The likelihood of triggering is a description of how likely it is to encounter a particular avalanche problem. It combines the spatial distribution of the problem with the sensitivity or ease of triggering avalanches. The spatial distribution states how likely it is to encounter the problem in the terrain (altitude and aspect) indicated in the advisory. Sensitivity indicates how easy it is to trigger an avalanche once you have encountered it, or how abundant natural avalanches are expected to be.
As already mentioned, one of the points to consider when we establish the likelihood of triggering a given problem (for example, wind slabs) is what its distribution is in the mountains. The following are our working definitions:
Widespread: the problem is present all throughout the indicated terrain, affecting most avalanche terrain. Evidence of instability is everywhere and is easy to find.
Specific: The instability is present in specific portions of the mountains and relates to elements of the terrain that are described in the text of the advisory. Evidence of instability exists, but it is not always obvious.
Isolated: The instability is punctual and very localized, located in few places. Evidence of instability is rare and difficult to find.
It must be born in mind that the description of the distribution corresponds to the indicated terrain. A problem can thus be “Widespread” at “high altitudes facing N and NE” and at the same time “isolated” for “very steep slopes at all altitudes and shady aspects ” (this might seem awkward but it is conceptually possible).
Danger in isolated terrain: shady and very steep slopes, near the ridges and peaks.
Danger in specific terrain: shady aspects favoring the accumulation of snow by effect of the wind.
Widespread danger: affects all avalanche terrain.
This concept somehow replaces the old idea of “weak overload” and “strong overload”, which in light of the new way of understanding the triggering of avalanches is a little obsolete.
It is very easy to understand when it comes to accidental triggering. How easy is it to trigger a slab with a given distribution scenario? Are slabs triggered when overloaded at any point or must the triggering agent act on a very specific spot? With regard to natural avalanches, sensitivity indicates the number of avalanches that will occur.
What we use to measure this parameter is, above all, the occurrence of avalanches. In the case of human triggering, and particularly when the sensitivity starts to drop from “Fragile” to “Difficult” and “Unreactive”, stability tests are very helpful.
Sensitivity: very fragile. Human triggering is almost sure.
Sensitivity: fragile. Slabs are easily triggered by overload.
Sensitivity: unreactive. Slabs are not sensitive to a person’s overload.
An example might be useful to illustrate the typical evolution of the likelihood of triggering. Let us suppose there is a snowstorm with North wind, falling (to make it easier) on a very crusted snowpack at all heights and aspects.
– The distribution of the slabs will depend on the amount of snowfall and the force of the wind. If the wind is very strong and the amount of snowfall is scarce, surely the slabs will be limited to the well protected downwinds (isolated terrain). If there is little wind and a lot of snow, slabs will only be present on exposed places, near hills and ridges (specific terrain). With a significant amount of snow and a moderate to strong wind, slabs may be widespread at the downwinds. The combinations between these two parameters (wind and snow) are many, and even more so if we bring into play the previous surface of the snow.
– Sensitivity will typically decrease over the course of days. While they are still being formed, slabs will be fragile or very fragile. After a day or two they will become difficult to trigger and will become unreactive after less than a week (provided there are no persistent weak layers to be added to the picture)
Case 1: heavy snowfall with moderate to strong wind
Sensitivity + Distribution = LIKELIHOOD
We finally have to combine these two parameters to obtain the rating that you will find in the bulletin. Truth is to say, we do this intuitively and not with a systematic approach, and, based on our experience, we assign a likelihood to each problem after having reflected on its sensitivity and distribution. If we were to put it on a table, it would look something like this:
(Note: this table is only estimative, by no means is it definitive)
Almost certain: we expect numerous avalanches to occur throughout the terrain indicated by the icons, both natural and human triggered, including distance triggered ones.
Very likely: Cwe believe there might be natural avalanches, linked to certain very specific features of the terrain, very likely on a particular type of terrain or less so on a more widespread distribution.
Likely: human triggered avalanches may occur, but generally speaking, it is not so likely since triggering requires very specific terrain features or loading certain sparse trigger points.
Unlikely: only in certain features of the terrain and with bad luck is it possible to trigger an avalanche.
(Note: When the sensitivity is “unreactive”, the avalanches no longer appear in the bulletin, although we do keep track of them).
LIKELIHOOD OF TRIGGERING + AVALANCHE SIZE = AVALANCHE DANGER