Ice formed when falls of snow fail to melt from one season to another. As further snow accumulates, its weight presses on earlier snow, compacting and melting it to a mass of globular particles of ice with interconnecting air spaces. Where temperatures are around 0 °C, snow can turn to firn within five years, but the process takes much longer in very cold conditions. See Parry et al. (2007) Annals Glaciol. 46 on firn within the Greenland ice sheet. The firn line is the line, close to the equilibrium line, at which firn forms, and varies with aspect (Menzies (1951) J. Glaciol. 1, 9). See König et al. (2000) Procs EARSeL-SIG-Workshop on equilibrium and firn-line detection.