Sea ice
Sea ice is frozen seawater that forms, grows, and melts in the ocean. It differs from glaciers and ice sheets, which form on land.
Frazil ice: Small, loose ice crystals that form in supercooled water and can accumulate into a surface layer. It is an early form of sea ice in turbulent, freezing seawater.
Freezing point of seawater: The temperature at which seawater begins to freeze, varying with its salinity and pressure. Salt lowers seawater’s freezing point below that of pure water.
Sea-ice concentration: The fraction of a sea-surface area covered by sea ice. Satellite concentration records track the changing extent and distribution of ice.
Sea-ice albedo: The fraction of incoming sunlight reflected by sea ice and its snow cover. Bright ice reflects sunlight that dark ocean water would absorb.
Glacier: A persistent mass of land ice that flows under its own weight. Glaciers form on land, unlike sea ice, which freezes directly from seawater.
Nilas: A thin, elastic crust of newly formed sea ice, generally less than 10 centimeters thick. It develops when frazil crystals consolidate into a continuous surface sheet.
Brine rejection: The expulsion of concentrated salty water as sea ice forms from seawater. Growing ice rejects most salt, leaving brine within and beneath the ice.
Sea-ice thickness measurement: The observation of sea-ice thickness using satellites, aircraft, ships, or instruments deployed on ice. Thickness measurements reveal changes that ice-covered area alone cannot show.
Polar amplification: The tendency for polar regions to warm faster than the global average. Declining sea ice exposes darker ocean, reinforcing regional warming.
Iceberg: A large fragment of freshwater ice that has broken away from a glacier or ice shelf and floats in the sea. Icebergs originate on land or floating land ice rather than freezing from seawater.