| What it is | Old, compacted granular snow |
| Stage | Between snow and glacier ice |
| Where | Glacier accumulation zones, permanent snowfields |
| Density | Denser than seasonal snow, not yet ice |
From German/Swiss German for 'last year's (snow)'.
Firn is old, dense, granular snow that has survived at least one melt season and been compacted, representing the intermediate stage between fresh snow and glacier ice. Found in glacier accumulation zones and on permanent snowfields, firn is denser than seasonal snow but not yet solid ice, and is part of how glaciers form over years of compaction.
Snow becoming ice
The step toward the ice of glacier travel; spring firn is prized terrain for ski mountaineering.
Frequently asked questions
What is firn?
Firn is granular, compacted snow that has lasted through at least one summer melt season without turning to ice yet. It's the transitional material between fluffy fresh snow and dense glacier ice — older and denser than seasonal snow but still air-permeable and not fully solid ice. It's also called névé.
How does firn turn into glacier ice?
Over years, successive snowfalls bury and compress firn, squeezing out air and increasing density until the firn recrystallizes into solid glacier ice. This slow process, taking years to decades, is how the accumulation of snow in a glacier's upper zone eventually becomes the flowing ice of a glacier.
Why does firn matter to skiers and mountaineers?
Firn fields and the firn in a glacier's accumulation zone are common terrain for ski mountaineering and glacier travel. Spring firn can offer good, supportive snow for climbing and skiing, while understanding firn helps mountaineers read glacier structure and travel safely over snow-covered ice.
Sources
- Glacier formation — USGS
- Snow & ice — The Mountaineers