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Ablation The loss of mass from snow or ice, most commonly by melt but also by sublimation or scour.

Age-Hardening A characteristic property of disturbed snow to gain strength with time through bond growth among the rearranged snow particles.

Artificial Avalanche Triggered by an animal, human or his/her equipment.

Avalanche, Loose Snow A sliding mass of snow that originates at a point on a slope as a small surface clump of snow and sweeps up larger and larger snow quantities as it falls.

Avalanche, Slab A sliding mass of snow that breaks away as a discrete, cohesive layer from the clearly defined line of failure (the fracture line, or crown face).

Bed Surface The main sliding surface of the slab, usually quite smoothed and compacted by the sliding blocks.

Boot Packing Trampling the snow by boot to increase density and strengthen the snow in avalanche starting zones.

Calorie The quantity of heat needed to raise the temperature of one gram of water one degree centigrade. (1 BTU = 252 calories)

Chinook Warm dry wind caused by descending air flowing down the lee side of a mountain range.

Climax Avalanche Avalanche which involves layers of older snow.

Coarse Grained Old Snow Old snow which is at or near the end product of the equitemperature metamorphism process plus melt metamorphism.

Cohesion The condition where individual particles are united or stuck together to form a coherent unit.

Concave Curved like a segment of the interior of a circle or hollow sphere.

Condensation The phase change from gas (water vapor) to liquid (water) or to solid (ice).

Convex Having a surface that is curved or rounded outward.

Cornice An overhanging mass of snow deposited on the lee of a ridge or other obstacle to drifting snow.

Creep Internal, viscous deformation of the snow cover under the influence of gravity resulting in downhill displacement on a slope.

Creep Tension Tensile stress in snow caused by variations in creep velocity.

Crown The snow which remains on the slope above the crown surface.

Crown Surface The top fracture surface of the slab, usually a smooth clean cut, 90 degrees to the bed surface.

Crystal Any domain of ice which has a common orientation of the orderly array of molecules which makes up the solid structure.

Delayed Action Avalanches Avalanches which occur other than during or immediately after a storm.

Density Mass per volume, in scientific units, Kg M-3 (kilograms per cubic meter). The density of water is 1000 Kg M-3. New snow density is often 60 to 80 Kg M-3.

Deposition The direct formation of ice from the vapor phase.

Depth Hoar Large-grained, faceted, cup-shaped crystals near the ground. Depth hoar is caused by large temperature gradients within the snowpack, usually in the early winter, by large temperature differences between the warm ground and the cold snow surface.

Direct Action Avalanches Avalanches which occur during or immediately after a storm.

Diurnal Recrystallization Faceted snow created by large temperature gradients in the surface of the snow from strong heating and cooling of the snow surface between day and night.

Dust Cloud Mixture of air and snow particles accompanying an avalanche.

Elastic In the case of snow, capable of returning, to some limited extent, to original shape after being deformed.

Elastic Deformation The temporary change in shape produced in an elastic substance by a stress that is less than the elastic limit of the substance.

Equilibrium Process or Equilibrium Metamorphism Process whereby snow crystals tend in time to become rounded particles of ice. This process takes place in bodies of snow not far from a uniform temperature. If this type of metamorphism continues, the end result is a stronger, coherent snow structure due to sintering between the grains.

Evaluation Assessment of current snow stability.

Evaporation The phase change from liquid (water) to gas (water vapor).

Faceted Snow Angular, larger-grained snow with poor bonding created by large temperature gradients within the snowpack. Different kinds of faceted crystals include depth hoar, diurnal recrystallization, melt layer recrystallization, and radiation recrystallization .

Failure Plane The surface between two usually cohesive snow layers (slabs) and often the sliding surface of a slab avalanche.

Fine Grained Old Snow Snow in middle to advanced stages of ET metamorphism.

Firn Snow, usually on a glacier, that has survived a complete season of ablation.

Firnspiegel A thin, usually fragile layer of smooth, clear ice that forms over a melting snow surface with just the right heat-balance conditions. Sometimes called "glacier fire" when it reflects sunlight like a mirror.

Flank The side boundary of a slab.

Forecast To predict the occurrence of an avalanche event(s).

Fracture Cracking of snow under stress.

Front A discontinuity between air masses.

Glide The slow sliding of the snow cover along the ground under the influence of gravity on a slope. The amount of glide is highly dependent on ground roughness and character. It is not the same as creep. The total displacement of the snow surface on a slope is the sum of creep, glide, and settlement.

Grain A mechanically separate particle in the snow cover, may consist of several crystals.

Ground Avalanche A slab avalanche in which the ground is the bed surface.

Ground Surface Bottom boundary of the snow pack.

Hard Depth Hoar Dense, fine-grained old snow cemented by tiny vapor-deposited crystals between the grains. Like depth hoar, it is generated by strong temperature gradients, but results in a gain instead of loss of strength.

Hazard The risk of avalanche accident when man or his works is exposed to snow avalanches.

Heat Of Fusion The amount of heat needed to melt a unit mass of a substance at its normal melting point. For ice = 80 calories/gram.

Heat Of Vaporization The heat required to vaporize a unit mass of a substance (e.g., water). For water = 600 calories/gram.

Homogeneous Similar throughout (referring to the snow pack).

Inversion Cold air near the ground with warmer air above.

Isothermal Same temperature throughout.

Kinetic Growth Metamorphism of snow crystals by recrystallization under the influence of strong temperature gradients.

 

Kinetic Process or Kinetic Metamorphism Snow which, because of a steep enough temperature gradient is metamorphosing into faceted grains.

Lee The side of a mountain protected from the wind.

