The presence of snow in the mountains has to be one of their most evocative components. Its ability to transform landscapes into blinding oceans of white fuels our imagination, wonder, and respect for the natural world. The constant changes that are happening within the snow and its sensitivity to complex and transient environmental conditions are the very things that make snow metamorphism so interesting and seemingly difficult to understand.
Despite these complexities, there are a number of processes that can be relatively easy to identify. This article is a combination of things I’ve learned, read about, and experienced over the years. It’s intended as a summary of some of the natural processes that affect snow, as well as other environmental factors that influence its character and distribution.
All of the processes described here are relevant wherever there is snow lying, falling or being redistributed. Some are more prevalent in certain areas due to prevailing weather and general climate. Wind, temperature, and humidity are three of the main drivers of the perpetually changing snow, but there are of course complex thermodynamic processes at work too.
It’s ultimately the interplay of these processes, and the surrounding environment that will determine the character of snow and its relative stability. A basic understanding of these processes, combined with information from experts in the form of avalanche forecasts, and a systematic pattern of evaluating potential risk should enable us to make more informed decisions and ultimately have more rewarding and safe days out enjoying the mountains in winter.
I feel it’s important for anyone reading this to understand that I am not a qualified snow scientist. There are people far more qualified. However, like many climbers and mountaineering instructors, I have more than a passing interest in the subject. Although the science is very well established, there may well be more contemporary research that a snow scientist could expand on. As such I welcome any comments or feedback.
Snow is almost constantly being moved around in the mountains. Wind speeds as low as 15km/hr can be enough to move snow already lying on the ground and deposit it on more sheltered ‘lee’ slopes. This movement of snow can often be very turbulent, causing fragile snow crystals to break down into smaller components. As they get battered together and re-deposited they can form relatively dense and homogenous layers, producing what is characteristically described as windslab.
Windslab can form in isolated pockets as well as across whole mountainsides and often demonstrates good cohesion to itself but not always to any underlying layers of snow or ground features. It is well documented as having a ‘chalky’ white appearance and sometimes ‘squeaking’ when walked upon. Cohesive chunks or blocks can often be seen to fracture and break apart underfoot. This is certainly true of dense windslab but it can be much less obvious to identify in certain conditions when ‘soft-slab’ may prevail. Soft-slab is sometimes mistakenly identified as loose powdery snow.
It’s windslab’s internal cohesion that also creates tension and potential stress points as it forms across complex ground shapes and less cohesive underlying layers. When these stress points are triggered, a domino effect can ensue causing large fracture lines to propagate, sometimes over hundreds of meters and with the optimum slope angle (usually but not exclusively between 30-45°) the slope may fail, initiating a slab avalanche.
A ‘Crown Wall’ which marks the upper fracture line of a large slab avalanche.
A temperature gradient (or vertical temperature gradient) is a term used to describe the relative difference between the ground temperature under a snowpack and the ambient air temperature. In temperate climates, the insulating properties of snow always retain ground heat at around 0° Celcius. The depth of a given snowpack also influences the temperature gradient. A shallow snowpack will cool or warm up at a greater rate throughout its depth compared to a deeper snowpack subject to the same ground and air temperatures. The presence of a temperature gradient has a number of implications on crystal size and shape that affect all of the following processes to a greater or lesser extent.
Above: A very simple illustration of a vertical temperature gradient within a snowpack showing diurnal and nocturnal fluctuations. In reality, the temperature gradient may not be so smooth. If the same ground and air temperatures were present within a shallower snowpack the temperature gradient would become more pronounced (stronger).
This process effects snowpacks that are subject to cold and dry conditions, typically but not exclusively found in arctic and alpine environments. The character of the snow in this kind of snowpack is influenced by vapor pressure which is driven by the presence of a vertical temperature gradient. Differences in pressure affect individual crystals and cause them to become rounded through a process called ‘sublimation’ (similar to evaporation, except solids turn into gasses without going through a liquid state).
