The Basic Ideas Behind Snow Metamorphism

S.C.Colbeck

CRREL, 72 Lyme Road, Hanover NH 03755

The basic concepts of thermodynamics and crystal growth that account forthe metamorphism of snow are pre-
sented. First, the differencesbetween wetand dry snow are emphasized since these materials even appear differ-
entlytothe naked eye.Then thebasic environmentalfactorsare introduced and theirimportanceexplained.
These include temperature, temperature gradient, liquid water content, humidity, solar radiation and wind. The
basic concepts of thermodynamics are introducedqualitatively just to explain how they influence ultimate crys-
tal shapes. These include thermodynamic equilibrium among the water, vapor and solid phases, the equilibrium
shape of ice crystals, the kinetic growth form of ice crystals, minimum surface energy, andcapillary. Finally, these
environmental factors and basic physical principles are combined to explain the basic shapes that are seenin the
seasonal snow cover. The problem with applying these basic ideas to snow metamorphism is that there are many
scenarios where the shapes of snow crystals that one sees are complicated by combinations of these factors.For
example,melt-freeze particles result from cycles of dry-wet-dry and thus are not simplywet snow ordry snow.
There are several intermediate stages of dry snow between purely rounded andpurely faceted crystals. The origin
of these partly faceted - partly rounded crystals in any particular situation depends on the path that the crystals
tookintheirgrowthcycle and thatin turn depends predominatelyonrecent weather patterns.Forexample,
similarlookingcrystalswithsomefacetsand somerounding can ariseiftheirgrowthrate has notbeen high
enough to make them purely faceted but is high enough to produce some facets. Alternately, they can form once
the high growth rates of purely faceted crystals have ceased and the corners and edges of these crystals begin to
roundoff.Finally,theimplicationsforthe physicalpropertiesofsnowarediscussed asthe evolutionofthe
shapes is explained.For example, sinteringoccurs withthe developmentof the equilibrium form but is minimized
in the kinetic growth form of dry snow. Likewise, wet snow is either ice-bonded or cohesionlessdepending on the
amount of water present.