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NOAA NWS

An above freezing layer (AFL) is a horizontal layer of air at some altitude in the atmosphere with a temperature above 0 C. It lies between two layers of sub-freezing air. AFLs are a common result of atmospheric subsidence and incoming warm fronts overriding cold air entrenched within valley bottoms. A temperature inversion is always present at the bottom of the layer.

Note that warm air at valley bottom is not referred to as an above freezing layer because air is normally warmest at low elevations.

Elevated layers of above freezing air play an important role in the type of precipitation observed at a given altitude. Above an AFL, precipitation falls as snow. It then transitions to rain within the AFL (assuming the layer is of sufficient depth), and then to ice pellets or freezing rain below the AFL.

Credit

Nolan Atkins (Lyndon State College)

An air mass is a large region of air with minimal horizontal variation in temperature and moisture. An air mass forms when air remains stationary for enough time (usually many days or weeks) to exchange energy (absorbing heat or cooling) and moisture with the surface below. 

Air masses are classified in two ways, based on where they originated: 

• By moisture: Air masses that form over land are called continental (dry), while ones that form over oceans are called maritime (moist).
• By temperature: Whether they are from tropical (warm), polar (cold) or arctic (extremely cold) regions.

There are a total of six air mass types: continental tropical (cT), maritime tropical (mT), continental polar (cP), maritime polar (mP), continental arctic (cA), and maritime arctic (mA). Maritime arctic air masses affect BC’s coast when continental arctic air pushes through coastal fjords. Strong to very strong northerly/northeasterly outflow winds occur and localized bands of heavy snow (known as streamers) pummel communities and mountains downstream (e.g. eastern Vancouver Island).

The alpine consists of wide expanses of open, exposed terrain with few or no trees. Where it exists, it is the highest elevation band of a mountain or range. Not all mountains have alpine terrain, but it may be the dominant terrain in other mountains or ranges, particularly in northern Canada.

Alpine elevations contain the terrain that is usually the most exposed to sun, wind, cold, and precipitation. 

The lower extent of the alpine elevation band is highly variable. It transitions to the treeline elevation band where vegetation, especially trees, overcomes the more open and rocky terrain of the alpine.

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Silas Patterson

Aspect refers to the direction a slope faces. It is an important terrain attribute as it relates directly to both sun and wind exposure, as well as to snow metamorphism. The presence of sun crusts, surface hoar, near-surface faceted snow, solar warming, and wind effect can all be highly dependant on aspect.

Sunny, or solar, aspects:

• Are more likely to see temperature and sun crust formation than shaded aspects;
• Often hold a shallower snowpack than shaded aspects;
• Can experience rapid warming during periods of strong sunshine;

Shaded aspects:

• Generally face north and east.
• Often hold and preserve persistent weak layers for longer than solar aspects.
• Are more likely to see surface hoar or faceted snow form.

The solar effect on slopes is dependent on aspect. Early in winter, the sun mostly impacts south and west facing slopes. As the winter progresses and days lengthen, eastern aspects will also see sun affects in the morning, while the effect on west-facing slopes is enhanced in the afternoon.

Avalanche forecasts specify which aspects are affected by each avalanche problem. This helps  forecast readers manage their exposure to avalanche hazard in the backcountry.

There are several ways to determine what aspect you’re on: 

• Compare your position to the sun’s position in the sky;
• Use a compass;
• Check your position on a GPS device or smart phone app.

Credit

Wikihow.com, modified by Lisa Erven

Atmospheric convergence refers to two or more streams of air flowing into one another, or when stronger winds flow into slower winds. This piling up of air at low levels of the atmosphere results in atmospheric lift, or rising air. As air converges and lifts, cooling takes place, leading to condensation, cloud formation and, ultimately, precipitation. 

Atmospheric convergence is one of the key processes involved with storm development and precipitation. In the mountain environment, convergence occurs on windward slopes and at the heads of valleys and fjords, leading to stronger winds and enhanced precipitation in those locations.

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Cengage Learning

Atmospheric lift is a general term referring to a group of processes that cause air masses to rise upwards in the atmosphere. It is significant because it leads to the formation of precipitation. Rising air cools, losing its capacity to hold water vapour, and the vapour turns into water droplets or ice crystals. Clouds form and, when the weight of water droplets or ice crystals becomes too great, they fall as precipitation. 

The four processes responsible for atmospheric lift are convection, convergence, interaction with topography, and the movement of weather fronts.

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Penn State University Meteorology Department

Credit

Environment and Climate Change Canada

Atmospheric pressure is a fundamental weather element and refers to the weight of a column of air pressing down on a given area. It is commonly expressed in units of millibars (mb). A surface weather map shows the pressure pattern as measured by weather stations (converted to sea level pressure to eliminate changes in elevation) across the globe, with isobars connecting points of equal pressure. 

Not only does the current pressure pattern act as a useful indicator of likely weather, but changes in the pressure pattern indicate the weather that is likely to come.

If pressure is falling, it usually signifies the approach of a low pressure system, and its associated deteriorating weather.

Rising pressure comes with many meanings. Following a passing front or low pressure system, surface pressure rises and conditions generally improve. However, a rapid increase in pressure may bring a burst of strong winds. In winter, an arctic ridge of high pressure seeping down from the north may bring harsh temperatures and persistent valley cloud, but sunshine in the alpine.

Credit

Environment and Climate Change Canada

Atmospheric Rivers (ARs) are long, narrow plumes of moisture originating from the tropics that deliver heavy amounts of precipitation and mild air. A well-known example is the “Pineapple Express” that originates near Hawaii and delivers heavy rain and snow to the west coast of North America.

ARs are responsible for 30-50% of our annual precipitation on the west coast. The strongest ones can deliver over 200mm of precipitation in a period of 36 to 48 hours. Given their warm source region, freezing levels rise significantly with the arrival of atmospheric rivers--typically over 2,500m along the coast and over 1,500m in the interior of BC.

About 25 to 35 ARs hit BC’s coast each year, primarily during the late fall and winter seasons. The most intense produce damaging floods, landslides, and widespread natural avalanche cycles. While the onset of an AR can deliver copious amounts of mountain snow, rising freezing levels typically result in rain to almost all elevations by the end of the storm. Freezing rain is another common occurrence in the valleys of the BC Interior, particularly when ARs are preceded by well-entrenched Arctic air.

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Wikihow.com, modified by Lisa Erven

Atmospheric subsidence is the downward movement (sinking) of air parcels in the atmosphere. It is the opposite of atmospheric lift. This movement compresses the air, which increases both the temperature of an air mass and its capacity to hold water vapour. The net effect is a lowering of an air mass’s relative humidity, and the dissipation of clouds and precipitation. 

While subsidence often leads to clear skies, a caveat is the formation of valley cloud and the associated drizzle or light, fine snow (frozen drizzle), and/or extensive valley fog. This happens because subsiding air acts as a lid, gradually lowering overtime, trapping moisture and pollution into a smaller and smaller volume. Additional water vapour content may be added from evaporation off open water bodies. When the weather is stagnant, subsidence and evaporation, combined with nighttime cooling, can lead to saturation (the formation of fog or valley cloud). This is common during fall and winter in the mountains. 

Timing and success of dissipation, or “burn off”, depends on the strength of the sun (solar angle, length of day), strength of the subsidence inversion, depth of the cloud/fog layer, and local wind patterns.

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Staff photo

An avalanche airbag is a piece of protective equipment designed to keep its wearer on or near the surface of a flowing avalanche. Airbags are normally integrated into a backpack and are deployed by pulling a trigger in the case of an avalanche incident. 

When the trigger is pulled, the airbag inflates, increasing the size of the wearer relative to other debris. Larger debris tends to rise above smaller debris, so this helps keep the wearer on or near the surface.

An avalanche cycle is a period of heightened avalanche activity. Avalanche cycles are often associated with weather changes such as snow, rain, wind events, or atmospheric or solar warming. Obviously, avalanche danger is increased during an avalanche cycle.

The North American Public Avalanche Danger Scale is a system that rates the avalanche danger based on the likelihood, size, and distribution of avalanches. It consists of five levels, from least to highest amount of danger: low, moderate, considerable, high, and extreme. In Avalanche Canada Forecasts, ratings are provided for alpine, treeline, and below treeline elevations.

Avalanche distribution refers to where an avalanche problem exists in a given area. It is referred to as either widespread, specific, or isolated:

• Widespread avalanche problems exist in most terrain.
• Avalanche problems with specific distribution only exist on certain aspects, at certain elevations, or in certain types of avalanche terrain.
• Isolated avalanche problems exist in limited terrain. A problem may be isolated due to limited distribution of the problematic snowpack structure, such as a patchy weak layer of surface hoar; or because it is confined to very specific terrain, such as steep leeward slopes.

The avalanche essentials are a transceiver, probe and shovel - the three key pieces of rescue gear that everyone should carry for winter backcountry recreation. They enable other members of a group to find and rescue a person buried in an avalanche using companion rescue techniques, maximizing the chances of a positive outcome following an avalanche incident. 

 These tools need to be carried on your person. The transceiver should be worn using its harness and switched on at the start of the day, and the probe and shovel should be carried in a pack. Training in the use of these devices as well as other elements of companion rescue is critical. 

Click here for a list of licensed Avalanche Canada Training providers.

An avalanche path is the area involved in an avalanche or a location where an avalanche can occur. Avalanche paths are subdivided into three sections: the start zone, the track, and the runout zone: 

• The start zone is where avalanches typically start. Significant characteristics of the starting zone include the slope incline, orientation to wind, orientation to sun, roughness of ground, forest cover, elevation, and trigger points.
• The track is the area covered by an avalanche in motion. It connects the start zone to the runout zone. 
• The runout zone is the part of an avalanche path where an avalanche slows down and stops. 

At treeline and below, large avalanche paths are frequently evident from their impact on forests. They can damage or destroy forests, resulting in treeless paths through the forest where avalanches regularly occur. 

