Firn is partially-compacted snow that has been left over from past seasons and has been recrystallized into a more dense substance. It is ice that is at an intermediate stage between snow and glacial ice. Firn has the appearance of wet sugar, but has a hardness that makes it extremely resistant to shovelling. It generally has a density greater than 0.55 and is often found underneath the snow that accumulates at the head of a glacier.

Snowflakes are compressed under the weight of the overlying snowpack. Individual crystals 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.

The minimum altitude that firn accumulates on a glacier is called the firn limit or snowline.

References

• USGS Glossary of Selected Glacier and Related Terminology
• Fundamentals of Physical Geography