AVALANCHE/SNOWPACK
GLOSSARY OF USEFUL TERMS

artificial avalanches - equipment (skier, explosives,those avalanches triggered directly by man or his equipment (skier, explosives,etc.).

artificial control - the stabilization of avalanche areas by hand charges (explosives, artillery, ski testing or other non-natural means. This control method usually results in slope stability through reduction or elimination of stresses within the snowpack by either avalanche release, sluffing, or snow settlement.

aspect - the direction toward which an (avalanche) slope faces. For example, north aspect slopes face toward the north. Slope aspects are particularly important when considering the effects of solar radiation or wind loading on the snowpack.

avalanche hazard - measure of the probability of avalanche release in a given area at a particular time, considered together with the threat to people and property.

avalanche path - areas in mountainous terrain where avalanches are know or suspected to occur These areas include, but are not limited to, steep, open slopes, gullies and bowls.

avalanche potential - measure of the probability of avalanche release in a given area at a particular time, regardless of threat to people or property.

climax avalanche - an avalanche which occurs at the culmination of slow load buildup during several storms and/or results from metamorphism in the snow cover. Generally, this avalanche involves snow layers from more than one storm.

compression zone - an area of compression at the base of a slide path where terrain steepness decreases. This zone is concave in profile, and subject to gravitational pressure from the snow above.

cornice - an overhanging snow structure resulting from the accumulation of (large) quantities of wind-drifted snow over and in the lee of sharp terrain bends. Natural cornice releases during warm-ups often trigger slab avalanches on the slopes below.

creep - the slow, continuous, glacier-like downhill deformation of the snow cover, as a result of gravity-induced internal snow motion. This does not include the downhill motion of the snow cover relative to the ground, which is known as glide.

creep tension - tensile stress in snow caused by variations in creep velocity.

delayed action avalanche -avalanches which occur other than during or immediately after a storm.

density - the mass per unit volume of a given quantity of snow, usually expressed in grams per cubic centimeter (gm/cm³). The density of water, 1 gm/cm³, (or 1000 kg/m⁵), is a convenient reference. In the absence of wind, new snowfall densities usually range from .07 to .12 gm/cm³, (70 to 120 kilograms per cubic meter), while in areas exposed to wind densities are often from .20 to .30 gm/cm³ (200-300 kg/m³). In general, high densities of new snowfall correlate with warm air (rimed crystals) or high winds (breakage of crystals), while low densities correlate with cold air (no riming) or low winds (crystals intact). Snowpack settlement also results in increasing snow density.

depth hoar - end product of temperature gradient metamorphism. Large cohesionless grains with facets and stepped surface. Also called facets.

deposition zones - see lee slopes (as relating to wind transport); see runout (as related to avalanche paths).

direct-action avalanche - an avalanche occurring during or immediately after a storm, which involves only the snow deposited during that storm.

equitemperature (ET) metamorphism - the process of changes in snow texture from complex crystal shapes toward rounded snow (ice) grains in the absence of large temperature gradients. Technically this is known as destructive metamorphism (it destroys crystal shapes), and it results in a strengthening of the ice skeleton and a general rounding of snow grains largely through a preferential transfer of water vapor within the snowpack.

exposure - see aspect

fracture line - a well-defined line where the moving snow cover breaks away from the stable snow in a slab avalanche release.

fracture-line profile - a snow profile obtained by excavating a snowpit at a recent slab avalanche release site. Normally, measurements of temperature, density, ram resistance and stratigraphy at various depths are taken in order to understand the snow layering leading to the avalanche release.

free water - liquid water present in a snow layer.

glide - The slow, downhill movement of the snow cover over the ground surface. May be accelerated by the presence of water in the form of rain.

ice lens, ice layer - a very hard layer in the snowpack produced by freezing of meltwater into solid ice.

isothermal - same temperature throughout.

lee slopes - those (avalanche) areas on the down-wind side of ridges and other terrain obstacles, where deceleration of wind flow often deposits deep accumulations of snow. Also, usually refers to those slopes sheltered or protected from the wind. An east-facing slope is in the lee of a west wind.

