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:

TYPE	ALSO CALLED	LOOKS LIKE	WHERE FOUND	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 (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, 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