Melt Change of state from a solid to a liquid.

Melt-Freeze Process or Metamorphism Large-grained, rounded, and clustered crystals formed by the repeated melting and freezing of the snow. It typically forms in spring conditions near the surface of the snow. Also called clustered snow or corn snow.

Melt Layer Recrystallization Faceted snow created by large temperature gradients between a wet, warm snow layer and the overlying colder new snow. This typically occurs when a cold storm deposits snow on top of a wet, warm rain crust.

Metamorphism Changes in size and shape of snow crystals or grains governed by the internal environment of the snow cover.

Negative Radiation Balance The snow surface loses heat by infra-red radiation faster than it is gained by radiation at all wave lengths.

 

Névé The partially compacted granular snow that forms the fine grained old snow.

Orographic Lifting Forcing of air up and over terrain barriers.

Point Release A loose avalanche originating at a point and spreading out as it descends. The snow structure involved is cohesionless.

Positive Radiation Balance The snow absorbs more radiant heat than is lost to space.

Precipitation Intensity Rate of precipitation measured in amount of water per unit time.

Radiation Heat that is emitted as electromagnetic radiation from any body not at absolute zero.

Radiation Balance Algebraic sum of all the radiant heat inputs and losses at a surface (e.g., a snow surface).

Radiation Cooling Loss of heat from a snow surface when the radiation heat balance is negative for that surface. Loss of energy in the long-wave (infrared) part of the spectrum dominates this kind of cooling. It is especially strong with dry air, clear skies, and high altitudes.

Radiation Recrystallization A thin layer of faceted snow created in the top centimeter or two of the snow surface by strong heating by the sun combined with strong surface cooling from outgoing radiation. This is usually a high elevation phenomenon.

Relative Humidity Ratio (in percent) of actual amount of water vapor in a body of air to the maximum amount that body can hold at a given temperature. Relative humidity varies with temperature for a given amount of water vapor.

Release Zone The part of the slope, usually the upper part, where the avalanche breaks loose. This may apply to both loose snow and slab avalanches.

Rime A dense, fine-grained deposit built up from the freezing of supercooled cloud droplets onto exposed objects, including falling snow crystals.

Saturation A parcel of air at a given temperature is said to be saturated with water vapor at that temperature when the addition of any more water(or a decrease in the temperature) will lead to condensation.

Saturation Vapor Pressure Water vapor pressure in the atmosphere at which saturation is achieved for a given temperature.

Settlement Compaction of the snow cover through metamorphism and compression. On a sloping surface, additional deformations occur from creep and glide.

Sintering The process of vapor diffusion which joins individual snow grains together forming an ice skeleton of connected grains. The eventual effect is a stronger snow layer.

Slab A cohesive layer or layers of snow.

Slab Avalanche An avalanche involving a discrete, cohesive layer of snow. The presence of a crown surface, or fracture line, is the key indicator.

Snow Climate The characteristic combination for a given area of winter weather parameters, such as temperature, wind, snowfall amounts, and percentage of clear skies.

Snow Crystal An individual particle of snow, often with some form of hexagonal symmetry, whose water molecules are arranged in a common crystallographic orientation.

Snow Density The weight of snow mass per unit volume. Very low density snow "fluff" may weigh 20 to 40 kilograms per cubic meter. Typical new snow densities are 70 to 120 Kg M-3, while dense old snow may reach 400 to 500 and firn 600 Kg M-3. The density of bubble-free ice is 917 Kg M-3.

Snow Flake An aggregation of individual snow crystals.

Snowslide Synonymous with avalanche.

Snow Stake Flat square board which lies on the snow with a measuring stick fastened vertically. Used to measure increments of new snow falling on an old snow surface.

Snow Surface Top boundary of the snow pack.

Stable A snow slope which is well anchored and possesses sufficient internal strength so as not to be susceptible to avalanching.

Starting Zone The area near the top of an avalanche path.

Stauchwall The downslope boundary of the slab, often difficult to identify since it is plowed over by the sliding blocks.

Stress The physical pressure, pull, or other force exerted on a substance.

Strain Mechanical deformation within a material as the result of stress.

Study Plot Flat, sheltered clearing used for gathering snow and weather data.

Sublimation The phase change from solid (ice) directly to gas (water vapor) at subfreezing temperatures without an intermediate liquid (water) phase.

Substratum Layer or surface of snow within the snow pack.

Supercooled Water that remains liquid when cooled below the freezing point is said to be supercooled. Tiny droplets, such as those that make up clouds, are much more easily supercooled than larger masses of water.

Supersaturation Amount of water vapor in excess of saturation.

Sunballs Balls of wet or damp snow which roll down a snow slope.

Surface Avalance An avalanche involving the surface layer or layers of the snowpack.

Surface Heat Balance The algebraic sum of heat inputs (plus) and losses (minus) to a surface. For snow, these include sensible heat by conduction from below or turbulent transfer from the atmosphere, condensation or evaporation, and both long (infrared) and short (visible) radiation. The snow surface heat balance can be either positive (gains heat) or negative (cools) according to circumstance.

Surface Hoar Deposition of water vapor from the air as ice crystals on to a cold surface (e.g., a snow surface).

Temperature Gradient Change of temperature per unit change in depth (referring to a snowpack)

Trigger A force or event which initiates an avalanche.

Viscosity The internal friction of a fluid. Snow is, in part, a viscous substance.

Wind Blast The air pressure wave which may precede (or accompany) an avalanche.

Wind Scoop A hollow space formed in a snowdrift on the windward side of an obstacle.

Windward The side of a mountain exposed to a wind.