Resources
Publications:
Trip Planning:
Glossary:
Related Organisations:
Be kept in the loop:
Email Sign up
Getting the goods on the weather forecast is never easy. Get your mountain weather forecast right now!
Met Service
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. |
