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Snow Stability and Avalanches

Adapted from a 5 part article for the Wasatch Mountain Club "Rambler" which was published in 1995

Jim Frankenfield
Director, Avalanche-Center.org

Part 1 of 5 - Introduction and Overview

I hope that this series will help backcountry skiers and other interested persons to better understand the factors which contribute to mechanically unstable snowpacks and avalanches.

Part 2 will discuss the effects of weather on the snowpack. This will include the effects of new snowfall, the redistribution of snow by winds, and heat exchange at the snow surface. Part 3 will focus on the stability of a seasonal snowpack - how snow crystals in the snowpack change form and how this and other factors lead to layering. Part 4 will cover terrain and routefinding in light of the factors discussed in the preceding sections. The concluding part will discuss spring hazards.

For readers interested in a more in-depth study of the topic there are many resources available. Most outdoor shops carry books on the subject. It is best to start with one or more of the standard basics. For the more technically inclined I would recommend the following:

McClung, David and Schaerer, P.; "Avalanche Handbook" - A classic reference which is thorough but quite technical. This book should be easy to find in the local shops.

Gray and Male, ed.; "Handbook of Snow"; Pergamon Press - This is a more detailed and technical book which covers most aspects of snow science, not just avalanches.

LaChapelle, Edward; "Field Guide to Snow Crystals"; Univ. of Wash. Press - Listed because it has many good photos of different types of snow crystals. People interested in natural science in general as well as those interested primarily in avalanches may want to check this out. [No longer by the U of Wash and now difficult to find. Last I knew, it was available through the International Glaciological Society.]

Frankenfield, James; "Snow and Avalanche Physics"; USU Thesis 1989 - This is a technical treatment of the subject and includes a large bibliography of scientific journal articles.

It is difficult to cover field methods such as snowpit analysis and rescue methods in a short written series such as this. There are numerous field courses offered in the area throughout the winter. Skiers, snowboarders, and other winter recreationists are encouraged to attend one of these field classes and also to practice with a rescue beacon often.

Part 2 of 5 - Weather

When most people think about weather and its relation to snowpack stability they think of new snow. The weather affects the snowpack in many ways, even when it is not currently snowing. Following the weather between outings is helpful in interpreting snow pit data and other observations in the field. There is a certain satisfaction in digging a snowpit and not only being able to identify layers but knowing when and how they formed. This is a good reason to review your local avalanche forecast frequently between trips.

New Precipitation

The vast majority of avalanches occur during or shortly after a storm. Given enough time, most snowpacks will adjust to the new load of fresh snow. However, the amount of time which constitutes 'enough' can vary widely and depends on the structure of the snowpack. Since this structure varies with aspect (direction), elevation, and other factors, some slopes will stabilize more quickly than others.

Important factors concerning the new snow layer include its bond with the old snow surface and density changes which occur within the new snow as the storm progresses. Crusts and slabs are examples of surfaces which new snow sometimes bonds poorly with. New snow density typically decreases as a storm progresses. In those cases where the density increases or fluctuates widely heavier snow can be deposited on top of lighter snow, leading to instability within the new snow.

If the precipitation comes in the form of graupel or rain it generally has an adverse effect on stability. Graupel consists of snowflakes which accumulated so much rime ice on their journey through the atmosphere that they are spherical. They are more stable, thermodynamically, than dendrite crystals and take longer to stabilize. Rain wreaks havoc when it percolates into the snowpack. Free water tends to make weak layers weaker. It can also flow along planes parallel to the ground, such as crusts, lubricating bed surfaces.

Wind

Wind is one of the most important factors affecting the mountain snowpack. It can cause large accumulations of snow through redistribution of existing as well as new snow. The wind takes snow from the windward side of ridgelines and deposits it on the lee slopes. These ridgelines may be along ridgetops or they may extend downward (such as the ridges separating the slide paths along the north side of Little Cottonwood Canyon). Rates of wind deposition of at least 45cm/hr have been observed.

Cornices often form over these lee slopes where 'wind slabs' form. If part or all of the cornice collapses it often triggers the slope below. Cornices often fracture back further than most people expect. They have caused numerous accidents and should be given as much leeway as possible.

Heat Exchange at the Snow Surface

The radiation balance at the snow surface is usually the primary factor in heat exchange. Snow is a very efficient radiator of energy in the visible and infrared part of the spectrum. One effect of this is the formation of surface hoar on cold clear nights. Surface hoar is the equivalent of dew. It typically forms as large flat crystals which sparkle in the sunlight. Large accumulations of surface hoar often build up on shaded aspects during temperature inversions.

During the day the snow absorbs energy from short-wave UV radiation. The ratio of the amount of this radiation reflected to the amount incident is called the Albedo, which varies from 60% to 85% for clean snow. (Albedo is an important geophysical parameter in general, particularly in remote sensing.) On sun exposed slopes the combination of short-wave heating and long-wave cooling results in very steep temperature gradients near the snow surface, causing a variety of weak surface layers involving recrystallized snow and/or crusts.

