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

From the instant snow hits the ground, it begins an endless process of metamorphism. No commonly-occurring substance in nature undergoes such dramatic and rapid changes because snow exists near its “triple point”, meaning that solid, liquid and vapor phases all exist at the same time. In other words, small, 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:

Type Also called: Looks like: Where you find it How it’s formed
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 (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 (more than 1 deg C per 10 cm)
Surface Hoar Frost,
Feathers
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

Additional Terms:
Anchors Hard Slab Avalanche Slide
Aspect High Danger Sluff
Avalanche High Marking Snowpit
Avalanche Path Isothermal Soft Slab Avalanche
Avalanche Transceiver Layer, Snow Stability
Bed Surface Leeward Stability Test
Collapse Loading Starting Zone
Concave Slope Loose Snow Avalanche Stepping Down
Considerable Danger Low Avalanche Hazard Sun Crust
Convex Slope Melt-Freeze Snow Surface Hoar
Cornice Metamorphism, Snow Sympathetic Trigger
Corn Snow Moderate Danger Temperature Gradient
Couloir Persistent Weak Layers Terrain Trap
Cross Loading Point-Release Track
Crown Face Probe Trigger
Danger Ratings Propagation Trigger Point
Deep Slab Avalanche Rain Crust Upside-Down Storm
Density, Snow Remote Trigger Weak Layer
Depth Hoar Rime Weak Interface
Dry Snow Avalanche Runout Zone Wet Snow Avalanche
Extreme Danger Sastrugi Windward
Faceted Snow Settlement Wind Loading
Fracture Ski or Slope Cut Wind Slab
Glide Skinning, Skin Track Whumpf
Graupel Slab