firn, partially compacted granular snow that is the intermediate stage between snow and glacial ice. Firn is found under the snow that accumulates at the head of a glacier. It is formed under the pressure of overlying snow by the processes of compaction, recrystallization, localized melting, and the crushing of individual snowflakes. This process is thought to take a period of about one year. Annual layers of firn may often be detected by thin films of dust or ash that accumulate on the surface during each summer.
Further compaction of firn, usually at a depth of 45 to 60 m (150 to 200 feet), results in glacial ice, distinguished by its impermeability to air and water. The density of firn is generally accepted as 0.4 to 0.84 grams per cubic cm, and its grain size ranges from 0.5 to 5 mm.