with the slightly warmer and deeper water that lies immediately below. Without the strong density gradient in the surface waters, the mixed layer would progressively deepen as it cooled. Once the surface waters are chilled to the freezing point, the very first ice crystals begin to form. Microscopic spheres of pure ice form first.