Recognizing avalanche paths and the areas they cover is an important part of managing avalanche terrain.

The full length of an avalanche path. Photo by Jennifer Coulter.

Credit

Jen Coulter

An avalanche probe is a long, thin, collapsible rod used by a rescuer to pinpoint the location of a burial subject beneath the snow surface. 

Avalanche probes are normally made of sections of rugged, lightweight material, such as aluminum or carbon fibre, and threaded by a cable that is used to quickly guide and lock the sections together when the probe is deployed. 

Avalanche probes are also used to make point observations of the snowpack’s depth, as well as the depth and hardness of snowpack layers.

Avalanche problems are eight types of snowpack instabilities commonly encountered in the mountains and referenced in avalanche forecasts. They are diverse and include instabilities such as persistent slabs and wind slabs. Some characteristics and management strategies are shared between problems while others are more specific to particular problems. The eight problems are:

• Storm slab
• Wind slab
• Wet slab avalanches
• Persistent slab
• Deep persistent slab
• Loose dry avalanches
• Loose wet avalanches
• Cornices

Avalanche Canada’s daily forecasts discuss up to three avalanche problems, in decreasing order of concern. The forecast will indicate where each problem might be found (elevation and aspect), the likelihood of triggering or being exposed to an avalanche, and the size of avalanche expected if triggered. By understanding what kind of avalanche danger is present and what kind of terrain it can be found in, you can better match your terrain choices with avalanche conditions.

Credit

Avalanche Canada South Rockies field team

An avalanche shovel is a specialized shovel which has a handle that can be removed from the blade, making it easy to store in a backpack. It is one of three avalanche safety essentials. Its main use is for companion rescue, though it is also used to dig pits for performing snowpack tests.

Avalanches are categorized into five size classes. As a general indication of their destructive potential:

• Size 1 avalanches are relatively harmless to people.
• Size 2 avalanches can bury, injure, or kill a person.
• Size 3 avalanches can bury and destroy a car, damage a truck, destroy a small building, or break a few trees.
• Size 4 avalanches could destroy a railway car, large truck, several buildings, or a forest area up to four hectares (~10 acres).
• Size 5 avalanches are the largest snow avalanches known. They could destroy a village or a forest area up to 40 hectares (~100 acres).

Avalanche survival techniques are actions a person can take to increase their chances of survival if caught in an avalanche. Check out Avy Savvy, our online avalanche tutorial, to learn about these techniques.

Video: Researcher Pascal Haegeli runs us through simple, challenging and complex terrain.

The Avalanche Terrain Exposure Scale (ATES) classifies routes and areas of terrain according to the severity of exposure to avalanche hazard.   It has three terrain classes: simple, challenging, and complex.

ATES ratings are available for many popular recreational areas in Canada. ATES rated terrain is displayed on the Online Trip Planner. ATES ratings are often available on signs posted at trailheads and on trail maps and guides.

ATES ratings are compiled by professionals who consider 11 weighted terrain parameters in ranking a trip or region:

• Slope angle
• Slope shape
• Forest density
• Terrain traps
• Start zone density
• Avalanche frequency
• Runout zone characteristics
• Interaction with avalanche paths
• Route options
• Exposure time
• Glaciation

Photo: An example of ATES-rates terrain.

See avalanche path.

The Avaluator™ is a portable decision-making aid designed by Avalanche Canada to help backcountry travelers make critical decisions before and during backcountry trips in avalanche terrain. It has three elements: the Trip Planner, the Slope Evaluation Card, and an accompanying booklet.

The Trip Planner (printed in the booklet) is designed to facilitate pre-trip route selection and works by combining the danger rating for the day with the ATES-rating of the terrain in the area under consideration.

The Slope Evaluation Card is designed to help make slope-specific decisions. It is printed on a card that can be easily carried on a trip. The card works by giving a recommendation (travel under normal caution, travel under extra caution or travel not recommended) that follows a localized assessment of avalanche conditions and terrain factors.

The booklet describes how to use the Trip Planner and Slope Evaluation Card. It also provides additional avalanche safety advice. Its four main sections are:

• Trip planning
• Identifying avalanche terrain
• Slope evaluation
• Good travel habits

The term basal facets refers to faceted snow found at the base of the snowpack. Basal facets are a common feature of thin snowpack areas exposed to prolonged cold temperatures. These conditions create a strong temperature gradient that encourages the faceting process in the lower snowpack.

Once formed, basal facets tend to persist for some time (frequently the whole season), they usually act as a persistent weak layer, and are often associated with deep persistent slab avalanche problems.

Credit

File photo

The bed surface is the surface on which an avalanche runs. The presence of a firm bed surface below a weak layer can contribute to the likelihood of slab avalanches. The bed surface is not to be confused with the failure plane, which is the weak layer found above the bed surface and below the slab. 

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Raveneye Photography

Below treeline (BTL) is the elevation band of a mountain or range that is covered by forest. From its upper to lower limits, it often covers the greatest area and the greatest variation in snowpack conditions of any elevation band. Below treeline areas usually see less sun, wind, cold, and precipitation than higher elevation bands.

See wind transport.

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K. Klassen

Challenging terrain is one of three levels of the Avalanche Terrain Exposure Scale (ATES). It involves exposure to well defined avalanche paths, starting zones, or terrain traps. In challenging terrain, options exist to reduce or eliminate exposure with careful route-finding. Any required glacier travel is straightforward but crevasse hazards may exist.

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Environment and Climate Change Canada

Credit

M. Pidwirny

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M. Pidwirny

The interface where a cold air mass advances towards a warm air mass is called a cold front. Cold fronts generate atmospheric lift as their associated cold, dense air mass wedges under and lifts the warmer, less dense air it advances upon. Cold fronts are represented on weather maps by a solid blue line with blue triangles pointing in the direction the front is moving.

Companion rescue refers to the rescue efforts made by the members of a group involved in an avalanche incident. The rescue group organizes and begins by watching the avalanche in progress, observing the path and the last seen point of anyone caught, scanning the debris for any signs of victims, and checking for any remaining avalanche hazard. The next steps are transceiver searching, pinpoint searching (or probing), and rescue digging.

Self rescue is often used as a synonym for companion rescue.

Credit

Mark Bender

Complex terrain is one of three levels of terrain classification described by the Avalanche Terrain Exposure Scale (ATES). It involves exposure to multiple overlapping avalanche paths, large expanses of steep, open terrain, multiple avalanche starting zones, and terrain traps below. Minimal options exist to reduce exposure. Complicated glacier travel through extensive crevasse bands or icefalls may be required.

This video explains how to conduct numerous snowpack tests the section on compression tests begins at 1:32. The video is by Mike Conlan, a forecaster for Avalanche Canada.

A compression test is a snowpack test that allows someone to test the strength of weak layers in the upper snowpack. It involves applying an incremental load to a narrow column of snow and then monitoring it for its tendency to fracture, as well as for the character of any fracture that occurs. A sudden result indicates greater instability than a resistant result.

Photo: Conducting a compression test. By Mark Grist

Considerable is the third of five levels on the avalanche danger scale. Under considerable danger, natural avalanches are possible and human-triggered ones are likely. Avalanche conditions are considered dangerous and careful snowpack evaluation, cautious route-finding and conservative decision making is essential. Small avalanches can occur in many areas, large avalanches in specific areas, and very large avalanches in isolated areas.

Decision making under considerable danger can be challenging. While conditions are dangerous, avalanches may be less widespread, smaller, or less likely than under high avalanche danger, potentially making the danger less obvious. Many slopes should be avoided when avalanche danger is rated considerable. Use the Avaluator Trip Planner and Slope Evaluation Tools to help decide on appropriate areas to travel. 

Historically, the highest number of avalanche fatalities have occurred when the danger was rated considerable.

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Versageek/Wikipedia Creative Commons

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Brockmann Consult

Convection is the vertical movement of air in which warm air rises (picture bubbles rising to the top in a boiling pot of water) and cool air sinks. Like other forms of atmospheric lift, convective lift causes rising air to cool and reach saturation, forming tall clouds and showery precipitation. Precipitation resulting from convective storms is known for being variable in location and duration, with locally intense, short-bursts of precipitation possible. This, in turn, creates greater uncertainty in forecasting precipitation. 

Strong convective lift can result in very tall cloud formations called cumulonimbus clouds, which are associated with thunder and lightning.

See Atmospheric Convergence.

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Wren McElroy

Credit

James Floyer

A convex slope is a slope that steepens as it descends. It is also known as a convexity, roll, or rollover. Avalanches often fracture at or just below the roll in the slope.

A cornice is an overhanging ledge or shelf of snow that usually forms over the lee side of ridges. A cornice forms as loose snow is transported over a ridge, where it collects and grows. Cornices have an unstable structure and they grow more unstable with loading or warming. When a cornice breaks, the resulting fall is usually powerful enough to trigger an avalanche.

Photos

Above: Cornices along a ridge. By Martina Halik

Below: It's important to stay far back from the edge as cornices can break surprisingly far in from the edge. By Avalanche Canada

See shooting cracks.

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Raven Eye Photography

Critical loading happens when new precipitation or windblown snow overloads a slab sitting on a weak layer. A slab that has reached its critical load may avalanche naturally or with a light trigger.

The suggested threshold for identifying critical loading is 30 cm or more of new snow, or significant wind, or rain during the 24-hour period prior to and up to the end of your day.

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Raven Eye Photography

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Mark Bender

Cross-loading is the result of wind transporting snow across a slope. During cross-loading, snow is picked up from the windward side of ribs and outcrops and is deposited in lee pockets. Cross-loading commonly contributes to wind slab formation.

Credit

Avalanche Quebec

The crown is the uppermost part of the fracture of a slab avalanche, where the slab breaks away from the snowpack overhead. It connects to the flanks of the avalanche on either side and sits perpendicular to the bed surface of the avalanche. The height of the crown is generally used when measuring the dimensions of an avalanche.

