This chapter is much better talked about in Chapter 3 of the Avalanche Handbook, by David McClung and Peter Schaurer. I’ve combined Chapter 6’s notes with Chapter 3’s notes of the Avalanche Handbook. I have omitted anything already discussed in the previous chapters.
To understand when, where, and why avalanches form, it is necessary to understand the effects of weather parameters on snow and mountain topography. There are four primary weather parameters related to snow formation and mountain topography:
Wind Speed and Direction. Wind speed and direction depend on the balance of forces of the wind velocity. The horizontal component determines wind speed and direction, whereas the vertical component determines the amount, rate, and distribution of precipitation.
As discussed in Chapter 1, there are four mechanism which cause air to rise. The vertical rate of air ascent strongly depends on which mechanisms are responsible. The following table described the typical values for precipitation characteristic of major lifting types. As a rough estimate, the following percentages apply to winter precipitation totals for the mechanisms: 10% cyclonic, 30% frontal, 50% orographic, and 5% convection. These values will change with the season, with the mountain climate, and from year to year. In most cases, more than one mechanism is at play.
|Characteristics of Major Lifting Types (Typical Values)|
|Type||Vertical Wind Speed||Duration of Precipitation||Horizontal Scale|
|Cyclonic||1-10cm/s||Tens of hours to several days||1,000km|
|Frontal||1-20cm/s||Up to tens of hours||100km width, 1,000km length|
|Orographic||10-200cm/s||Up to tens of hours||10-100km|
|Convective||100-1000cm/s||Minutes to hours||0.1-10km|
Precipitation Patterns and Intensity. The local terrain features on a mountain have very important effects on precipitation patterns and intensity. Trees, rock, ridges, and mountain contours can all affect the distribution of windblown snow, which result in varied avalanche conditions across the slopes.
Typically, a pressure difference of 1mb results in an increase in wind speed of 14-18km/h over a ridge-crest. It is common for the compression effect on an exposed ridge-crest to exceed the free-air speed. Sharp breaks of slope can also cause more turbulence in the air passing over them, and this can cause the airflow to separate from the ground on the lee-side. Vertical eddies form in this instance, and a reversal of flow-direction at the snow-surface takes place. It is common that minor rolls or changes in slope angle may significantly affect the character and distribution of wind-deposited snow.
Cornices are created by the wind, forming on the leeward side of ridge crests where there is a steep change in slope angle. They grow outwards as an overhang of snow away from the direction of the wind. Cornices indicate the direction of the prevailing wind, as well as likely areas of wind deposition. Cornices are significant avalanche hazards for two reasons: cornices can collapse if you’re standing on top of them; and the weight of a fallen cornice could trigger an avalanche on the slope below.
Relative humidity is a consideration when estimating wind-loading potential. Low relative humidity may contribute to drier, low-density snow more susceptible to wind transport, whereas high relative humidity may decrease the likelihood of wind-loading.
Heat Exchange at the Snow Surface. Temperature influences the metamorphism of snow within the snowpack. Rapid fluctuations in temperature throughout winter have an immediate effect on snow stability, particularly if the temperature rises rapidly to 0oC. Prolonged cold temperatures create a strong temperature gradient within the snowpack, resulting in the growth of facets. Mild temperatures consolidate and strengthen the snowpack by promoting rounding, but may result in layers becoming future slabs.
An important example with respect to heat exchange at the snow surface via condensation is surface-hoar formation. Surface hoar forms when relatively moist air over a cold snow-surface becomes oversaturated with respect to the snow-surface. The results are feathery crystals (the ice-equivalent of dew) varying in thickness from 1mm to several centimetres, and once buried, can become a considerable weak-layer. Surface-hoar tends to form at night, in clear, cool, and calm conditions
Falling precipitation can also warm or cool the snow-surface, but the amount of heat exchanged is very small (for either rain or snow). More important than the heat-exchange between the snow-layers however, is the potential mismatch in layer properties between the layers. Rising temperatures in snowstorms can cause cold, unstable layers to be buried; and if rain freezes on the snow-surface or below it, additional heat will be realised by the freezing process.
Radiation at the Snow Surface. Radiation interacting with the snow surface is primarily of two basic types: Short-wave radiation from the sun, and long-wave radiation from the Earth and clouds. 99% of solar radiation is composed of shortwave radiation, and 99% of terrestrial radiation is composed of long-wave radiation. The changing balance between these two types of radiation is what’s responsible for quick temperature changes near the surface of the snow. The result may be a formation of weak layers by cooling, or avalanches by heating. In avalanche prediction methods, it is the radiation balance that matters most, not the individual components.
