Sea ice: types and forms
Sea ice comes in a variety of types and forms, depending on the stage of development and the meteorological, atmospheric, and other physical conditions present.
Sea Ice Types
There are various types of sea ice, according to its stage of development. Within each stage below, various sub-types also exist, depending on the internal structure of the ice. Please refer to the Glossary for more details.
New Ice: A general term for recently formed ice which includes frazil ice, grease ice, slush and shuga. These types of ice are composed of ice crystals which are only weakly frozen together (if at all) and have a definite form only while they are afloat.
Nilas: A thin elastic crust of ice, easily bending on waves and swell and under pressure growing in a pattern of interlocking “fingers” (finger rafting). Nilas has a matte surface and is up to 10 cm in thickness and may be subdivided into dark nilas and light nilas.
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.
First-year Ice: Sea ice of not more than one winter’s growth, developing from young ice; 30 cm or greater. It may be subdivided into thin first-year ice - sometimes referred to as white ice -, medium first-year ice and thick first-year ice.
Old Ice: Sea ice which has survived at least one summer’s melt. Topographic features generally are smoother than first-year ice. It maybe subdivided into second-year ice and multiyear ice.
Sea Ice Forms
Ice can take on many forms, depending on external conditions and other physical considerations. Except in sheltered waters, an even sheet of ice seldom forms immediately:
The thickening slush breaks up into separate masses under wind and wave action, the masses taking on a characteristic pancake form due to the fragments colliding with each other. The slush layer dampens down the waves, and if freezing continues, the pancakes will adhere together, forming a continuous sheet. Here are some of the more common forms of ice:
Pancake Ice: Predominantly circular pieces of ice 30 cm to 3 m in diameter, up to 10 cm in thickness, with raised rims due to the pieces striking against one another. It may form on a slight swell from grease ice, shuga or slush or as a result of the breaking of ice rind, nilas or, under severe conditions of swell or waves, of grey ice. It also sometimes forms at some depth at an interface between water bodies of different physical characteristics where it floats to the surface. It may rapidly form over wide areas of water.
Brash Ice: Accumulation of floating ice made up of fragments not more than 2 m across, the wreckage of other forms of ice.
Ice Cake: Any relatively flat piece of ice less than 20 m across.
Floe: Any relatively flat piece of ice 20 m or more across. Floes are subdivided according to horizontal extent as follows:
- Small: 20-100 m across
- Medium: 100-500 m across
- Big: 500-2,000 m across
- Vast: 2-10 km across
- Giant: Greater than 10 km across
Fast Ice: Ice which forms and remains fast along the coast. It may be attached to the shore, to an ice wall, to an ice front, between shoals or grounded icebergs. Vertical fluctuations may be observed during changes of sea level. It may be formed “in-situ” from water or by freezing of floating ice of any age to shore and can extend a few metres or several hundred kilometres from the coast. It may be more than one year old in which case it may be prefixed with the appropriate age category (old, second-year or multi-year). If higher than 2 m above sea level, it is called an ice shelf.
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