Crusts are hard layers of snow usually created by liquid freezing on or near the snow surface. They can also result from strong winds. 

Crusts can act as a firm bed surface for slab avalanches after they are buried in the snowpack. The presence of a weak layer on the crust can contribute to the likelihood of slab avalanches. If a crust is strong enough to support the weight of a person or machine, it may reduce the chance of triggering the weak layers beneath it.

Types of crust include:

Melt-freeze crust 

• Caused by surface snow melting and then refreezing as a result of warm air temperature.

Rain crust 

• A crust caused by rain falling on the snow surface and freezing.
• Can form anywhere rain falls and, unlike sun crusts, it can form on all aspects.
• Forms as a thin, breakable crust on the snow surface.

Sun crust

• A crust resulting from solar radiation.
• Unlike rain crusts, sun crusts are limited to sun-exposed slopes.
• Can form in below-freezing air temperatures by heat from the sun melting the snow.
• It generally results in a thin, breakable crust on the snow surface.

Wind crust

• Caused by the wind compacting snow into a hard surface.
  
  
  
  

The Dangerator™ is a tool to help estimate an avalanche danger rating for your local conditions when there is no forecast to advise you. The Dangerator guides you through a two-step process of combining weather data and field observations to assess whether the danger is moderate, considerable or high. The Dangerator does not include low or extreme avalanche danger.

Credit

Curtis Pawliuk

Deep persistent slabs are an avalanche problem defined by the presence of a weak layer, usually at or near the base of the snowpack, that resists bonding to an overlying slab over an extended time period. 

This weak layer is normally a product of snow grain metamorphism within the snowpack rather than the accumulation of new snow or formation of surface hoar on the snow surface. Once formed, a deep persistent slab can last for an extended period, sometimes throughout the entire season. They can survive numerous avalanche cycles and are inherently difficult to forecast.

A deep persistent slab problem often leads to a low-probability/high-consequence scenario, where the chances of triggering an avalanche are slim, but the destructive potential of any that are triggered is great. Managing this problem involves avoiding large avalanche paths and avoiding terrain where the problem exists.

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Alex Cooper

Dense trees are forested areas where the canopies of the trees are touching. Dense trees shelter the snow from the effects of wind and sunlight, they modify the snowpack by shedding snow (think tree bombs), and they inhibit the growth of surface hoar. 

In some cases, dense trees also work to anchor the snowpack in place, limiting the potential for a slab to dislodge from the snowpack. Traveling in dense trees can be a good choice in periods of elevated avalanche danger.

Depth hoar is an advanced, generally larger and weaker form of faceted snow crystal usually found near the bottom of the snowpack. Like basal facets, depth hoar exists as a persistent weak layer in the snowpack and is frequently associated with deep persistent slab avalanche problems.

Depth hoar crystals show visible striping known as striations and in later stages often form hollow cup-like shapes.

The dew point is the temperature to which air must be cooled to become saturated with water vapour. When the dew point is reached and the temperature is below freezing, water vapour is deposited on surfaces like the ground as frost.

This is how surface hoar forms on the snowpack in the mountains. Surface hoar is notoriously problematic and frequently becomes a persistent weak layer once it is buried in the snowpack.

Layers in the snowpack are always changing. Sometimes they are gaining strength, other times they become weaker.

A weak layer is said to become ‘dormant’ when its strength increases to a degree that a slab avalanche on that layer is considered unlikely.

Like the name suggests, a layer becoming dormant is not necessarily a permanent change. Over time, some combination of increasing load or stress on the dormant layer, and/or loss of strength at the layer, can cause it to “wake up”.

The chance of a dormant layer waking up is a serious consideration during storms (when load on the snowpack is increasing), during times of rapid or prolonged warming, and especially if these kinds of changes are preceded by a period where the snowpack has weakened, such as a cold snap.

Explosives are an effective and widely adopted tool in numerous public and industrial avalanche safety programs around the world. Explosives act as a large trigger that can be placed in a precise location without exposing workers to avalanche hazards. They are used to remove snow from slopes before it can build up enough to create very large avalanches. 

Avalanche occurrences, whether natural or artificial, offer observers high quality information about local avalanche problems. For this reason explosive-triggered avalanches figure prominently in public avalanche forecasts.

Photo: A large avalanche is triggered using explosives. From the Canadian Avalanche Association

Credit

Parks Canada

The extended column test is a snowpack test that is similar to the compression test but uses a much wider column of snow that gives an observer a better indication of fracture propagation. It involves applying an incremental load to the column and then monitoring it to see the point at which it fractures, and how far the fracture spreads along the weak layer.

Extreme is the highest of five levels on the avalanche danger scale. Natural and human-triggered avalanches are certain; large to very large avalanches are expected in many areas; and all avalanche terrain should be avoided.

Extreme avalanche danger is used rarely in avalanche forecasts. It is employed when the likelihood and size of avalanches could present conditions that are outside the realm of many people’s experience. Conditions such as avalanches running beyond the existing bounds of their path, avalanches running into forested terrain, avalanches running across valleys and up the other side, or avalanches impacting simple terrain are possibilities under extreme danger.

Faceted snow refers to snow grains within the snowpack that have transformed into larger, angular grains. Facets have weak bonds with neighbouring snow grains. It is often referred to as sugary snow.

When present, faceted snow frequently exists as a persistent weak layer in the snowpack and it is commonly associated with persistent slab avalanche problems. 

A strong temperature gradient is the condition that promotes the faceting process. For this reason facets often form at the base of the snowpack, near the surface of the snowpack, near crusts, in shallow snowpack areas, and in areas where rocks or trees perforate the snowpack.

Credit

Raven Eye Photography

The combination of facets and crust in close proximity is a common feature of snowpacks that contain crusts. Facets can grow either above or below the crust. 

Facets commonly grow above crusts. For example, when rain wets the snow and cold snow falls on top of it, it produces a very strong temperature gradient from the wet layer to the snow surface. Once the wet layer freezes and forms a crust, facets can form above the crust and act as a weak layer while the crust acts as a hard sliding surface below. This increases the likelihood of avalanches. 

Facets can also grow below a crust as the crust inhibits the movement of water vapour through the snowpack. As the flow of water vapour halts near a crust, vapour is deposited as ice on nearby snow grains, creating facets. This is less problematic than facets forming above a crust, but avalanches can still happen if the crust slides along with an overlying slab.

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Raven Eye Photography

The fracture that releases a slab avalanche spreads along a weak snowpack layer called the failure plane. The bed surface usually lies immediately below the failure plane.

The term flank refers to the side of an avalanche. Two flanks, one on each side, are connected by the crown, or upper fracture line, which runs across the top.

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Curtis Pawliuk

The line that forms the outer extent of the slab fracture of a slab avalanche. Fracture lines outline the top of the start zone and often part of the flanks of a slab avalanche after it has released. They often connect a line between likely trigger points and locations of heightened strain on the snowpack, such as convexities, as well as thinner or perforated snowpack areas. The upper part of a fracture line is commonly referred to as a crown.

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Lisa Erven

The freezing level is the altitude at which the air temperature reaches 0 C (zero degrees Celsius). In the atmosphere, temperature typically decreases with height so that the temperature is above 0 C at elevations below the freezing level, and below 0 C at elevations above the freezing level. 

However, warm air associated with Pacific frontal systems frequently invades British Columbia during winter, causing warm air to overrun cold arctic air settled in the valley bottoms. This process creates a temperature inversion, where temperature increases with height, and can lead to an above freezing layer with multiple freezing levels.

Rising freezing levels are generally synonymous with rising temperatures and a sign of a warming snowpack.

Credit

M. Pidwirny, modified by Lisa Erven

Frontal lift is one of four primary mechanisms of atmospheric lift. Cold fronts, warm fronts, and occluded fronts all force air to rise, and if sufficient moisture is present, form clouds and precipitation.

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Raven Eye Photography

Glide cracks are openings in the snowpack that are created when a glide slab moves slowly down slope to expose the bed surface beneath the slab. As the crack increases in size, they can indicate increasing instability of the slab.

Credit

Mark Bender

A glide slab is a cohesive slab of snow, often consisting of the entire snowpack, that lacks significant support of friction from the bed surface beneath it. The slab “glides” slowly downslope as a whole unit, slipping over a smooth ground surface, or sometimes a hard ice crust or glacier. Grass slopes, smooth rock slabs, or areas exposed to geothermal heat are common areas for glide slabs to occur. 

Glide slabs are often visible by the presence of glide cracks, which form as the slab begins to tear away from the surrounding snowpack. When the strength of the slab is finally overcome, it releases as an avalanche.

A hand shear test is a snowpack test that allows you to test the strength of a shallow weak layer. It involves isolating a small column of snow with your pole and then pulling on it with your hand. It is a quick way to test how a surface snow layer is bonded with the underlying snowpack. Hand shear tests work well to test out shallow or new snow instabilities, but are not useful for testing deeper weak layers.

Credit

Phil Hein

Hangfire is a term used to describe unstable snow that is left on a slope above the fracture line of a slab avalanche. This is because slab avalanches don’t always involve all of the unstable snow on the slope; what’s left behind poses a risk to anyone traveling below it, particularly rescuers.

Hangfire is a serious concern when coordinating avalanche rescue because the potential exists for it to release on the rescuers. If hangfire threatens a rescue—and resources allow—the best practice is to station someone in a safe spot where they can monitor the hangfire and warn other rescuers if a second avalanche releases. As well

, those actively searching for the victim should plan an escape route so they can quickly leave the scene.

A hard slab consists of dense wind-packed snow or old snow that has hardened after a long period of settlement. Hard slabs are usually more than a few days old and can support the weight of a person or machine. Hard slabs are more difficult to trigger than soft slabs, but they tend to propagate much farther and therefore have greater destructive potential. Hard slabs are also more likely to fracture above a person or machine, rather than below, making the likelihood of becoming involved in the resulting avalanche much higher.

