2  Sea Ice

2.1 Overview

Sea ice is any form of floating ice found at sea that has formed from the freezing of sea water. Sea ice is distinct from lake ice, which forms at the surface of freshwater lakes; river ice, which forms on the surface of rivers; and glacier ice that originates on land and may be found floating at sea as icebergs and ice shelves. Here we focus on sea ice.

Sea ice is split into two broad categories: fast ice, which forms at and remains along the coast; and drift ice, which includes all ice that is not fast ice. The term drift, refers to the ice’s ability to move freely. When concentrations are greater than 7/10, drift ice is normally referred to as pack ice.

2.2 Growth of Sea Ice

Sea ice starts to form when the entire mixed layer has cooled to the freezing point at -1.8 \(^\circ\\\)C.

2.3 Decay of Sea Ice

2.4 Stages of Development

The descriptions of the stages of development are taken from the WMO Nomenclature of Sea Ice and the Canadian ECCC MANICE. Many of the descriptions are based on the visual appearance of the ice, as well as the tickness. This reflects the history of ice observations being ship- or shore-based, and later from reconnaisance aircraft.

2.4.1 New Ice

Recently formed ice that icludes frazil ice, grease ice, slush, and shuga. The types of ice are formed from ice crystals that are only weakly frozen together.

  • Frazil Ice: fine spicules or plates of ice suspended in water
  • Grease Ice: a later stage of freezing where frazil ice crystals have coagulated to form a soupy layer of the surface. Grease ice does not reflect much light and gives the water surface a matte appearance.
  • Slush: Snow that is saturated and mixed with water on the sea ice surface or as a floating mass just after a heavy snowfall.
  • Shuga: spongy white ice clumps a few centimeters across. They are generally formed from grease ice or slush.

2.4.2 Nilas

A thin elastic “skin” (crust) of ice that flexes on waves and swell that grows as interlocking fingers. Nilas has a matte surface and may be up to 10 cm thick. Nilas is separated into dark nilas and light nilas.

  • Dark Nilas: nilas up to 5 cm in thickness and dark in color.
  • Light Nilas: nilas that is more than 5 cm thick and lighter in colour that dark nilas.
  • Ice Rind: a brittle, shiney crust of ice formed on a calm surface, either from direct freezing or from grease ice. It often occurs in water with low salinity. Ice rind is about 5 cm thick and easily broken into, usually, rectangular pieces by wind or swell.

2.4.3 Young Ice

Young ice is the transitional stage between nilas and first-year ice. It is between 10 and 30 cm thick. It is divided into grey ice and grey-white ice.

  • Grey Ice: Young ice between 10 and 15 cm thick. Grey ice is less elastic than nilas and breaks in swell. It usually rafts under pressure.
  • Grey-white ice: Young ice that is 15 to 30 cm thick. Under pressure, it is more likely to ridge than raft.

2.4.4 First Year Ice

First-year ice is sea ice that has no more than one winters growth. It develops from young ice and is more than 30 cm thick. First-year ice can be divided into _thin first-year ice (sometimes called white-ice), medium first year ice, and thick first year ice.

  • Thin First-year Ice/White Ice - First Stage: 30 to 50 cm thick
  • Thin First-year Ice/White Ice - Second Stage: 50 to 70 cm thick
  • Medium First-year Ice: 70 to 120 cm thick.
  • Thick First-year Ice: Greater than 120 cm thick.

2.4.5 Old Ice

2.5 Resources

WMO Sea Ice Nomenclature (WMO-No 259) ECCC Manual of Ice (MANICE)