Provided by the
AMERICAN AVALANCHE INSTITUTE
Wilson, Wyoming 83014
GLOSSARY OF TERMS
AGE HARDENING The strengthening of snow after mechanical compacting (e.g., boot packing and ski packing).
ARTIFICIAL AVALANCHE Triggered by an animal, man or his equipment.
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 densify 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.
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.
CONVEX Having a surface that is curved or rounded outward.
CREEP Viscous deformation which takes place within the snow cover under the influence of gravity.
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.
COARSE GRAINED OLD SNOW Old snow which is at or near the end product of the equitemperature metamorphism process plus melt metamorphism.
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 End product of temperature gradient metamorphism. Large cohesion-less grains with facets and stepped surface.
DIRECT ACTION AVALANCHES Avalanches which occur during or immediately after a storm.
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.
EQUITEMPERATURE (ET) 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.
FINE GRAINED OLD SNOW Snow in middle to advanced stages of ET metamorphism.
FIRN SPIEGEL Thin layer of clear ice on the surface which permits sunlight to pass through to cause melt in the subsurface layers. A highly-reflective, mirror-like surface.
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, downhill movement of the entire snow cover over the ground surface.
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.
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.
LEE The side of a mountain protected from the wind.
MELT Change of state from a solid to a liquid.
METAMORPHISM Changes in the snow texture caused by pressure and temperature conditions.
NEGATIVE RADIATION BALANCE The snow surface loses heat by infra-red radiation faster than it is gained by radiation at all wave lengths.
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 RECRYSTALIZATION Recrystalized snow on or near the snow surface caused by an extreme temperature gradient induced by radiation processes.
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.
RIME Accretion of frozen supercooled water droplets on a snow crystal or any exposed surface.
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 The decrease in thickness of a snow layer due to gravity and metamorphism.
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 FLAKE Aggregation of several 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 To pass directly from the solid to the gaseous state.
SUBSTRATUM Layer or surface of snow within the snow pack.
SUPERCOOLED Pure water that is cooled below the normal freezing point but remains liquid.
SUPERSATURATION Amount of water vapor in excess of saturation.
SUNBALLS Balls of wet or damp snow which roll down a snow slope.
SURFACE AVALANCHE An avalanche involving the surface layer or layers of the snowpack.
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)
TEMPERATURE GRADIENT (TG) SNOW Snow which, because of a steep enough temperature gradient is metamorphosing into depth hoar.
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.
WINDWARD The side of a mountain exposed to a wind.