Snowpack stability & frequency

Definition

Snowpack stability is a local property of the snowpack describing the propensity of a snow-covered slope to avalanche (Reuter and Schweizer, 2018). Snowpack stability is described using four classes: very poor, poor, fair, and good.

Remarks

1. Depending on the avalanche type, snowpack stability is described by:

Failure initiation, crack propagation and slab tensile support (slab avalanche) (Reuter and Schweizer, 2018)
Loss of strength/bonding (loose-snow avalanche) (e.g., McClung and Schaerer, 2006)
Loss of basal friction and slab tensile and/or compressive support (glide-snow avalanche) (e.g., Bartelt et al., 2012).

2. Snowpack stability is inversely related to the probability of avalanche release. Snowpack stability describes the snowpack to fail given a specific trigger (Statham et al., 2018), as for instance a person skiing a slope. Table 1 shows the relation between snowpack stability and how easy it is to trigger an avalanche.

3. The term local refers to a point which ranges in size from a potential trigger location or stability test to a starting zone.

4. All snowpack stability assessments may refer to either future (forecast) or present (nowcast) based on observations or models. E.g., if the snowpack in a release area is considered fair today, and until tomorrow a layer of new snow is expected, the stability of tomorrows snowpack including the new snow layer needs to be assessed. Likely, it has changed to poor or even very poor by that time.

Table 1: Stability classes, and the type of triggering typically associated with these classes. For more examples relating to these classes refer to Figures A1 – A3 in Appendix A.

Stability Class	How easy is it to trigger an avalanche?
Very poor	Natural / very easy to trigger
Poor	Easy to trigger (e.g., a single skier)
Fair	Difficult to trigger (e.g.,explosives)
Good	Stable conditions

frequency distribution of snowpack stability

Definition

The frequency distribution of snowpack stability describes the percentages of points for each stability class relative to all points in avalanche terrain. Thus, the frequency f for all points with stability class i (ni) compared to all points (n) is f(i) = ni/n. The frequency distribution of snowpack stability is described using four classes: many, some, a few, and none or nearly none (Table 2).

Remarks

1. The frequency distribution of snowpack stability refers to (many) points (i.e., stability tests, snowpack models or potential triggering locations) or avalanche starting zones.

2. The frequency must be assessed for a warning region which must be equal to or larger than the reference unit.

3. The definition asks, in theory, for a percentage. However, this is often impossible to assess since the frequency distribution must often be inferred from sparse data in a real situation. Percentages or thresholds for many, some, a few, or none or nearly none differ depending on the measurement/evidence used or available. E.g., the percentages for slopes that produce spontaneous avalanches might be lower than the percentage of points with stability tests that indicate very poor stability.

Table 2: Classes of frequency distribution of snowpack stability.

Frequency Class	Description	Evidence (e.g., observations)
many	Points with this stability class are abundant.	Evidence for instability is often easy to find.
some	Points with this stability class are neither many nor a few, but these points typically exist in terrain features with common characteristics (i.e., close to ridgelines, in gullies).
a few	Points with this stability class are rare. While rare, their number is considered relevant for stability assessment.	Evidence for instability is hard to find.
none or neary none	Points with this stability class do not exist, or they are so rare that they are not considered relevant for stability assessment.