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

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.

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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. )

Polymorphism
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.

Conclusion
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.

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