Northern Climate Change Schools Program
Lesson Plan

Radiation from the Sun influences snow temperature that, in turn, plays a role in determining the strength of the snow. (Snow at the melting point is usually weaker than colder snow.)

Sun exposed slopes tend to have higher temperatures than shaded slopes. In the northern hemisphere shady, cold slopes facing north and east tend to have weaker snow between December and March, because surface hoar and faceted grains (which often form weaker layers in the snowpack) are more likely to form and linger there than on sunny slopes. In the late winter and spring, however, sunny, south-facing slopes are more likely to contain weak snow due to strong warming.

The wind exposure of slopes is a primary factor in avalanche formation.

Lee (downwind) slopes are more likely to produce avalanches than are other slopes with equal incline because they receive much greater accumulations of dense, slabby snow. Lee slopes are found behind high ridges, fall line ribs, rows of trees, hills, convex parts of slopes, and gully walls.

Snow on slopes exposed to the wind (windward) is often shallow and/or irregular due to scouring, creating a potential weak snowpack.

While local wind is significant, it is important to remember that ridge-top wind speed and direction may be quite different from winds experienced locally.

Snow Crystal Grain Form

When identifying snow crystal type (or grain form) such as when making observations in a snow profile or "snow pit" there is a standard graphical way of recording what you have observed. In Canada the "International Classification for Seasonal Snow on the Ground" (Colbeck, et al, 1990) is generally used to record the snow crystal type.

Chart from "Observation Guidelines and recording Standards for Weather, Snowpack and Avalanches"
Copyright © 1995 Canadian Avalanche Association

• Precipitation Particles are any freshly fallen "new snow" type. This can include a wide variety of specific forms that are sub-classified in the next section below. The size of the individual grains can vary wildly. When the snow crystals have significant "rime" attached the small letter "r" is added behind the graphical symbol.
   
  
• Decomposing and fragmented Particles are those snow crystals that have in some way been changed from their original form. This change can be through mechanical action (such as with wind) or through natural processes such as rounding (in which a crystal loses its original form and branches of a crystal can become detached from the original crystal). Parts of the original crystal forms may still be discernible. Decomposing forms are often smaller than the original particles from which they formed.
   
  
• Rounded Grains are those in which the original form is no longer discernible and the crystal takes on a generally rounded amorphous appearance. Rounded grains typically tend to be smaller grains and are usually significantly smaller than the original grain from which they formed. Typical grain size is often less than 1mm. Commonly referred to as "rounds" these grains tend to be found in harder or stronger snowpack layers. Rounded grains typically form when there is a weak temperature gradient in the snowpack (<1 degree / 10 cm). This weak temperature gradient occurs most often in areas where the snowpack is deep and temperatures are mild. (see also Metamorphism).
   
  
• Solid Faceted Crystals are those in which the original crystal form is often no longer discernible and the crystals begin to take on an angular, faceted or striated appearance with straight edges and angular corners beginning to predominate. Faceted grains typically tend to be larger grains and are often significantly larger than the grain from which they formed. Typical grain size is often more than 1mm. Commonly referred to as "facets" or "sugar snow" these grains tend to be found in softer or weaker snowpack layers. Faceted grains typically form when there is a strong temperature gradient in the snowpack (>1 degree / 10 cm). This strong temperature gradient occurs most often in areas where the snowpack is shallow and temperatures are cold. (see also Metamorphism). According to Avalanche Accidents in Canada (Geldsetzer and Jamieson), faceted grains and surface hoar are the weak layer in the failure plane in 78% of fatal accidents. These crystals have poor bonds with each other or adjacent layers and persist in the snowpack for a long time until there is adequate load to make them fail.
   
