label |
Definition |
Brash ice |
Accumulations of floating ice made up of fragments not more than 2m across; the wreckage of other forms of ice. Brash is common between colliding floes or in regions where pressure ridges have collapsed. |
Brown ice |
Brown sea ice occurs when algal material within the ice becomes highly concentrated. This may occur in any layer of the ice. |
Fast ice |
Sea ice which forms and remains fast along the coast, where it is attached to the shore, to an ice wall, to an ice front, between shoals or grounded icebergs. Vertical fluctuations may be observed during changes in sea level. Fast ice may be formedin situfrom sea water or by freezing of pack ice of any age to the shore, and it may extend up to several hundred kilometres from the coast. Fast ice may be more than one year old and may then be prefixed with the appropriate age category (old, second-year, or multi-year). |
First-year ice |
Sea ice of not more than one winter's growth, developing from young ice; thickness (typically) 30 cm ? 2 m. May be subdivided into thin first-year ice/white ice, medium first-year ice and thick first-year ice. |
Floe |
A floe is any contiguous piece of sea ice. Floes may be described in terms of several size categories: Vast: >2 km across, Large: 500-2000 m across, Medium: 100-500 m across, Small: 20-100 m across. Floes less than 20 m across are called "cake ice". |
Flooding |
Flooding of the ice surface may occur in summer due to intense incoming radiation, particularly if the ice is snow free. |
Fracture |
Any break or rupture through very close pack ice, compact pack ice, consolidated pack ice, fast ice, or a single floe resulting from deformation processes. Fractures may contain brash ice and/or be covered with nilas and/or young ice. Length may vary from metres to kilometres. Fractures are narrower than leads and do not aid the navigation of surface vessels. |
Frazil ice |
Fine spicules or plates of ice, suspended in water. Frazil ice formation represents the first stage of sea ice growth. The frazil crystals are usually suspended in the top few centimetres of the surface layer of the ocean and give the water a soupy appearance. In the open ocean the crystals may form or be stirred to a depth of several metres by wave induced turbulence. |
Grease ice |
A later stage of freezing than frazil ice when the crystals have coagulated to form a soupy layer on the surface. Grease ice reflects little light, giving the surface a matt appearance. Grease ice behaves in a viscous fluid-like manner, and does not form distinct ice floes. |
Grey-white ice |
Grey Ice: Young ice 10?15 cm thick. Less elastic than nilas and breaks on swell. Usually rafts under pressure. Grey-white Ice: Young ice 15?30 cm thick. Under pressure more likely to ridge than to raft. |
Hummock |
Point scale roughness features caused by the convergence of ice floes. Similar to ridges but not linear. |
Lead |
Any fracture or passage-way through sea ice which is navigable by surface vessels. A more general description of a lead is an area of open water or new ice between ice floes, although the term is generally applied to linear features. If the open area is very large it may be called a polynya. A lead between the shore and the pack ice is called a coastal lead or shore lead, and a lead between the fast ice and the pack ice is called a flaw lead. |
Multi-year ice |
Old ice up to 3 m or more thick which has survived at least two summers' melt. Hummocks are typically smoother than in second-year ice, and the ice is almost salt-free. Colour, where bare, is usually blue. |
New ice |
A general term for recently formed ice which includes frazil ice, grease ice and shuga. |
Nilas |
A thin elastic crust of ice, easily bending on waves and swell and under pressure, thrusting in a pattern of interlocking |
Pack ice |
Term used in a wide sense to include any area of sea ice, other than fast ice, no matter what form it takes or how it is disposed. The pack can be described as: ?very open - with an ice concentration of 1/10 to 3/10 ?open - 4/10 to 6/10, with many leads and polynyas and the floes generally not in contact with one another ?close - 7/10 to 8/10, composed of floes mostly in contact ?very close - 9/10 to less than 10/10 ?compact - 10/10, with no water visible, called consolidated pack ice if the floes are frozen together |
Pancake ice |
Predominantly circular pieces of ice from 30cm - 3m in diameter, and up to 10 cm in thickness (unrafted). Pancakes often have raised rims due to the pieces striking against one another. Pancake ice may be formed on a slight swell from grease ice, shuga or slush or as the result of the breaking of ice rind, nilas or, under severe conditions of swell or waves, of grey ice. |
Rafting |
Pressure process whereby one piece of ice over-rides another. Most common in new and young ice. |
Second-year ice |
Second year ice is the most common form of old ice in Antarctica. It is old ice that has survived only one summer. Because the snow cover does not completely melt during summer, second year ice typically has much thicker snow than first year ice. |
Shuga |
An accumulation of spongy white lumps of ice, a few centimetres across; they are formed from grease ice or slush and sometimes from anchor ice rising to the surface. |
Slush |
Snow which is saturated and mixed with water on land or ice surfaces, or as a viscous floating mass in water after heavy snowfall. |
Young ice |
Ice in the transition stage between nilas and first-year ice, 10-30 cm in thickness. May be subdivided into grey ice and grey-white ice. |