Technical information, news, research, and opinion on avalanches, snow safety, and winter backcountry travel.

Wednesday, December 15, 2010

Q & A

I have been hyperbusy with personal stuff, so thanks for your patience. Today, I'm going to answer a few questions that have been sent my way over the past few weeks.

( Promise a full post tomorrow or the next day-I've managed to read through most of the ISSW 2010 papers, and I've got a few additional remarks that I'd like to make about some recent conversations in which I have participated. )

Do avalanches originate in tree-covered areas?

Good question. Trees definitely alter the snowpack by anchoring snow, intercepting snowfall, intercepting radiation, and by depositing "bombs" and moisture that break up the snowpack.
  1. According to The Avalanche Handbook, avalanches in tree covered areas are infrequent, but they do happen. If you travel in mountainous terrain during the winter, you can rely on very thick tree cover for safety only when there is no avalanche terrain anywhere above the trees.
  2. Avalanches frequently initiate above tree-covered areas and flowing snow travels easily through the trees, even if the trees are thick. Have you ever noticed snow plastered on the uphill side of a tree? This is a good indicator of avalanche activity, and can also be used to determine the height of flowing snow. Remember that forest clearings may be especially dangerous because of instantaneous changes in snow conditions. This means that you can go from safe to unsafe in just a few steps.
  3. Gladed areas are, for all intents and purposes, the same as open slopes. The consequences of taking a ride through the trees are severe. How severe? Think about what might happen if you have a high-velocity encounter with a tree. You can express such an encounter with the following formula: NOTFUN.
  4. You may not be safer in forested areas, and if conditions are very poor, you might actually be less safe. As always, you must use the current terrain, weather, and snowpack to determine the likelihood of avalanche formation, and you must choose terrain appropriate for conditions.
  5. According to researchers, people in North America are much more likely to suffer traumatic injuries during avalanches than their counterparts in Europe. Trees are definitely a contributory factor.
  6. Don't forget about tree wells.
Other facts about trees and the snowpack.
  1. Thick trees anchor the snowpack, which can prevent avalanche formation. On the other hand, trees often serve as trigger points or fracture points. It's a good idea to keep this in mind.
  2. Trees intercept incoming shortwave radiation, which explains why you can find decent snow in the trees several days after a storm.
  3. Trees intercept outgoing longwave radiation and reflect it back into the snow cover, which prevents heat loss. This is why surface hoar doesn't form readily beneath trees, and it's also why you're less likely to find near surface facets underneath the trees unless the temperatures are really really cold.
  4. Trees drop bombs and liquid water onto the snowpack. Large snow bombs break up slabs, and liquid water turns the snowpack into solid concrete when it freezes.
How are buried facets formed?

This is a bit more complicated.
  1. Facets can form inside the snowpack when the air is much colder than the ground on which the snow sits. The strong temperature difference means that water vapour moves quickly toward the snow surface, and a constant replenishment of water vapour means that crystals can grow rapidly. Rapid growth produces angular crystals.
  2. Facets can form inside the snowpack itself when a crust creates a vapour barrier that traps moisture. Over time, with vapour supply and the right temperature gradient, fast crystal growth occurs. Once again, rapid growth produces angular crystals.
  3. Facets can also form at the snow surface. This happens with very cold ambient air temperatures, or when the snow surface becomes intensely cool from longwave radiation loss.
  4. Radiation recrystallisation can also produce facets. This happens when sunlight strikes very cold snow, and usually happens on south faces at at very high elevations.
  5. Finally, facets can form when cold, dry snow falls on wet snow. The wet snow contains heat that sets up a strong temperature difference when it comes into contact with cold, dry snow. Again, as with above, the strong temperature gradient moves moisture rapidly, and constant replenishment of water vapour means that crystals can grow rapidly.

Wednesday, December 1, 2010

The Life of a Snowflake

Time is a wheel in constant motion always, moving us along—Lee Ann Womack

Double Post Wednesday. I'm going to be busy for the next few weeks, so I'm posting twice today. The first post, below, is a brief essay on the importance of understanding snow metamorphism- from crystal formation in the atmosphere, through deposition on the ground, and inside the snowpack itself. The second post is a snow metamorphism exam based on Chapter 3 of The Avalanche Handbook. It's ridiculous, but some people will enjoy it.


NOTE: This is the third post that will address the general question of Why Is It So Complicated? This time we're going to talk about snow metamorphism. I don't really know how to teach anyone about snow metamorphism, but hopefully this post will provide a basic understanding of why snow metamorphism is important and how it can be applied in the field. In my own experience, I literally cannot imagine making safe decisions without having significant knowledge of snow metamorphism, but this statement applies to me and me alone. As Ed LaChapelle said, there are many ways to forecast avalanches.

If you read this blog often, you may notice that I try to describe things in very literal terms. On first read, this post is going to seem like a departure, because I do use some highly subjective language. Please keep in mind that I do not approach backcountry avalanche forecasting from a mystical perspective; I actively observe the environment—right down to the way the air feels on my nose after a few rapid breaths*—and I draw my observations from the science, not the psychic.

( *A very chilled nose after three rapid breaths means extra caution delivers good skiing. )

The term polymorphism is the ancient Greek word for many forms and it very neatly describes the heart of the complexity and uncertainty that many people experience when they think about snow crystals. But you and I, we aren't so different from snowflakes, because over our lives, we also react to the dynamic conditions around us, and we too change as a result. Sometimes these changes are for the better, and sometimes they're for the worse.

Ask yourself the following question: Am I the same person I was 5 years ago?

Of course not.

So the simple fact of the matter is that, as with people, snowflakes change with the passage of time. The principles of snow metamorphism are incredibly useful, and in this post I refer to snow metamorphism in the holistic sense: changes to snow crystals in the clouds, changes to snow crystals on the ground, and changes to snow crystals in the snowpack.

The Life of A Snowflake
Snowflakes are born in clouds of supercooled water vapour. Both the temperature and the amount of moisture available for crystal formation vary over time and space in the atmosphere, which means that the neither the temperature, nor the amount of moisture, are consistent. This is responsible for variations in crystal form as the snow falls, and variations in crystal form are very important if you're a backcountry skier.

But let's get back to our snow storm. As we all know, eventually gravity pulls the snowflakes to Earth, where they form a layer. On the ground, the amount of water vapour and the temperature are different than in the clouds, so the snowflakes begin to change. The first observable changes are the loss of branches as the fine details of the snow flake melt. Soon thereafter, as the crystal size decreases, the weight of the snowpack produces overburden pressure that pushes the snowflakes into each other, effectively crushing them and further reducing the pore space. These changes happen in a relatively short amount of time, from a few hours to a few days.

Over the longer term, the temperature gradient determines how quickly water vapour moves through the snowpack, which determines whether or not the snowpack becomes stronger or weaker. Thick, deep snowpacks are strong because moisture transport is slower and weaknesses are crushed and rounded out of existence. Thin snowpacks are weak because moisture transport is fast, which leads to rapid crystal growth and angular shapes with cavernous pore spaces and fewer bonds. Furthermore, cold temperatures preserve existing weaknesses such as crusts and buried surface hoar.

What I Think About
Okay, I've rattled on and on about the delights of snow metamorphism, but what are the practical concerns? Well, having a strong working knowledge of snow metamorphism leads me straight to the following questions:
  • Are there instabilities in the new snow itself? ( Timeframe: 24-72 hours )
  • Are there instabilities in the bond between the new snow and the old snow? ( Timeframe: Up to 7 days )
  • Are there older instabilities and how will they react to loading by snow or skiers? ( Timeframe: Entire winter )
At first glance, this list probably seems quite thin, but continue reading if you'd like to see how a strong working knowledge of snow metamorphism allows me to integrate some very useful details into these broad questions. The first concern is, of course, given the questions above, how can you identify snowpack with dangerous characteristics?

Applying The Principles Of Snow Metamorphism In The Field
If you know what to look for, you can observe these conditions in the snowpack. Recently, I was out on a ski tour the morning after a significant storm. In the Cascades, this generally means that it's time to watch your ass, as snow often arrives in large amounts, and with significant wind. Consider the following list of observations from multiple locations. These observations were based on, or related to, a strong working knowledge of snow metamorphism.
  • Constant presence of decomposing crystals at the snow surface.
  • Uniform crystal density in the new snow.
  • Uniform increase in hardness with depth.
  • Loss of branches on new snow crystals.
  • The crystals at the bottom of the snow had fewer branches than the crystals at the top.
  • The pore space at the bottom was smaller than the pore space at the top.
  • The bond between the old snow and the new snow was bomber.
  • Zero slab avalanches.
  • Very little sluffing.
  • Few drift lines in the alpine.
  • Good snow coverage on trees regardless of orientation.
Going Behind The Scenes
Instabilities in new snow are often represented by soft slab avalanches that form in the new snow itself, or by avalanches that form when dense wind slab overlies softer snow below. An understanding of snow metamorphism can help you learn what to look for when you need to determine whether either type of avalanche is likely.