  
• Cup Shaped Crystals (Depth Hoar) are those in which a faceted form continues to grow or mature in the presence of a strong temperature gradient. These crystals are heavily striated and can have cup or scroll shapes. The grains typically tend to be very large and are often significantly larger than most other grains that would be found in a snowpack. Typical grain size can be more than 3 -- 5mm. Commonly referred to as "depth hoar" or "sugar snow" these grains tend to be found in very soft or very weak snowpack layers usually near the bottom of a weak, shallow snowpack. Depth Hoar forms when there is a strong temperature gradient in the snowpack for a protracted period of time (>1 degree / 10 cm). This strong temperature gradient occurs most often in areas where the snowpack is shallow and temperatures are moderate or cold. (see also Metamorphism). According to Avalanche Accidents in Canada (Geldsetzer and Jamieson), faceted grains and surface hoar are the weak layer in the failure plane of slab avalanches in 78% of fatal accidents. Depth Hoar crystals have very poor bonds with each other or adjacent layers and can persist in the snowpack until the end of winter or until there is adequate load to make them fail. These grains can be the weakness in avalanches that fail right at the ground, in which the entire winter snowpack is involved in a large climax type of avalanche.
   
  
• Wet Grains are just as the name implies... WET. They form when temperatures are above freezing, when solar radiation is intense, when rain falls onto the snowpack or when any of these factors occurs individually or together. The original crystal shape is often rapidly changed and rounded. While grains are warm and wet they tend to be weak and can fail in wet loose avalanches or wet slab avalanches. (see also Metamorphism).
   
  
• Feathery Crystals (Surface Hoar) form on the surface of the snow under conditions where there is a generally clear sky with high humidity and little or no wind. These crystals are the same as the "hoar frost" seen sparkling on trees and on the snow surface after a clear cold night. They can be large in size (>10mm) or smaller (<1mm). As a broad generalization, the larger the size of surface hoar the more likely that it will form a poor bond to the adjacent snow layer. Once buried under a new snow layer they can be difficult to detect without diligent testing. According to Avalanche Accidents in Canada (Geldsetzer and Jamieson), faceted grains and surface hoar are the weak layer in the failure plane in 78% of fatal accidents. These crystals have poor bonds with each other or adjacent layers and persist in the snowpack for a long time until there is adequate load to make them fail. (see the last section below for a more detailed discussion.)
   
  
• Ice Masses often form when a wet layer cools and hardens.
   
  
• Surface Deposits and Crusts is a broad category that includes sun crust, wind crust, rain crust, rime and other melt / freeze crusts. This can include a variety of specific forms that are sub -- classified in a section below.

Precipitation Particles

Precipitation Particles are any freshly fallen "new snow" type. This can include a wide variety of specific forms that are sub-classified here. The size of the individual grains can vary wildly. When the snow crystals have significant "rime" attached the small letter "r" is added behind the graphical symbol. When snow forms under varying conditions of temperature and humidity the actual crystal type can change during a storm. Different atmospheric conditions favour crystal growth in quite different ways.

Chart from "Observation Guidelines and recording Standards for Weather, Snowpack and Avalanches"
Copyright © 1995 Canadian Avalanche Association

• Columns are new snow crystals consisting of a six -- sided hollow or solid prism. A capped column has flat plates attached to the ends.
   
  
• Needles are thin, long, needle -- like crystals of new snow.
   
  
• Plates are thin, plate -- like, usually hexagonal crystals of new snow. Layers of plates can form a failure plane for an avalanche as the plates may not bond well to one another. Plate crystals become rounded slowly and may form weak layers for long periods of time.
   
  
• Stellar Crystals are what many people commonly think the classic snowflake should look like in that they are star-like new snow crystals with unbroken arms. Dendrites are star-like new snow crystals with numerous side branches, often three-dimensional.
   
  
• Irregular Crystals are those which defy categorization into the other classes or sub-classes.
   
  
• Graupel is formed when a snow crystal falls through a layer of air that is supersaturated with moisture. As the crystal falls through the air the water droplets instantly freeze onto the crystal surface forming miniature dull colored balls of "rime". As the rime continues to accumulate on the crystal, its original shape becomes obscured and is no longer recognizable. The grain now simply looks like a dull colored tiny ball of snow.
   
  
• Hail forms when a precipitation particle falls through a layer of moist air and becomes coated with a layer of ice. Convective winds then blow the precipitation particle back up into the cloud where it falls through the moist air again and grows in size with an additional layer (s). This process may continue through several cycles. Hail particles exhibit a layered or laminated internal structure when carefully sliced open. In essence, hail is a ball of ice whose size has increased by added successive layers of ice on the outside.
   
  
• Ice Pellets may form when rain falls though a very cold air mass. As a result the rain drops may freeze into pellets. Other processes may form pellets as well.

