Development of sea ice

Frazil ice built up at the shore amongst rocks
Frazil ice (Photo: Frederique Olivier)
A large iceberg in background with water in foreground which shows grease ice formingSun reflecting off water and slushy ice known as frazil icePancake ice in the Southern Ocean taken from aboveQuad sinking into the slush on the fast iceAn accumulation of spongy white lumps of ice, a few centimetres acrossFingers of thin nilas ice covering dark blue water viewed from the airSea ice meeting in the open water and melting

Sea ice development takes place over several stages, and comprises different subtypes of ice forms which change depending on time and environmental conditions.

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. 

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 an oily 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 sea a matt appearance.

Grease ice behaves in a viscous fluid-like manner, and does not form distinct ice floes. 

Slush

Snow which is saturated and mixed with water on land or ice surfaces, or as a viscous floating mass in water after a heavy snowfall.

Shuga

An accumulation of spongy white ice lumps, a few centimetres across; they are formed from grease ice or slush and sometimes from anchor ice rising to the surface. 

Nilas

A thin elastic crust of ice, easily bending on waves and swell and under pressure, thrusting in a pattern of interlocking 'fingers' (finger rafting). Has a matt surface and is up to 10 cm in thickness. May be subdivided into dark nilas and light nilas. 

Dark nilas

Nilas which is under 5 cm in thickness and is very dark in colour.

Light nilas

Nilas which is more than 5 cm in thickness and rather lighter in colour than dark nilas.

Ice rind

A brittle shiny crust of ice formed on a quiet surface by direct freezing or from grease ice, usually in water of low salinity. Thickness to about 5 cm. Easily broken by wind or swell, commonly breaking in rectangular pieces.

Pancake ice

Predominantly circular pieces of ice from 30 cm - 3 m in diameter, and up to about 10 cm in thickness, with raised rims due to the pieces striking against one another. It may be formed 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, from where it floats to the surface; its appearance may rapidly cover wide areas of water. 

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. 

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.

First-year ice

Sea ice of not more than one winter's growth, developing from young ice; thickness 30 cm - 2 m. May be subdivided into thin first-year ice/white ice, medium first-year ice and thick first-year ice.

Thin first-year ice / white ice

First-year ice 30-70 cm thick

Thin first-year ice / white ice first stage

30-50 cm thick. 

Thin first-year ice / white ice second stage

50-70 cm thick.

Medium first-year ice

First-year ice 70-120 cm thick.

Thick first-year ice

First-year ice over 120 cm thick.

Old ice

Sea ice which has survived at least one summer's melt; typical thickness up to 3m or more. Most topographic features are smoother than on first-year ice. May be subdivided into residual, second-year ice and multi-year ice.

Residual ice

First-year ice that has survived the summer’s melt and is now in the new cycle of growth. It is 30 to 180 cm thick depending on the region where it was in summer. After 1 January (in the Southern hemisphere after 1 July), this ice is called second-year ice. 

Second-year ice

Old ice which has survived only one summer's melt; typical thickness up to 2.5 m and sometimes more. Because it is thicker than first-year ice, it stands higher out of the water. In contrast to multi-year ice, summer melting produces a regular pattern of numerous small puddles. Bare patches and puddles are usually greenish-blue.  

Multi-year ice

Old ice up to 3 m or more thick which has survived at least two summers' melt. Hummocks (hillocks of broken ice that have been forced up by pressure) even smoother than in second-year ice, and the ice is almost salt-free. Colour, where bare, is usually blue. Melt pattern consists of large interconnecting irregular puddles and a well-developed drainage system.