In a perfect blanket of new snow, you should observe uniform increases in hardness with depth. This is often referred to as right side up snow. However, it's not the only possibility. In addition to upside down snow, any new layer of snow could contain internal weaknesses. These usually take the form of a band of heavier snow crystals overlying a band of lighter snow crystals inside the layer of new snow. Suffice it to say, although the new snow might be mostly right side up, it needs to be completely right side up.

Weaknesses of this type form because the atmosphere in which snow crystals form is inconsistent with respect to temperature and the amount of water vapour. This means that the mass and shape of snow crystals can vary. A storm may at first lay down a few centimetres of of light crystals, followed by a few centimetres of heavy crystals, followed by a few more centimetres of light crystals.This creates a complex layering that is highly suitable for avalanche formation; even though the snowpack is mostly right side up.

Wind slab is upside down snow, but it forms for different reasons. Snow crystals shatter when moved by the wind, and they form a fine dust-like mist that accumulates on lower density snow in sheltered areas. Over the course of a few hours, this fine mist turns into a thick layer of tiny crystals. The tiny grains have a high number of bonds per unit volume, and this enables them to knit together very rapidly. However, the new snow below may not be able to support the weight of the slab, and avalanches form quite easily when you crush the weaker snow below by applying a dynamic load to the wind slab. In an instant you have something on top of nothing, and an instant later you have an avalanche.

For older weaknesses, there are two concerns over the long term: favourable metamorphism that comes with depth and relative heat, and overburden pressure that slowly crushes weaknesses and fills in the pore spaces. Obviously, without a working knowledge of snow metamorphism, it's going to be pretty hard to know what to look for. I'm not comfortable writing much more about older weaknesses, because research also shows that they're not very manageable. Therefore, unless you have professional-grade knowledge, you should avoid trying to diagnose these problems.

This might seem complicated, but it's much easier to understand if you have a decent working knowledge of snow metamorphism across various scales of space and time. On the day of our trip, we had deep, stable snow that was well-bonded to the old snow below. I was very comfortable skiing in steep avalanche terrain without constantly feeling the need to look over my shoulder.

Much of the information used during my recent trip was pulled directly from the principles of snow metamorphism. I sometimes hear people say that introductory avalanche education should focus less on snow metamorphism and more on decision-making. This makes me scratch my head in confusion.

How can you make critical decisions about snow when you know almost nothing about it? Continuous avoidance of avalanche terrain simply isn't compatible with realistic human behaviour, and you can't travel safely in avalanche terrain if you don't understand snow.

Perhaps this winter I will turn my brain toward developing a realistic, usable framework for understanding snow metamorphism. Of course, it might not be possible to develop this framework, in which case, I'll point you straight at This Little Monster.

Snow Metamorphism Exam

255 questions from chapter 3 of The Avalanche Handbook. Questions start with green headings and answers start with red headings. I'm not sure how long it will take to complete this exam, but I can guarantee that you will gain professional-level knowledge about snow metamorphism if you master 90% of this material.

Yes, I am fully aware that this exam is ridiculous, but it was a lot of work, so have fun.


This chapter discusses snow formation from cloud to ground, including in-depth information on snow metamorphism and its links to avalanche formation.

Snow Crystal Formation And Growth in The Atmosphere

  1. Variations in snow crystal type are responsible for some avalanches. True or False?
  2. Explain this relative to potential instability in new snow and old snow.
  3. Clouds are composed of what elements?
  4. Inside an air mass, what specific process leads to cloud formation?
  5. List two types of particles that serve as condensation nuclei?
  6. What is the typical diameter of such a particle?
  7. Condensation nuclei are rare. That's why you don't see clouds everywhere. True or False?
  8. When does formation of ice crystals become possible?
  9. Ice crystals always form when the temperature reaches 0 degrees Celsius. True or False?
  10. What is the typical size of a water droplet in a cloud?
  11. What is the typical concentration of water droplets in a cloud?
  12. What two elements are required for ice crystal formation above -40 degrees Celsius?
  13. Freezing always takes place at constant temperatures in clouds. True or False? Explain.
  14. At what temperature will water droplets freeze by themselves?
  15. What two processes determine subsequent growth once an ice crystal has formed?
  16. Describe the initial process of crystal growth.
  17. Describe the secondary process of crystal growth.
  18. Branches form from which mechanism?
  19. Branches are destroyed by which mechanism?
  20. What process creates graupel?
  21. What process forms hail?
  22. Describe a novel use for graupel ( Be creative. Invent something. ).
  23. What is the most important variable with respect to crystal form?
  24. What are the two primary crystallographic axes?
  25. How many intrinsic axes are found along the base axis?
  26. Describe the symmetry of the basal plane.
  27. Along which axis does heat flow most efficiently?
  28. Platelike crystals form from growth along which axis?
  29. Needlelike crystals form from growth along which axis?
  30. Why are crystals six-sided?
  31. Is rate of growth a strong factor in final crystal shape?
  32. At low excess vapor density, what shape is produced regardless of temperature?
  33. What vapor quantity condition is implied by a complex crystal?
  34. Describe vapor and temperature regimes.
  35. Crystals that fall through a cold atmosphere are larger. True or False?
  36. Name one crystal type that often serves as a future sliding layer?
  37. Changes between crystal type or riming can result in poor bonding between layers. True or False?
  38. ________ ________ are expected at low vapor density; ________ ________ are expected at higher vapor density.
  39. Explain the concept outlined by the previous sentence.
  40. Define the terms behind the acronym "ACMG"

Classification Of Newly Fallen Snow Crystals

  1. How many levels of classification are typically used to describe new snow?
  2. Describe each level.
  3. How many crystal classifications are used in each?
  4. Describe the typical crystal composition of new snow in simple terms ( not by specific crystal type ).
  5. What effect does this have on avalanche forecasting?
  6. What crystal type is of particular importance? Why?
  7. Describe the correct process for crystal identification at the snow surface.
  8. How do you measure and record the size of snow crystals in the field?
  9. List the levels used for crystal grain size classification, including name and size.

Surface Hoar: Formation And Growth Conditions

  1. Provide the common definition for surface hoar.
  2. Under what condition does surface hoar form? Be very specific.
  3. What is the result of this condition? Be very specific.
  4. Provides a visual description of surface hoar.
  5. What are the two conditions for continued growth of surface hoar?
  6. Surface hoar usually forms on ________ ________ ________ with ________ or ________ ________ conditions in the ________ ________ of ________.
  7. Slight air movement destroys surface hoar. True or False?
  8. Describe the rate of air motion necessary for vapor replenishment.
  9. How would an observer regard these conditions?
  10. What happens if air motion is turbulent?
  11. What usually creates the temperature gradient necessary for surface hoar formation?
  12. ________ % humidity is usually required for rapid growth.
  13. Below the % of humidity specified above, surface hoar growth is not possible. True or False?
  14. Describe one scenario during which formation of surface hoar might be expected.
  15. Clouds, especially high, thin clouds, enhance surface hoar formation by contributing moisture. True or False?
  16. If true, explain. If false, explain.
  17. If clouds are present, what factor determines their effect on surface hoar formation?
  18. Describe a terrain feature that might inhibit surface hoar formation.
  19. Why does this terrain feature inhibit surface hoar formation? Be very specific.
  20. Surface hoar forms readily under forest because forest has a sheltering effect. True or False?
  21. If true, explain. If false, explain.
  22. Mountain guides often call logged clear cuts "lunch counters". True or False?
  23. If true, explain. If false, explain.
  24. Avalanches are rare in clear cuts. True of False?
  25. If true, explain. If false, explain.
  26. In what snow climate is surface hoar found? Be very specific.
  27. Once formed, surface hoar is extremely strong. This is why it persists for such long periods. True or False?
  28. Describe three conditions that might destroy surface hoar.
  29. Surface hoar is sometimes a more significant problem at lower elevations. True or False?
  30. If true, explain. If false, explain.
  31. In addition to being pretty, surface hoar is good at producing ________ ________ ________.
  32. What is the term used to describe the mechanical strength of surface hoar?
  33. What does this term mean?
  34. Why is this important?
  35. What provides energy to drive a fracture forward when surface hoar is disturbed?
  36. It is only safe to travel across buried surface hoar on flat terrain. True or False?
  37. If true, explain. If false, explain.
  38. How might surface hoar gain strength?
  39. What layer variable determines how long surface hoar remains unstable?
  40. What is the range of size for surface hoar crystals?
  41. Surface hoar is often described as a ________ ________. Or ________ ________ ________ ________.
  42. Unlike facets, surface hoar is easily destroyed, and therefore not responsible for major avalanches. True or False?
  43. Wild-harvested, Selkirk surface hoar is highly prized as both a recipe ingredient and ice cream garnish at fancy restaurants in Japan. True or False?