Surface Deposits and Crusts

Surface Deposits and Crusts is a category that includes sun crust, wind crust, rain crust, rime and other melt / freeze crusts.

Chart from "Observation Guidelines and recording Standards for Weather, Snowpack and Avalanches"
Copyright © 1995 Canadian Avalanche Association

Rime is the deposition of super-cooled water droplets on any object. It may build up into the wind on days when there is an "ice fog" of super cooled water droplets in the air. Firnspiegel is a thin, often highly reflective, type of sun crust formed by solar radiation on cold clear days. It acts like a "greenhouse" to enhance melting just below the surface. It may even appear to be suspended above the rest of the snow surface by a millimetre or more.

Surface Hoar

Under certain conditions, a new snow crystal referred to as surface hoar forms directly on the surface. Surface hoar is created when air is cooled to below its dew point (dew point is the temperature at which air becomes completely saturated and must begin releasing water vapour) and deposits water vapour as ice onto a colder surface.

Diagram from "Avalanche Safety Course Overheads"
Copyright © 1998 Canadian Avalanche Association

For this to occur, the following conditions must be present:

• moist air (provides the vapour that will create the surface hoar crystals)
• clear skies (allowing the surface to cool by radiation)
• a snow surface with a temperature lower than the dew point of the adjacent air (which brings adjacent air to below its dew point)
• calm or light wind conditions (wind will disturb the air at the surface and prevent it from being cooled to the dew point).

Metamorphism of Snow

Once new snow crystals are added to the snowpack they begin to metamorphose (change). From this point on, snow crystals are technically referred to as grains (although the word crystal is still used by many practitioners). Through metamorphism, the form and size of snow crystals and grains inside a snowpack change continuously, altering the strength characteristics of the snowpack.

Metamorphism of snow in a seasonal snowpack is the result of sublimation and deposition. (Sublimation is the process of ice becoming vapour without going through a liquid state and vice versa.) During metamorphism in the snowpack, ice from grain surfaces changes into water vapour that is then deposited as ice at other grain surfaces as follows:

1. Vapour moves from warm surfaces to cold surfaces. Because the snowpack is usually warm (at or near 0 degrees C) at the ground and cold near the surface, ice sublimates from lower, warmer grains and is deposited as ice at other grains sites as shown in the diagram below.
   
   
   
   "Diagram from "Avalanche Safety Course Overheads"
   Copyright © 1998 Canadian Avalanche Association
    
   
2. Vapour typically moves from convex surfaces (points) to concave surfaces (hollows). The sharp ends of new snow crystals becomes blunt and the space between the branches is filled. In the same manner, large grains with broad curvatures grow at the expense of small grains with sharp curvatures.

The Temperature Gradient

The temperature gradient is the most important factor determining the type of metamorphism, the resulting grain form, and the rate of growth of the grains. Temperature Gradient is the difference in snow temperature across a given vertical distance in the snowpack. In practice it is expressed in degrees Celsius per 10 centimetres. As a general rule, a temperature gradient less than 1 degree / 10cm is considered weak. A strong temperature gradient is greater than 1degree / 10cm. Strong Temperature gradients promote greater vapour movement than weak gradients.

The nature of the temperature gradient influences the type of metamorphic process that will be dominant in a given portion of the snowpack. The primary processes are faceting and rounding. (For a detailed explanation of faceting and rounding see the sections that follow below.) Faceting and rounding take place in the snowpack interchangeably. When the temperature gradient is strong and the snow density is low, the faceting process dominates. When the temperature gradient shits from strong to weak (usually the result of warming at the snow surface), faceted grains, depth hoar and surface hoar grains begin rounding. These large angular grains resist rounding much more than branched new snow crystals and may remain weak for long periods.

