firns

Firns are a crucial stage in the formation of glaciers. They are a type of metamorphosed snow, distinct from fresh snow and glacial ice. Firns form when snow accumulates over multiple years in a cold environment, such as high mountain altitudes or polar regions. The weight of overlying snow causes the individual ice crystals to transform. Initially, the snowflakes lose their delicate structures and become more granular and rounded. With continued pressure and slight melting and refreezing, air is expelled from the snowpack, and the ice grains begin to fuse together.

This process of densification and recrystallization creates a granular ice layer that is much denser than fresh snow. Firn typically has a density between 0.4 and 0.8 grams per cubic centimeter, whereas fresh snow is much less dense, around 0.1 grams per cubic centimeter. Glacial ice, the final stage, is even denser, often exceeding 0.85 grams per cubic centimeter. Firn acts as an intermediate step, a partially compacted snow that is on its way to becoming solid glacial ice.

The presence of firn is a strong indicator of glacier formation or the potential for glacier growth. It is a common feature on the accumulation zones of glaciers, where more snow falls than melts or ablates. Understanding firn is important for glaciologists studying climate change, as changes in firn accumulation and density can reflect long-term temperature and precipitation trends. In some contexts, the term "névé" is used interchangeably with firn, particularly in alpine regions.