The ice season lasts about eight months in Lake Kilpisjärvi, from November to June (Table 2). The ice cover consists of three principal layers: congelation ice, snow-ice and snow. Occasionally there may be slush sub-layers in the snow or snow-ice layer. In slush formation, snow is compacted so that the thickness of slush is less than the original snow thickness. In the ice seasons in 2007–2009, the mean snow and snow-ice thickness corresponded to snow accumulation by about 70 cm, which is less than snow accumulation on the ground (on average 90 cm, from FEI data archives), the difference is largely due to drifting of snow from the lake. The snow in snow-ice corresponded to 30 cm snow accumulation due to snow compression in snow-ice formation (see Leppäranta & Kosloff 2000), and when the snow cover on ice is added to this, we have altogether a representative snow accumulation of 70 cm.