Strong Temperature Gradient

The cross section of a snowpack at left shows a strong temperature gradient. The height of the snowpack is 100cm and the snow temperature near the top of the snowpack is -15 degrees C. This creates an "average" temperature gradient within this snowpack of 1.5 degrees / 10cm. The actual gradient in any particular layer varies and may be greater or less than this average, but it can be expected that in this sample snowpack the faceting process will be predominating. (This process has gone by other names including temperature gradient metamorphism, TG metamorphism, Constructive metamorphism, recrystallization and kinetic growth. The term faceting is preferred.) This example is fairly typical of a snowpack that you may find in early winter in many regions or in the Canadian Rockies even during mid-winter or later. If this temperature gradient does not change, the snowpack will continue to lose strength over time and a base of weak depth hoar will continue to develop. Faceted grains and depth hoar formed in this way will persist in the snowpack and can cause cycles of avalanche activity for the rest of the winter and even into the spring or, in some cases, summer.

Diagram from "Avalanche Safety Course Overheads"
Copyright © 1998 Canadian Avalanche Association

Weak Temperature Gradient

The cross section of a snowpack at left shows a weak temperature gradient. The height of the snowpack is 200cm and the snow temperature near the top of the snowpack is -15 degrees C. This creates an "average" temperature gradient within this snowpack of 0.75 degrees / 10cm. The actual gradient in any particular layer varies and may be greater or less than this average, but it can be expected that in this sample snowpack the rounding process will be predominating. (The rounding process has gone by other names including equi-temperature metamorphism, ET metamorphism, destructive metamorphism, and equilibrium growth. The term rounding is preferred.) In this sample snowpack, the temperature gradient is weaker near the base and stronger near the top. There is no place in this sample snowpack that the faceting process will be predominating.

Diagram from "Avalanche Safety Course Overheads"
Copyright © 1998 Canadian Avalanche Association

This example is fairly typical of a snowpack that you may find in early winter in a deep snowpack region with moderate climate (such as the Coast Ranges of British Columbia or the US). Similarly this type of snowpack may exist in the Columbia Mountains in the Interior of British Columbia in early winter during a heavy snowfall winter and certainly by mid winter in an average winter. The Canadian Rockies would typically only have this type of condition later in winter or spring or in a good snow year.

If this temperature gradient does not change, the snowpack will continue to gain strength over time and any base of weaker facets as shown in this example will continue to strengthen. Even with a weak temperature gradient which promotes rounding and strengthening of the snowpack, hidden weak layers may exist. In this sample snowpack, a layer of surface hoar is buried just above 130cm. Buried surface hoar may persist in the snowpack and can cause cycles of avalanche activity for the next several weeks or more. The weak temperature gradient will eventually round out the surface hoar and promote bonding with the layers above and below but this gain in strength of this insidious layer can take a very long time in some cases.

Rounding

The rounding process builds rounded grains (rounds) which bond well to one another creating a snowpack (or layer) that is generally increasingly strong. In weak temperature gradients(<1 degree / 10cm) sublimation typically moves ice from convex surfaces (points) to concave surfaces (hollows) in 2 stages:

1. In the initial stage of rounding, the sharp ends of new crystals and the points of faceted grains sublimate and the resulting water vapour is deposited in concave areas. At high temperatures, molecules also glide along the grain surface from convexities to concavities. As well, large grains with broad curvatures grow at the expense of small grains with sharp curvatures. The result is a concentration of mass with a minimum surface area.
2. Under weak temperature gradients, water vapour moves from warm areas to cold, but the rate of movement is much slower than in strong temperature gradient environments. Slow moving vapour is deposited on the colder surfaces in a more homogenous manner and the faceted, stepped pattern associated with a strong temperature gradient does not occur.

The following conditions promote rounding:

• A weak temperature gradient, generally less than 1 degree C per 10 cm (which moves water vapour slowly from warm areas to cold)
• Dense, tightly packed snow
• Small grains (which produce denser snow)
• A high snow temperature, typically above -10 degrees C (which promotes weaker temperature gradients)

Faceting

The faceting process builds angular grains (facets) which bond relatively poorly to one another and other grains creating a snowpack (or layer) that is generally increasingly weak. When the temperature gradient is strong (> 1 degree / 10cm) water vapour moves rapidly from warm grain surfaces to colder surfaces. Because the snowpack usually is warm (at or near 0 degrees C) at the ground and colder at the surface, ice sublimates from lower, warmer grains and is deposited onto colder grains higher up in the snowpack. These colder grains first develop sharp corners, then stepped facets.

If the faceting process continues, large, six -- sided hollow or filled cup shaped grains called depth hoar are formed. Depth Hoar is common in Rocky Mountain climates, around large rocks and high shrubs, and where the snowpack is thin.