Snowpack Temperatures And Temperature Gradients

  1. What serves as the upper and lower boundaries for winter snowpack?
  2. What is the temperature of the lower boundary and why?
  3. What is the temperature of the upper boundary and why?
  4. Which boundary is usually cooler?
  5. Define diurnal fluctuation.
  6. Define the two elements of a temperature gradient in terms of a bipartite vector.
  7. How is the temperature gradient expressed?
  8. What is the term for a snowpack that lacks a temperature gradient?
  9. What can you assume about this snowpack?
  10. When does this occur?
  11. Describe, in very simple terms, the quality of the temperature gradient in a maritime climate.
  12. Why is this the case?
  13. Describe, in very simple terms, the quality of the temperature gradient in a continental climate.
  14. Why is this the case?
  15. What explains the very general difference in character of avalanching in these climates?
  16. As shown above, crystals are dependent on climate, not physical processes. True or False?
  17. Certain crystal types are not found in some snow climates. True or False?

Snowpack Temperatures And Temperature Gradients

  1. Once deposited on the ground, how many minutes must pass before snow crystals begin to change form?
  2. Why do these initial changes cause direct-action, loose-snow avalanches?
  3. Why do crystals change form?
  4. What is the range of typical values for supersaturation in the atmosphere?
  5. What is the range of typical values for supersaturation in the snowpack?
  6. New snow crystals are highly stable. True or False?
  7. What ratio and quantity describes unstable crystals?
  8. In very general terms without regard to specific crystal type, which crystals are most stable?
  9. In very general terms without regard to specific crystal type, which crystals are least stable?
  10. What shape has the minimum surface area to volume?
  11. What effect might crystals of this shape have on stability of crystal shape?
  12. What causes the disappearance of intricate crystal branches?
  13. Vapor pressure over a ________ surface is higher than vapor pressure over a ________ surface.
  14. Therefore, what crystal shape quality promotes sublimation?
  15. Where does this water vapor go?
  16. What is the implication of the production of water vapor via sublimation?
  17. Saturation vapor pressure increases by what percent across what temperature range?
  18. What is the implication of this?
  19. List the three factors that influence metamorphism in dry snow.
  20. Define each factor.
  21. Where is the temperature gradient usually highest?
  22. This region is defined as the ________ ________ ________ ________ ________.
  23. What are the two key factors that determine rate of metamorphism for dry snow?
  24. What is the initial result of branch disappearance with respect to crystal size?
  25. What is the name for this process. Be very specific.
  26. What happens next? Why?
  27. ________ particles grow at the expense of ________ particles.
  28. Average particle size ________ when a mixture of sizes is present.

Dry Snow Metamorphism In The Seasonal Snow Cover

  1. Why are the variety of crystal forms limited inside the snowpack?
  2. The term metamorphism includes changes to form induced by these two factors.
  3. What snowpack-related force rearranges grains in the snowpack?
  4. Relative to crystal configuration in the snowpack, what is the result of this rearrangement process?
  5. How might this affect both stability and instability in the short and long term?
  6. What process dominates shape change in glaciers or firn snow?
  7. What is the primary difference between crystal formation in the atmosphere -vs- in the snowpack?
  8. How does the temperature gradient influence vapor diffusion? Be very specific.
  9. What is the general result with respect to motion of water vapor?
  10. Specifically, how does mass transport occur under this effect?
  11. What determines crystal forms that develop under this recrystallization process?
  12. With respect to water vapor, rate of motion increases as these three factors increase.

Crystal Forms In Dry, Seasonal Alpine Snow

  1. Growth rate and crystal form are more dependent on pore size than temperature gradient. True or False?
  2. If true, explain. If false, explain.
  3. High temperatures, large temperature gradients, and large pore spaces result in what growth rate?
  4. Low temperatures, small temperature gradients, and tiny pore spaces result in what growth rate?
  5. What is the critical temperature gradient required to produce faceted crystals?
  6. How do you measure a temperature gradient?
  7. Faceted forms and highly angular snowflakes develop because of similarities between vapor saturation conditions in the atmosphere and snowpack. True or False?
  8. Why does depth hoar only form near the ground? Be very specific.
  9. Why do facets form slowly in very cold snow? Be very specific.
  10. At high growth rates, what crystal forms are expected?
  11. At low growth rates, what crystal forms are expected?
  12. What affect might growth rate have on snowpack instability?
  13. List three examples of crystals types produced at high growth rates.
  14. High growth rate crystals form preferentially in what type of snow climate?
  15. Connect the number of avalanche fatalities in Colorado with its snow climate.
  16. Which are the top five states with respect to avalanche fatalities?
  17. Large pore spaces present favorable conditions for what type of crystal?
  18. What pore space quality inhibits development of facets?
  19. Describe a method used to decrease pore space in depth hoar and when this method must be implemented.
  20. Depth hoar is always weak. True or False?
  21. What is near-surface faceted snow?
  22. What are the persistent forms? Explain each type.
  23. What are mixed forms?
  24. Crystals developing under the slowest growth rates are referred to as ________.
  25. Crystals developing under the highest growth rates are referred to as ________.
  26. Draw the symbol for "new snow or precipitation particles".
  27. Draw the symbol for "decomposing and fragmented precipitation particles".
  28. Draw the symbol for "rounded grains ( monocrystals )".
  29. Draw the symbol for "faceted crystals".
  30. Draw the symbol for "cup-shaped crystals; depth hoar".
  31. Draw the symbol for "wet grains".
  32. Draw the symbol for "feathery crystals".
  33. Draw the symbol for "ice masses".
  34. Draw the symbol for "surface deposits and crusts".
  35. Why was the term "equitemperature metamorphism" discarded?
  36. Why was the term "temperature gradient metamorphism" discarded?
  37. What is the name for the crystal type that constitutes the largest size found in the snowpack?
  38. Why are these crystals the largest?

Growth Of Crystals Around Crusts In Dry Snow

  1. How does a crust influence crystal formation?
  2. With respect to crusts, what is the most important feature for avalanche formation?
  3. Why do facets sometimes grow directly beneath a crust?
  4. What leads to dry/wet faceting?

Bond Formation In Dry Alpine Snow

  1. Formation of bonds is also referred to as ________.
  2. How does bond formation occur?
  3. Where do bonds form?
  4. What is found at the boundary between two crystals?
  5. Describe the perspective from which experimental work on bond formation in dry snow has been conducted.
  6. What is the final angle of the dihedral?
  7. Stress along the grain boundary is constant. True or False?
  8. If true, explain. If false, explain.
  9. Describe the process of bond formation relative to rate of bond formation over time.
  10. Discuss how differences in crystal size affect stability with respect to bond formation.
  11. Thermodynamic processes occur ________ at ________ temperatures.
  12. What is the temperature threshold for persistence of instability in new snow?
  13. In theory, what exists at the surface of crystals? What happens to this theoretical region as temperature increases?
  14. When the temperature gradient is high, mass transport is ________.
  15. Discuss the spatial characteristics of vapor deposition under a high temperature gradient.
  16. If avalanches released easily on depth hoar, what conclusion could be drawn about travel in continental climates?
  17. What conclusion is reached instead?
  18. When snow falls at high temperatures, rapid bond formation decreases what?
  19. Discuss bonds / unit volume with respect to grain size.
  20. What is necessary for bond formation between adjacent layers of snow?
  21. How does the degree of riming effect bond formation?
  22. Broken crystals are usually small. What effect does this have on bond formation?
  23. List the integrated elements of bond formation and layer strength for dry snow.
  24. Fractures originate at what scale relative to the size of an individual bond?
  25. Why is this important?
  26. What is the most important temperature effect on dry slab-formation relative to avalanche forecasting?
  27. Discuss the nature of bonding and strength increases for the persistent forms.
  28. Mismatches in crystal type enhance bond formation because opposites attract. True or False?
  29. Where do persistent forms originate? Name the exception.
  30. What do studies of avalanche fracture lines suggest about layering and bonding?
  31. What information about layering and bonding can one derive from instability tests?

