Snowpack Information (A very basic primer)
INTRODUCTION: In most situations the snowpack is rather dynamic and constantly changing in response to a variety of factors. Such snowpack changes may be very slow (a particular danger level may persist for some time), occur very quickly (e.g., rain or strong sunshine on recent snow) or evolve at a rate somehwere in between. Weather and terrain are the primary factors that determine these snowpack changes, with the magnitude and location of resultant changes also influenced by the internal structure or layering of the snowpack itself. This state of transition-either toward more stability (most common) or less stability- may be inferred through a thorough review of recent weather conditions affecting the slope in question (meteorological reconstruction). However, a series of snow profiles or snowpits (snowpack excavations to view, test and analyze the relative strengths and weaknesses of the layering) often provide better and more specific information about the magnitude and direction of such temporal changes in snow stability and related danger from avalanches.
Nevertheless, despite the very best snow profile sampling programs, marked differences in snowpack strength are often common over small distances. This spatial variability in snow layering and resultant stability is a fact in most alpine environments, with the practical result that snow profile information should only be considered as part of any stability evaluation process. For optimal results, this process should be ongoing from the start of the winter, or at least from the start of the trip (if armed with the most recent regional or local avalanche center information). Go or No-Go decisions should be constantly refined and evaluated or re-evaluated as weather, terrain or snow cover changes dictate. Although significant stability changes may be observed in adjacent locations on a relatively uniform open slope, even a minor change in slope aspect or slope angle can affect the magnitude and direction of the stability trend.
The following highlights present a brief summary of some of the factors that favor stability or instability of a given snowpack. By no means should these highlights be considered inclusive, as they represent only the tip of the stability iceberg which includes a rather daunting array of possible combinations that affect avalanche danger.
STABILITY: In the absence of strong vertical temperature variations within the snowpack, relatively consistent and moderate ambient air temperatures, and no further weight or loading by current snow or rain, a transition toward greater stability is typical for a settling snowpack. Such stabilization in which decreasing snow depth occurs at relatively uniform temperatures is driven both by gravity (including overburden pressure) and by a decrease in crystal or grain sizes (through mass transfer via equilibrium metamorphism or the "rounding" process). This "compacting" of the snowpack allows more ice grains to come into closer contact with each other and more bonds to develop per unit volume (sintering)...which result in a generally strengthening snowpack. Weather factors that are most likely to result in a stabilizing snowpack include:
- No recent precipitation (no loading of existing weak layers)
- Moderate air temperatures (favor settlement, rounding, sintering and stabilization of the snowpack)
- Light winds (favor deposition of intact crystals and a lower density snow layer)
Of course, new snowfall does not have to result in significant danger if accompanied by light winds and low temperatures which produce little internal cohesion, minor layering differences and primarily point release or small, loose avalanches. But beware that this relatively benign situation of great powder can quickly change if winds or temperatures increase, thus changing a previously fluffy layer into a much more cohesive and slab-like surface snow layer now lying over a relatively weak and fluffy substratum.
INSTABILITY: As briefly alluded to in the Introduction, trends toward increased avalanche danger can be driven by many, many factors, and due to this there are truly no rules of thumb. But if there were, the following might be stated:
Increasing avalanche danger is often produced by:
Increasing precipitation rates (either snow or rain-result in higher rates of loading and stress to underlying layers)
Increasing temperature (heavier, denser snow or rain stressing weaker and lower density layers)
Increasing winds (favors crystal destruction and more cohesive and denser snow deposited over and stressing weaker and less dense layers)
That said, other factors that can and do produce increased danger and a more unstable snowpack include:
- Shallow snowpacks and cold temperatures (produce kinetic growth or squaring--typically occurs at the base of the snowpack where temperatures are warmest and temperature variations are strongest, but may also occur near crusts or other significant density changes)
- Clear skies, light winds and cold temperatures (favor surface hoar formation or near surface faceting)
- Cold temperatures (in deeper maritime snowpacks, associated faceting and weakening is concentrated near crusts or other layers with marked density changes)
- Variations in wind speeds and precipitation rates (may produce thin, weak layers where stress may concentrate)
Given a particular current stability or danger level of the snowpack and a set of weather conditions that are beyond our control, TERRAIN and TERRAIN MANAGEMENT become the most important factors determining our risk in such a situation. We can control and limit our movement through avalanche terrain in response to the changing danger level, minimizing our exposure where appropriate and "nibbling away" at the edges of avalanche paths or progressively steeper terrain in order to gain information about avalanche danger and snow stability.
SOURCES OF CURRENT SNOWPACK INFORMATION:
In the avalanche forecast product, NWAC forecasters distill snowpack information gathered from a variety of sources and try to paint a regional or local picture of snow layering and related stability in each day's detailed "Snowpack Analysis". This site specific snow profile and avalanche information comes from personal snowpack visits by forecast staff as well as other snow safety professionals and back country travelers. Professional ski patrols, highway avalanche control programs, park and forest rangers, ski guides and others are in relatively constant contact with NWAC staff throughout the winter in order to ensure that the best and most pertinent snow safety information is made available.