High is the second highest level on the avalanche danger scale. During periods of high danger, avalanche conditions are very dangerous and travel in avalanche terrain is not recommended. Natural avalanches are likely and human-triggered avalanches are very likely. Large avalanches are expected in many areas or very large avalanches in specific areas.

Under high danger, all avalanche terrain should be avoided. Use extra caution when travelling in simple terrain and stick to very mellow slopes or dense trees that are free of overhead hazard. 

Credit

Environment and Climate Change Canada

High pressure systems generally result in clearer skies and reduced precipitation. Air within an area of high pressure spirals downward towards the earth’s surface and then spreads outwards in a clockwise manner towards areas of lower pressure, resulting in atmospheric subsidence. 

An area of relatively high atmospheric pressure surrounded on all sides by lower pressure is referred to as a high. An elongated region of high pressure extending from a high is called a ridge.

It is common for surface hoar and facets to form on the snow surface during periods of high pressure, particularly those that last several days. These become a weak layer when they get buried by subsequent storms and can be responsible for persistent slab problems.

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Raven Eye Photography

These are avalanches triggered by a person or machine. The majority of avalanches that involve a person are triggered either by the victim or a member of their party, and 90% of fatal avalanche incidents are human-triggered. An avalanche is considered to be machine-triggered if the person triggering is on a snowmobile or other motorized vehicle.

Credit

Nicole Matei

An inclinometer (or clinometer) is a tool that measures the angle of a slope. Because most avalanches take place on slopes between 30-45 degrees, an inclinometer is a valuable tool in determining whether or not a slope is steep enough to slide.

See Signs of Instability

Isothermal snow is snow that is at the same temperature throughout a given depth range. In practice, the term isothermal is most often used to describe a snowpack that has reached its melting point, 0 Celsius, throughout its depth. As a snowpack reaches its melting point, the bonds between snow grains transform from ice to water, resulting in a loss of cohesion throughout the snowpack. 

Loose wet avalanches and wet slab avalanches often result from surface layers reaching 0 degrees. As deeper layers of the snowpack begin to melt, larger avalanches become possible.

Credit

James Floyer

The term leeward, or lee, refers to slopes that are oriented away from the wind. If winds are strong enough, snow from windward slopes can be redistributed or blown to leeward areas, causing them to become loaded. Wind slabs often form on lee aspects as a result of this wind transport.

See Atmospheric Lift.

Credit

Grant Helgeson

Loose snow avalanches are avalanches that start from a point on the snow surface, gather mass progressively in a fan-like shape, and are composed of snow that lacks cohesion.

They differ from slab avalanches in that they do not have a fracture line or release at a failure plane within the snowpack. Instead, they are usually confined to surface layers and have relatively narrow propagation. In very steep terrain, loose snow avalanches are more prevalent than slab avalanches.

Loose snow avalanches are described as loose wet or loose dry avalanches depending on whether they are composed of wet or moist snow, or dry snow.

Small loose snow avalanches are often referred to as sluffs. Since loose snow avalanches start from a point and fan out, they are also called point releases.

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Raven Eye Photography

Loose dry avalanches are a type of loose snow avalanche composed of dry snow. These types of avalanches are most likely to occur - either naturally or with a human trigger - when surface snow layers are composed of deep, low density snow.

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Raven Eye Photography

Loose wet avalanches are a type of loose snow avalanche composed of wet or moist snow. They occur when surface snow loses strength due to melting. 

Because of their high density, wet loose avalanches tend to contain greater mass and are often more difficult to fight against than loose dry avalanches

In periods of prolonged melt or rainfall, loose wet avalanches can become very large and destructive. These events are most common later in the winter and spring, and are associated with warm temperatures, strong solar radiation, and rain. 

Check out this video of a powerful loose wet avalanche that had enough mass to shake trees as it went past.

The lowest of five levels on the avalanche danger scale. Under low danger, avalanche conditions are considered generally safe but avalanches can still be triggered on isolated features and in extreme terrain. Natural and human-triggered avalanches are considered unlikely.

While many people hold the attitude of “green means go” when the danger is low, one still needs to be cautious and exercise smart travel habits when entering steep slopes. Watch for any signs of instability and avoid obvious trigger points such as shallow rocky start zones.

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Pearson Education Inc.

Low pressure systems generate cloudy skies, precipitation, and, over time, drastic changes in temperature and wind. Air flows from areas of high pressure to areas of low pressure, spiraling inwards and upwards in a counterclockwise manner, resulting in atmospheric lift. 

An area of relatively low atmospheric pressure surrounded on all sides by higher pressure is referred to as a low. An elongated area of low pressure extending from a low is called a trough.

Low-probability/high-consequence refers to occasions where triggering an avalanche is unlikely but would result in a very large slide. Management of low probability/high consequence situations requires a strong grasp of the associated avalanche problem’s distribution, as clues are unlikely to be visible on the surface. It also requires a patient and diligent mindset, shown through the willingness to avoid suspect slopes - and adjacent areas that could be impacted by a high consequence avalanche - for extended periods of time.

Snow metamorphism refers to the change of snow crystals over time. Snow crystals begin to metamorphose from the instant they begin to fall. After new snow is added to the snowpack, the form and size of snow crystals (now labeled snow grains) changes continuously. As snow grains change, layers within the snowpack strengthen and weaken. 

One type of snow metamorphism is the result of sublimation and deposition. These are physical processes that transform ice to water vapour (without a liquid phase) and vice versa. In the snowpack we see this as ice from grain surfaces transforming into water vapour which is then deposited as ice on other grain surfaces.

Sublimation and deposition processes are driven by the temperature gradient of a given snowpack layer. Together, temperature gradient and the movement of water vapour determine whether rounding or faceting processes will dominate within the snowpack.

Melt-freeze is another type of metamorphism that changes the composition of snow grains through melting and refreezing, rather than through sublimation and deposition.

See Crusts

Moderate is the second of five levels on the avalanche danger scale. Under moderate danger there is heightened avalanche conditions on specific terrain features. Features of concern should be identified and terrain and snow should be evaluated carefully. Natural avalanches are unlikely and human-triggered avalanches are possible. Small avalanches can happen in specific areas and large avalanches in isolated areas.

Decision-making under moderate avalanche danger can be challenging, since avalanche danger is present, but is limited in distribution, likelihood, and/or destructive potential. It can apply to a variety of avalanche problem scenarios, including low-probability/high-consequence situations, when sporadic yet very large avalanches are possible. Pay careful attention to the avalanche problems in the forecast to avoid slopes where avalanches may be triggered.

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Raven Eye Photography

An avalanche is said to be natural if the trigger is not a result of the actions of a person or animal, or from another avalanche on an adjacent slope. Natural triggers include:

• Loading of the snowpack from snowfall, wind, or rain;
• Rapid warming from the sun or rising temperatures;
• Cornice, rock, or ice fall.

Credit

Lisa Gardiner

Credit

National Oceanic and Atmospheric Administration (NOAA)

An occluded front occurs when the faster, trailing cold front of a low pressure system catches up to the slower moving warm front. This process lifts the wedge of warm air originally ahead of the cold front, cutting it off from the ground, and also lifts the warm front itself. A temperature difference across the occluded front is still observed, but it is smaller (cold to cool air instead of cold to warm air). It is represented on international weather maps by a purple line with alternating purple triangles and semicircles pointing in the direction the front is traveling in. 

On Canadian surface weather maps, the occluded front is not typically analyzed. Instead, the elevated front, known as a TROWAL (trough of warm air aloft), where the cold front has lifted the warm front off the ground, is drawn. 

The heaviest precipitation occurs at the point where the cold front has caught up to the warm front. Here, the temperature contrasts are greatest and lift is amplified.

Credit

Jonathan Reich

Open trees describes forested areas where the tree canopy is open, or where the forest cover is uneven or patchy. These areas don’t receive the same protection from sun and wind as densely treed areas, snow shedding from branches in these areas has a minimal impact on snowpack structure, and they can be a ‘hot spot’ for the growth of surface hoar crystals.

Open trees do not anchor the snowpack in place in the same way that dense trees do. Instead, their perforation of the snowpack forms a network of weak points that are more likely to aid the propagation of a fracture line than to hold a slab in place. Open trees in avalanche terrain may also be indicative of historical avalanche activity. 

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Raven Eye Photography

An organized rescue is one carried out by trained professionals or volunteers such as a search & rescue organization, ski patrol, or guiding company using a prepared rescue plan. It can involve numerous personnel and extensive resources such as helicopters and avalanche rescue dogs, and can last several days. 

Backcountry travelers should not depend on organized rescue teams to come to their assistance, as such an effort usually arrives too late to save someone’s life if someone is buried.

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Encyclopedia Brittanica

Orographic lift takes place when a moving mass of air encounters a  physical barrier such as a mountain range and is forced upwards. The rising air cools, condenses and forms clouds. With sufficient lift and moisture, precipitation generally falls on windward slopes and/or near mountain peaks. On the leeward side of mountains (downwind), air descends, warms, becomes drier, and creates an area of lower precipitation, commonly called a rain shadow. 

Since the predominant wind direction in the mid-latitudes of North America is from west to east, orographic lift is a key component of precipitation patterns and snowpack depth. Consequently, western-facing (windward) slopes experience higher annual precipitation accumulations than eastern-facing (lee) slopes. Pacific frontal systems will also wring out a larger percentage of their moisture content over the Coast Mountains, with less moisture available to fall over the Columbia and Rocky Mountain ranges.

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Raven Eye Photography

Overhead hazard refers to an avalanche slope or other hazard such as cornice or loose rock that threatens the area below it. When traveling in avalanche terrain, it is important to maintain awareness of overhead hazards and the places they threaten. It is wise to avoid exposure to overhead hazards when possible, or to minimize the number of people exposed as well as the duration of their exposure when avoidance isn’t possible.