Persistent And Non-Persistent Weak-Layer Forms

  1. Who coined the term "persistent forms"?
  2. What are the characteristics of the persistent forms?
  3. What are the characteristics of the non-persistent forms?
  4. Discuss human perception with respect to persistent forms.
  5. Discuss human perception with respect to non-persistent forms.
  6. When do non-persistent forms originate?

Metamorphism Of Wet Snow

  1. What happens to heat flow and metamorphism in wet snow?
  2. Wet snow can consist of what three materials?
  3. Discuss the relationship between melting point and particle size.

Snow With High Water Content

  1. Discuss particle separation in wet snow. Include the percentage of water content by bulk volume required for complete grain separation.
  2. What drives metamorphism in water-saturated snow?
  3. In theory, when does metamorphism stop?
  4. Since differences in melting temperature due to curvature is very small in wet snow, why does metamorphism occur rapidly?
  5. Define "dry snow".
  6. Define "moist snow".
  7. Define "wet snow".
  8. Define "very wet" snow.
  9. Define "slush".
  10. What is a pendular regime?
  11. What is a funicular regime?

Snow With Low Water Content

  1. At what level of water content does grain growth occur through vapor flux?
  2. Relative to the pore space, what develops as water content in wet snow decreases?
  3. What is the result of this?
  4. What is the distinguishing visual feature of moist snow?
  5. What is responsible for this occurence?
  6. How can you differentiate between wet and dry snow?
  7. What is the most accurate method of measuring the water content of snow?
  8. As water content increases, what can you expect about grain growth?

Classification Of Wet Snow

  1. Does the grain classification of faceted snow change when it becomes wet? Explain.

Bond Melting And Formation In Wet Snow

  1. What has the highest heat conductivity of all common substances?
  2. Describe what happens to the melting point of grains when grains touch in water saturated snow.
  3. Describe the process of melt-freeze metamorphism.
  4. What is the colloquial term for snow crystals created by melt-freeze metamorphism?
  5. How do melt-freeze crusts contribute to avalanche formation?

Snow Crystal Formation And Growth in The Atmosphere

  1. True
  2. Variations in crystal size and type can lead to poor between layers of new snow. This causes new snow instability.
    Later, these variations can result in grain size mismatch and poor bonding between layers of old snow. This
    causes deep instability.
  3. Water droplets
  4. Condensation of water molecules onto condensation nuclei.
  5. Salt, dust, soil, spores.
  6. 1 micrometer.
  7. False. Condensation nuclei are always in abundant supply.
  8. At 0 degrees Celsius.
  9. False
  10. 20 micrometers.
  11. Several hundred per cubic centimeter.
  12. Freezing nuclei and and temperatures below 0 degrees Celsius.
  13. False. Water droplets can remain in liquid phase ( supercooled ) at temperatures below 0 degrees Celsius.
  14. -40 degrees Celsius.
  15. Vapor pressure gradients and riming
  16. Water molecules are deposited directly on tiny ice crystals by vapor pressure gradients in the cloud.
  17. Riming occurs as the enlarged crystal falls through the atmosphere and collides with water droplets.
  18. Vapor deposition.
  19. Riming
  20. Riming, long growth period, multiple passes through clouds on thermal updrafts.
  21. Multiple passes through freeze-thaw cycles in clouds.
  22. Graupel makes a festive decoration for mountain martinis or other cold beverages.
  23. Temperature
  24. A and C or planar and vertical.
  25. Three. Separated by 120 degrees.
  26. Hexagonal
  27. Along the C ( vertical ) axis.
  28. A
  29. C
  30. Hexagonal symmetry of the A axis.
  31. Yes
  32. Columns
  33. High quantity of water vapor or high supersaturation.
  34. Vapor regime is a specific area with a specific level of saturation. A temperature regime is a specific area at a specific temperature. Clouds often have multiple vapor and temperature regimes.
  35. False
  36. Stellars
  37. True
  38. Rounded forms are expected at low vapor density; edges and corners are expected at higher vapor density.
  39. Slow growth, from low vapor density, produces rounded forms. Fast growth, from high vapor density, produces angular forms.
  40. Attractive Canadian Men on Glaciers.

Classification Of Newly Fallen Snow Crystals

  1. Two
  2. The first level uses the "+" symbol to describe all new precipitation in a single category. The second level uses five crystal types, and three irregular crystal types, to sort precipitation into specific forms.
  3. 1, 8
  4. Mixed, or a variety of types are mixed together.
  5. This can complicate decision-making.
  6. Stellars - because they're flat and often form sliding layers.
  7. Use a loupe to determine the predominant crystal type in a sample.
  8. Place crystals on a millimeter grid and provide a range of sizes, i.e. 0.2 to 0.5 millimeters.
  9. Very Fine 0.2mm, Fine 0.2-0.5mm, Medium 0.5-1mm, Coarse 1-2mm, Very Coarse 2-5mm, Extreme >5mm.

Surface Hoar: Formation And Growth Conditions

  1. Frozen dew
  2. When the water vapor pressure of air exceeds the water vapor pressure of ice crystals on the snow surface.
  3. Sublimation of water directly onto the snow surface.
  4. Large, glittering, feathery crystals.
  5. Sufficient water vapor and temperature gradient at the snow surface.
  6. Surface hoar usually forms on clear, cold nights with calm or nearly calm conditions in the lowest meter of air.
  7. False
  8. A few centimeters per second.
  9. As calm or similar to air motion in an enclosed room.
  10. Destroys the temperature gradient.
  11. Long wave radiation loss.
  12. 70 % humidity is usually required for rapid growth.
  13. False
  14. If a cold front passes after an overcast day.
  15. False
  16. Clouds can destroy the temperature gradient by preventing loss of long wave radiation.
  17. The temperature difference between the cloud and the snow surface.
  18. Concavity
  19. Long wave radiation is reflected back onto nearby snow ( parabolic / anti-diffusion effects ), destroying the temperature gradient.
  20. False
  21. Forest cover inhibits loss of long wave radiation, which prevents formation of temperature gradient.
  22. False
  23. Mountain guides often call logged clear cuts "surface hoar farms".
  24. False
  25. Surface hoar formation and clear ground suitable for loading result in avalanches that release in clear cuts.
  26. Any climates, provided conditions necessary for formation are met.
  27. False
  28. Wind, sun, rain.
  29. True
  30. Often, surface hoar is easily destroyed by high winds and harsh conditions found at higher elevations.
  31. In addition to being pretty, surface hoar is good at producing propagating shear fractures.
  32. Anisotropic
  33. Weaker in shear than compression.
  34. When surface hoar is loaded, force is transferred from compression to shear.
  35. Collapse
  36. False
  37. Even on flat terrain, movement over buried surface hoar can result in shear fractures that travel uphill to release an avalanche above the traveler.
  38. Bond formation with adjacent layers.
  39. Thickness
  40. 1 millimeter to several centimeters.
  41. Surface hoar is often described as a persistent form.Or pain in the ass.
  42. False. Surface hoar is easily destroyed before burial but once buried persists for a long time because its compressive strength means it resists strength by overburden.
  43. True or false are acceptable answers.

Snowpack Temperatures And Temperature Gradients

  1. Lower boundary = ground. Upper boundary = air.
  2. 0 degrees because of stored heat from summer ( most important ) and geothermal heat from the Earth's core.
  3. Variable. The air temperature is set by the atmospheric conditions.
  4. Upper
  5. Daytime warming, nighttime cooling.
  6. A vector having both magnitude and direction.
  7. In degrees Celsius per meter.
  8. Isothermal
  9. It contains water.
  10. Spring
  11. Weak
  12. Predominantly warm temperatures and deep snowpack.
  13. Strong
  14. Predominantly cold temperatures and shallow snowpack.
  15. The crystal forms produced are very different.
  16. False
  17. False

Snowpack Temperatures And Temperature Gradients

  1. Zero
  2. Loss of branches causes loss of cohesion through reduction in static friction.
  3. Differences in supersaturation/temperature between the snowpack and the clouds.
  4. Tens of percents
  5. 1%
  6. False
  7. Large ratio between surface area and volume, i.e. dendrites -vs- spherical crystals such as graupel.
  8. Round forms.
  9. Angular forms.
  10. Sphere
  11. Spherical crystals retain their original form for a long time.
  12. Vapor pressure over sharply curved branches is very high.
  13. Vapor pressure over a convex surface is higher than vapor pressure over a concave surface.
  14. Sharpness, Angularity, Edges, Creases
  15. Into the surrounding air.
  16. Water vapor is available for additional crystal development.
  17. 300% between -15C and 0C.
  18. This is the primary factor in metamorphism, rather than curvature effects.
  19. Temperature gradient, grain curvature, overburden pressure.
  20. Definition of each factor:
    • Temperature Gradient. Difference in temperature between snow at depth and surface of snow.
    • Grain curvature. The curvature of each grain in terms of convexity or concavity.
    • Overburden Pressure. The effect of the weight of snow above rearranges grains and produces contact points for bond formation.
  21. Surface regions of the snowpack.
  22. This region is defined as the top few tens of centimeters.
  23. Temperature of snow and the temperature gradient.
  24. Decrease in crystal size.
  25. Destructive metamorphism.
  26. Destructive metamorphism supplies water vapor, which in the presence of a temperature gradient moves through the snowpack and provides a means for ongoing changes to crystal form.
  27. Larger particles grow at the expense of smaller particles.
  28. Average particle size increases when a mixture of sizes is present.