loose snow avalanche (L) - a progressive rupture of snow cover, starting at a point and fanning out downhill. Loose snow grains start to slip from a point near the surface in this type of avalanche, sweeping progressively more grains with them as they move downhill leaving an inverted V-shaped scar. Loose slides may be present.

lubricating layer - the snow layer involved in avalanche release which, due to its weak internal strength and/or poor bonding to adjacent layers, facilitates the mechanical failure of a snow slab. Two examples of this lubricating layer are graupel or light, wind-deposited snow sandwiched between two more cohesive slab layers. A clearly defined lubricating layer may not always be present in a slab avalanche release.

melt-freeze (MF) crust - a usually hard layer within the snowpack which has undergone at least one melt-and-freeze cycle, and has gained strong intercrystalline bonds through refreezing of interstitial liquid water.

metamorphism - as applied to a mountain snowpack, metamorphism refers to changes in snow texture caused by pressure and temperature conditions. The temperature of the snow layer determines the rate of metamorphism, and the temperature change (gradient) across the layer largely determines the type of metamorphism.

natural avalanches - those avalanches not triggered directly by man or his equipment (e.g., cornice fall, earth tremors, etc.).

percolation - the downward motion of meltwater through interstitial air
spaces in a snowpack due to gravity.

pocket of instability - an isolated area of potentially unstable snow.

rain crust - a melt-freeze crust where the source of liquid water is rain.

ram resistance (ram number) - a measure of the relative mechanical strength of snow layers. This number is obtained by utilizing a device known as the ram penetrometer.

riming - the deposition of supercooled water droplets directly on snow crystals or terrestrial objects. Riming on snow crystals may also play an important role in avalanche formation through either its higher density or its promotion of a more slablike snow texture.

runout - the bottom boundary of an avalanche path, often identifiable by forest damage or avalanche deposition.

settlement - the progressive densification (consolidation) of a snowpack due to gravity, overburden pressure (of overlying snow) and metamorphism. In general, substantial settlement (25%) of new snow layers is a stabilizing influence on a mountain snowpack.

shear strength - in a snow slab, the slope parallel component of gravity tends to pull the slab downhill while friction and cohesion between snow surfaces act to hold the slab in place. Slippage between the slab and its undersurface can result, and avalanching can result if gravity induced shear stress between layers exceeds shear strength bonding layers together. Snow layers composed of surface hoar, graupel, low-density snow, etc., have very low shear strengths.

sintering - the process of vapor diffusion which joins individual snow grains together forming an ice skeleton of connected grains.

slab - a layer of snow held together by internal cohesion between snow grains.

slab avalanche - the simultaneous rupture of a coherent mass of snow over an extended area. A distinct fracture line is left at the upslope limit of the avalanche, and a clearly defined sliding surface is often revealed. Slab avalanches may be sub-classified as either soft slab (SS), if during motion the avalanche breaks into a formless mass, or hard slab (HS), if hard angular blocks of snow are left in the final avalanche debris. Wet slabs are slab avalanches that have free water present at the fracture line and generally result from rain or appreciable surface snow melt.

slide cycle - a period of time during which instability in the snowpack is high and substantial avalanching occurs naturally or artificially. A given slide cycle may be variously referred to as a wet slab cycle, soft slab cycle, etc., depending on the nature of the predominant avalanching.

sliding surface - the usually hard snow surface below a possible lubricating layer upon which a slab avalanche slides. This may be a sun crust, a rain crust, an ice layer, a wind-slab surface or other strong snow surface.

sluffing - the progressive stabilization of steep snow slopes by small, usually harmless avalanches of either point or slab origin. Technically, a sluff is any snow slide that moves less than 150 ft. (50 m) slope distance.

slope loading - the increase in stress (shear and tensile) within an inclined snowpack by the addition of new snowfall.

snow decomposition - the mechanical weakening of a melt-freeze crust by the action of a strong temperature gradient, where the gradual separation of previously bonded snow grains into individual loose snow grains, or the recrystallization of new snow grains, often occurs.

snowpack instability - see unstable snow conditions.

snowpack stratigraphy - layering within the snowpack.

snowpit - a hole dug into the snow surface to obtain certain physical properties of the snowpack.

