Snow and Snowpack evolution in a mountain environment

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Important Note:  As with other topics presented here about snow, weather and avalanches, this short outline of snow and some of its behavior in a mountain environment only scratches the surface of the myriad of possibilities that exist and presents at best only the very tip of a very large iceberg of knowledge and wisdom about the complex nature of snow (and resulting avalanches). As you learn more about snow, weather and avalanches, you may be surprised by how snowpack evolution unfolds and what those results might be…the best to hope for is that you are surprised less often, you can minimize the consequences of those surprises, and that you can find safe routes through this always changing and highly variable medium.

 

SNOWFALL and SNOWPACK

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Snow falls every season in mountainous and non-mountainous locations, and it is deposited in any number of countless forms and crystal shapes and sizes. Such snowfalls are intermittently layered with its liquid cousin, rain, melt, or a variety of icy relations like graupel, hail and sleet. This inevitable combination creates an often complex and profoundly layered snowpack, some weak and some strong layers…and all with differing adhesions to each other and differing internal strengths. Adding to this intricate combination is the fact that any of these layers, and especially those near the ground or an upper layer interface like a rain crust, may radically change and weaken in situ through the effects of internal temperature gradients (local temperature differences over distance). Any of these layers may also weaken from the effects of liquid water (rain or melt driven) which breaks down bonds and increases downward motion (creep and glide).

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While most snow layers…if given time and a lack of strong, externally applied changes like temperature shifts or added layer stresses (e.g., more snow, melt or rain) …will slowly settle and strengthen through rounding and sintering (bonding to adjacent snow crystals) to form stronger and denser layers, sometimes destabilizing transformations may occur. Such internal weakening may be driven by temperature differences in the snowpack (which can produce faceting or snow grain growth without the formation of significant crystal bonding), or by rapid snow grain rounding and bond weakening (e.g., by liquid pooling near denser layers and saturating interstitial spaces) or additional shear stresses thru enhanced creep or downhill motion (stress directed along or parallel to layer boundaries) induced by rain or melt.

 

Thus, the snowpack that is deposited and draped over terrain through the action of snowfall, temperature and wind is only the start of the snowpack evolution on a given slope that ultimately forms and reforms itself many times over the course of a season (fall, winter and spring…sometimes extending well into summer).

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Remember too that snow is a visco-elastic material, meaning that it has both the ability to slowly deform (without failure or fracture) and store energy internally as bonds (between crystals and grains) are stressed and stretched or deformed. Hence, if given enough time (without more stresses), snow can and will slowly settle, densify and strengthen in place (stabilize); on the other hand, if stresses amplify or are applied unequally within or between layers on variable terrain, increased stress concentrations may result in enhanced susceptibility to layer failure and fracture when subjected to external triggers (i.e., explosives, skiers, snowboarders, snowshoers, hikers, climbers, etc.).

 

Finally, you need to remember that a large amount of soft, lower density snow (lighter, powder) accumulating in the mountains in forest covered terrain can also generate its own danger if deposited loosely around trees, where unaware travelers may experience a difficult extraction while struggling to keep from being asphyxiated.

 

TERRAIN

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Of course if there were no slopes or terrain involved, snow layers and variations in bonding and strengths both within and between layers, would not be such a significant cause for concern. However, when terrain becomes involved and is highly variable (like on most mountain slopes that invites recreationists), the combined effects of gravity (vertical downward directed force) and shear strength or stress (along or between layers) and tensile strength or stress (within individual layers) become important and are worthy of our consideration.

 

Stress concentrations develop within the snowpack from both the juxtaposition and high variability of originally deposited snow layering over terrain (e.g., consider snow crystal changes during storms and wind effects on falling snow) and both small and large variations in underlying terrain (stress concentrated near terrain changes). In order to maximize safety in avalanche terrain, one needs to become very aware of and mindful of subtle changes in topography that might help to tip the safety balance toward failure and fracture of the affected snow layers. A few degrees of slope change might not seem like a lot to the uninitiated in the battle between stress and strength, but to the snowpack it may make all the difference necessary between relative stability and avalanche release.

 

TRIGGERS

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Avalanche triggers can be both natural and artificial. For example, slides can be triggered by naturally increasing stress…the weight of additional snow fall, increased temperature, more wind deposits…or they can be released thru artificially induced stresses like skiers, snowboarders, snow machines, or even a hiker / climber / snowshoer adding stress or initiating snow layer failure (and maybe subsequent fracture) in the wrong place at the wrong time. When we are out and about in snow covered avalanche terrain, it must be part of our life’s mission to remain aware of all of the potential ways that our passage through such slopes maybe interacting with the underlying snow structure. As a result, it much be up to us to recognize snow layering and the underlying terrain which might be susceptible to the tipping point of our passage… and proceed cautiously and with great focus.

 

HUMAN INTERACTION

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It is the over-arching mission of avalanche educators and forecasters to increase user and recreational knowledge about avalanches and their environment, and to enhance their ability to apply this knowledge when making safer terrain and route-finding choices in snow covered back country. Such decisions should be made with full knowledge of the level of avalanche danger (how unstable the current or future snowpack might be), and with better understanding of how we as social beings may change our behavior when we become part of a (larger) group. Even with a head full of knowledge and experience, unless we choose to apply that information in a reasoned manner, an adage may still apply: “We have met the enemy, and the enemy is us.”

 

While snow safety training has come a long way with the dramatic increase in back country travel, we tend to overlook key indicators and safe travel protocol when taken up in the heat of the “powder” moment with friends. Much of our recent research toward maximizing safe back country travel has focused on the” human element” and some of the common issues that turn up in analyses of avalanche accidents.

 

SNOW RESOURCES

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Fortunately for back country users and enthusiasts, there are a host of avalanche education resources available, ranging from AAA (American Avalanche Association) approved courses to a great many excellent on-line sites that offer an incredible amount of avalanche, snowpack and weather-related information. Just be sure that once you have started on the journey to learn about avalanche safety that you always take this information with you and apply it all the time in the back country.

 

Some of the best sites for beginning or continuing your journey of snow knowledge, avalanche education and being safe in the back country are given below. Also be aware that all the regional avalanche forecast centers in the US offer knowledge and snow safety courses to help make your back country journey a safe one.

 

• Avalanche.org (a partnership between the American Avalanche Association and the Forest Service National Avalanche Center)

• AIARE (The American Institute for Avalanche Research and Education)

• Mountaineers (Avalanche safety courses offered by be Mountaineers for a wide range of mountain users)