Dry Snow Metamorphism In The Seasonal Snow Cover

  1. Crystals are insulated by neighbors and physical conditions are slow to change.
  2. Temperature and overburden pressure.
  3. Overburden pressure
  4. Pore space is decreased, crystals touch each other and form bonds.
  5. Affect On Instability- This is is how slabs are formed. In the short term, this may result in enough cohesion for slab formation and release if a weak layer/interface is present. Affect On Stability- This process usually increases the hardness ( and therefore strength ) of the snowpack.
  6. Overburden pressure
  7. The amount of supersaturation in the air surrounding the crystals.
  8. Warmer air holds more water vapor than colder air. Therefore water vapor pressure is higher at the bottom of the snowpack. Water vapor moves up through the snowpack in a hand-to-hand process.
  9. Water vapor moves up through the pore space.
  10. Water molecules move from the top of one crystal to the bottom of the crystal above.
  11. Rate of motion
  12. Temperature gradient, temperature, and pore space.

Crystal Forms In Dry, Seasonal Alpine Snow

  1. False
  2. While all both factors are important, temperature gradient is more important than available pore space ( second order effect ).
  3. High
  4. Low
  5. 10 degrees Celsius / meter
  6. Measure temperature at bottom of snowpack and air temperature above. Or measure temperature of layers.
  7. True
  8. Temperatures are high and a lot of water vapor is available for fast crystal growth.
  9. Temperatures are low and water vapor is in short supply. This means slow crystal growth. High supersaturation and high temperatures are required for fast crystal growth.
  10. Facets
  11. Rounds
  12. Faceted crystals are weak. Therefore a high growth rate produces crystals that foster/increase instability.
  13. Surface hoar, depth hoar, facets.
  14. Continental
  15. Much of the Colorado snowpack is composed primarily of weak forms such as facets, depth hoar, and surface hoar because of high temperature gradients created by cold surface temperatures.
  16. Colorado, Alaska, Washington, Utah, Montana ( CAWUM )
  17. Facets
  18. Small pore spaces, tightly packed crystals.
  19. Bootpacking during the early season.
  20. False
  21. Facets produced near the top of the snowpack by temperature gradients, radiation recrystallization, crust influence, or dry/wet snowfall mixes.
  22. The persistent forms are surface hoar, facets, depth hoar, and combinations of these with crusts.
    • Surface hoar. Feathery crystals produced by high humidity and long wave radiation cooling that creates a large temperature gradient at the snow surface.
    • Facets. Angular crystals produced by high temperature gradients.
    • Depth Hoar. Angular, cup-shaped crystals produced at the bottom of the snow by high temperatures and larger quantities of water vapor.
    • Crusts. Crusts are a persistent form once buried. You can find combinations of persistent crystal forms with crusts such as facets above/below a crust, or surface hoar above a crust.
  23. Crystals with rounded and angular elements, usually transitions between round-to-facet or facet-to-round. These transition processes are called "faceting" or "rounding".
  24. Crystals developing under the slowest growth rates are referred to as rounds.
  25. Crystals developing under the highest growth rates are referred to as facets.
  26. +
  27. / ( or this symbol with a small break in the line )
  28. o ( but it should be filled in )
  29. [] ( should be a hollow square )
  30. /\ ( upside down "V" )
  31. o ( hollow circle )
  32. \/ ( right side up "V" )
  33. - ( thick, elongated dash )
  34. \/ ( with a curved line through the top, like an upside down "A".)
  35. Equiptemperature implies all snow is at the same temperature which is rare in alpine snow.
  36. All metamorphism happens under a temperature gradient of some magnitude. With respect to crystal shape this term is not specific enough since it basically refers to all crystals.
  37. Depth hoar
  38. They have been growing the longest, under warm temperatures and higher supersaturation than crystals elsewhere in the snowpack.

Growth Of Crystals Around Crusts In Dry Snow

  1. Provides a barrier for vapor transport.
  2. Weak bonding of snow above or below the crust.
  3. Vapor transport is prevented, which provides a relatively high level of supersaturation for rapid crystal growth.
  4. Dry on wet snowfall combinations involve "dry" snow falling on "wet" snow. Latent heat in the water fosters a temperature gradient.

Bond Formation In Dry Alpine Snow

  1. Formation of bonds is also referred to as sintering.
  2. Diffusion of water vapor through pore space and molecular motion at grain boundaries.
  3. At grain boundaries where crystals touch.
  4. Grain boundary groove.
  5. Generally from the perspective of ice spheres with a boundary where the spheres touch.
  6. 145 degrees
  7. False
  8. Highest stress is found on the interior of the grain boundary groove.
  9. Bond formation is initially quite fast, slowing gradually as time passes.
  10. Differences in crystal size ( mismatch ) result in poor bond formation.
  11. Thermodynamic processes occur faster at higher temperatures. Thermodynamic processes occur more slowly at lower temperatures.
  12. -5 degrees Celsius
  13. A mobile, liquid-like layer that thickens as temperature increases.
  14. When the temperature gradient is high, mass transport is rapid.
  15. Vapor molecules are not longer preferentially deposited at necks and growth occurs on sides and corners of crystals.
  16. Travel in continental climates would be extremely dangerous.
  17. Avalanches are not easily released on depth hoar because of its depth below the surface.
  18. Instability
  19. Large grains have fewer bonds per unit volume; smaller grains have more bonds per unit volume because of tighter packing.
  20. Similar geometry / crystal grain type/size.
  21. Differences in degree of riming can inhibit bond formation.
  22. These crystals pack very tightly and bond very well. This is how slabs form.
  23. List of integrated elements:
    • Temperature
    • Temperature gradient
    • Applied load
    • Pore-space configuration
    • Crystal type/size
    • Interface geometry
  24. About 100 times individual bond size.
  25. The individual bonds don't matter as much.
  26. High temperature and load work together to increase fracture toughness.
  27. Bonding and strength increase slowly for persistent forms because of large grain size, loose packing, and resistance to strength increases from overburden due to anisotropy.
  28. False
  29. In near surface snow. Depth hoar is the exception.
  30. Weak bonding between layers may be more important than low strength in a weak layer.
  31. Shear quality.

Persistent And Non-Persistent Weak-Layer Forms

  1. Canadian researcher Bruce Jamieson.
  2. Anisotropic: weaker in shear than compression, low-fracture toughness, may persistent for long periods of time.
  3. Fracture toughness and bonding increase fairly rapidly due to overburden slab load and temperature. However instability may persist at temperatures below -5 degrees Celsius.
  4. Human perception is poorer because of forgetfulness, spatial variability, or depth of burial.
  5. Human perception is generally good because instabilities are near the surface.
  6. During storms.

Metamorphism Of Wet Snow

  1. Conditions change greatly.
  2. Water, ice, and air.
  3. Smaller particles have a lower melting point than larger particles.

Snow With High Water Content

  1. At 15% water, ice particles may become completely separated from each other.
  2. Heat flux through liquid water.
  3. When all particles are the exact same size.
  4. Heat flux through liquid water is extremely efficient.
  5. Snow with 0% water content by volume.
  6. Snow with < 3% water content by volume.
  7. Snow with 3-8% water content by volume.
  8. Snow with 8-15% water content by volume.
  9. Snow with > 15% water content by volume.
  10. This is another term for wet snow.
  11. This is another term for very wet snow.

Snow With Low Water Content

  1. 8%
  2. Capillary pressure in the pore space.
  3. Liquid water is forced out of the pore space, usually draining down.
  4. Grain clustering that occurs as a result of surface tension.
  5. Surface tension of water between grains.
  6. Take its temperature, examine it with a lens, squeeze a handful.
  7. Dielectric measuring device.
  8. Grain growth increases because heat transfer through water is efficient.

