A Review of Sintering in Seasonal Snow
SAMUEL C. COLBECK
about the geometry of the bonds nor about the
INTRODUCTION
processes that form them. Models describing the
Snow on the ground consists of three basic
behavior of snow are often based on an assumed
components: air, ice grains, and liquid water when
geometry or observations of the bonds from sur-
the snow is wet. Ice bonds form between the grains
face sections (e.g., Brown and Edens 1991.)
in most, but not all, types of snow. Much atten-
The strength of snow is not the only property
tion has been paid to the size, shape, and growth
that is controlled by the size of the bonds. Some
of the grains, but the bonds are of equal impor-
properties depend primarily on the grains, such
tance to the grains themselves, and relatively little
as the optical properties, where scattering and
attention has been paid to them. This probably
absorption can depend on grain size and shape.
occurs for two reasons. First, the bonds are smaller,
However, some properties are most sensitive to
partly hidden by the grains, and much harder to
the narrow constrictions between grains where
see with a hand lens. Second, the physics of their
stresses are larger but heat and electrical flow
growth is not well understood, partly because
paths are reduced. Thus the size, shape, and fre-
their basic geometry is usually misunderstood. In
quency of grain bonds greatly affects many of the
fact, the literature describes sintering in snow as
most important properties of snow. Sintering is
if the snow were a noncrystalline material with
the process by which these bonds form and the
no imposed temperature gradient. The widely
study of their size, shape, and number density.
used approach to sintering in dry snow could be
To study the bonds in snow, it must be recog-
applied to glass beads held in an adiabatic cell,
nized immediately that wet snow and dry snow
but not to ice grains in a seasonal snow cover.
are basically two different materials. While
When snow pits are dug to look for weak
changes do occur more rapidly in wet snow be-
layers in the snow profile, the grains are gener-
cause of the higher temperature, the fundamental
ally examined rather than the bonds. For ex-
difference is that the introduction of a third phase,
ample, depth hoar is known to be weak due to
liquid water, causes major reconfigurations of both
poor bonding, so the existence of these highly
grains and bonds. The geometries of wet and dry
faceted crystals is taken as evidence for the pres-
snow are markedly different, and their properties
ence of a weak layer. In the International Classifi-
differ for several important reasons. Wet snow is
cation System for Seasonal Snow on the Ground
active thermodynamically because of the high
(Colbeck et al. 1990), there are photographs of the
temperature and presence of the liquid phase, but
grains, but information about the bonds is only
vapor flow due to a macroscopic temperature gra-
inferred. The size and shape of the grains are
dient can only occur in dry snow.
recorded in snow-pit logs, even when it is only
Within each of these categories there are also
the bonding that is of concern. Instead of direct
two important divisions: wet snow at low and
examinations of the bonds, stereological methods
high liquid contents and dry snow at low and
have sometimes been used to infer information
high growth rates. Wet snow is cohesionless and
about the bonds, including their size and an as-
slushy at high liquid contents, but well-bonded at
sumed shape (e.g., Keeler 1969, Alley et al. 1982).
low liquid contents. Rapidly growing grains in
Direct information about the degree of bonding
dry snow lack bonding, whereas strong bonds
comes from strength tests (e.g., Keeler 1969, Gow
form when the grains grow slowly. To under-
1975), but these tests do not provide information
stand the formation of bonds in snow, it is first
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