The following conditions promote faceting:

• A strong temperature gradient, generally greater than 1 degree / 10cm (which quickly drives water vapour from warm areas to cold)
• Loose, low density snow (which facilitates the free movement of water vapour between grains)
• Presence of crusts (which concentrate water vapour, promoting vapour transfer in the concentrated area)
• Moderate snow temperature (which maximizes the amount of vapour the snowpack can hold but does not reduce the overall temperature gradient significantly)

Text and diagram from "Advanced Avalanche Safety Course Manual"
Copyright © 1998 Canadian Avalanche Association

Sintering

Sintering is usually associated with the rounding process. Water Vapour is deposited at the contact points between snow grains forming necks. These necks create strong bonds between grains, increasing snow strength.

Mechanical Hardening

Compaction from any mechanical disturbance such as boots, skis, snowmobiles, groomers, wind and avalanches breaks up large grains and brings grains into close contact, producing rapid sintering.

Melt-Freeze Metamorphism

The change in snow grains as the snowpack becomes wet (snow temperature reaches 0 degrees C) and subsequently refreezes is known as melt-freeze metamorphism. This process usually occurs during late winter and spring when air temperatures are high, solar radiation is high, and cycles of melting and refreezing are common. Melt-freeze crust layers that exist in the freeze part of the cycle can be very strong.

Slope Incline and Avalanche Frequency

A primary requirement for avalanche formation is a slope incline that is steep enough for avalanches to initiate and then accelerate. The following guidelines for using slope incline to predict avalanche size and frequency have been developed from experience.

Avalanches are rare on slopes with an incline greater than 55 degrees because the snow sluffs off frequently in small amounts.

Diagram from "Avalanche Safety Course Overheads"
Copyright © 1998 Canadian Avalanche Association

A minimum slope angle is required to initiate a slab failure, however, a fracture may propagate to an area with less incline after initial failure on a steeper slope has occurred.

In "Avalanche Accidents in Canada -- Volume 4" by Geldsetzer and Jamieson it was reported that in a sample of 184 recreational avalanche accidents, 83% of them occurred on slopes between 25 and 40 degrees and half of these fell in the range from 31 to 35 degrees. (From talk given by T. Geldsetzer, Edmonton, AB, November, 1997)

Text from "Advanced Avalanche Safety Course Manual"
Copyright © 1998 Canadian Avalanche Association

Loose Snow Avalanches

Loose avalanches are usually confined to surface layers, and therefore are often small. Loose snow avalanches:

• start from a point
• gather mass progressively in a fan-like shape
• require loose cohesionless snow
• may contain dry or wet snow

Small loose snow avalanches (size 1) are often referred to as "sluffs". Since loose snow avalanches start from a point and fan out, they are also called "point releases".

Slab Avalanches

Large Hazardous avalanches are usually slab avalanches. Slab Avalanches:

• leave a fracture line
• can release simultaneously over a large area, setting large volumes of snow into motion
• may start as a shallow surface layer
• may consist of a thicker layer(s) deeper in the snowpack
• may involve a number of layers
• range from new snow (soft slab) to hard wind -- packed snow (hard slab)
• may contain dry or wet snow

Failure of a Snow Slab

For a slab avalanche to start, there must be:

• A cohesive, relatively strong layer
• A stronger layer overlying a weak layer or a weak bond between layers
• A closely balanced stress -- strength relationship between the weak layer and the overlying snow
• A trigger which upsets the balance (Triggers may be natural factors, such as: heavy snowfall, rapid depositing of snow by wind, a rapid rise in temperature, fall of a cornice, ice fall, earthquake. Triggers may also be artificial factors such as: skiers, snowboarders, snowshoers, snowmobiles, hikers, vibrations from machinery and traffic, and explosives.)
• A mechanical condition that allows the condition to propagate (spread). Hard snow allows wide propagation and tends to produce large slab avalanches.

The most important characteristic of the snowpack with respect to formation of slab avalanches is the existence of a weak layer underlying a stronger layer or layers and / or a weak boundary between layers.