Overnight freeze, re-freeze, or overnight recovery refers to the snowpack returning to a frozen state after a period of daytime warming. Re-freezing can occur as a result of below zero air temperatures, radiative cooling of the snowpack, or both.

Overnight re-freeze is an important consideration during spring and summer travel in avalanche terrain because the refrozen snowpack is much more stable than a snowpack that remains moist or wet overnight. Good overnight recovery of the snowpack ensures relatively safe conditions from the early morning until the heat of the day begins to melt the snowpack once again.

During periods of prolonged warming when there is no overnight freeze, it is wise to avoid avalanche terrain.

A persistent slab is an avalanche problem that is defined by a slab formed over a persistent weak layer. New snow that accumulates and consolidates over a persistent weak layer is normally first labeled a storm slab until the persistent nature of the weak layer becomes apparent over time.

Persistent slabs are most likely to result in dangerous human-triggered avalanches when the weak layer is buried 30-100 cm below the snow surface. When the layer is buried more than one metre deep, it usually becomes harder to trigger, but can still result in a very large, destructive avalanche. 

Management of persistent slab problems requires a strong grasp of the problem’s distribution, as clues about the problem are unlikely to be visible on the surface. It also requires a patient and diligent mindset, shown through the willingness to avoid suspect slopes for extended periods of time.

Credit

Raven Eye Photography

A persistent weak layer is a weak layer in the snowpack that resists forming a strong bond to neighbouring grains in the snowpack over an extended time period. They can be composed of surface hoar, facets, or depth hoar.

Credit

Wren McElroy

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James Floyer

Pinpoint search is the act of using a probe to pinpoint the location of a victim buried in an avalanche. It follows a successful transceiver search to the point where the victim was buried. A successful probe strike is normally felt as a soft area at the probe tip, which is either the victim’s body or equipment.

Probing is done in a spiral or square, starting at the point where the strongest transceiver signal is found and spiraling outwards in 25 cm increments until a strike is made. 

A successful probe strike is followed by rescue digging.

A synonym for loose snow avalanche.

Avalanche Canada’s Mountain Weather Forecast tracks four different types of precipitation: snow, rain, freezing rain, and ice pellets. Each one has a different impact on snowpack structure.

• Snowfall builds the snowpack. Recently fallen snow is normally easily transported by wind. Heavy snowfall of more than 2 cm per hour or 30 cm within the last 48 hours, increases avalanche danger. Consistent light snow (5-10 cm per day) makes avalanches less likely.
• Rain adds moisture and heat to the snowpack and causes melt. Rain crusts form if surface snow refreezes after rainfall. Any amount of rain on snow can make avalanches more likely. If the rain freezes into a thick crust (3 cm or more), it makes avalanches less likely.
• Freezing rain adds less heat to the snowpack than rain and causes minimal snowpack melt. It can rapidly form surface crusts.
• Ice pellets form when snow melts into rain droplets and then they fall into arctic air trapped in valley bottoms causing it to change into frozen rain droplets. They are much larger than snow grains and may resist bonding to surrounding snow grains.

Precipitation of any kind adds load to the snowpack and stresses the weak layers within it. Critical loading of weak layers present in the snowpack can occur during rain or heavy snowfall.

The video illustrates how precipitation particles fall to the ground as snow, sleet (ice pellets), freezing rain, or rain depending on the vertical temperature structure in the atmosphere. If the air temperature remains below zero throughout the particle’s descent, it falls as snow. If the air temperature rises above and remains above zero, rain occurs. When a mid layer of warm air overrides cooler, valley bottom air, ice pellets or freezing rain form, the deciding factor being the depth of the cold air in the valley bottoms.  Video by The Comet Program.

See Atmospheric Pressure.

Credit

Wren McElroy

A probe line consists of a group of searchers lined up in a row to look for someone buried in an avalanche, each person with an avalanche probe. Probe lines are normally employed when a transceiver search is either unsuccessful or not possible. 

To set up a probe line:

• Establish the most likely area of burial.
• Line up searchers in a straight line, spaced approximately 1.5 metres apart (wrist to wrist).
• Searchers probe three times, once in front of the sternum, then reaching left and right about 50 cm on either side of the middle hole.
• Searchers take one step forward and repeat the process.

See pinpoint search.

See Avalanche Problem.

Credit

Mark Bender

Propagation refers to the spreading of a fracture in the snowpack. Slab avalanches occur when a fracture propagates along a weak layer on a slope that is steep enough to avalanche. The fracture then propagates through the slab as it separates and slides away from the surrounding snowpack. 

The farther these fractures propagate, the larger the resulting avalanche will be. For this reason, propagation propensity, or the tendency for a fracture to spread, is an important indication of the destructive potential of a slab. Thicker and/or harder slabs tend to propagate wider than thinner or softer slabs.

The extended column test and propagation saw test are ways to test the propagation propensity of a slab over a weak layer.

The propagation saw test is a form of snowpack test that involves drawing the back edge of a snow saw up the length of a known weak layer in a column of snow. During the test, the weak layer is monitored and the test is halted the moment it begins to independently propagate the fracture being initiated by the saw. The results of the test give a point observation of a weak layer's tendency to propagate a fracture once it has been initiated.

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Credit: Environment and Climate Change Canada

Quantitative Precipitation Forecast (QPF) refers to the quantity (or amount) of precipitation (usually conveyed in millimetres of water) forecast to accumulate at a given location over a specified period of time. Common time intervals are one hour, three hours, 12 hours, or per storm event.

See Crusts.

See Whumpf

Credit

Frozen Pirate Snow Services

Remote-triggered avalanches are avalanches that occur away from the point where they are triggered. They happen when a slab fractures the weak layer below it but does not produce an avalanche at the site of the fracture. Instead, the fracture propagates along the weak layer until it reaches a portion of the slab that rests on a slope that is sufficiently steep to avalanche.

Remote triggered avalanches are a strong sign of an unstable snowpack. If you have observed or caused a remote triggered avalanche, a good course of action would be to avoid avalanche terrain. Instead, seek out simple, low angle, and/or densely forested terrain that is free of overhead hazards.

Video: Doug Latimer and Jordy Shepherd demonstrate the strike team conveyor shovelling method.

Rescue digging refers to the digging and extrication phase of an avalanche rescue. Digging for a person buried in avalanche debris is physically demanding and often takes longer than the search, so an efficient approach to recovering a burial subject is an important means of maximizing the chances of a positive outcome.

The strike team method is a systematic rescue digging technique for multiple shovelers and one of the best approaches for rescue digging. To employ this technique:

• Shovelers line up behind one another downhill of the probe, spaced out a shovel-length apart (about 80 cm). 
• First, each digger shovels down, moving snow to the side until they’ve dug out a trench about a shovel-blade deep. 
• Shovelers then move snow back to the next person in line, who clears it out behind them.
• Shovelers rotate positions periodically to ensure the digger at the front doesn’t become too tired.

Once the head is reached, one rescuer administers first aid while the additional diggers remove snow from around the subject’s body and prepare a ramp for extrication.

Photo: People practice rescue digging in an AST 1 course. By Brent Strand.

Reverse loading occurs when winds blow counter to the prevailing wind direction, transporting snow to normally wind-scoured slopes. Reverse loading creates irregular and often complicated patterns of wind effect in exposed areas.

For example, in an area where winds normally blow from west to east, the snow would be blown from west-facing (windward) slopes onto east-facing (leeward) slopes. If a system brought winds from the east, the situation would be reversed, and snow would be deposited on slopes that are normally wind-scoured.

Credit

Derek Vanderkooy

A rib is a vertical ridge of snow that runs down a slope. On slopes that are cross-loaded by the wind, the leeward side of ribs will be loaded with snow, making them likely to contain wind slab problems.

Credit

Raven Eye Photography

Ridgecrest and ridgetop are terms used interchangeably to describe terrain at or near the top of a ridge. Ridgetop terrain is significant for a number of reasons:

• Ridges often exist as a boundary between windward and leeward slopes.
• Ridges often exist as a safe spot in avalanche terrain, reducing or eliminating exposure to hazards from overhead or adjacent slopes.
• Ridges often offer a more efficient and lower-angle route to higher elevation when compared to gullies or draws.
• Ridges offer better sightlines on surrounding terrain than do gullies or draws.
• Cornices are most commonly formed along ridgetops.

Credit

Raven Eye Photography

Rime is a deposit of ice from super-cooled water droplets. It can accumulate on the windward side of rocks, trees, and structures; and on falling crystals of snow. When snow crystals cannot be recognized because of rime, the grains are called graupel.

Rounded snow refers to snow grains within the snowpack that have transformed into small, increasingly spherical grains. Rounded snow grains are packed tightly in the snowpack and bond well to each other, creating a snowpack (or layer) that is increasingly strong.

A weak temperature gradient is the condition that promotes the rounding process. Dense, tightly packed snow; and small snow grains are other contributing factors.

Credit

Raven Eye Photography

See Avalanche Path

Credit

Wren McElroy

A rutschblock test is a form of snowpack test that involves applying the weight of a person in increments to a large column of snow. Like the compression test, the rutschblock test gives a point observation of weak layer strength as well as fracture propagation propensity. One advantage of the rutschblock test is its unique and impactful approximation of an avalanche involvement through its use of a human trigger and a large slab of snow.

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Cam Campbell

A safe spot is a location within avalanche terrain that either minimizes or eliminates a person’s exposure to avalanche hazard. The use of safe spots is an important part of safe travel in avalanche terrain. For example, if your route takes you into avalanche terrain, it is wise to group up beforehand in a safe spot, then enter the area one at a time in order to limit your group’s exposure, re-grouping later in another safe spot. Safe spots are also targeted when performing a ski cut.

