PACK ICE

floating mass of ice formed from seawater in the Earth's polar regions. Pack ice expands during winter to cover about 5 percent of the northern oceans and 8 percent of the southern oceans. When melting occurs in spring and summer, the margins of the pack ice retreat. As seawater freezes, minute pools of salty water called brine pockets are entrapped. Once an ice field has formed, brine pockets migrate downward through the ice block in response to gravity and thermal gradients. After a few weeks to a few months, the surface of the ice becomes lower in salt content than the deeper layers. In the summer, when the ice temperature rises, there is a rapid increase in the migration of salt out of the ice. The sea-ice surface becomes potable and, in fact, is used by Eskimos as a source of fresh water. Also characteristic of frozen salt water is that, even after freezing, the water below the ice will continue to turn over, or circulate. As the surface water near the ice becomes colder, it becomes heavier and sinks, resulting in a continuous turnover or vertical circulation of the water beneath the ice. In the Northern Hemisphere during September and October, the air temperature lowers sufficiently to form a thin sheet of ice on some polar seas. The freezing temperature for average northern ocean water of about 3.5 percent salt composition by weight (usually designated 35 parts per thousand) is -1.8 C (28.7 F). Initially the ice film is entirely fresh, but as more ice crystals form, brine pockets become entrapped between lamellae (very fine layers) of tiny ice plates. Owing to slight breezes and water motion, the thin sheets of ice jostle about and, after a few hours, form a field of ice paddies; these disks of ice are known as pancake ice. If the temperature remains below freezing, the pancake ice coalesces as more ice forms, and within a few days the ice cover can be 810 cm (34 inches) thick with a slightly corrugated surface. As seawater continues to freeze at the bottom edges and sides of ice floes and fields, pressures associated with the stresses and strains caused by water and wind movement result in a hummocking and ridge development in some places and open water in others. With alternating freezing, partial melting, snow, and wind and swell, the ice field develops over a matter of a few weeks to a month into ocean cover 1560 cm (6 inches2 feet) deep. At this point the ice field is still navigable by most large vessels; however, if a vessel finds itself in the far north two weeks after the commencement of active surface freezing (in late October), it is in peril of being locked in for the remainder of the winter. The pack ice of the Northern Hemisphere covers an average area of about 10,600,000 square km (4,100,000 square miles), filling the Arctic Ocean Basin and adjacent North Atlantic Ocean. The polar ice field consists of about 4,700,000 square km (1,800,000 square miles) of 36-metre- (1020-foot-) thick polar ice that never melts. The motion of the polar ice follows a giant clockwise eddy with a centre at approximately 85 N, 170 W. Together with Arctic Basin seasonal sea ice, this Arctic pack exits into the northern Atlantic through two ice streams. The major exit of drifting pack ice from the Arctic Basin is along the eastern side of Greenland. The second icy arm of the north consists of a discharge through the Canadian-Arctic Archipelago and along the eastern North American shore. Approximately twice as much pack ice forms in the oceans surrounding Antarctica as is found in the Arctic. The maximum area of Antarctic pack ice is about 20,000,000 square km (7,700,000 square miles). The Antarctic pack ice forms a fairly constant band of drifting sea ice around the continent in a giant clockwise eddy, with the farthest northern extent occurring at the end of the austral (southern) winter in October. The greatest extension of pack ice in the South Pacific sector is found in about latitude 62 S, and in the South Atlantic pack ice extends to 52 S. floating mass of ice formed from seawater in the Earth's polar regions. Pack ice expands during winter to cover about 5 percent of the northern oceans and 8 percent of the southern oceans. When melting occurs in spring and summer, the margins of the pack ice retreat. Additional reading The published literature on icebergs is found mainly in U.S. journals and in atlases prepared by the U.S. Coast Guard and U.S. Navy for Arctic and Antarctic military and scientific maritime resupply expeditions. A classic discussion of icebergs and sea ice in the Arctic Ocean is The Marion Expedition to Davis Strait and Baffin Bay, vol. 3 by E.H. Smith, Arctic Ice (1931). Also of interest is the treatment of the oceanography of the north polar basin by Fridtjof Nansen, The Norwegian North Polar Expedition, 18931896: Scientific Results, vol. 3 (1902, reprinted 1969), and Farthest North, 2 vol. (1897, reissued 1967). The importance of ice in the water budget of the planet may be studied by consulting James L. Dyson, The World of Ice (1962, reissued 1972). Robert C. Pritchard (ed.), Sea Ice Processes and Models (1980), a collection of proceedings papers, reports field observations and the development of models in an effort to establish usable flow laws for sea ice. Two reference works are United States Hydrographic Office, A Functional Glossary of Ice Terminology (1952); and World Meteorological Organization, Sea-Ice Nomenclature (1970). Thomas F. Budinger The Editors of the Encyclopdia Britannica