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Sea Ice Glossary

Arctic ice can be divided into two different types, classified by their method of formation. Icebergs are chunks of floating ice that have "calved" (broken off) from a glacier. Since they are formed from compacted snow, they are composed entirely of fresh water, like big floating ice cubes. The Greenland ice sheet and the Ellesmere, Devon, and Baffin Island glaciers in Canada account for the vast majority of arctic bergs. Since 90% of an iceberg is below the surface of the water, they travel with ocean currents and not the winds. Most bergs formed in Canada and Greenland stay in the eastern Arctic, eventually being carried into the Atlantic via Davis Strait and melting away.

The other kind of arctic ice is pack ice. This kind of ice is formed by the freezing of seawater. Seawater, because of the salt it contains, freezes in a different manner compared to fresh water. The density of seawater steadily increases as it cools, until reaching the freezing point of around -2°C (although this depends on how salty the water is). At the onset of winter, cold air chills the surface waters until a relatively large layer of water at -2°C has formed on the surface. A soupy cristalline mixture known as frazil ice begins to form in the upper layer. As this soup thickens, a thin film of ice known as grease ice forms. This film is strong enough to support the weight of a seabird, and can ripple with the waves passing beneath. As the surface temperature continues to drop, the ice forms a solid layer and is then called pack ice.

Ice that forms each winter and melts each summer is known as annual ice, and is usually about six feet thick. In the central Arctic, the ice never thaws completely in summer, and thus it is known as multi-year ice. This ice ranges in thickness from 15 feet to 25 feet. Why doesn't it just keep getting thicker and thicker? Multi-year ice eventually enters an equilibrium where the amount of new ice being formed on the bottom of the layer in winter is exactly balanced by the loss due to melting off the top in summer.

Mariners have adopted a number of different names for icebergs and pack ice. The following glossary of ice terms is from Bowditch's Glossary of Marine Navigation.

Anchor ice. Submerged ice attached or anchored to the bottom, irrespective of the nature of its formation.

Bergy bit. A large piece of floating glacier ice, generally showing less than 5 meters above sea level but more than 1 meter and normally about 100 to 300 square meters in area. It is smaller than an ICEBERG but larger than a GROWLER. A typical bergy bit is about the size of a small house.

Blue ice. The oldest and hardest form of glacier ice, distinguished by a slightly bluish or greenish color.

Brash ice. Accumulations of floating ice made up of fragments not more than 2 meters across, the wreckage of other forms of ice.

Bummock. A downward projection from the underside of an ice field; the counterpart of a HUMMOCK.

Close pack ice. Pack ice in which the concentration is 7/10 to 8/10, composed of floes mostly in contact.

Compact pack ice. Pack ice in which the concentration is 10/10 and no water is visible.

Consolidated pack ice. Pack ice in which the concentration is 10/10 and the floes are frozen together.

Fast ice. Sea ice which forms and remains 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. Fast ice may be formed in situ from the sea water or by freezing of pack ice of any age to the shore, and it may extend a few meters or several hundred kilometers from the coast. Fast ice may be more than 1 year old and may then be prefixed with the appropriate age category (old, second-year or multi-year). If it is thicker than about 2 meters above sea level, it is called an ICE SHELF.

Fast-ice boundary. The ice boundary at any given time between fast ice and pack ice.

Fast-ice edge. The demarcation at any given time between fast ice and open water.

First-year ice. Sea ice of not more than one winter's growth, developing from young ice, with a thickness of 30 centimeters to 2 meters. First-year ice may be subdivided into THIN FIRST YEAR ICE, WHITE ICE, MEDIUM FIRST YEAR ICE, and THICK FIRST YEAR ICE.

Floe. Any relatively flat piece of sea ice 20 meters or more across. Floes are subdivided according to horizontal extent. A giant flow is over 5.4 nautical miles across; a vast floe is 1.1 to 5.4 nautical miles across; a big floe is 500 to 2000 meters across; a medium floe is 100 to 500 meters across; and a small floe is 20 to 100 meters across.

Floeberg. A massive piece of sea ice composed of a hummock, or a group of hummocks frozen together, and separated from any ice surroundings. It may float showing up to 5 meters above sea level.

Firn. Old snow which has recrystallized into a dense material. Unlike snow, the particles are to some extent joined together; but, unlike ice, the air spaces in it still connect with each other.

Frazil ice. Fine spicules or plates of ice, suspended in water.

Glacier ice. Ice in, or originating from, a glacier, whether on land or floating on the sea as icebergs, bergy bits, or growlers.

