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			Sea Ice Naval Meterological and Oceanography Command 03.19.01 The polar regions possess the surreal beauty of science fiction scenery. Miles of snow deserts. Enormous fortress-like icebergs. The reality of sea ice, however, lies in its threat to shipping lanes. In 1912, the ocean liner Titanic struck an iceberg near Greenland and sank with 1,500 passengers aboard. The ship billed as "unsinkable" never completed its first - and last - voyage. Sea ice is commonly found in the Arctic and Antarctic where low air and water temperatures provide ideal conditions for ice formation and growth. However, currents have pushed ice to such unlikely locales as Bermuda. Salinity, the amount of salt in seawater, determines the temperature at which sea water freezes. Although sea water is usually very salty, sea ice is practically salt-free. Why is this? When the water temperature dips to its freezing point (about 28.6°F), tiny ice particles form. Salt is not part of the ice crystals' molecular structure, so it is gradually released into the surrounding water. However, in extremely cold water, ice freezes rapidly, so salt cells are sometimes trapped before they are discarded. The cells then "migrate" through the ice, a process which takes up to a year. As ice crystals multiply, the water turns slushy and the surface freezes into thin sheets of ice. Wind and wave action break the sheets into pieces, push them together, and freeze them into thick solid masses of pack ice. Depending on the severity of the winter, the pack's thickness is usually nine to ten feet. Pieces breaking from the pack are called ice floes. These present the greatest threat to ships transiting through polar waters. Not all ice in the sea is spawned by ocean pack ice. Some icebergs break away from glaciers and floating ice shelves on land masses. Land ice is comprised entirely of fresh water and may be hundreds of feet thick. Pinnacle icebergs are irregularly shaped and usually originate from glaciers in the Northern Hemisphere. Icebergs in the Antarctic are much larger, more uniformly shaped, and flat-topped. The Arctic produces 10,000 to 50,000 icebergs annually. The amount produced in the Antarctic regions is inestimable. Icebergs normally have a four-year life-span; they begin entering shipping lanes after about three years.


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