Credit

Mark Bender

Sastrugi are convoluted formations of snow scoured out of the snow surface by wind. Sastrugi are often found in areas where a significant depth of loose, wind-transportable snow is exposed to a strong wind event.

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Alex Cooper

Satellite communications devices offer a range of communication capabilities for users outside of areas covered by cellular service. These devices have a number of valuable uses for outside communication while in the backcountry, and especially for emergency response. Satellite communication devices may require an open line-of-sight to a specific point in the sky, or may be subject to operating windows determined by the movement of satellites. Terrain that obstructs line-of-sight to the sky may limit the operation of satellite communications devices.

• Satellite phones can be used to call out to emergency services or other telephone contacts or to receive calls. These devices are used for spoken two-way communications only. 
• Satellite messengers, such as InReach devices, can be used to send and receive text communications as well as location and other information in areas where there is no cell phone service. Older models of some brands may only support one-way communication. Most messenger devices also have an emergency SOS function to rapidly alert emergency services.
• Personal Locator Beacons, or PLBs, are used to transmit location information. They are one-way communications devices designed to alert emergency services.

*Note: None of the above devices can function as, or be substituted for, an avalanche transceiver.*

Credit

Gavin Lang

Sea level pressure is a calculated value determined by measuring the actual pressure at a given weather station and correcting it for altitude. 

Atmospheric pressure is measured by surface weather stations across the globe. Since the elevation of these stations can vary significantly from the coast of British Columbia to the Rocky Mountains, station pressure (the actual pressure measured at the station) is converted to sea level pressure (the pressure that would exist if that station was at sea level). Doing so simplifies the pressure analysis, allowing meteorologists to compare pressure at different locations.

The mathematics take into consideration station altitude, station temperature, and minor instrumentation corrections. It also includes several assumptions, such as how temperature changes with altitude, which at times, can lead to incorrect values. Erroneous data, if identified, can be ignored or smoothed over while doing a hand analysis of isobars on a weather map.

Self-arrest is the act of stopping oneself from moving downslope during a sliding fall. Self-arrest is one way of staying out of an avalanche after being caught. It can be achieved by anchoring oneself to a set of ski poles or ice axe while using body weight to drive their tips into the snow surface, or by hanging onto a tree or other object not caught in the flow.

See Companion Rescue.

Settlement refers to the process that consolidates the snowpack and diminishes its depth over time. It occurs as a result of snow metamorphism combined with gravity’s compression of grains in the snowpack.

Settlement is synonymous with strengthening of snow, but it can result in both stable and unstable snowpack structure. A snowpack described as ‘settled’ suggests snow grains are well bonded throughout the snowpack. Conversely, a layer of new snow that has ‘settled’ into a slab over a weak layer suggests an unstable snowpack structure.

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Mark Bender

A shallow rocky start zone is an area of the snowpack where avalanches are more likely to be triggered. Areas where the snowpack is shallow and perforated by rocks are known to promote the formation of weak, faceted snow. Weak layers present in a shallow snowpack are also normally closer to the snow surface than in deeper snowpack areas. This places these layers closer to the triggering forces of a person or machine traveling on the snow above. Avalanches triggered in shallow snowpack areas often have fracture lines that propagate to deeper snowpack areas.

These areas should be avoided when traveling in avalanche terrain.

Shooting cracks are a sign of instability in the snowpack. They appear as cracks propagating outwards through the snowpack under the weight of a person or machine. Shooting cracks are indicative of an unstable snowpack, often illustrating the transition of low density surface snow into a consolidating slab. If you see shooting tracks, you should avoid any slopes steep enough to avalanche.

Photo: Shooting cracks propagate from the tips of skis. By Raven Eye Photography

The following signs of instability indicate an elevated risk of triggering avalanches:

• Recent avalanche activity, particularly slab avalanches that have occurred in the last 48 hours.
• Remote-triggered avalanches. This is where an avalanche occurs on a slope some distance from where it was triggered. Often, remote-triggered avalanches are triggered from flatter terrain adjacent to avalanche slopes, such as ridge crests, benches or areas immediately below avalanche paths.
• Whumpfs are the collapsing of a weak layer in the snowpack. It often has an accompanying loud 'whumpf' sound.
• Shooting cracks that appear in the snow from under your sled, skis, board, snowshoes or feet.
• Hollow or drum-like sounds caused by moving over the snow surface.
• Snow shedding naturally from trees, which suggests the snowpack is warming rapidly.
• Snow pinwheeling or snowballing down slopes, which are other signs of a warming snowpack.

These signs of instability indicate the snowpack structure is primed for human-triggering. The weight of a person or machine moving over the snow could be sufficient to release an avalanche. If you notice any of these signs, the safest course of action is to avoid exposure to avalanche terrain. Stick to low angled, simple terrain, or routes that stay within densely forested terrain. 

The absence of signs of instability does not indicate the absence of avalanche danger.

Credit

Mark Grist

Simple terrain is one of three levels of the Avalanche Terrain Exposure Scale. It involves exposure to low angle or primarily forested terrain. Some forest openings may involve the runout zones of infrequent avalanche paths. Many options are available to reduce or eliminate exposure to avalanche danger. No glacier travel is required.

Sintering, or pressure sintering, is the physical process of bond formation between grains in the snowpack. It usually accompanies the rounding process. Sintering occurs when water vapour is deposited at the contact points between snow grains, forming necks. These necks then create strong bonds between grains, increasing the strength of the snow.

Hardening or compaction from any mechanical disturbance such as boots, skis, snowmobiles, groomers, wind, and avalanches produces rapid sintering by breaking up large grains and bringing grains into close contact.

A ski cut, or slope cut or slope test, is an intentional attempt by a skier, snowboarder, or snowmobiler to safely trigger a small avalanche. Ski cuts can be used to test the strength of weak layers in the upper snowpack, but they should only be attempted on small slopes where the consequences of an avalanche are limited. They are not useful for testing deeper weak layers.

Effective ski cutting requires the skier or rider to enter the slope at or above the start zone and target the highest elevation, most likely trigger point before quickly traversing to a safe spot adjacent to the targeted slope.  A ski cut that is executed poorly or in the wrong situation can be ineffective and dangerous.

Because the individual performing a ski cut is exposing themselves to possible avalanche involvement, their partners should observe the cut from a safe spot and be ready to perform a companion rescue if necessary.

Credit

Jennifer Coulter

A slab is one or more cohesive layers of snow overlying a comparatively weak snowpack layer. Slab formation takes place as low density new snow densifies and stiffens from the effects of settlement. Wind transport is another common contributor to slab formation. The presence of a slab over a weak layer is one of the preconditions for a slab avalanche.

Slab avalanches are formed when a slab of cohesive snow releases at a weak layer in the snowpack and slides over an underlying bed surface. They are the most dangerous types of avalanche for people as they can result in large amounts of snow rapidly breaking up around you, causing the ground to move as if someone just pulled the rug out from under your, and engulfing you before you have a chance to react. 

Slab avalanches:

• Leave a fracture line where the slab breaks away from the surrounding snowpack;
• Can release simultaneously over a large area, setting large volumes of snow into motion;
• Involve one or more snowpack layers;
• Range from new snow (soft slab) to hard, wind-packed snow (hard slab);
• May contain dry or wet snow;
• Are generally more dangerous than loose snow avalanches.

Large, destructive avalanches are usually slab avalanches.

Slab consolidation takes place as low density new snow densifies and stiffens from the effects of settlement and sintering. The chance for slab avalanches is introduced when slab consolidation occurs above a weak layer.

The Slope Evaluation Card is the component of the Avaluator decision-making aid that deals with slope-scale decisions. It employs a scoring system for avalanche conditions and terrain characteristics variables. After calculating the scores for each, the user applies these scores to a grid which suggests whether or not the slope in question is appropriate to travel on.

The incline of a slope is a significant factor in whether or not it can avalanche. Most avalanches occur on slopes that have an incline of 30-45 degrees ‒ about the steepness of a black diamond run at a ski hill, and favourite terrain for backcountry skiers and riders. However, avalanches  can happen on slopes as flat as 25 degrees and as steep as 60 degrees. Below 25 degrees, slopes aren’t steep enough to avalanche and above 60 degrees, new snow sluffs frequently and slab avalanches are rare.

The following guidelines for using slope incline to predict avalanche size and frequency have been developed from experience.

• 60 to 90 degrees: Avalanches are rare; snow sluffs frequently in small amounts. 
• 50 to 60 degrees: Frequent loose snow avalanches. 
• 45 to 55 degrees: Frequent small slab avalanches. 
• 30 to 45 degrees: Slab avalanches of all sizes. 
• 25 to 30 degrees: Infrequent (often large) slab avalanches; wet loose avalanches. 
• 10 to 25 degrees: Infrequent wet snow avalanches and slush flows.

A minimum slope angle is required to initiate a slab fracture, however, a fracture may propagate to a flatter slope after an initial failure on a steeper slope has occurred. Traveling on lower-angled terrain and minimizing exposure to steep slopes is an effective way to limit exposure to avalanche terrain.

A sluff is synonymous with a loose snow avalanche but normally refers to a small release and is often associated with human triggering. Sluff is common while riding on steep slopes. While getting caught in sluff may be avoided with proper riding technique, sluffs can knock a person off their feet or machine, they can have high consequences if paired with a terrain trap, and can even bury a person if they gather enough mass.

Sluff management is the practice of anticipating sluff while skiing or riding and taking steps to avoid being caught in its flow. One of the most common forms of sluff management involves skiing short pitches and pulling over to safe spots to escape the flow of sluff while it passes.

Credit

Curtis Pawliuk

Small slopes are fairly short, generally only 10-30 metres long, and are not exposed to terrain traps. They are considered low consequence terrain and are generally only able to produce small avalanches (up to size one). By using them to test for instabilities, small slopes can be a good place to safely gain information about the presence and reactivity of avalanche problems in an area.

