New Zealand Mountan Safety Council


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Technical Descriptors 

Age Hardening  The strengthening of snow after mechanical compacting (e.g., boot packing and ski packing). 
Artificial Avalanche  Triggered by an animal, human or their 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. 
Bridging  The ability of a relatively stiff slab to spread a person's weight over a wider area, making that person less likely to trigger an avalanche. 
Calorie  The quantity of heat needed to raise the temperature of one gram of water one degree centigrade. (1 BTU = 252 calories; 1 Joule = 0.2388 calorie) 
Climax Avalanche  Avalanche that involves layers of older snow. 
Course Grained Old Snow  Old snow, which is at or near the end product of the rounding 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. 
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 that remains on the slope above the crown surface. 
Crown Wall  The top fracture surface of the slab, usually a smooth clean cut, 90 degrees to the bed surface. 
Crystal  Any domain of ice or snow, 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/m3 (kilograms per cubic meter). The density of water is 1000 kg/m3. 
Deposition  The direct formation of ice from the vapour phase. 
Depth Hoar  Large-grained, faceted, cup-shaped crystals near the ground. 
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. 
Evaluation  Assessment of current snow stability. 
Faceted Snow  Angular, large-grained snow with poor bonding created by large temperature gradients within the snowpack. 
Fine Grained Old Snow  Snow in middle to advanced stages of rounding processes resulting from weak temperature gradients in the snowpack. 
Firnspiegel  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 which may consist of several crystals. 
Graupel  Rimed precipitation particles that looks like little Styrofoam balls. 
Ground Avalanche  A slab avalanche in which the ground is the bed surface. 
Ground Surface  Bottom boundary of the snowpack. 
Hazard  The risk of avalanche accident when humans or their works are 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 = (MJ/kg; 334 J/g at 0C). 
Heat of Vaporization  The heat required to vaporize a unit mass of a substance (e.g., water). For water = (MJ/kg; 2501 J/g at 0C). 
Homogenous  Similar throughout (referring to the snowpack). 
Inversion  Cold air near the ground with warmer air above. 
Isothermal  Same temperature throughout - usually referring to 00C for a melting snowpack. 
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 vapour pressure and temperature conditions. 
Norwester  Warm dry wind caused by descending air flowing down the lee side of a mountain range. (Föhn in Europe, Chinook in North America). 
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 Recrystalisation  Recrystalised snow on or near the snow surface caused by an extreme temperature gradient induced by radiation processes. 
Rain Crust  A clear layer of ice formed from rain on the snow surface, which later freezes. 
Relative Humidity  Ratio (in percent) of actual amount of water vapour 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 vapour. 
Rime  Accretion of frozen super cooled 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 vapour at that temperature when the addition of any more water (or a decrease in the temperature) will lead to condensation. 
Saturation Vapour Pressure  Water vapour 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 vapour 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 Board  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 Flake  Aggregation of several snow crystals. 
Snow Slide  Synonymous with avalanche. 
Snow Surface  Top boundary of the snowpack. 
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 ploughed 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. 
Subtratum  Layer or surface of snow within the snow pack. 
Sun Crust  A thin, clear layer of ice formed by radiation from the sun followed by refreezing. 
Supercooled  Water that is cooled below the normal freezing point but remains liquid. 
Supersaturation  Amount of water vapour in excess of saturation. 
Sunballs (Rollerballs)  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 vapour from the air as ice crystals on to a cold surface (e.g., a snow surface). 
Temperature Gradient  The change of temperature over a certain distance within the snowpack. 
Threshold  The amount of snow depth required in a start zone, relative to the terrain anchoring features, for avalanching to occur. 
Trigger  A force or event that initiates an avalanche. 
Viscosity  The internal friction of a fluid. Snow is, in part, a viscous substance. 
Whumpf or Whumpfing  Refers to the sound or feeling associated with fracture propagation due to a skier's weight, a.k.a. rapid settlement. 
Wind Blast  The air pressure wave that may precede (or accompany) an avalanche. 
Windward                                                                         The side of a mountain exposed to a wind. 

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