Glaze. A coating of ice, generally clear and smooth but usually containing some air pockets, formed on exposed objects by the freezing of a film of super cooled water deposited by rain, drizzle, fog, or possibly condensed from super cooled water vapor. Glaze is denser, harder and more transparent than either rime or hoarfrost Also called GLAZE ICE, GLAZED FROST VERGLAS.

Grease ice. Ice at that stage of freezing when the crystals have coagulated to form a soupy layer on the surface. Grease ice is at a later stage of freezing than frazil ice and reflects little light, giving the sea a matte appearance.

Grounded ice. Floating ice which is aground in shoal water.

Growler. A piece of ice smaller than a BERGY BIT or FLOEBERG, often transparent but appearing green or almost black in color. It extends less than 1 meter above the sea surface and its length is less than 20 feet (6 meters). A growler is large enough to be a hazard to shipping but small enough that it may escape visual or radar detection.

Hummock. 1. A hillock of broken ice which has been forced upwards by pressure. It may be fresh or weathered. The submerged volume of broken ice under the hummocks, forced downwards by pressure, is called a BUMMOCK; 2. A natural elevation of the earth’s surface resembling a hillock, but smaller and lower.

Hummocked ice. Sea ice piled haphazardly one piece over another to form an uneven surface. When weathered, hummocked ice has the appearance of smooth hillocks.

Iceberg. A massive piece of ice greatly varying in shape, showing more than 5 meters above the sea surface, which has broken away from a glacier, and which may be afloat or aground. Icebergs may be described as tabular, dome shaped, pinnacled, drydock, glacier or weathered, blocky, tilted blocky, or drydock icebergs. For reports to the International Ice Patrol they are described with respect to size as small, medium, or large icebergs.

Ice-blink. A whitish glare on low clouds above an accumulation of distant ice.

Icefoot. A narrow fringe of ice attached to the coast, unmoved by tides and remaining after the fast ice has moved away.

Land sky. Dark streaks or patches or a grayness on the underside of extensive cloud areas, due to the absence of reflected light from bare ground. Land sky is not as dark as WATER SKY. The clouds above ice or snow covered surfaces have a white or yellowish white glare called ICE BLINK.

Lead. A fracture or passage-way through ice which is navigable by surface vessels.

Nilas. A thin elastic crust of ice, easily bending on waves and swell and under pressure, thrusting in a pattern of interlocking “fingers.” Nilas has a matte surface and is up to 10 centimeters in thickness. It may be subdivided into DARK NILAS and LIGHT NILAS. See also FINGER RAFTING.

Nipped. Beset in the ice with the surrounding ice forcibly pressing against the hull.

Old ice. Sea ice which has survived at least one summer’s melt. Most topographic features are smoother than on first-year ice. Old ice may be subdivided into SECOND-YEAR ICE and MULTI YEAR ICE.

Open pack ice. Pack ice in which the concentration is 4/10 to 6/10, with many leads and polynyas, and the floes generally not in contact with one another.

Pancake ice. Predominantly circular pieces of ice from 30 centimeters to 3 meters in diameter, and up to about 10 centimeters in thickness with raised rims due to 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 gray 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.

Polynya. A non-linear shaped area of water enclosed by ice. Polynyas may contain brash ice and/or be covered with new ice, nilas, or young ice; submariners refer to these as SKYLIGHTS. Sometimes the POLYNYA is limited on one side by the coast and is called a SHORE POLYNYA or by fast ice and is called a FLAW POLYNYA. If it recurs in the same position every year, it is called a RECURRING POLYNYA.

Rafted ice. A type of deformed ice formed by one piece of ice overriding another.

Rotten ice. Sea ice which has become honeycombed and is in an advanced state of disintegration.

Sastrugi, (sing. sastruga). Sharp, irregular ridges formed on a snow surface by wind erosion and deposition. On mobile floating ice, the ridges are parallel to the direction of the prevailing wind at the time they were formed.

Sea ice. Any form of ice found at sea which has originated from the freezing of sea water.

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

Stranded ice. Ice which has been floating and has been deposited on the shore by retreating high water.

Tabular iceberg. A flat-topped iceberg with length-to-height ratio greater than 5:1. Most tabular bergs form by calving from an ice shelf and show horizontal banding.

Tongue. A projection of the ice edge up to several kilometers in length, caused by wind or current.

Very close pack ice. Pack ice in which the concentration is 9/10 to less than 10/10.

Water sky. Dark streaks on the underside of low clouds, indicating the presence of water features in the vicinity of sea ice.

Last updated: March 8, 2012