Fresh snow is an interlocking structure of new snow crystals with a high air content. Metamorphosis of the snow cover begins immediately it is deposited. Due to variable water vapour concentrations in the snowpack, when the temperature gradient is low the snow crystals are transformed slowly by destructive metamorphosis into old granular snow with a particle diameter of <0.5 mm. The snowpack then settles and consolidates.
When the temperature difference within the snowpack is medium or high
constructive metamorphosis - an enlargement of the snow crystals - takes place. Angular, square-edged structures are formed and may then metamorphose into hollow crystalline shapes called cup crystals. Layers of snow containing cup crystals are weak and not very resilient. In wet snow metamorphosis can take place when the snow warms to above 0°C and a snowmelt film is formed on the particles. This reduces the stability of the snow.