Classification Of Wet Snow

  1. No. The snow is still faceted because that is the actual grain type by morphological classification.

Bond Melting And Formation In Wet Snow

  1. Water
  2. The melting point decreases as area of contact increases.
  3. During the day, snow melts, producing liquid water. At night, snow freezes. ( Diurnal temperature fluctuations. ) After several repetitions, the result is large grain sizes that lose most of their cohesion when wet.
  4. Corn snow
  5. A melt-freeze crust can serve as a future sliding layer if buried. A wet melt-freeze crust can produce wet/loose avalanches.

Wednesday, November 24, 2010

Living In The Moment

It was a long and dark December, from the rooftops I remember there was snowColdplay

NOTE: This is the second post that will address the general question of Why Is It So Complicated? This time we're going to talk about cold weather, avalanche forecasting, and persistent weak layers. Instead of engaging in endless speculation over the state of the winter snowpack, I'd like to take this opportunity to discuss the basic elements of forecasting, and what you can and cannot accomplish with forecasting.

Seeing The Future
Recently there have been a few online discussions about snow and weather conditions in the Pacific Northwest. With the current cold weather, and a generally colder winter forecast, a lot of people are wondering if persistent weaknesses will plague the snowpack this winter.

The straight answer to the above question is as follows: no one really knows what will happen in the Cascades this winter. What we know right now is that the current cold temperatures are almost certainly producing instability wherever the snowpack is shallow, and there is a high likelihood that the cold temperatures are producing surface instability in areas where the snowpack is relatively deep. It is very likely that faceting is widespread during clear, cold nights when the snow loses heat through long wave radiation loss. On solar aspects at high elevations, you might find radiation recrystallisation.

Armed with the knowledge that instability is developing in the snowpack, we can start to speculate about what might happen next.

In some areas, the instability may persist through the next couple of storm cycles, whereas in other areas, the first big avalanche cycle will clean things up. There's also the spectre of a pineapple express, which would produce widespread instability in many places, but not everywhere, while simultaneously producing a fantastic layer of "glue" to heal surface instabilities. On the other hand, a strong rain event could melt the facets and turn the snowpack into a layer of bulletproof concrete.

Of course, in the event any faceted snow is buried to a depth of about 1 metre, the existence of moderate temperatures would allow rounding to prevail in the snowpack...effectively healing instability in areas with deep snow cover. It could take a week in some areas, a couple of weeks in other areas, and in some areas the problem could indeed persist for the entire winter.

When you get down to it, just about anything could happen at this point.

Why Is It So Complicated?
In simple terms, it's complicated because no one knows the future interactions between terrain, snowpack, and weather. Therefore, no one knows whether or not persistent weaknesses will develop. As I often write on this blog, the chaotic interaction between terrain, snowpack, and weather is responsible for much of the uncertainty in backcountry avalanche forecasting. Rich Marriot writes, how can you forecast avalanches if you can't forecast the weather. The short answer is that you definitely can forecast avalanches, you just can't forecast avalanches very far into the future.

So, if the chaotic interaction between terrain, snowpack, and weather is responsible for a lot of uncertainty, it might be useful to understand why this is the case. To do this, we have to examine the discipline of forecasting. In the context of avalanche forecasting, wanting to know why there is so much uncertainty leads us directly to the following principles:
  • Information Types & Relation to Perception
  • Scales in Space and Time
These are two of the Elements of Applied Avalanche Forecasting discussed in The Avalanche Handbook. We'll discuss them next. ( Please see The Avalanche Handbook for a complete discussion of these elements. It is dangerous to issue avalanche forecasts using these elements by themselves. )

Why Forecasting Is Difficult
Forecasting is concerned with producing an accurate picture of future events. For avalanches, we can consider information types such as Class I data, Class II data, and Class III data. We can also consider the relationship between perception and data from a specific class. To define the scope of the problem, we can consider scales of space and time, or in very simple terms, we want to know where ( space ) and when ( time ).

To issue any type of forecast, you start by gathering data, and then you subject this information to some form of analysis. If you happen to be out in the backcountry on a particular afternoon, you have the tremendous luxury of hindsight made available by knowing the previous weather, and by knowing something about the current mechanical structure of the snowpack. You can also make very specific observations of the environment, including observations of the terrain and current snow deposition patterns.

On our theoretical afternoon, you also know the current weather. So while it is generally more difficult to issue a precise forecast, you also just happen to have access to an incredible amount of information on which to base any such forecast. To make things even easier, you're only concerned with issuing a forecast for a very short time, and for a very limited number of places. On the other hand, long-range forecasts are based on theoretical weather data ( Class III ), and there is always high uncertainty associated with such data. High uncertainty means that you're much more likely to make errors and blow the forecast as result.

For this reason, forecasts for the immediate time frame and for a small geographic area, often called nowcasts, are the most accurate. A forecast for a few days ahead is less accurate, and a long-range forecast might not be accurate in any sense. Nowcasts are more accurate because uncertainty is lower when we actually know something about the variables affecting the current situation. Unfortunately, we usually can't know the variables that will create or affect situations in the future.

This means that for any date far enough in the future, for a large area, uncertainty is essentially unlimited. If you want to see how this works, issue a forecast for your life over the next minute. What about a forecast for the next hour. What about the next 24 hours. What about next week? What about next month? Three months from now? Three years from now?

Bayesian Logic
The dynamic, integative process humans that use to conduct avalanche forecasting can be referred to as a Bayesian activity. We say that this process is dynamic, because it is active, and because it changes as information is collected. The process itself is integrative because you must consider the evidence as a whole, rather than as separate pieces. This is reflected by the following formula:
  • Prior × Likelihood = Posterior ( or, what follows )
If we convert that formula to human-compatible terms, we get the following:
  • The combination of Past Conditions and Current Conditions = Forecast
If you want to issue a forecast for February 14th, 2011, you'll notice a rather glaring lack of information about conditions leading up to that date. The reason why is obvious: you have no information about terrain, weather, and snowpack for that date because it hasn't happened yet.

The most important characteristics of Bayesian revision is the ability for a single piece of data to change the forecast. That means, you throw out the old forecast as you acquire additional data. If you observe unstable snow, your forecast must change.

Try It Out
Here are some exercises:
  • Forecast snowpack instability for the Cascade Mountains during winter 2010-2011. ( You can also choose your home mountain range if you don't live in the Cascades. )
  • Forecast snowpack instability for Phantom Trees backcountry ski run on November 30th, 2010 at 2:30pm. ( You can also choose your own favourite backcountry ski run. )
  • When you're finished with your forecasts, write a short snippet about which forecast is more precise and why it is more precise.
  • Remember, it's easy to confuse accuracy with precision, but they are not the same thing at all.
With respect to the Cascades, it seems pretty likely that another big dump will produce significant instability. But such general forecasts are easy to issue because it's well known that large dumps of snow produce significant snowpack instability. When we consider the uncertainty of long range forecasting, it's very easy to see why it's all but impossible to say anything about an entire winter.

As usual, we'll just have to wait and see.

But remember, theoretical avalanches aren't dangerous. It's the real avalanches that you have to watch out for, and real avalanches always happen at a very specific place and time.

Regardless of the specific place and time in which you find yourself, Happy Holidays to all my readers.

Wednesday, November 17, 2010

There Are No Magic Bullets

Chances are, when said and done, Who'll be the lucky ones, Who make it all the wayFive For Fighting

A complete backcountry safety system uses a mix of elements, including thorough planning, safe travel habits, rescue gear, and good judgment. Using multiple risk management elements allows you to reduce risk in a variety of different places, which is the same as not putting all your eggs in one basket. ( Putting all your eggs in one basket is referred to as risk concentration. )

To this point, many recreational skiers go lite on the trip planning, and by doing so they miss out on important opportunities to reduce risk. Then, perhaps due to poor planning, or lack of skill, the party makes a few poor decisions, which again represent missed opportunities to reduce risk.

As opportunities to manage/reduce risk are discarded, more pressure is put onto the rescue gear component of your backcountry safety system. Unfortunately, the rescue gear component of your backcountry safety system is really only designed to give you a chance at live recovery in the event of a complete burial. Rescue gear is not a comprehensive backcountry safety system.

And it's certainly not a magic bullet.

If you travel somewhere frequently, you may be tempted to avoid planning. But remember, even though the terrain remains the same, both environmental conditions and humans are subject to frequent changes. For this reason, it's a good idea to have a set of stock trip plans that you can pull out and review from the perspective of current conditions.

