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
Fig. 7f.1 - A snow
pit in Manning
Park, BC backcountry. (Credit: West)
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