ICE

solid substance produced by the freezing of water vapour or liquid water. At temperatures below 0 C (32 F), water vapour develops into frost at ground level and snowflakes (each of which consists of a single ice crystal) in clouds. Below the same temperature, liquid water forms a solid, as, for example, river ice, sea ice, hail, and ice produced commercially or in household refridgerators. Ice occurs on the Earth's continents and surface waters in a variety of forms. Most notable are the continental glaciers (ice sheets) that cover much of Antarctica and Greenland. Smaller masses of perennial ice called ice caps occupy parts of Arctic Canada and other high-latitude regions, and mountain glaciers occur in more restricted areas, such as mountain valleys and the flatlands below. Other occurrences of ice on land include the different types of ground ice associated with permafrostthat is, permanently frozen soil common to very cold regions. In the oceanic waters of the polar regions, icebergs occur when large masses of ice break off from glaciers or ice shelves and drift away. The freezing of seawater in these regions results in the formation of sheets of sea ice known as pack ice. During the winter months similar ice bodies form on lakes and rivers in many parts of the world. This article treats the structure and properties of ice in general. Ice in lakes and rivers, glaciers, icebergs, pack ice, and permafrost are treated separately in articles under their respective titles. For a detailed account of the widespread occurrences of glacial ice during the Earth's past, see the articles geochronology and climate. See also glacial landform for the effects of glaciation. solid substance produced by the freezing of water vapour or liquid water. At temperatures below 0 C (32 F), water vapour develops into frost at ground level and snowflakes (each of which consists of a single ice crystal) in clouds. Below the same temperature, liquid water forms a solid, as, for example, river ice, sea ice, hail, and ice produced commercially or in household refrigerators. See also glacier; pack ice; permafrost. Ice consists of compact aggregates of many crystals. This fact, however, is not readily apparent, because ice crystals grown from liquid water, unlike those from water vapour, do not develop crystal faces. Typical specimens of ice derived from liquid water have crystals that measure roughly 1 to 20 mm (0.04 to 0.78 inch) in size. In old glacier ice, the constituent crystals have grown larger by long-continued recrystallization and reach about 50 cm (20 inches) in diameter. To melt one gram (0.04 ounce) of ice, 79.8 calories of heati.e., the latent heat of fusionmust be supplied. This is high compared with the latent heat of most substances and makes ice useful as a refrigerating agent or heat sink. Melting ice remains at a constant temperature of 0 C. Ice has a density of 0.919 g/cm-3, as opposed to 0.9998 g/cm-3 for water. Consequently, a mass of ice occupies 9 percent more volume than an equal mass of water at 0 C. This condition accounts for the bursting of water pipes on freezing and for the fact that ice floats in water, with about 1/10 of its volume above the surface. Because of the increase in volume on freezing, the melting point of ice decreases under pressure. It falls by 0.0075 C per atmosphere of applied pressure. The low sliding friction of ice surfaces, as witnessed in ice skating and in the skidding of automobiles on icy roads, results from melting and the consequent lubrication that occurs where the sliding object increases the pressure on the ice. Additional reading Samuel C. Colbeck (ed.), Dynamics of Snow and Ice Masses (1980), includes chapters on valley glaciers, ice sheets, snow packs, icebergs, sea ice, and avalanches, emphasizing the basic physics. P.V. Hobbs, Ice Physics (1974), treats all aspects of the physics and chemistry of ice. W. Richard Peltier (ed.), Ice in the Climate System (1993), is a modern review of the past, present, and future interactions between ice and climate. George D. Ashton