In hydrologic terms, old snow on top of glaciers, granular and compact and not yet converted into ice. It is a transitional stage between snow and ice. Also called Neve. Rounded, well-bonded snow that is older than one year; firn has a density greater than 550 kilograms per cubic-meter (35 pounds per cubic-foot); called n (1) Snow that has survived at least one ablation season but has not been transformed to glacier ice. This sense prevails in the study of mass balance. Snow becomes firn, by definition, at the instant when the mass-balance year ends. See zone. (2) Structurally, the metamorphic stage intermediate between snow and ice, in which the pore space is at least partially interconnected, allowing air and water to circulate; typical densities are 400830 kg m3. In this sense, the firn is generally up to a few tens of metres thick on a temperate glacier that is close to a steady state, and up to or more than 100 m thick in the dry snow zone on the ice sheets. Old snow that has recrystallized into a dense material. Unlike ordinary snow, the particles are to some extent joined; but, unlike ice, the air spaces in it still connect with each other. Sea ice terminology that describes old snow which has re-crystallized into a dense material. Unlike ordinary snow, particles are (to some extent) joined together; but, unlike ice, the air spaces in it still connect with each other. Well-bonded and compacted snow that has survived the summer season, but hasnot been transformed to glacier ice. Typical densities are 400-830 kg m3 (perennialsnow, n An intermediate stage in the transformation of snow to glacier ice. Snow becomes firn when it has been compressed so that no pore space remains between flakes or crystals, a process that takes less than a year. Old snow which has become granular and dense under the action of various processes of melting and refreezing, including sublimation and crystallization.