Accumulation zone, ablation zone, firn line and equilibrium line,

A glacier consists of ice, which moves slowly downhill like a viscous liquid. That is, it flows to lower elevations where temperatures are higher than on the mountain peaks. In these higher temperatures, the ice melts.

In order for a glacier not to lose mass, about 1/3 of the glacier must be covered with snow at the higher altitudes in autumn (end of September). This snow turns into firn and later into glacier ice, which in turn flows downhill.

Firn line
In winter, snow usually falls on the glacier. This is also the case at lower elevations. In early spring, the snow begins to melt and the glacier tongue gradually becomes apertured. This means that the snow melts away completely and the glacier ice becomes visible. During the summer and autumn months, this process is gradually repeated at higher and higher altitudes. The line that marks the transition from snow/firn to impermeable glacier ice is called the firn line. The firn line cannot always be seen as precisely as in the photo above.

Equilibrium line
When the snow has melted down to the equilibrium line around mid-October, so much firn and ice is formed from the remaining snow that the glacier does not lose mass. The firn line and equilibrium line are then more or less congruent.

If the firn line is above the equilibrium line around mid-October, the glacier will lose mass and length.
If the firn line is below the equilibrium line around mid-October, the glacier will increase in mass and length.

Since glaciers have a reaction time of a few years to about 50 years, depending on their size, the consequences of a firn line above or below the equilibrium line are not visible in the following year. This requires several years in a row with a firn line above or below the equilibrium line.

The equilibrium line changes its position annually.

Accumulation zone
The area above the equilibrium line is called the accumulation zone.

Ablation zone
The area below the equilibrium line is called the ablation area.