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Glossary

Avalanche forecasts and backcountry safety bulletins are fraught with tricky words which, to a newcomer, can seem overwhelming. But they needn't be — here's our handy guide to demystifying the terminology so you know your terrain trap from your depth hoar.

Anchors

Trees, rocks, or bushes that help hold the snowpack in place.

Aspect

The direction a slope faces with respect to sun and wind. For instance, a slope may be north-facing, or it may be a leeward slope.

Avalanche Air Bag

Victims caught in an avalanche pull a rip cord to deploy this air bag mounted on their packs to help the victims rise to the surface of avalanche debris.

Avalung™

A device that delays the dangerous buildup of carbon dioxide for a buried avalanche victim. Buried victims breathe through a tube that exhausts carbon dioxide on the back side of their bodies and allows them to breathe in air from the front side.

Beacon

An electronic device used to locate buried avalanche victims. Also called transceivers or locators.

Bed Surface

The snow surface on which an avalanche slides.

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. However, if an avalanche is triggered, it is often larger and more dangerous than an avalanche with a softer slab.

Cirque

A large bowl-shaped concavity formed by glaciers in mountainous terrain.

Climax Slab Avalanche

An avalanche that involves the entire season’s snowpack and slides either on a weak layer near ground (usually depth hoar), or firn, or on a glacier.

Clustered Snow

See melt freeze snow.

Collapsing

Also called “whoomphing”. A snowpack collapsing onto a buried weak layer, which is an obvious sign of instability. It often produces an audible whoomphing sound. etimes incorrectly called “settling.”)

Concave Slope

A slope shaped like the interior of a circle or sphere. When descending it becomes less steep. A double concave slope is shaped like the inside of a bowl.

Continental Climate

The snow climate found in mountains far from the influence of the ocean’s weather. Characterized by thin snowpacks, cold temperatures, and more persistently unstable snowpack.

Convex Slope

A slope shaped like the exterior of a circle or sphere. When descending it becomes steeper. A double convex slope is shaped like the outside of a basketball.

Corn Snow

See melt freeze snow.

Cornice

An overhanging mass of snow created by the wind, usually near a sharp terrain break such as ridge.

Couloir

A steep gully in alpine terrain. In winter, a couloir is usually filled with snow bound by rocks on either side.

Cross Loaded

Snow blown by the wind across a slope, depositing drifts on the sides of gullies or other terrain features.

Crown

The snow that remains on the slope above the crown face of an avalanche.

Crown Face

The top fracture surface of a slab avalanche. Usually smooth, clean-cut, and angled 90 degrees to the bed surface. See fracture line.

Deep Slab Avalanches

Avalanches that break deeply into old weak layers of snow that formed some time ago. Often persistent weak layers such as facets or surface hoar are the culprit weak layer.

Delayed Action Avalanches

Avalanches that occur a day or more after a storm, usually with persistent weak layers such as faceted snow or surface hoar as the culprit.

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 caused directly by storms, usually from loading of new, wind-blown snow or rain.

Diurnal Recrystalization

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

Dry Avalanches

An avalanche that occurs in snow of below-freezing temperature.

Equalibrium

A situation that occurs when the snowpack has a low temperature gradient, which metamorphoses rounded crystals instead of faceted crystals. Usually, less than 1°C per 10cm is considered to be equilibrium conditions.

Faceted Snow

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

Firn

Snow that did not melt in the previous summer. After one or more seasons, firn can become glacial ice.

Flagged Trees

Trees with the branches ripped off on the uphill side, indicating that they have been hit by an avalanche in the past.

See induction search.

Fracture

The separation of a solid body into two parts under the action of stress. Fracture usually requires both initiation and propagation of the fracture. Fracturing must occur for a snow avalanche to release.

Fracture Line

The visible crack in the snow after a slab avalanche has released. The fracture line is composed of the crown face, flanks, and a staunchwall of the avalanche.

Glide

The entire snowpack slowly moving as a unit on the ground, similar to a glacier. This is usually caused by melt water lubricating the snow-ground interface. Slabs of snow can release catastrophically at random intervals.Not to be confused with climax slab avalanche.

