The avalanche world contains many specific terms used in the discussion of weather, snowpack, terrain, avalanches, and safety. This glossary has been created to help you better understand our avalanche and weather forecasts, and further your avalanche education.
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.
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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
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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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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:
Near-surface facets are a persistent weak layer that forms at or near the snow surface during periods of cold, clear, and dry weather. Strong temperature gradients between warm, sunny days and cold, clear nights cause snow crystals near the surface to recrystallize into small, sugary grains that bond poorly to surrounding layers. This soft, weak surface snow can sometimes be called “recycled powder.”
Once buried by new snowfall, near-surface facets can persist for weeks to months, sometimes at any depth in the snowpack, and are responsible for many persistent slab avalanches. They are often thin, composed of very small grains, and can be difficult to identify in a snow pit, especially compared to larger weak layers like depth hoar or surface hoar. When associated with a crust, they tend to be even more persistent.
Near-surface facets are typically smaller and less developed than basal facets, giving them a somewhat shorter lifespan. Their formation and survival can be disrupted by sun, rain, wind, or dense wind-packed surface snow, meaning their distribution often varies widely across terrain.
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.
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.
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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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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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.
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.
A guide to probing, from Backcountry Access.
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.
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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.
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.
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See Atmospheric Pressure.
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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:
See pinpoint search.
See Avalanche Problem.
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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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.
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See Whumpf
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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:
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.
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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.
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:
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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.
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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.
See Avalanche Path
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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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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.
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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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.
*Note: None of the above devices can function as, or be substituted for, an avalanche transceiver.*
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.
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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.
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See Companion Rescue.
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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.
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.
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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:
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.
Simple terrain is one of four 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.
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:
Large, destructive avalanches are usually slab avalanches.
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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.
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The Slope Evaluator card is a decision-making aid that deals with slope-scale decisions. Part of the Avaluator 3.0 Avalanche Accident Prevention Method, 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.
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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.
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.
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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.
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.
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.
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.
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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:
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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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.
This icon is used when melt-freeze conditions are prevalent. In melt-freeze conditions, danger is relatively low when surface snow is frozen and a thick crust at or near the surface of the snowpack. Danger may increase when the air temperature rises and the crust breaks down. This cycle is usually diurnal with lower danger late at night and in the morning, followed by greater danger in the afternoon and evening. Generally, once a region is considered to be in a melt-freeze state, the spring conditions icon will stay in place for the remainder of the season.
In spring conditions, danger can change from Low to High very rapidly, sometimes in a matter of minutes. The actual danger levels, timing and speed of changes, and the type of avalanche problem that might be encountered are driven by solar radiation, air temperature, sky cover, precipitation, and snowpack characteristics. Keep in mind that conditions are likely to be highly variable when the snowpack has entered a diurnal cycle, depending factors such as aspect, terrain features, and elevation. Be aware that slopes above or around you may have significantly different avalanche conditions or danger.
See avalanche path.
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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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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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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.
Snow that fell during a period of continuous or almost continuous snowfall.
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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.
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See Avalanche Survival.
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.
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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 grdepositione 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.
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.
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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
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.
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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
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:
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.
The trigger point is the location where a person or machine initiates an avalanche. Possible trigger points include:
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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TROWAL is an acronym for "trough of warm air aloft". It is used ow Canadian weather maps, instead of an occludealfront, to illustrate the position of the warm front pinched off from the surface by the faster-moving cold front.
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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
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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.
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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.
Virga is a wisp, streak, or shaft of precipitation falling from the base of a cloud that evaporates or sublimates before reaching the ground.
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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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.
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.
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.
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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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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.
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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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.
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.
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