Slab avalanches start when the weight of snow layers and a trigger combine to create forces which exceed the strength of the snow. Slab avalanches are thought to occur as follows:

1. an initial shear or tension failure occurs.
2. Failure then propagates along a shear plane (usually the weak layer) to a location where the snow is under tension (convex roll, rock outcrop, etc.)
3. A tension failure occurs when the tensile strength of the slab is exceeded.
4. The fracture line forms. (Due to propagation, the point of initial failure may be a considerable distance from the fracture.

Compression Test

The test as described here was developed by Parks Canada Wardens working in the Canadian Rockies in the 1970s. Similar tests were developed elsewhere. The test identifies weak layers and is most effective at finding weak layers near the snow surface. Manual taps applied to a shovel blade placed on top of a snow column cause weak layers within the column to fail. These failures can be seen on the smooth walls of the column. The test can be performed on level or sloping terrain.

A pit is dug in undisturbed snow to expose a smooth snow wall on a safe slope representative of the slopes of interest. The pit is dug to ground or until well below any possible significant weak layers (often as much as 2 metres deep). The column is not dug down to very weak layers of facets or depth hoar if these layers are likely to fail before upper weak layers of interest.

A column of snow 30cm wide (across the slope) and 30 cm upslope is created as in the diagram at left. A snow saw can assist in creating the column and making the subsequent backcut that is required. Be sure the visible walls of the column are smooth so that subtle failures can be easily seen.

A shovel is placed squarely on the surface of the column and progressively harder taps are applied to the shovel blade. Any failures are recorded. Collapse of very thin layers may be subtle and hard to detect. In most cases it is good to have a second person observe for these failures while the first person applies the taps. The size and type of crystals at the failure plane (often from the underside of the block) are also recorded. Another backcut is now made an additional 70cm below the first and the process is repeated to the bottom of the pit. The test may be repeated to verify the results or a Shovel Shear Test may be done alongside the first test location.

The amount of effort required to cause the failure is recorded as follows:

• Very Easy (CV) -- fails during cutting of column
• Easy (CE) -- fails with 5 -- 10 light taps using finger tips only
• Moderated (CM) -- fails with 5 -- 10 moderate taps from elbow using finger tips
• Hard -- (CH) -- fails with 5 -- 10 firm taps from whole arm using palm or first
• Collapse (SC) -- block settles when cut

The primary objective of the compression test is the identification of weak layers. Deeper layers are generally less sensitive to taps on the shovel resulting in higher ratings. Similarly, deeper layers are less sensitive to human triggering. Experience in the Canadian Rockies suggests that layers with "very easy" or "easy" failures are more often associated with human or explosive triggering than are "moderate" or "hard" failures. Sudden failures that show up on the column wall as distinct lines seem more likely to indicate potential failure planes than rough or indistinct failures.

Caution: While the rating of effort needed to have the snow fail in compression may assist with a decision concerning snow failure, it is an inaccurate measurement of slope stability. The ratings of effort are subjective and depend on the strength and stiffness of the slab, on the size and shape of the shovel, the experience of the tester and on whether or not the test site is truly representative of the slope of interest for which the test is being applied.

NOTE: If the top surface slopes, test the near surface layers then remove a wedge of snow to level the top of the column. Once level. place the shovel blade squarely on top of the column and continue testing.

Tapping forces are not transmitted efficiently down through the column, particularly through soft layers within the column. Harder taps are generally required to cause failure in deep layers, particularly if the layers between the shovel and the weak layers contain soft snow.

Snow below the shovel that crushes and fails to support the shovel squarely should be removed. The tends to reduce the force required to cause failures in the remaining column.

Text modified after "Advanced Avalanche Safety Course Manual"

Rutschblock Test

The "rutschblock" (or glide-block) test is a slope test that was developed in Switzerland in the 1960s. This section is based on a recent Swiss analysis of rutschblock tests (Fohn, 1987) and on Canadian Research experience (Jamieson and Johnston, 1993).

Test sites should be safe, representative of the avalanche terrain under consideration and undisturbed. For example, to gain information about a wind blown slope, find a safe part of a similarly loaded slope for a test. The site should not contain buried ski or snowmobile tracks, avalanche deposits, etc. or be within about 5 m or trees where the buried layers might be disturbed by wind action or by clumps of snow which have fallen from nearby trees. Although Dr. P. Fohn (1987) recommends slope angles of at least 30 degrees, rutschblocks of 25 -- 30 degrees may also give useful information (discussed below). Be aware that near the top of a slope, snowpack layering and hence rutschblock scores may differ from the slope below.

