Snow metamorphism refers to the change of snow crystals over time. Snow crystals begin to metamorphose from the instant they begin to fall. After new snow is added to the snowpack, the form and size of snow crystals (now labeled snow grains) changes continuously. As snow grains change, layers within the snowpack strengthen and weaken.
One type of snow metamorphism is the result of sublimation and deposition. These are physical processes that transform ice to water vapour (without a liquid phase) and vice versa. In the snowpack we see this as ice from grain surfaces transforming into water vapour which is then deposited as ice on other grain surfaces.
Sublimation and deposition processes are driven by the temperature gradient of a given snowpack layer. Together, temperature gradient and the movement of water vapour determine whether rounding or faceting processes will dominate within the snowpack.
Melt-freeze is another type of metamorphism that changes the composition of snow grains through melting and refreezing, rather than through sublimation and deposition.