Credit

Richard So

The snow-to-liquid ratio (SLR) is a measure used in weather forecasting to describe the water equivalent of forecast new snow. The most basic rule of thumb to follow would be to apply a SLR of 10:1 (i.e., for a snow forecast of 10cm, 1cm (10 mm) of liquid water is expected). 

In reality, SLRs vary from 5:1 or 7:1 (commonly found in the westernmost coastal ranges) to 25:1 or more (common in the Arctic and on eastern slopes of the Rockies). The variability of SLR depends on the temperature of the surrounding air throughout the ice crystal’s life cycle (from growth to its descent to earth), the presence of supercooled water droplets within the cloud, shape of the resulting ice crystals, and wind strength within and below the cloud layer.

Numerous weather products predict the quantitative precipitation forecast (QPF, in mm), which requires forecasters and other users to convert to centimetres of snow by applying an appropriate SLR. Since SLR is linked to temperature, colder air tends to produce higher SLR (15:1 or 20:1), resulting in lighter, fluffy powder snow, while warmer temperatures usually mean a lower SLR (7:1 or 5:1) and denser snow, which is excellent for making snowballs.

For example, if 10 mm of precipitation is forecast with mild temperatures, one might apply an SLR of 10:1. Thus, the forecast would call for 10 cm (100 mm) of snow. If precipitation was occurring during a cold snap, one would apply a higher SLR of 20:1. This means the same 10 mm of precipitation will create 20 cm (200 mm) of snow.

Credit

Simon Fraser University Avalanche Research Program/Stamen Design

The idea of a snow climate is a way of grouping geographical areas according to broad trends in weather and snowpack composition. Three snow climates have generally been used to categorize different regions in Canada:

Maritime Snow Climate

A maritime snow climate is found in mountain ranges closest to the ocean. In western Canada, it is found in the Coast Mountains and Cascades. In eastern Canada, it is found in coastal Quebec and Newfoundland. Maritime snowpack regions are characterized by frequent intense storms, deep snowpacks, and warm weather. Avalanche danger in maritime snowpack regions is typically connected with storm events where snowfall rapidly accumulates and forms unstable storm slabs or wind slabs. A maritime snowpack often stabilizes within one or two days of a storm. Persistent weak layers are less common in a maritime snowpack, but they are not rare. The perception they are rare may in fact increase the chance of someone getting caught by surprise by a persistent slab avalanche.

Continental Snow Climate

A continental snow climate exists in mountains a long distance from the ocean, such as the Rocky Mountains. A continental snow climate is characterized by cold weather and thin snow cover from relatively infrequent storms. These conditions frequently lead to the formation of persistent weak layers such as facets, depth hoar, and surface hoar. As a result, avalanche danger in a continental snow climate can rise sharply with only light snowfall and often persists long after storms.

Transitional Snow Climate

The snowpack in a transitional snow climate shares characteristics of both maritime and continental snowpacks. In Canada, it is found in the Columbia Mountains of the BC Interior. The depth of a transitional snowpack can be similar to a coastal snowpack, but it is generally composed of less dense snow. Weather conditions are often conducive to the formation of persistent weak layers that can last for weeks or even months.

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Raven Eye Photography

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Jim Steenburgh

The snow level is the altitude at which precipitation changes from snow to rain. How far below the freezing level it is located depends on the amount of cooling that takes place due to sublimation and melting. 

If the air mass is dry in the low levels, falling snow will sublimate (change from frozen water to water vapour) and cool the atmosphere. The snow level will quickly plummet from near the freezing level to several hundred metres below.

If the atmosphere is moist, little cooling from sublimation will occur but melting snow (which also cools the atmosphere) still occurs. The harder it is precipitating, the more melting occurs, leading to greater cooling and lowering of the snow level. 

A general rule of thumb to determine snow level is to subtract 200-300m from the freezing level, but depending on the atmospheric conditions, snow level may be substantially lower.

This video by the Canadian Avalanche Association documents snowpack test procedures.

A variety of standardized snowpack tests have been developed to aid in assessing snowpack structure and stability. Common examples include:

• Compression test
• Extended column test
• Propagation saw test
• Rutshblock test

Avalanche forecasts that discuss weak layer reactivity and slab propagation propensity in relation to snowpack tests normally refer to these tests. Test results are generally more concerning and indicate greater instability when classified as 'sudden' or 'easy' and less so when labeled 'resistant' or 'hard'.

It is important to not put too much faith in a single test result. The safest way to interpret the results is to use them to potentially rule out skiing a slope, and to never use the result to rule-in skiing a slope.

For training in proper application and interpretation of these tests in the field, please consult an avalanche professional or attend a recognized training course.

Soft slabs are composed of light, low-density, recent snow. Soft slabs form as this snow settles and consolidates over time or with redistribution by wind. Soft slab avalanches tend to be triggered easily by the weight of a person or machine. They typically fracture at or just below the person or machine triggering them, and they tend not to propagate as widely as hard slabs ​​unless they are associated with a critical avalanche layer.

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Raven Eye Photography

Solar warming is warming caused by radiation from the sun. As a result of solar warming, east to west aspects often see increased rates of snowpack settlement and more frequent melting of snow at the surface. This can result in accelerated slab formation, the creation of surface crusts, and wet loose avalanches.

Even when air temperatures are below freezing, solar radiation can bring snow to its melting point. When air temperatures are above zero, solar radiation can greatly increase snow melt. 

The exposure of slopes to the sun changes over each day and throughout winter according to their incline and aspect. Slopes that saw little solar warming early in winter experience substantially more in spring as the sun shines on the snow for longer periods and from higher up. Slopes receiving direct sunlight are most affected by solar warming.

Being aware of the sun’s path and anticipating the effects of solar warming is an important part of decision-making in avalanche terrain.

Photo: Snow pinwheels down a slope, a result of solar warming. By Mark Bender

Spot probing is conducted if a transceiver search is unsuccessful or not an option. It is done in likely burial areas around trees, rocks, and depressions in the terrain. Effective spot probing requires rapid, continuous, and methodical probing of these areas. The more holes poked in the snow, the more likely the searcher will be to find the person buried.

See avalanche path.

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Environment and Climate Change Canada

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Damon Stokes

A stationary front is an interface between two air masses that are not moving or moving very slowly. Surface winds blowing parallel to the front can help it remain in place for longer.

Weather along the frontal zone depends on the two air masses at play. If both air masses are relatively dry, cloud cover will be scattered with little or no precipitation. If one or both air masses is sufficiently moist, widespread cloud cover with intermittent to steady light precipitation can be expected. 

A stationary front is represented on weather maps by alternating blue and red line segments, with blue triangles that point towards warm air and red semicircles that point towards cold air. 

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Curtis Pawliuk

A slab avalanche is said to step down if the motion of the initial slab causes deeper layers to fail, resulting in a larger avalanche than the one that initially released and a second bed surface deeper in the snowpack. A step in the bed surface may be visible, or in many cases, may be scrubbed away by the motion of the slab release.

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Curtis Pawliuk

A storm slab is one or more layers of recent storm snow that has consolidated into a slab above a weak layer. Storm slab distribution is generally much wider than that of wind slabs due to the slab forming as a result of snowfall rather than wind effect. It is one of eight avalanche problems identified in avalanche forecasts.

Management of storm slab problems involves paying close attention to slope angle and slope size, as the widespread nature of the problem means that it tends to affect most slopes where avalanches can occur. 

Storm slabs form a strong bond with the previous snow surface over a relatively short time period. If they remain unstable for more than a few days, they may be re-labeled as persistent slabs.

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Raven Eye Photography

Snow that fell during a period of continuous or almost continuous snowfall.

See Atmospheric Subsidence.

See Crusts.

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Raven Eye Photography

Surface hoar consists of feathery-shaped frost crystals that grow upward from the snow surface when the air just above is cooled to the dew point.  Once buried, layers of surface hoar are slow to gain strength, sometimes persisting for a month or more as a weak layer. 

Surface hoar grows most easily on cold and relatively clear nights, when the wind is calm, though it can also grow during the day on shady slopes. It can be identified by the feathery, sparkly crystals that grow on the snow surface as it forms.

See Avalanche Survival.

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Mark Stanley

An avalanche is said to have been triggered sympathetically if it occurs as a result of another avalanche on an adjacent slope. Sympathetic avalanches are a strong indicator of an unstable snowpack.

Temperature gradient is the difference in snow temperature across a given vertical range in the snowpack. In practice, it is usually expressed in degrees celsius per 10 centimetres. 

The strength of temperature gradient dictates the rate of sublimation and deposition processes in the snowpack. The stronger the temperature gradient, the more rapid the sublimation and deposition.

As a general rule, a temperature gradient less than 1 C per 10 cm contributes to strengthening bonds between snow grains. This process is called rounding. 

A temperature gradient greater than 1 C per 10 cm weakens bonds between snow grains. This is called faceting.

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Jen Coulter

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pataga.net/WhetherToFLy.html

A temperature inversion occurs when air at higher elevations is warmer than air at valley bottom. This is a reverse of normal conditions, where air temperature decreases with height. 

Temperature inversions form due to many processes, two of which are warm air flooding in aloft and subsidence. In the first case, a warm front moving into British Columbia will cause warm air to rise up and over cooler, denser air settled in valley bottoms. If this elevated air mass is above 0 C, rain may occur in the alpine or treeline, with freezing rain or ice pellets falling in valley bottoms.

Alternatively, when high pressure is in place for multiple days, air descending to the surface of the Earth (subsiding) can create a temperature inversion up high known as a subsidence inversion. Warm, sunny conditions may exist at mountain top, while cool, often cloudy conditions are found at valley bottom. 

Surface-based inversions also form due to overnight heat loss at ground level, and the subsequent cooling of adjacent layers of air.

During a temperature inversion, dramatic warming of the snowpack can occur during the day at alpine elevations. This warming may not be obvious to a person traveling at lower, cooler elevations, and may increase threats posed by overhead hazards. 

