ATSC 113 Weather for Sailing, Flying & Snow Sports
Stable and Unstable Snowpacks
Learning Goal 7f: Describe the properties of a stable and an unstable snowpack and how to assess stability
In various Learning Goals within section 7 we talk about snow crystals, and how they evolve once they are on the ground in the snowpack. In this section we will discuss how these different snow crystals may layer to form a stable or unstable snowpack. This is one of the most crucial pieces of information you need to evaluate the likelihood of avalanches.
IMPORTANT NOTICE: This course on snow sports is not an avalanche course, and should not be taken in lieu of an actual avalanche skill training course.
Measuring Snow-layer Conditions
Snow pits
Backcountry skiers and avalanche professionals dig snow pits to find out information about snow layers inside the snowpack. Snow pits are typically 1-2 metres deep and are deep enough to analyse potential layers of concern, i.e. unstable sections of the snowpack. Analysis includes layer identification, snow crystal type, hardness tests, and stability tests. Crystal types are covered in later modules, and we do not cover stability tests in this course (for those interested in learning about these, an avalanche skills training course is recommended). Next we discuss snow hardness, which is important to this section.
Hand hardness test
The hardness of a layer is related to the density and cohesiveness of the layer. The hand-hardness test is done by applying light pressure to each snowpack layer. The hardness levels from softest to hardest are: fist, four fingers, one finger, pencil, and knife. The last thing that you are easily able to push into the layer is the hardness rating of the layer. Often a pencil or knife is not available and these hardness ratings are estimated.
Types of Snowpack Layers
- Weakly-bonded snow layers
Weak snow within the snowpack, or a weak layer, is typically snow that has undergone faceting (Learning Goals 7de). This layer tends to be lower density and have fewer bonds between crystals. Since there are fewer bonds, the layer is less cohesive, weaker, and more likely to collapse or fail. Another type of weak layer is surface hoar (Learning Goal 7i). It forms at the surface of the snowpack, but becomes a weak layer once it's buried by more snow.
Yet another type of weak layer is newly-fallen, low density snow. This type of weak layer is short-lived, as once on the ground the snow will start evolving, undergoing settling (deforming, compacting and sagging due to gravity) within hours of falling. In a snow pit, weak layers typically have fist hardness or four-finger hardness.
- Strongly-bonded snow layers
Strong snow within the snowpack, or a strong layer, is most commonly comprised of old stellar dendrites (picture-perfect looking snowflakes with branches) that are now rounded crystals (a.k.a. rounds). These layers are more dense and hard (typically four-finger hardness if found in the upper snowpack, one-finger hardness in mid to lower parts of the snowpack), with more bonds and stronger bonds.
Newly-fallen snow that's higher density, e.g. that fell during warmer and windier conditions, may create a relatively strong layer (four-finger hardness). Yet another type of strongly-bonded snow layer is wind slab (typically one-finger or pencil hardness). These types of layers are less likely to fail or collapse.
- Crust layers
When a rain-on-snow event occurs (rain falling on a snowpack) and is followed by below-freezing temperatures, the rain-saturated snow freezes, forming a rain crust. Another way that a crust can form is when above-freezing temperatures occur, melting the snow surface, and it subsequently refreezes. This is known as a melt/freeze crust. Sometimes it is the radiative heating from the sun that leads to above-freezing temperatures, and this melt/freeze crust is then referred to as a sun crust.
In the northern hemisphere, it is common for sun crusts to form on more southerly facing aspects, while northerly-facing aspects remain crust-free. Crusts are typically very thin (millimeters to a few centimeters), but quite dense and hard (pencil or knife hardness). The crusts themselves don't usually fail but they may lead to failure. For example, crusts can block the flow of water vapour within the snowpack, causing a weak layer of facets to form immediately above them in the snowpack. You do not need to know how this happens, just that it does.
Stable and unstable snowpacks
Individual strong or weak layers by themselves are only part of the equation. It's the layering of multiple layers that determines if the whole snowpack is stable or unstable. As a general rule, a stronger/harder/denser/more cohesive layer on top of a weaker/softer/less dense/less cohesive layer is an unstable configuration. A common metaphor frequently used by well known avalanche forecaster Bruce Tremper is that this configuration is like trying to balance a book on a house of cards. You might get it to balance like that, but it needs only a small trigger to collapse.
You only need one unstable configuration anywhere within the snowpack to create dangerous avalanche conditions. It doesn't matter if there are several layers above or below that are in stable configurations, i.e. a weaker layer on top of a stronger layer.
The relative strength of layers is also important. A layer with four-finger hardness may be considered a relatively weak layer if it's below a one-finger hardness layer (stronger over weaker), yielding an unstable configuration. However, a four-finger hardness layer may be viewed as a relatively strong layer if it's below a fist hardness layer (weaker over stronger), yielding a stable configuration.
Layers like facets and buried surface hoar are particularly dangerous, since they are weak compared to virtually any other that may be on top of them. In other words, if there's a very faceted or surface hoar layer, almost any layer you put on top of it will lead to an unstable snowpack. Further, facets and surface hoar have a tendency to persist for long periods of time in a snowpack.
Storm snow is defined as snow that is currently falling or newly-fallen within the last ~48 hours. Subtle snow density changes can occur in storm snow due to temperature or wind changes during the storm, leading to unstable setups that are likely to avalanche (this will be covered in more detail in a later module). These instabilities are typically short-lived, lasting only 12-48 hours as the new snow rapidly settles.
Keywords: hand-hardness test, melt/freeze crust,
rain crust, rounded crystals/rounds, settling, snow pits, stellar
dendrites, storm snow, strong layer, sun crust, weak layer, wind slab, surface hoar
Figure Credits: Stull: Roland Stull, West:
Greg West, Howard: Rosie Howard, Wagner: Wendy Wagner, Chugach
National Forest Avalanche Information Centre