The lower topographic limit of multi-annual snow cover is called the snow line or equilibrium line altitude (ELA). It is an irregular line (altitude) located along the ground surface above which snow does not entirely melt in summer. At the ELA the accumulation of snowfall equals ablation (melting and evaporation). This line varies greatly in altitude. The annual equilibrium line separates the accumulation and ablation zone and differs annually. Average altitudes of the snow line taken over large areas can be used to derive a climatic snow line, which rises or falls in altitude in response to worldwide climatic zones:
- Near the poles~0 m a.s.l.;
- Polar zone ~200–1000 m a.s.l.;
- Temperate zone ~800–3200 m a.s.l.;
- Tropical zone ~5000–6000 m a.s.l.;
- Near the Equator ~4500–5000 m a.s.l.
Above ELA if enough snow accumulates to transform into ice – glaciers may form.