Grain

The smallest distinguishable ice particle in a snowpack. Synonymous with crystal in avalanche applications.

Graupel

New snow that looks like little Styrofoam. Mechanically, it behaves like ball bearings but it can also form slabs.

A rescue technique with beacons in which the rescuer uses a series of perpendicular grids to find the transmitting beacon.

Hang Fire

Snow that remains above a crown face after an avalanche. Hang fire seldom avalanches naturally but if disturbed it can sometimes release, which is a danger to rescuers below or to avalanche geeks doing a fracture line profile.

Hard Slab

A snow slab having the density of 300kg/m³ prior to avalanching. Hard slabs usually include layers of older, harder snow but they can be formed within new snow by strong winds. Hard slabs are considered difficult to manage because because they tend to break above you instead of at your feet as soft slabs do.

Humidity

The amount of water contained in air. See also relative humidity.

Icefall

The glacial equivalent of a waterfall. A glacier slowly moves over a drop-off, such as a cliff or bulge, creating jumbled ice that can calve off ice blocks.

Icefall Avalanche

An avalanche of ice falling from an icefall.

A rescue technique with beacons in which the rescuer follows the curving magnetic lines of force that emanate from a transmitting beacon. Also called tangent line search or flux line search.

Intermountain Climate

The snow climate commonly found in intermountain areas midway between maritime climate and continental climates. Characterized by intermediate snow depths and intermediate temperatures.

Isothermal

The state of equal temperature, that is, temperature does not change with depth. This usually occurs in spring after the entire snowpack warms up to 0°C.

Leeward

The downward side of an obstacle such as a ridge. Wind can deposit snow onto leeward terrain, creating wind slabs or wind pillows, which are often dangerous.

Loading

The addition of weight on top of a snowpack, usually from wind, new snow, transport, or rain.

Loose Snow Avalanche

An avalanche of loose snow—not a slab avalanche. Small loose snow avalanches are called stuffs. They often start from a point and fan out. Also called point releases.

Maritime Climate

The snow climate near the oceans, characterized by deep snow and warm temperatures. Also called coastal climate.

Melt Freeze Snow

Large-grained, rounded, and clustered crystals formed by the repeated melting and freezing of the snow. It typically forms in spring conditions near the snow surface. 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. Also called melt layer recrystallization.

Persistent Weak Layers

Weak layers within the snowpack that continue to produce avalanches several days after they were subjected to a rapid change such as loading of new or wind-blown snow or a rapid temperature rise. Persistent weak layers include faceted snow and surface hoar. These layers account for about 80 percent of avalanche fatalities in North American and Europe.

Point Release

See sluff.

Probe

A rod used to probe avalanche debris for buried victims.

Propagation

The spreading of a fracture or crack within the snowpack. During very unstable conditions, fractures can propagate for long distances.

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 at lower latitudes.

Rain Crust

A clear layer of ice formed from rain on the snow surface, which later freezes. Not to be confused with sun crust of melt-freeze crust.

Relative Humidity

The amount of water air holds compared with the amount of water it can hold at certain temperature.

Remote Trigger

When a person triggers an avalanche some distance away. Sometimes incorrectly called sympathetic trigger pathetic trigger is when one avalanche triggers another avalanche some distance away.) Remote triggers are usually seen as a sign of very unstable conditions.

Rime

A Styrofoam-textured snow that forms on solid surfaces during storms. Rime forms when supercooled water droplets in the clouds freeze upon contact with a surface.Rime can form on the snow surface, trees, and other snowflakes at they fall.

Runout

The zone where an avalanche loses speed and deposits debris.

Runout Angle

The angle, measured from horizontal, between the toe of an avalanche and the crown. Also called the “alpha” angle.

Sastrugi

Wind-eroded snow, which often looks rough as if it were sandblasted.

Settlement

See settling.

Settling

The slow deformation and densification of snow under the influence of gravity. Sometimes settlement is incorrectly used to describe collapsing or whoomphing snow.