An example of combining the effects of warming and cooling by radiation is what avalanche workers call ‘radiation recrystallization’. It occurs on southern-facing slopes at low latitudes and high elevation in clear and calm weather. During the day, incoming radiation warms the first few centimetres of the snow-surface. Daytime cooling by loss of long-wave radiation then cools and produces very cold air near the snow-surface. This combination of events gives rise to a tremendous temperature gradient in the top few centimetres of snow, which result in rapid recrystallization of the snow-surface to form a weak layer of crystals. Sometimes the transmitted short-wave radiation can produce a melt zone several centimetres below the surface, which later freezes to form an ice crust under the weak layer of recrystalized grains. This combination makes an ideal failure surface when buried by subsequent loads.
A second example is when short-wave radiation simply melts and weakens the snow-surface, creating a sun-crust when it freezes overnight. When a sun-crust is buried, it can become a smooth sliding layer for avalanches. This is especially true if faceted crystals grow above or below it.
A third example is encountered during wet-avalanche formation. Under thin fog or low-cloud conditions (whiteout), sunlight can penetrate through to warm the snow-surface. Long-wave radiation is emitted by the snow-surface, but it is absorbed back by the clouds and re-radiated back again to the snow surface (referred to as the greenhouse effect). The result is a tremendous heat-input and meltwater production at the snow’s surface.
Once heat is applied or subtracted to the snow-surface, it is transferred within the snowpack primarily by two mechanisms:
The effective thermal conductivity of low-density snow (100kg/m3) is about 1/25 that of ice. Heat is transferred mainly though vapour diffusion. At a density of 600km/m3 however, the thermal conductivity increases tenfold to about half that of solid ice, making heat transferred mainly by conduction. In the usual range of densities of slab formation (between 100-400kg/m3), both mechanisms share the heat transfer.
An important point about heat transfer with respect to avalanches is that it is very slow. Avalanches brought about directly by conduction or vapour diffusion heat-transfers are usually only observed in very thin slabs less than 50cm thick. Most of the temperature-changes in snow are brought about by short and long-wave radiation.
The following table is designed to draw links between the primary meteorological factors and their influences on the snowpack. It is important to note that this is not a complete analysis of snow instability factors. Factors commonly used in evaluating snow instability, i.e. Class I – Instability or Direct Evidence Factors, are completely absent from the table, and Class II – Snowpack Factors, appear only in the context where snowpack properties are strongly influenced by the weather.
The rules are generalized, and need to be localized for any kind of practical application. They should be considered only as a starting point for analysis and discussions.
Trending more unstable
Trending more stable
|Precipitation||· Rapid loading
o Accumulation of ≥30cm (25mm SWE) in the past 24 hours
o ≥2cm/h (2mm/h SWE) for 10+ hours
· Gradual loading when there is a persistent weak layer present
· Localized convective precipitation
|· Deep snowpacks are often more stable than shallow snowpacks
· Consistent small or medium-sized storms that build the snowpack steadily but not too rapidly, provided there is no persistent weak layer
|Wind||· 25-50km/h = critical loading rate
· Lee areas behind exposed features
· Loading enhanced with presence of dry new snow
· Higher wind speeds (>50km/h) may cause wind slabs to form at tree-line or BTL
|· Windward scoured slopes typically become more stable, although subsequent faceting of shallow areas is possible
· Very strong winds may deposit less snow in alpine areas (caution: very high variability means you should approach this factor with caution)
|Temperature||· Rapid rise in temperature often promotes avalanche activity, particularly if it is the first warming following a major snowfall event
· Rapid cooling may cause instabilities under specific conditions, but is far less common than rapid warming
· Prolonged cold temperatures with shallow snowpack builds facets or depth hoar
· Melt freeze crusts are associated with weak layers
· Temperature inversions are often associated with surface hoar growth
· Storm trends that begins cold and end warm
|· Cold temperatures typically increase snowpack strength in the short term
· Prolonged warm temperatures promote rounding
· Melt-freeze crusts can increase strength provided they are not associated with a weak layer
· Storm trend: begins warm and ends cold
|Solar Radiation||· Loose snow avalanches can occur due to loss of cohesion
· Cold clear weather is associated with surface hoar growth
· May warm the upper snowpack significantly, even in mid-winter
· 1st day with intense radiation of the season (typically early February for mid-latitudes in the Northern Hemisphere)
· 1st day with intense radiation after a cloudy period
|· Melt-freeze cycles can promote longer-term stability
· Solar radiation during the day breaks down surface crystals