Once in place, this stagnant weather pattern is challenging to break.

Terrain traps are features that increase the consequences of being caught in an avalanche. Terrain traps that increase the risk of physical injury include trees, rocks, cliffs, and open water. Terrain traps that increase burial depth include gullies, flat sections, and crevasses.

It is important to assess slopes for the presence of terrain traps when evaluating terrain. Even a small slope can be dangerous if it is perched above a cliff or gully. 

Photo: Skiers spread out while traveling through a terrain trap on the way to Bow Hut in Banff National Park. By Jordy Shepherd

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Mark Bender

Thin spots are locations in the snowpack that are less deep than the surrounding area. They are locations where avalanches are more likely to be triggered and are one example of spatial variability in the snowpack

There are two reasons for this. First, areas where the snowpack is thin promote the formation of faceted snow that acts as a weak layer within the snowpack. Secondly, weak layers in the snowpack will usually be closer to the surface than in places where the snow is deeper. This places these layers closer to the triggering forces of a person or machine traveling on the snow surface.

Thin spots should be avoided when traveling in avalanche terrain. Avalanches triggered from a thin spot often have fracture lines that propagate to deeper snowpack areas. Examples of thin spots include areas where rocks protrude through the snow and the edges of wind-loaded areas.

Threshold for avalanches refers to the minimum depth of snowpack needed to produce an avalanche. On most slopes, especially below treeline, a certain depth of ground roughness needs to be exceeded by the snowpack before it has established a sufficiently flat surface upon which an avalanche may run.

See avalanche path.

An avalanche transceiver is an electronic device that sends (transmits) a signal, and searches for signals from other transceivers to enable rescue in the event of a burial. It is considered one of the three avalanche essentials, along with a probe and shovel.

In send/transmit mode, it constantly emits a radio signal that is stronger at close range. If someone with a transceiver is buried, the other members of the group can switch their transceivers into search mode and follow a search pattern to locate the strongest signal. 

To be effective, a transceiver must always be worn and in send/transmit mode when traveling in avalanche terrain. Transceivers are often referred to as beacons.

Photo: A transceiver check to see if everyone's transceivers are turned on and working should be performed at the start of any trip into avalanche terrain. By Mark Bender

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Curtis Pawliuk

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James Floyer

A transceiver search is the phase of a companion rescue that involves using an avalanche transceiver to locate a burial subject wearing another transceiver. Transceiver searching is divided into three phases:

• Signal Search: Beginning from the point where the person buried was last seen, the transceiver searcher moves across the avalanche path in a methodical pattern in an effort to locate a transmitting signal with their transceiver.
• Coarse Search: Once a signal has been acquired, the searcher orients their transceiver to point in the direction indicated by the device and moves quickly to reduce the distance indicated. Care is taken to slow down and approach the snow surface as the distance indicated approaches the subject's possible burial depth. This phase of the search ends when the point of the strongest signal is reached and then exceeded by the searching transceiver.
• Fine Search: When the strongest transceiver signal is reached, the searcher makes a mark on the snow surface as a visual clue to prevent searching beyond this point. Without re-orienting the transceiver, the searcher moves directly in reverse, again passing the point of the strongest signal and again marking the snow surface when the closest distance is exceeded. The searcher can then proceed to a pinpoint search or, if the point of the strongest signal is suspected to be inaccurate, they can perform the same actions on a side-to-side axis beginning from the point of the strongest signal.

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Karl Klassen

Treeline elevation is characterized by sparse forest cover and is the transition between uniform forest cover below it and the alpine above it. Treeline areas are distinct for sharing features of both alpine and below treeline elevations. Exposure to sun, wind, cold, and precipitation is generally less here than in the alpine but greater than below treeline.

Avalanches can be triggered either naturally or artificially. Natural triggers include heavy snowfall, rapid depositing of snow by wind, a rapid rise in temperature, fall of a cornice, ice fall, or earthquake. 

Artificial triggers include skiers, snowboarders, snowshoers, snowmobiles, hikers, machinery, and explosives. 

An avalanche is said to be triggered remotely when the trigger initiates the avalanche from a distance.

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Mark Bender

The trigger point is the location where a person or machine initiates an avalanche. Possible trigger points include: 

• The top of a steep slope; 
• Thin areas of the snowpack such as a shallow rocky start zone; 
• Locations where the snowpack has reached critical loading through windloading or new precipitation;
• Places where a weak layer is especially weak;
• Places where the snowpack is perforated by rocks or trees. 
• Combinations of the above

Fracture lines often connect these areas of weakness or increased stress on the snowpack.

The Trip Planner card is the component of the Avaluator™ decision-making aid that helps recreationists plan their day in the backcountry. Using a simple graph, it connects the avalanche danger rating with the terrain rating of the area they plan on visiting to provide a recommendation on what level of caution should be taken given the forecast avalanche conditions. The user can then decide whether or not to proceed or go somewhere else.

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Environment and Climate Change Canada

TROWAL is an acronym for "trough of warm air aloft". It is used on Canadian weather maps, instead of an occluded front, to illustrate the position of the warm front pinched off from the surface by the faster-moving cold front.

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Grant Helgeson

Two-way radios are a communications tool that allows for communication between parties in remote areas. They allow for communication when out of earshot of one’s partners and are especially useful for snowmobilers, who have limited verbal communication while riding

Upside-down snow refers to a snowpack where denser snow sits on top of lighter snow. This structure can indicate slab properties - a cohesive slab resting on a weak layer - existing in the snowpack. Conversely, right-side-up snow increases in density with depth and suggests a more stable snowpack structure.

Since weather maps and charts often display the time in UTC (coordinated universal time) or "zulu" time, this chart will allow you to convert UTC / zulu time to your local time. On weather charts, UTC time is often abbreviated or written in several ways, for example 12 noon UTC time may be written 1200Z or sometimes simply 12Z. Note that since some provinces change back and forth between daylight savings time, there are both summer and winter conversion charts.

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Ron Ratliff

Virga is a wisp, streak, or shaft of precipitation falling from the base of a cloud that evaporates or sublimates before reaching the ground.

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Environment and Climate Change Canada

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M. Pidwirny

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M. Pidwirny

A warm front is the interface that occurs when a warm air mass replaces a retreating cold air mass. Warm fronts generate atmospheric lift as their associated low-density air mass climbs up and over the cooler, denser air it advances upon. This gradual overrunning of warm, moist air produces clouds and precipitation well ahead of the warm front’s surface position. Warm fronts are represented on weather maps by a solid red line with red semicircles pointing in the direction the front is moving.

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Grant Helgeson

Weak layers are layers within the snowpack composed of snow crystals that are poorly bonded with the snow above or below them. They are the layer that fails in response to a trigger, allowing an avalanche to slide downslope on the bed surface. 

Weak layers can consist of new or recent snow, buried surface hoar, or facets that are formed in the snowpack. When a weak layer fractures, it is referred to as the failure plane of the resulting avalanche.

Weak layers can be active anywhere from a few hours to months. Ones that last about a week or more are known as persistent weak layers.

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Environment and Climate Change Canada

A weather front is the interface between two air masses with differing characteristics such as the border between a cold, dry air mass, and a warmer, wetter one. They are named for their abrupt transitions in temperature (e.g. warm front, cold front), but significant changes in other weather elements such as moisture content, wind speed and direction, and cloud clover also tend to occur. The interface itself is not a vertical wall, rather it’s a sloped surface tilting toward the cold air.

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Raven Eye Photography

Wet slab avalanches are caused by a thick cohesive slab of snow losing its bond to a weaker layer after becoming damp, moist, or saturated with water. The slab that initially fails can be very firm or even hard, but once moving the debris generally becomes a dense, mushy mass. Wet slab avalanches are generally slower moving than dry slab types, but are often highly destructive due to their greater mass.

A whumpf is a collapse of a weak layer in the snowpack, normally triggered by the weight of a person or machine, and often accompanied by a very loud “whumpf” sound. If you hear a whumpf, it’s an indication of an unstable snowpack. In terrain steep enough to avalanche, whumpfs often result in slab avalanches.

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Saul Greenberg

Wind has numerous effects on the composition of the snowpack. It impacts the snowpack by transporting snow from windward to leeward slopes, breaking snow crystals into smaller particles, pressing loose snow crystals together, forming wind slabs, and forming hard wind crusts. It can also insulate the snow surface from solar warming.

The transport of snow by wind is a primary factor in avalanche formation. Evidence of recent wind transport can often be observed by the existence of formations like sastrugi, from a firm, textured snow surface, or from visible accumulations of wind transported snow. These visible surface features are referred to both independently and collectively as wind effect.

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Raven Eye Photography

A wind slab is one or more layers of stiff, wind deposited snow. Wind slabs usually consist of snow crystals broken into small particles by the wind and packed together. Snow accumulation from wind is variable across the landscape, so management of wind slab avalanche problems requires an awareness of this variability. Wind slab is one of eight avalanche problems identified in avalanche forecasts.

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James Floyer

Wind transport refers to snow being lifted from the surface and blown elsewhere. It often leads to snow being transported from the windward to the leeward side of a mountain, or across the face of a mountain (known as cross-loading). Snow accumulation from wind transport can be much higher than from snowfall, making the associated wind loading a major factor in avalanches.

Wind transport varies greatly depending on wind strength, the hardness of surface snow, and the orientation of terrain to the wind. When surface snow is loose and transportable, moderate winds (25-40 km/hr) typically result in transport that is the most conducive to the creation of wind slabs. Light winds (1-25 km/h) are less effective at transporting snow and strong winds (41-60 km/h) often result in snow being spread over a wider area or even sublimating (transforming into water vapour) snow into the atmosphere.

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James Floyer

The term windward refers to slopes that face the wind. If winds are strong enough, snow on windward slopes can be redistributed or blown away to leeward areas, it can be stiffened into a wind slab, or it can form a wind crust.