According to the Glossary of Glacier Mass Balance and Related Terms, sea ice is ice formed at the sea surface by the freezing of sea water. Because the oceans consist of salt water, this occurs below the freezing point of pure water, at about -1.8°C (28.8°F). Where it forms ridges, sea ice can be up to several metres thick (Cogley et al., 2011).
Sea ice loses much of its salt during the process of formation from sea water: sea ice becomes less salty the longer it remains frozen, thus old sea ice (multi-year ice) is more like frozen fresh water than salt water. This occurs in the Arctic Ocean where sea ice can remain frozen for several years before it melts again.
Sea ice may be contrasted with icebergs, which are chunks of ice shelves or glaciers that calve into the ocean. Icebergs are compacted snow and hence are fresh water.
References
Sea Ice Links
Material on this page was edited and provided by Maren Pauly, Department of Geography, University of Waterloo. For photograph references, hover over image.