necessary to understand the growth of the grains
of the clusters (see Fig. 1), and the remaining pore
between which the bonds develop. Thus, for each
space is filled with air. The basic unit of a cluster
category of snow, the growth of the grains is re-
is the well-rounded single crystal of ice: the grain,
viewed first to put the growth of the bonds in
or the minimum observable unit. These single
context. More details of the growth of grains in
crystals join in groups of two or more and are
snow are given in an earlier review of the physics
tightly bonded by ice-to-ice contacts, not by capil-
of snow metamorphism (Colbeck 1987a) and a
larity as is often supposed; the ice-to-ice grain
more recent review from a more practical point of
boundaries are depicted in Figure 2 for a three-
view (Colbeck, in press).
grain cluster. Their large size gives the snow con-
siderable strength. The liquid-filled veins form at
the junctions of three crystals, and more water is
held at the junctions of four veins. The geometry
WET SNOW
of these veins and junctions can be most easily
Wet snow contains an observable quantity of
visualized by examining the lines joining soap
liquid water in one of two basic modes of satura-
bubbles that have had time to grow to a size of
tion. First, at low liquid contents where air is
about 10 mm.
While the growth of individual grains at low
continuous throughout the pore space, the liquid
liquid contents is not as rapid as grain growth in
is held by "grain clusters" in a mode of liquid
slush, the clusters do form rapidly by the collect-
saturation known as the "pendular regime." Sec-
ing together of existing grains into clusters. Fully
ond, at high liquid contents where the liquid is
developed clusters arise from drained slush in
continuous throughout the pore space, the air
about 24 hours, which is remarkably fast com-
occurs only in isolated bubbles trapped in the
pared with the growth of particles by any other
pores. This is "slush," where the liquid is in the
process in snow. This happens in part because the
"funicular regime" of saturation. The sintering of
clusters are at the melting temperature and thus
these two modes is markedly different because
transport through the liquid phase is possible. As
grain clusters develop strength quickly whereas
a result, vapor diffusion is probably not the rate-
slush is cohesionless. The first mode occurs when
the snow is free to drain, while the second mode
limiting process that it is in grain growth in dry
occurs in snow overlying a surface that impedes
snow. Of course, these clusters are multicrystal-
water flow.
line collections, so their growth processes are
different from the processes that lead to the growth
of grains of single crystals.
Grain clusters
At low liquid contents, all of the liquid is held
Clusters form in this manner because this con-
by capillarity in the crevices, veins, and junctions
figuration of the vapor/ice, vapor/water, and ice/
water interfaces minimizes the to-
tal surface free energy (Colbeck
1979a). Neighboring clusters are
well bonded to each other, with
ice-to-ice bonds forming between
two grains, one ice grain from each
cluster. These ice-to-ice bonds are
strong enough to give this form of
snow some considerable strength,
both within the clusters and within
the snow cover as a whole. In fact
the strength is well-known to vary
significantly with the liquid-
water content of snow (Kinosita
1963, Colbeck 1979b).
Another form of well-bonded
snow is also common, one that
forms a transition between the cat-
Figure 1. Cluster of ice grains in wet snow at a low liquid content. The
egories of wet and dry snow.
individual ice grains are single crystals, usually 0.5 to 1.0 mm in size.
Amorphous, multicrystalline par-
2
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