Lake Ice and 
Snow Science 
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| Formation | Metamorphism | Conductive Heat Flow | Other Properties |

SNOW: Temperature Gradient Metamorphism (TG)

Diagram of temperature gradient metamorphism
TG metamorphosis occurs in response to a strong temperature gradient (> 10°C /m).
As water vapour is deposited on the grains, they grow larger, and can eventually form the large crystals known as depth hoar.
Depth hoar normally forms near the base of the snow pack, where the vapour pressure gradient is strongest and most persistent.

Sublimation: The transition of a substance from the solid phase directly to the vapor phase, or vice versa, without passing through an intermediate liquid phase. Sublimation is a phase transition that occurs at temperatures and pressures below the triple point (see H2O phase diagram).
Well rounded crystals (left) and rounded crystals with developing facets (right)
Well rounded crystals Rounded crystals with developing facets
Soild faceted crystal (l) and depth hoar (r)
Soild faceted crystal Depth hoar
Large depth hoar (l) and Cup and striated crystals (r)
Large depth hoar Cup-shaped striated crystals (depth hoar)
All photographs by E. Akitaya
Four electron microscope images of temperature gradient metamorphism snow crystals
Electron microscope images of temperature gradient snow particles. Source: USDA

QUESTION: Why is temperature-gradient metamorphism called "constructive"?

Click here for a complete desciption of temperature gradient metamorphism snow grain types.