Sampling the surface of a glacier. There is increasingly dense firn between surface snow and blue glacier ice.Firn field on the top ofSäuleck,Hohe Tauern
Firn (/fɪərn/; fromSwiss Germanfirn "last year's", cognate withbefore) is partially compactednévé, a type ofsnow that has been left over from past seasons and has beenrecrystallized into a substance denser than névé. It isice that is at an intermediate stage between snow and glacial ice.[1] Firn has the appearance of wet sugar, but has a hardness that makes it extremely resistant to shovelling. Its density generally ranges from 0.35 g/cm3 to 0.9 g/cm3,[1][2] and it can often be found underneath the snow that accumulates at the head of aglacier.
Snowflakes are compressed under the weight of the overlyingsnowpack. Individualcrystals near the melting point are semiliquid and slick, allowing them to glide along other crystal planes and to fill in the spaces between them, increasing the ice's density. Where the crystals touch, they bond together, squeezing the air between them to the surface or into bubbles.
In the summer months, the crystal metamorphosis can occur more rapidly because of water percolation between the crystals. By summer's end, the result is firn.[3]
The minimum altitude that firn accumulates on a glacier is called thefirn limit,firn line orsnowline.
the more recent snow layers of a temperate, or "firned",glacier
used in skiing, the uppermost, soft layer of snow that is frozen overnight and, as a result of spring sunshine and high air temperatures, melts and reforms on an area of old snow orharsch (lit.'harsh', referring to the snow's rough texture)
As in the last context, a ski slope that experiences melting and refreezing intoharsch is said to "firn up". In Switzerland, these slopes are calledSulz, but in Germany,Sulz more often refers to a depth at which skiing downhill is no longer enjoyable.