Human conditions: Are you tired? Maybe a bit hungover? How's your hydration and calorie intake? Are you really jonesing for a fix? What about your friends? Environmental conditions: Is avalanche danger high? What do you think the snowpack is doing on your intended route? What does the public avalanche bulletin have to say?

Three major elements of a complete backcountry safety system:
  • Planning: Thorough pre-trip analysis of terrain, snowpack, weather, and people involved.
  • Traveling: Safe travel habits, avalanche forecasting, managing yourself, and good judgment.
  • Rescue: Beacon, shovel, probe, spotting, searching, extraction.

Wednesday, November 10, 2010

Dust In The Wind

All we are is dust in the windKansas

NOTE: This is the first in a series of posts that will address the general question of Why Is It So Complicated? While teaching often involves simplification, it's important to remember that you can also use complexity to teach. Despite conventional wisdom, complexity is not always the enemy of simple, and simplicity does not always improve understanding.

Today's post is going to discuss wind loading, more specifically the ins-and-outs of using wind speed and direction to forecast wind loading. The main difficulty in using a simple model, is that the simple model doesn't account for turbulence, and turbulence has an incredible influence on the "patterns" of wind deposited snow.

Video 1. You can't see clear air turbulence, but watch this time-lapse video. The presence of clouds makes the turbulence very easy to see. At about 1 minute, you can watch some backwards loading, where turbulent vortices load a slope that is facing against the wind. This is referred to by the very technical term wind slab where you least expect it.

The public avalanche bulletin often contains a forecast of aspects on which you might expect to find wind loading, but it's important to remember that a.) the public avalanche bulletin covers a very large area, b.) avalanche forecasters have a lot of knowledge about the interaction between terrain and weather, as well as a deep body of experience about a variety of such situations in the past, and c.) they also have access to very sophisticated computer models. Forecasters with years of experience at a specific location will also develop a very good sense of how a specific storm will influence loading in certain areas.

Since most of us normal folks don't have that experience, we need to stick with the tried and true.

Character Of The Data
Wind speed and direction is Class III information, which means there is high uncertainty about its relationship to avalanche formation. High uncertainty exists because there are many ways to interpret the data, and as a result, this data may or may not reveal useful information about instability. This is because the physics behind wind flow are intensely complicated and the specific outcomes are very possibly unknowable.

Prevailing Wind
When we talk about wind, there are two important elements: the prevailing wind and local winds. The prevailing wind is located high above rough mountain terrain, where airflow is unimpeded by obstacles. We can express the prevailing wind with a direction such as north or northwest, and with a speed such as 20 knots. Broadly, the prevailing wind is influenced by horizontal pressure differences and the rotation of air around pressure centres.

An additional complication of using prevailing wind is that the varying shape and behaviour of cyclonic weather systems means that the prevailing wind speed and direction can change during the passage of a storm, and it can be difficult to predict these changes because of the dynamic nature of weather systems. The reality is that each storm is a completely unique event that is driven by a set of completely unique parameters that will never happen again. Once you start to understand the complexity, it becomes easier to understand how to use this complexity to your advantage during the process of backcountry avalanche forecasting.

As usual, this complexity provides very useful clues about what not to do.

Local Wind
Anyway, things become awfully complicated when a large storm moves over the mountains and the air starts interacting with terrain. This is referred to as local wind. Local wind is formed from a complex stew of frontal lifting, orographic lifting, convective lifting, and convergence-based lifting, in addition to frictional forces.

Picture a mountain valley, along with all its nooks, crannies, and crevices. As air moves through complex mountain terrain, it hits obstacles that cause it to become turbulent. Large features block and channel the large scale flow, while airflow over, around, and through smaller barriers generates localised turbulence. On this blog I frequently refer to the chaotic interaction between terrain and weather, and these crazy wind patterns are a very large part of the chaos. Naturally, the mere presence of chaos raises uncertainty because chaos suggests that uncertainty is the only state of affairs.

Clouds, along with other sophisticated meteorological data, can provide a fairly good visual model of this turbulence at a large scale, but these data are almost no use at the small scales required for effective backcountry avalanche forecasting. At small scales, we can use weather stations to measure local wind, but because of resource limitations, our picture of local wind speed and direction is actually extremely limited.

Thankfully, despite these problems, there are a few really good techniques that we can use to determine if wind loading has occurred. In simple terms, even though the wind is invisible, and even though the chaos of turbulence makes it very hard to determine local wind flow, we can observe the physical environment for the signals of wind loading.

What do we know about the complexity of wind speed and direction at a single location? What about the chaotic complexity of turbulence?

Figure 1.1. Wind speed and direction at 1 hour increments for Crystal Mountain "Green Valley" weather station over the course of a single day. Since the weather station measures wind on an hourly basis, there is a lot of missing data here.

Figure 1.2. Wind speed and direction at 1 hour increments for Crystal Mountain "Green Valley" weather station over the course of a single day. Here, the pattern matching software in our brains sees two broad patterns that may be associated with a major wind direction change that occurs when a cyclonic weather system passes over the weather station. Or, maybe our brains are seeing a pattern where no pattern exists.

Figure 1.3. Wind speed and direction at 1 hour increments for Crystal Mountain "Green Valley" weather station over the course of a single day. Here our brain is tempted to view "clusters" of wind directions, but even if these "clusters" do occur, our brain is simply incapable of accounting for turbulence, and these "patterns" may very well arise from turbulence and contribute to additional turbulence. It's pretty crazy, isn't it?

Figure 1.4. Complex deposition and drift patterns near the toe of the Sulphide Glacier. Can you identify the wind slab? Does the wind slab have a simple shape? Is the layering simple or complicated?
Figure 1.5. How complicated is turbulence? I'll engage in a gross oversimplification, but you can clearly see the vortex generation that is one of the hallmarks of turbulent fluids such as air. In the mountains, this effect produces cornices in some situations, and in general it produces loading patterns that are incredibly intricate. These loading patterns produce wind slabs of varying size and depth. The depth of a wind slab often varies greatly across a very small area, which further raises uncertainty about the likelihood of avalanche formation.

Figure 1.6. From MIT. The difference between laminar and turbulent flow. Imagine this in three dimensions, and then imagine trying to relate this model to snow deposition patterns. By now it's pretty obvious that you can't actually do this. It's worth mentioning that a supercomputer with 1024 cores might be able to produce a relatively accurate simulation. However, small inaccuracies in the input data, including inaccuracies that are immeasurably small, and/or inaccuracies in the mathematics of the simulation, will produce severe errors at every scale. This means that you'll end up with a simulation that "looks accurate", but actually has no relationship to the real world.

So what's a guy or gal to do? Well, given the complexity and uncertainty that arises, observations of local weather and snow deposition patterns remain the gold standard for discerning the parameters of wind loading. You can safely assume that any combination of wind and snow means that some wind loading has occurred.
  • Significant amounts of snow are removed from windward slopes.
  • Significant amounts of snow can be deposited on lee slopes.
  • Drifting patterns. Drifts point in the same direction as airflow.
  • Snow build up, or the lack thereof, on trees and rocks.
  • Cornices point in the direction of airflow and indicate loading below.
  • Look for ripple patterns on the snow surface.
  • Observe the snow's texture, especially its hardness.
  • Wind slab often sounds hollow when you step on it.
  • Wind slab has an intricate shape and layering.
  • Complex stratigraphy raises uncertainty.
  • Delicate and/or intricate transitions between clean and dirty snow may exist.
  • Given the foregoing, never try to outsmart the snowpack. It simply can't be done.
Obviously, there's a theme: stick with tried and true methods of using visual observations to assess wind loading. These methods are far better than trying to model wind loading by using the prevailing wind direction.

Futher reading that merely hints at the unreal complexity of the problem:

Friday, November 5, 2010

Great Learning Resources

We Don't Need No EducationPink Floyd

Here are two absolutely wonderful resources. Free registration required.

Thursday, October 28, 2010

New From The Canadian Avalanche Centre

What we see and what we feelJonsi

The Canadian Avalanche Centre has published a great guide for advanced recreational skiers:

Avaluator 2.0 is also available:

Trip planning form:

These are great products from a wonderful organisation.

Friday, October 22, 2010

Union Creek

I keep dreamin', you'll be with me and you'll never goNickelback

This post is in memory of Kevin Carter, Devlin Williams, and Phillip Hollins.

February 2008
Almost three years ago I sat in a Seattle coffee shop and read an article in The Stranger. The sharp words pricked a nice hole in the warm bubble of what was an otherwise quiet afternoon: three snowboarders disappeared in Union Creek during early December, and they had not been seen, nor heard from since.