The radiation balance at the surface also effects the metamorphism (crystal changes) of the snow below the surface. Stability of the snowpack depends partly on the layering caused largely by metamorphism, surfaces formed between storms, wind slabs, and new snow. Metamorphism and stability will be discussed in the next part.

Part 3 of 5 - Snowpack Structure and Mechanical Stability

The most important aspect of seasonal snowpack structure pertaining to avalanche hazards is that it is layered. Layers form due to thermodynamic changes which occur within the snowpack, as well as from factors such as wind, sun and surface hoar deposition. The strength of these various layers varies from very weak to very strong. If the snowpack is on an inclined surface it is continuously subject to stress due to the gravitational force. Skiers, boarders, or snowmobilers add additional stress. If the total stress on some layer exceeds its strength then mechanical failure will result. The presence of a particularly weak layer will often be noted by a collapse the snowpack due to the weight of a skier - whumpf! Sometimes weak layers will have a hollow feel and/or sound to them, especially wind slabs. Don't ignore important clues like these.

Snow Metamorphism

Snow crystals change within the snowpack due to thermodynamic processes. These changes are commonly referred to as metamorphism. There are three types of processes which are important - temperature gradient (TG), equitemperature (ET), and melt-freeze. Only the first two will be discussed here. Melt-freeze processes are most common in spring.

A large temperature difference, or gradient, across the snowpack (or part of it) will cause the migration of water vapor from bottom to top. The warmer air down lower can hold more moisture, while the colder air up higher is dryer. This causes vapor to diffuse upward by leaving the top surface of one crystal and condensing on the bottom of another crystal above. The result is faceted grains which have low strength. Early in the season it is common to have a thin snowpack with a large temperature gradient imposed on it by cold nights. This causes the layer of "depth hoar" often found at the bottom of the snowpack. When the snowpack is thick temperature variations effect only the top section, where smaller facets form during spells of cold weather. This process is a form of "constructive metamorphism", a term preferred by some to TG.

In the absence of a temperature gradient the changes which occur are called equitemperature (ET) or "destructive metamorphism". These conditions promote the formation of rounded grains which bond together, resulting in high strength.

Constructive and destructive processes are not mutually exclusive, and there is typically some combination of the two occurring at once.

Glide and Creep

Snow is a viscoelastic material. This means that in some ways it behaves as a fluid (with very high viscosity) and in some ways it behaves as an elastic solid (such as rubber band). It is very difficult to model and/or predict the behavior of such materials.

The force of gravity causes two types of very slow motion in an inclined snowpack. Under creeping motion snow which is higher in the snowpack moves downslope slightly faster than snow which is lower in the snowpack. This introduces shear stress within the snowpack. Glide refers to the uniform motion downslope (on the order of millimeters per day) of the entire snowpack in relation to the underlying surface.

Summary

Many factors potentially leading to snowpack instability have been reviewed. All of these factors differ greatly from one location to another. The result is that the snowpack is very nonhomogeneous spatially. The structure found in one location may be very different from what exists in another location, even a fairly close distance away. It is very important to keep this in mind. One implication of this is that a slope is not necessarily safe just because it has been skied. It is possible for the second, third, or n-th skier to hit that spot which is structurally different and cause a failure. For this reason, safe skiing practices and good routefinding abilities are always necessary. That will be the topic of the next article in this series.

Part 4 of 5 - Terrain, Routefinding, and Safe Travel Techniques

Terrain

Major terrain factors include altitude, aspect, and slope incline. Conditions will vary largely with altitude due to differences in precipitation. In some cases precipitation will come as rain below a relatively high elevation. This is uncommon in Utah but does happen on occasion. In ranges such as the Cascades it is common. In Utah the amount of snow can vary widely with elevation, and a thinner low elevation snowpack differs greatly from a thicker high elevation pack.

Slope incline is one of the most important factors. If a slope is too steep snow tends to sluff off before accumulating enough to form a slab. If it is not steep enough a slab which fractures will not slide down the hill. The most dangerous slopes are generally between 30 and 45 degrees. However, there are exceptions under some conditions and large slides have been recorded on slopes ranging from 25 to 60 degrees. These angles pertain to the starting zone where a slab will fracture and move. Once in motion an avalanche will run over and/or onto flat terrain (or even uphill in some cases).

Slope aspect is also a very important factor. Cornices and wind transport have already been discussed. Different aspects also respond to the sun differently, which is particularly important in spring skiing. A slope can be frozen and icy early, have great corn snow for a while, then present a wet slide hazard. The 'corn window' varies with aspect, moving from east or southeast through west facing slopes as the morning progresses.

Other factors include slope convexity and terrain traps. The snowpack has more natural stress over convex slopes than straight or concave sections. This is mostly due to creep (see Part 3). It is possible for the weight of a skier to trigger a failure in such an area when other parts of the same slope do not fail. Terrain traps are gullies or short steep slopes where a very small slide can occur. Even though the slide is small it can bury an unwary traveler. Terrain traps are often either overlooked altogether or not given the respect they deserve.