spring avalanches - avalanches that typically occur after an extended period of warm weather saturates the snowpack with melt water. Usually this water will flow down through the snowpack until it either reaches the ground or an ice layer where it spreads out and lubricates the layer causing the snow above to slide. True spring avalanches are always wet snow avalanches.

starting zone - that portion of an avalanche path where a slide originates. Generally, starting zones are bare of trees, steeper than about 30 degrees and receive large amounts of snow. Gullies and bowls are particularly efficient collectors of snow (especially on lee slopes where wind transport occurs) and the tops of these areas make up a large portion of the most active starting zones. Many starting zones are also bounded by cliffs or rock outcrops.

steep slopes - those avalanche areas where large slab slides are most likely to start during conditions of high to extreme avalanche potential. In general, dangerous slabs most often occur on slopes in the 30 degree - 45 degree range. The upper limit of 45 degrees reflects the tendency of snow to sluff gradually off steep slopes. However, windpacked snow often accumulates on steeper terrain (45 degrees - 60 degrees) and here, too, slab slides may occur. Also, slab avalanches can propagate from high angle slopes to slopes of less than 30 degrees, and loose snow avalanches that would otherwise be harmless may spill onto lower slope, triggering dangerous slab avalanches.

sun crust - a melt-freeze crust where the source of liquid water is due to solar radiation.

surface hoar - also hoarfrost; the ice equivalent of dew. Surface ice crystals resulting from vapor deposition onto a cold surface. These crystals are quite intricate, extremely weak and cohesionless, and generally form on cold, clear nights.

temperature-gradient (TG) metamorphim - the variation of temperature per unit depth of the snow cover; also, constructive metamorphism of deposited snow crystals in response to a strong vertical temperature gradient (generally greater than 0.1 degree C/cm) in the snowpack and accompanying differences in vapor pressure with depth, where marked crystal growth generally has an adverse effect on mechanical properties of snow.

tensile strength - the slope-parallel component of strength in a snow layer which prevents it from fracturing across the slope. together with shear strength between the slab and the underlying snow surface prevents the slab from avalanching.

tension zone - a snow slab is placed in tension by the straining and stretching of the snowpack over terrain irregularities. The tension zone of a slab occurs at the top where the slab is trying to pull away from the stable snow, largely through the effects of varying snow creep.

track - that part of an avalanche path between the starting zone at the top and the runout zone at the bottom. In general, avalanche tracks have an inclination of at least 15 degrees more commonly 20 degrees - 25 degrees, and can be subdivided into channeled (gullies, gulches, couloirs, etc.) or unconfined (plane, open slopes) tracks.

trigger - the type of activating agent which results in an avalanche. The trigger types for a slide may be natural (cornice fall, snow from tree, internal stress build-up, etc.) or artificial (ski, explosives, etc.).

unstable snow conditions - physical characteristics of the snowpack which may result in avalanching. The presence of TG metamorphism, surface hoar or graupel may indicate an unstable snowpack in old snow. In new snow, instability often results from a heavy strong layer (e.g., wind slab) deposited over a relatively light (low density) or weak layer (e.g., surface hoar, low wind deposited snow). Rising temperatures, winds or snowfall intensity during a storm usually lead to unstable snow conditions.

wet-snow avalanche - (or wet avalanche) - an avalanche consisting of snow which contains liquid water. In many instances, an avalanche will begin as a dry snow avalanche but turn into a wet snow avalanche as it descends to lower elevations.

wind loading - the wind transport of snow onto lee slopes in addition to the accumulation due to snowfall. In this interpretation, wind loading may occur without precipitation, by scouring of snow on exposed windward slopes and sub sequent deposition of this scoured snow on lee slopes.

wind slab - a firm snow slab resulting from deposition of wind-pulverized or wind-transported snow. Although wind slabs usually occur on lee slopes, hard wind slabs may also occur on windward slopes. Wind slab deposited over weak, low density, snow layers represents a particular dangerous unstable snow condition.

wind transport - see wind loading.

windward slopes - those (avalanche) areas on the upwind (facing into the wind) side of ridges or other terrain obstacles, where accelerating wind-flow can erode surface snow, redepositing it in areas of low wind stress (lee slopes).