Greatly increased utilization and study of high latitudes by both military forces and civilian scientist research groups has brought to attention the phenomenon of floating sea ice. Supply of outposts by ship, maintenance of observing stations on floating ice masses, and submarine operation in the polar sea are activities influenced by sea ice. The oceanographer distinguishes sea ice, formed by direct freezing of ocean water, from icebergs, and ice islands, which are bodies of land ice broken free from tide-level glaciers and continental, ice shelves. Aside from differences in origin, a major difference between sea ice and floating masses of land ice is in thickness. Sea ice, which begins to form when the surface water is cooled to temperatures of about 28 1/2°F(-2°C) is limited in thickness to about 15 ft (5 m).
Pack ice is the name given to ice that completely covers the sea surface. Under the forces of wind and currents, pack ice breaks up into individual patches, termed ice floes. The narrow strips of open water between such floes are leads. Where ice floes are forcibly brought together by winds, the ice margins buckle and turn upward into pressure ridges resembling walls or irregular hummocks. The difficulties of travel on foot across the polar sea ice are made extreme by the presence of such obstacles. The surface zone of sea ice is composed of fresh water, the salt having been excluded in the process of freezing.
The Arctic Ocean, which is surrounded by landmasses, is normally covered by pack ice throughout the year, although open leads are numerous in the summer. The relatively warm North Atlantic drift maintains an ice-free zone off the northern coast of Norway. The situation is quite different in the Antarctic, where a vast open ocean bounds the sea ice zone on the equatorward margin. Because the ice floes can drift freely north into warmer waters, the Antarctic ice pack does not spread far beyond 60°S lat. in the cold season. In March, close to the end of the warm season, the ice margin shrinks to a narrow zone bordering the Antarctic continent.
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