Trees (and boulders) can serve as anchors which help hold the snowpack in place. A thickly forested slope is usually much safer than an open slope. It is important to remember, though, that moving avalanches can run right through such a slope. Small ones will flow through the trees and large ones will sever them. And in some conditions a slide can occur entirely within a slope which looks pretty densely forested.

Route Selection

Route selection often begins before leaving home. If traveling to a different region try to learn something about the snowpack in that geographical region. It might be inherently different from what you are accustomed to. Slope angles can often be estimated from topo maps. And avalanche bulletins or advisories should be obtained.

Once out in the backcountry various factors should be constantly observed. The most obvious sign of instability is recent avalanche activity. While this may seem like a statement of the obvious, it is overlooked (or ignored) with surprising frequency. Another thing to watch for are flag trees, or trees with no uphill branches below a certain level. While they are usually curved at the bottom, this feature alone is not indicative of previous avalanching since it can be caused by snowpack creep among other things.

Avoid open slopes whenever possible. While forested terrain is not always entirely safe it is much safer than open terrain. Also avoid runout zones and remember that it is possible to trigger an avalanche from below. Use ridgelines as much as possible, giving adequate leeway to cornices. Previously made tracks, even recent ones, do not imply stability. Since this is due in large part to spatial variations in the snowpack it is best to follow the same track as much as possible.

Safety Practices

When exposed to a potential hazard travel one at a time and keep at least one individual in a spot from which the entire slide path is visible. Remove pole straps and safety straps. Put on some layers and close them up. Ski the edges of that nice slope first, moving out onto it on additional runs only if you feel confident based on all of the information you have accumulated.

Part 5 of 5 - Human Factors, Spring Hazards

Human Factors

"Human Factors" refers to things such as enthusiasm, group dynamics, and risk acceptance. These three factors are not isolated from each other, but work together in various ways. Each of these will be demonstrated through an example.

Enthusiasm: A few weeks ago during the quasi-spring conditions of early February three skiers decided to go to Tanners Gulch. However, they got a late start and didn't arrive at the trailhead until about noon. One of the three was really excited about the skiing possibilities and started off quickly. The other two were not too sure and trailed behind a bit. The second one stopped at the mouth of the gully. When the third skier caught up (s)he said they would not continue, but would wait there. The second then decided to wait also. They ended up all returning to the car and coming back the next day at a much earlier hour. By noon the sluffs off the east facing slopes up in the gulch were getting pretty large! This story had a happy ending, but all too often the enthusiasm for that perfect run causes people to ignore all common sense practices and all factors which have been discussed earlier in the series.

Group Dynamics: A group was recently climbing in Colorado and found themselves out in a storm. On the way back they encountered a steep open slope on the lee side of a ridge. One person had recently taken an avalanche awareness course and wondered if this was a safe slope to cross. However, (s)he followed one of the other climbers out onto it since that person was far more experienced. The experienced person suddenly appeared coming back shouting "Slide!". Fortunately it was small enough that the group was not caught and this also had a happy ending. Individuals should not hesitate to speak up and question situations they feel uncertain about, despite the perceived experience of others in the group. Remember that even seasoned professionals have been known to get caught off-guard on occasion.

Risk Acceptance: In March of 1994 I was ice climbing with a friend between Banff and Jasper. On the sixth (and last) pitch of a classic called "Polar Circus" an avalanche came down over us. My partner was securely anchored at the belay, and I was on a steep section where it all went by overhead. Another party below was also lucky. So again we have a happy ending. It was clear that there was a risk, but the level of risk was difficult to assess. To make things more difficult, the question was one of how likely a natural release from above was, which is inherently different from digging a snowpit and questioning the odds of triggering something yourself. Partly because of these factors and partly because of the nature of the sport, ice climbers (and mountaineers) often tend to accept higher levels of risk than skiers and others.

Spring Hazards

Full spring conditions typically consist of an isothermal snowpack - One which is at 32 F throughout. The surface will refreeze overnight, and as it softens in the sun good "corn" snow makes great skiing. The hazard tends to follow a daily cycle of low in the morning and moderate in the afternoon. After softening too much wet slides can release, often starting as point releases and growing. A good rule of thumb is to head home when you start to sink to your boot tops.

Another hazard to keep in mind is that before softening up the snow can be icy and a fall can be dangerous. A few years ago a skier crossed into White Pine from Snowbird and tried to ski a long open slope a bit too early. A fall led to a long and fatal slide down the slope.

Large wet slides can be a hazard in the early spring, typically for a period of a few days when the snowpack first becomes isothermal. One reason for this is that free water can be introduced to snowpack too quickly if rapid warming occurs, especially if refreezing overnight does not occur. Initially this free water can flow along and lubricate buried bed surfaces, leading to wet slab releases. The effect of free water in the snowpack can be very difficult to predict. Ultimately, the free water and the melt-freeze process will hinder fracture propagation and break down the layering in the snowpack. After isothermal conditions prevail for a while the hazard of wet slabs becomes small.

Questions, comments, suggestions, and other feedback is welcome. I may be contacted by email at snowman@csac.org.

To learn more about avalanches, visit the Avalanche-Center.org

Jim Frankenfield.

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