After identifying weak layers (and potential slabs) in a snow profile, extend the pit wall until its width is larger than the observers skis or snowboard (2 metres minimum) across the slope. (Do not omit the profile unless the layering is already known.) Mark the width of the block and the length of the side cuts ( 1.5 m) on the surface of the snow with a ski, ruler, etc. The block should be 2m wide throughout if the block is to be dug with a shovel. However, if the side walls are to be cur with a ski, pole, cord or saw, the lower wall should be about 2.1 m across and the top of the side cuts should be about 1.9m apart. This flaring of the block ensures it is free to slide without binding at the sides. The lower wall should be a smooth surface cut with a shovel. Dig or cut the side walls and the upper wall deeper than any weak layers that may be active. If the side walls are exposed by shoveling, then one rutschblock test may require 20 or more minutes for two people.

If the weak layers of interest are within 60cm of the surface, time can be saved by cutting both the sides and the upper wall of the block with a ski pole (basket removed) or with the tail of a ski. If the weak layers are deeper than 60cm and the overlying snow does not contain any knife-hard crusts, both the sides and the upper wall of the block can be sawed with cord which travels up one side, around ski poles or probes placed at both upper corners of the block and down the other side.

The rutschblock is loaded, and failure recorded, in the following sequence:

• R1 -- the block slides during digging or cutting
• R2 -- the skier (or snowboarder) approaches the block from above and gently steps down onto the upper part of the block (within 35cm of the upper wall).
• R3 -- without lifting the heels, the skier drops from straight knee to bent knee position, pushing downwards and compacting surface layers.
• R4 -- The skier (or snowboarder) jumps up and lands on the same compacted spot.
• R5 -- The skier jumps again onto the same compacted spot.
• R6 -- for hard or deep slabs, remove skis and jump on the same spot. / -- for soft slabs or thin slabs where jumping without skis might penetrate the slab, keep the skis on, step down another 35 cm, almost to mid block and push once then jump three times.
• R7 -- none of the loading steps produced a smooth slope-parallel failure. Interpretation of rutschblock tests in the starting zone:
  • 1, 2 or 3: Block fails before the first jump. It is likely that slopes with similar snow conditions can be released by a skier, snowboarder, snowshoer or snowmobile.
  • 4 or 5: The block fails on first or second jump. The stability of the slope is suspect. It is possible that slopes with similar snow conditions can be released by a skier, snowboarder, snowshoer or snowmobile. Other observations or tests must be used to assess the slab stability. (Snowmobiles have increased risk or staring avalanches when compared to skiers.)
  • 6 or 7: the block does not fail on the first or second jump. There is a low (but not negligible) risk of skier, snowboarder, snowshoer or snowmobile triggering of avalanches on slopes with similar snow conditions. Other field observations and test as well as safety measures remain appropriate.

We should be very careful with interpreting slope tests since they overestimate the slope stability at least 10% of the time. (Jamieson, 2000)

The rutschblock is a good slope test but it is not a one stop stability evaluation. The test does not eliminate the need for snow profiles or careful field observations nor does it, in general, replace other slope tests such as ski cutting and explosive tests.

The rutschblock only tests those layers deeper than ski or snowboard penetration. For example, a weak layer 20 cm below the surface is not tested by skis which penetrate 20cm or more. Higher and more variable rutschblock scores are sometimes observed near the top of a slope where layering may differ from the middle and lower part of the slope (Jamieson and Johnston, 1993). Higher scores may contribute to an incorrect decision.

Rutschblock results are easiest to interpret if the tests are done in avalanche starting zones. However, since there is a general tendency for rutschblock scores to increase by 1 for each 10 degree decrease in slope angle (Jamieson and Johnston, 1993), scores for avalanche slopes can be estimated from safer, less steep slopes (as shallow as 25 degrees). Note that rutschblocks done on slopes of less than 30 degrees require a smooth lower wall and a second person standing in or near the pit to observe the small displacements (often less than 1 cm) that indicate a shear failure.