Water in solid state. Ice commonly occurs as hexagonal crystals. In Permafrost regions, Ice may occupy voids in soils and rocks and may develop in a variety of forms. Ice may be colourless to pale blu
e or greenish-blue. It may appear white due to included gas bubbles; in exposures, Ground Ice may also appear black.
Water substance in the solid phase. Ice can occur in many forms. At and near the Earth's surface, ice always crystallizes in the hexagonal system. This phase is designated ice Ih, the Roman numeral I
distinguishing it from more than a dozen other phases and the letter h distinguishing it from the metastable cubic phase ice Ic. See, among other articles, glacier ice and diamond dust.
The solid form of water in nature formed either by: (a) the freezing of water, (b) the condensation of atmospheric water vapour direct into ice crystals, (c) the compaction of snow with or without the
motion of a glacier, or (d) the impregnation of porous snow masses with water which subsequently freezes.
The solid, crystalline form of water substance; it is found in the atmosphere as snow crystals, hail, ice pellets, etc., and on the earth's surface in forms such as hoarfrost, rime, glaze, sea ice, gl
acier ice, ground ice, frazil, anchor ice, etc. This form of water is, strictly speaking, called ice I, the only one of the several known modifications of solid water substance that is stable at commonly occurring temperatures and pressures. (Some of the other forms have very unusual properties, ice VII, for example, being stable only at pressures above 22 400 kg/cm^2, but then existing at temperatures up to about 100C.) Ice has an open structure because the water molecules bond to their neighbors covalently only in four directions; it therefore floats on higher density water, where broken molecular bonds permit closer packing. All commonly occurring forms of ice are crystalline, although large single crystals are relatively rare except in glaciers. The ice crystal lattice possesses hexagonal symmetry that manifests itself in the gross forms of such single crystals as are sometimes found in snow. At an air pressure of one atmosphere, ice melts at 0C by definition of the Celsius temperature scale. (Strictly at equilibrium among water, ice, and vapor occurs at +0.01C, the triple point.) On the other hand, ice does not invariably form in liquid water cooled below this temperature; it has a tendency to supercool, more so in the absence of ice nuclei.
The process by which a layer of ice (icing) builds up on solid objects that are exposed to freezing precipitation or to supercooled fog or cloud droplets. At the earth's surface this usually refers to
glaze formation, and the amount of ice can be roughly measured by an ice-accretion indicator. For airborne objects, ice accretion refers to any type of aircraft icing.
An instrument used to detect the occurrence of freezing precipitation. It usually consists of a strip of sheet aluminum about 4 cm (1.5 in.) wide and is exposed horizontally, faceup, in the free air a
bout 1 m above the ground.
A cold period in Earth history when large ice sheets extend from the polar regions into temperate latitudes. The term is sometimes used synonymously with "glacial period" or "glacial period", or embra
ces several such periods to define a major phase of cold climate in Earth's climatic history.
A major interval of geologic time during which extensive ice sheets (continental glaciers) formed over many parts of the world. The best known ice ages are 1) the Huronian in Canada, occurring very ea
rly in the Proterozoic era (2700-1800 million years ago); 2) the pre-Cambrian and early Cambrian, which occurred in the early Paleozoic era (about 540 million years ago) and left traces widely scattered over the world; and 3) the Permo-Carboniferous, occurring during the late Paleozoic era (from 290 million years ago), which was extensively developed on Gondwana, a large continent comprising what is now India, South America, Australia, Antarctica, Africa, and portions of Asia and North America. The term ice age is also applied to advances and retreats of glaciers during the Quaternary era.