Ski Cut

See slope cut.

Slab

A relatively more cohesive layer of snow overlying a relatively less cohesive layer of snow. You can also think of a slab as strong snow sitting on top of weak snow. A slab avalanche is similar to a magazine sliding off an inclined table.

Slope Cut

Moving rapidly across an avalanche starting zone, aiming at safe terrain on the side, so that if an avalanche breaks, your momentum will hopefully carry you off the moving slab onto safe terrain. Skiers call them ski cuts.

Sluff

A small, loose snow avalanche—not a slab. Sometimes called “point releases.”

Slush Flow Avalanche

An avalanche composed of slush—very saturated snow. They usually occur in arctic climates on permafrost soil when dry cold snow becomes rapidly saturated with water in spring. Slush avalanches can run long distances on very gentle slopes.

Snow Profile

See snowpit.

Snowpit

A hole dug in the snow to examine snowpack properties. This is a very powerful tool used by avalanche forecasters and recreationists. Also called snow profile.

Soft Slab

A snow slab with a density less than 300kg/m³. Soft slabs are usually composed of new snow without strong winds. Soft slabs tend to be more manageable than hard slabs because they tend to break at your feet instead of above you.

Stepping Down

A slab avalanche sliding a short distance and breaking down into deeper weak layers, forming a stair-step pattern on the bed surface.

Sun Crust

A thin, clear layer of ice formed by radiation from the sun. Not to be confused with melt-freeze crust or rain crust.

Surface Hoar

Frost that forms on the snow surface during calm, clear humid conditions.When buried, surface hoar forms a thin, persistent weak layer within the snowpack—a very dangerous weak layer. Also called frost,hoar frost, or feathers.

Sympathetic Trigger

One avalanche triggering another avalanche some distance away. Sometimes incorrectly called remote trigger ote trigger is when a person triggers another avalanche some distance away.)

See induction line search.

Temperature Gradient

The change of temperature over a certain distance within the snowpack. Large temperature gradients erally more than 1°C per 10cm.) metamorphose crystals into weak, angular, faceted snow. Small temperature gradients erally less than 1°C per 10cm.) are called equilibrium conditions, which metamorphose the snow into more well-bonded, rounded crystals.

Terrain Trap

Terrain in which the consequences of an avalanche are especially hazardous. Common terrain traps include gullies, an abrupt transition, or an avalanche path that terminates in trees, a crevasse field, or a cliff.

Trigger

A disturbance that initiates fractures within the weak layer, allowing the slab to slide off a slope. In 93 percent of avalanche accidents, the victim or someone in the victim’s party triggers the avalanche. Natural triggers include new snow, cornice falls, win, rapid warming, or percolated water. Contrary to popular myth, avalanches are not triggered by noise.

Trigger Point

A place where a person can trigger an avalanche, usually in an area where the slab is thinner or is poorly bonded to the underlying snow. Sometimes called “Sweet spots” or “deceit areas” or “super weak zones.”

Upside Down Snow

New snow with relatively stronger snow on top of relatively weak snow.

Weak Interface

A poor bond between two layers of snow without a distinct weak layer.

Weak Layer

A relatively less cohesive layer of snow underlying a relatively more cohesive layer of snow. In a slab avalanche, the weak layer fractures, allowing the overlying slab to slide off the slope.

Wet Avalanche

An avalanche caused by snow losing its strength after becoming damp, moist, or saturated with water.

Whoomph

Snowpack collapsing on a buried weak layer—an obvious sign of instability.

Wind Loading

Loading of weight on top of a snowpack when wind drifts snow onto lee terrain. Wind can deposit snow much more radially than snow falling from clouds. Wind loading is a common denominator in most avalanche accidents.

Wind Slab

A slab of snow formed when wind deposits snow onto lee downwind) terrain. Wind slabs are often smooth and rounded and sometimes sound hollow.

Windward

The upwind side of an obstacle such as a ridge. Usually snow is eroded from windward slopes making them relatively safer.