Key takeaways
- Firn is granular, compacted snow that has survived at least one melt season.
- It's an intermediate stage between fresh snow and dense glacier ice.
- Melt-refreeze cycles and compaction round the crystals and densify the snow into firn.
- Encountered high on snowfields and glaciers — firm, supportive travel and good corn-like skiing.
From German firn / Swiss German, 'last year's' (old snow).
What firn is
Firn is granular, compacted, partly consolidated snow that has survived at least one summer melt season without turning fully to ice — an intermediate stage between fresh snow and dense glacier ice. It’s denser and more consolidated than seasonal snowpack, but not yet solid glacier ice.
How it forms
Through repeated cycles of partial melting, refreezing, and compaction. As snow sits through warm and cold periods, its delicate crystals melt and refreeze into rounded grains that pack and bond, while accumulating snow above compresses it. Snow that survives a melt season becomes firn; over many years of continued compaction, firn becomes glacier ice.
High on a glacier in early summer, a ski-mountaineer skins up firm morning firn that takes their edges securely — then, as the sun softens the top few centimeters by midday, skis down it like silky corn, before the snow gets too slushy.
Where you find it
Firn occurs high on permanent snowfields and the upper accumulation zones of glaciers, where snow persists year-round (and where seracs and crevasses also live). For glacier travel and ski-mountaineering it’s generally pleasant: firm and supportive, takes crampons and edges well, and when softened just right skis like excellent corn snow.
The bottom line
Firn is granular, compacted snow that has survived at least one melt season — the in-between stage from fresh snow to glacier ice, formed by melt-refreeze cycles and compaction. Found high on snowfields and glacier accumulation zones, it offers firm, supportive travel that takes crampons and edges well, and can ski like excellent corn when softened just right by the sun.
Frequently asked questions
What is firn?
Firn is granular, compacted snow that has survived at least one summer melt season without melting away or fully turning to ice. It's an intermediate stage in the transformation from fresh snowfall to dense glacier ice — denser and more consolidated than seasonal snow, but not yet solid glacier ice. The word comes from German for 'last year's' snow.
How does firn form?
Through repeated cycles of partial melting, refreezing, and compaction over time. As snow sits through warm and cold periods, its delicate crystals melt and refreeze into rounded grains that pack together and bond, while the weight of accumulating snow above compresses it. This makes the snow progressively denser. Snow that survives a melt season becomes firn, and over many years of continued compaction firn turns into glacier ice.
Where do you encounter firn, and what's it like to travel on?
You find firn high on permanent snowfields and on the upper, accumulation zones of glaciers, where snow persists year-round. For mountaineers and ski-mountaineers, firn is generally pleasant and useful: it's firm and supportive (you don't wallow in it), takes crampons and ski edges well, and when softened just right by the sun can offer excellent corn-snow-like skiing. Firm morning firn is also good for fast, secure travel.
Sources
- Glaciers & snow science — National Snow and Ice Data Center
- Glacier travel — The Mountaineers