As the crystals become more rounded they go through an initial period of instability as the interlocking arms and branches of individual crystals break down. If the snowpack survives this period of instability, the crystals begin to develop bridges or ‘necks’ to one another and become stronger and more cohesive (sintering). However, as a result, snowpacks that have been subject to these processes can also produce slab avalanches. In arctic and alpine environments, dry snow metamorphism ultimately leads to the formation of ‘firn’ snow and the development of glacial ice.
A second process called kinetic growth also occurs with the presence of a strong temperature gradient (greater than 10°C per meter). This process refers to the formation of crystals within the snowpack as well as on its surface, rather than snow falling out of the sky, and typically happens on cold still nights. Warmer air, nearest the ground surface, holds more water vapor than the colder air closer to the snow’s surface. Similar to condensation that forms on single glazed windows in a warm room on a cold night, this water vapor wants to diffuse to areas of lower concentration. As it migrates up through the snowpack, instead of condensing it freezes upon contact with colder surfaces resulting in the formation of angular ‘faceted‘ crystals that create weak layers within the snowpack (depth hoar), and on its surface (surface hoar).
Any obstructions within a snowpack such as a melt-freeze crust may prohibit this upward migration and result in depth hoar forming underneath the obstructive layer. Surface hoar can be considered as the frozen equivalent of dew. It forms large feathery and fragile crystals. If new or redeposited snow accumulates on top of it, these fragile layers can lead to instabilities within the snow which will only disappear during a period of thaw meaning they can persist for long periods of time in high alpine environments. Both of these crystal types display poor cohesion and can produce weak layers in the snowpack.
In the UK our temperate maritime climate means that our mountains are subject to regular temperature fluctuations which often lead to a regime of melt/freeze cycles in the winter. Whilst thawing conditions produce instabilities within a snowpack by weakening bonds between snow crystals, it also provides an opportunity for any particularly weak layers such as faceted grains, or buried graupel to melt away.
Through melting, snow and ice crystals break down and decompose into a relatively homogenous state. However, in these conditions, cornices can become unstable and may collapse under their own weight which can trigger avalanches on any underlying scarp slopes.
If a freeze cycle follows a period of thaw the snowpack can consolidate leaving relatively stable conditions that have been purged of any previous instabilities. If the mountains are exposed to several freeze-thaw cycles, snow ice can develop in large quantities and consolidates into what climbers refer to as névé which gives good climbing conditions. Melt/freeze cycles are less prevalent in alpine environments where temperatures are typically cold and stable meaning that any weaknesses can remain in the snow for long periods of time.
Like many youngsters, when I first started going out in winter snowboarding, skiing and climbing I justified decisions with ‘avalanches are unpredictable’ and ‘it won’t happen to me’. As time went on people I knew began to get hurt and die in avalanches. I was hit by a collapsing cornice with two friends on the side of Aanoch Mor. We were lucky enough to limp off with our tails between our legs but each one of us carried an injury, albeit minor. In hindsight, it was a well-deserved slap in the face.
I was later told by a very experienced mountain guide that staying safe in winter is about developing a pattern of behavior that is consistent every time you venture out. Justifying decisions with ‘avalanches are unpredictable’ and ‘it won’t happen to me’ might work one or twice, it might work for a year, or a decade, or even a few decades but it will catch you out in the end. The problem is that by doing or justifying something in this way creates positive feedback: ‘It was okay. I knew it would be’, and so the pattern continues. Sooner or later, this method of justification fails.
As a result, I decided to try and better educate myself and realized that through learning about these processes, it greatly enhanced my appreciation and respect for the natural world and opened my eyes to a heap of other related subjects including human behavior and heuristic traps from which none of us are immune. Although I feel I have only scratched the surface it is a fascinating subject that I will hopefully continue learning about for a long time to come.
B. Barton & B. Wright (2000): A Chance in a Million? Scottish Avalanches
R. Bolognesi (2002): Snow. Understanding, testing and interpreting snow conditions to make better avalanche predictions
R. Bolognesi (2003): Avalanche! Understanding and reduce the risks from avalanches