Firn

Firn is compacted granular snow that has persisted through one or more melting seasons and has begun to recrystallize but has not yet become glacier ice. It represents the intermediate stage between fresh snow and solid ice on a glacier or ice sheet.

Formation and properties: Firn forms as accumulated snow is buried and compressed by overlying layers. Its density increases from about 0.3–0.5 g/cm3 toward 0.4–0.8 g/cm3 as grains grow and interlock under pressure. The porosity, or air-filled void space, decreases with depth but remains significant in the upper part of the snowpack. Firn grains may undergo metamorphism driven by temperature and pressure, producing a hard, granular material with a grain structure distinct from freshly fallen snow.

Occurrence and transition: Firn is common in high mountains and polar regions, where prolonged cold and accumulation allow snow to persist across years. The transition from firn to glacial ice occurs with further densification, typically approaching a density around 0.9 g/cm3, after which most interconnected air is lost and the material behaves as glacier ice. The boundary on a glacier where snow persists year-round and becomes firn is referred to as the firn line or snow line.

Significance: Firn plays a key role in climate science. It contains trapped air and serves as a reservoir of atmospheric gases, which are later sealed as the firn densifies into ice. This makes firn an important medium for paleoclimate records, including isotopic measurements and gas concentrations, contributing to reconstructions of past temperatures and atmospheric composition.