The physical change of snow grains within the snowpack due to differences in temperature and pressure.

From the instant snow hits the ground, it begins an endless process of metamorphism. Few things in nature undergo such dramatic and rapid changes because water is the only naturally occurring substance that exists near its “triple point”, meaning that solid, liquid and vapor phases all exist at the same time. In other words, small and subtle changes in temperature, pressure, humidity and temperature gradient can have a dramatic effect on the type of snow crystal that forms. This makes snow one of the most complex and changeable substances on Earth. Here is a condensed list of the most common types:

New snow Powder, rime, graupel, etc. No two are alike On the snow surface Falls from the sky
Rounded snow Equilibrium snow
Old Snow
Fine-grained, chalky Old layers of snow Low temperature gradient conditions (typically less than 1 deg C per 10 cm)
Faceted Snow Sugar Snow
Kinetic Snow
Depth Hoar (when near the ground)
Sparkly, large-grained Anywhere in the snowpack Large temperature gradient conditions within the snowpack (typically more than 1 deg C per 10 cm)
Surface Hoar Frost,
Sparkly, large-grained On the snow surface or buried by more recent layers Winter equivalent of dew on the snow surface
Melt-Freeze Snow Corn snow
Spring snow
Wet snow
Corn snow
Spring snow
Wet snow
Snow surface or buried by more recent layers Repeated melting and freezing of the snowpack