It was a La Nina winter, and you can bet snow was on my mind: I'd already visited friends in Revelstoke once during early January, and my upcoming travel plans included another trip to Revelstoke, followed by a trip to Banff to see my aunt and uncle.

So, I won't really get into what was going on in my life at that time, but I think massive changes sums it up quite nicely. In the past three months I had finally started to escape from a very dark hole, and I wanted nothing more than to stay in British Columbia and spend the rest of the winter climbing my way out of the awful black.

Not yet ready to head back to the States, I extended my trip and spent some time skiing near Valhalla Provincial Park. Sitting alone in my hotel room one evening, I realised that computer graphics techniques already provided solutions to the certain types of perception problems. We look at maps, and we get an idea about where we should and should not travel in general. But the computer can transform a map from merely useful to truly useful.

Theory of Relativity
In addition to incredible ease of access, the backcountry near Crystal Mountain is middle ground terrain. This sets up a classic middle ground perception problem because Union Creek has a rather benign appearance relative to high alpine terrain. In other words, you can drive to the trailhead, skin from the car, but you won't see enormous snowfields below savage peaks. Instead, as mountain terrain goes, Union Peak is really sort of small, steepish, and extensively gladed. To this point, there are plenty of places that seem safe..

In a typical scenario, the skier examines the choices in front of them and selects what 'appears' to be a safer option. A common equation is as follows: trading steepness for trees, or trading open slopes for tree covered slopes. However, safety is relative, and safer is not the same thing as safe. When avalanche danger is High, statistically speaking, the safer option might not actually be any safer at all. Without hard numbers, how do you know if you're really reducing or exposure or if you're just trading horses?

Can you identify ski runs that are safe during high avalanche danger?

You can examine the terrain from the air, and can you stare at contour maps all day long, but the fact of the matter is that the human mind just isn't very good at certain types of tasks. Computers are wonderful tools, and they are quite happy to help us cut through the clutter of our minds and tell us the truth.

Want to take a look at what the computer sees?

Union Creek
Union Creek is popular backcountry terrain east of Crystal Mountain Ski Resort. Union Creek contains COMPLEX avalanche terrain. In poor conditions, line-of-sight is limited and safe travel may be difficult for anyone regardless of skill level. Start zones in Union Creek are capable of producing large, destructive avalanches.

Required Skills: Expert route finding and expert snow stability assessment.

Visualization of terrain > 25 degrees and < 25 degrees. Avalanches often run into blue terrain.

Avalanche Terrain!

Terrain Trap!

Elements of avalanche terrain at Union Creek.

HistoryPeople have been killed by avalanches in Union Creek. Large soft-slab avalanches run during and after storms. Surface hoar formation is widespread in this drainage. Faceted snow often develops near ridgeline and rocks. Wind-loading occurs during high winds. Local skiers report 1, 2, and 3 foot crowns. ( Crown size of 30cm to 1m. )
Avalanche Starting ZonesMultiple avalanche starting zones are found below ridgelines.
Avalanche PathsChanneled and unconfined avalanche tracks are found throughout Union Creek. Avalanches move very quickly in confined tracks. On the east and north slopes, several large, poorly defined paths exist below large, open start zones. Numerous, small avalanche paths run through trees on all the slopes of Union Creek. Avalanches in these narrow, tree-lined paths are extremely dangerous.
Avalanche Runout ZonesA network of gullies forms overlapping runout zones at the bottom of Union Creek. This region is a complex, dangerous terrain trap. Other runout zones extend into forested areas.
ShapeConvoluted, with very frequent changes between concave and convex slope shapes.
Large Surface AreaTerrain in this drainage has a very large surface relative to its size on a map. Large amounts of snow accumulate throughout Union Creek.
Steep TerrainMuch of the terrain in Union Creek is very steep and avalanche prone. In many areas, more than 90% of the terrain suitable for skiing is between 30-40 degrees.
Open TerrainUnion Creek has large areas of open terrain.
ConfinedTerrainSome locations in Union Creek feature highly enclosed terrain.
Line-Of-SightSome locations in Union Creek have limited line-of-sight. You may not be able to see overhead avalanche terrain because of terrain features or trees.
Terrain TrapsUnion Creek has numerous terrain traps such as convexities, trees, depressions, and gullies. Computer modeling finds hundreds of terrain traps.
Safe AreasRidge areas are safest. Terrain on the valley floor, if below open terrain above, is not safe. Many of the avalanche paths in Union Creek are enlarging every winter. There are safer areas in the valley below thick expanses of trees that run to the ridgeline.
Exposure Time
Many ski runs and travel routes are exposed to avalanches for their entire length and offer no chance to reduce exposure. Less dangerous routes do exist, but require expert route-finding relative to the current conditions. Deep snow and steep terrain often make uphill travel slow and difficult.

Tuesday, October 19, 2010

Dancing Around Uncertainty

But it's one missed step ... one slip before you know it—Sarah McLachlan

There is an interesting article on Friends of Berthoud Pass web site.

Our Friend Bob Berwyn at the Summit County Voice wrote recently about the growing need for basic avalanche awareness among “sidecountry” skiers in Colorado.

Avalanche awareness programmes are wonderful, but many of them do not present balanced thinking frames for beginner recreationists. Programmes that focus entirely on "avalanche awareness" do so at the expense of "avalanche uncertainty", which leaves students with only half the mental model that they need to make good decisions.

The winter snowpack is conditionally unstable, and very often this means that it can be difficult to determine the extent of instability and its parameters. Of course, this is often the point where human nature steps in and people find their own ways 'manage' the uncertainty.

Failing to proactively manage uncertainty is at the root of many avalanche accidents for the following reason: a highly uncertain mind is very susceptible to biases, speculation, rationalisation, and the disregarding of facts.

Since some degree of residual uncertainty always remains, and since the degree of uncertainty is often inversely proportional to the skill of the recreationist, it seems strange to focus so much effort on awareness without addressing the other side of the coin as well.

We've all seen this take place during online discussions, at avalanche awareness events and during level 1 classes. Someone asks a question and the instructor provides additional information along with a qualification or two. This leads to additional questions and qualifications, as the instructor and students dance around the uncertainty.

I believe avalanche awareness programmes and level 1 classes must explicitly address uncertainty and teach students that decisions should retain a conservative character when their uncertainty is high. This would present a much more balanced mental model to beginners, and it would also help smooth out the cognitive dissonance that arises when observations of the terrain, snowpack, and weather don't provide clear answers.

Further Reading

Wednesday, October 13, 2010

Q & A: La Nina

Look around, leaves are brown, There's a patch of snow on the ground... - Simon & Garfunkel

Several people have emailed me questions about La Nina in the last couple weeks. The general vibe: people want to know what a La Nina winter will mean for avalanche safety in Washington State.

From a meteorological perspective, a La Nina winter is colder and wetter, which means larger, more frequent storms, and lots of snow in the mountains. However, La Nina has no mysterious influence on the snowpack itself. Prolonged cold temperatures, which can occur during any winter, will either produce new snowpack instability or allow existing snowpack instability to linger for longer periods of time. Large storms, which can occur during any winter, always produce instability. Rising temperatures during storms always produces instability. Some or all of these events may happen more often in a La Nina winter, but the snow safety rulebook won't change. You can learn more details about La Nina years on Amar Andalkar's excellent web site.

What about avalanches during La Nina winters? The last La Nina winter was 2007-2008, and there were a record number of fatalities in Washington State. For example: there were 5 fatalities in Washington State during a single weekend in early December 2007. In North America, there were 14 fatalities during the month December. That's basically one fatality every other day.

So, where were we? ...Frequent storms and lots of snow in the mountains... Sounds perfect, right?

Yeah, it's perfect ... but like everything wonderful, there's a catch. According to The Avalanche Handbook, "direct loading by synoptic scale weather events causes most avalanches". In other words, lots of storms means lots of potential for skier-triggered avalanches. Avalanches that occur during storms are referred to as direct-action avalanches, and these avalanches occur during a storm or within 24-72 hours after the storm ends. It's a good bet that public avalanche bulletins in North America will often forecast High avalanche danger during the upcoming winter.

What's your strategy for touring on days when avalanche danger is High? Do you have a list of trips for different levels of avalanche danger? ( Feel free to post tips in the comment box. )

The Take-Away
  • There will be frequent storms.
  • Avalanche danger will be High during and after these storms.
  • Start thinking about terrain choices for the winter.
  • Always choose terrain appropriate for current conditions.
  • Develop a list of "go to" terrain that is appropriate for high avalanche danger.
  • Plan like your life depends on it. ( Because it might. )
  • Take an avalanche safety course if you haven't already done so. United States / Canada
  • Hire a guide for big trips.