Slush

Ice

Grain

Water

Ice

Grain

Boundary

Figure 2. Cross section of a three-grain clus-

Figure 3. Slush, which consists of well-rounded

ter in wet snow. Liquid is held in the crevices

grains 0.5 to 1 mm in size immersed in water.

between two grains, the veins among three

These do not bond, and therefore slush lacks

grains, and the junctions that join four veins.

cohesion.

Air fills the remaining pore space.

since the ice grains are surrounded by water (see

ticles arise from meltfreeze cycles, simply by the

Fig. 3), grain growth in slush is very rapid as first

freezing together of individual grains. When this

measured by Wakahama (1968) and later ex-

happens, it destroys the granular geometry of the

plained by Colbeck (1987b). Slush lacks inter-

grain cluster but probably increases the strength

granular bonding as do rapidly growing grains

of the snow cover. These particles are also ice-

in dry snow, but for very different reasons. In

bonded to their neighbors.

slush, the bonds are unstable because, when

stressed, they melt away by pressure melting,

Slush

whereas with clusters, the ice-to-ice bonds are

At higher liquid contents, the air is no longer

stable against pressure melting even though the

continuous throughout the pore space, but is lim-

snow contains liquid water (Colbeck 1979a). This

ited to isolated air bubbles trapped by constric-

is a very fundamental difference between the

tions in the pores. Since these bubbles occupy the

pendular and funicular regimes of water con-

largest part of the pore space, the volumetric air

tents since it leads directly to high strength at

content can still be higher than the volumetric

low liquid contents and low strength at high

liquid content, but only the liquid phase is mo-

liquid contents. This fundamental difference in

bile. In fact, the permeability to the liquid in-

the thermodynamics is due to the basic differ-

creases with liquid content, and slush is highly

ences in the geometry.

capable of conducting liquid water. In addition,

Figure 4. Geometry long assumed to de-

scribe rounded grains with necks in dry snow.

Actually, these grains are glass beads: ice grains

form a neck with a grain-boundary groove and, given

enough time, an equilibrium grain-boundary groove

angle of about 145.

3