Chapter 6: Weather and Mountain Snowpack

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.

Mountain Weather

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;
• Precipitation patterns and intensity;
• Heat exchange at the snow surface; and
• Radiation at the snow surface.

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.

• The Horizontal Component. Near the Earth’s surface there is a strong frictional component acting to slow the wind and alter its direction. Over flat terrain this frictional effect is largely absent, however most mountain ranges cause the wind to blow at an angle to the isobars. In some cases, mountain ranges can form barriers to surface-level winds, which result in surface-level air blowing parallel to the isobars. Wind-speed sensors are often placed on ridge tops, mountainsides, or in valleys, where they provide measurements of winds influenced by these surface frictional forces. Wind speeds are also typically increased with height; and through vertical compression as air is pushed against the mountains. On the leeward side of mountains, winds tend to decelerate. Key mountain winds to consider are katabatic winds, outflow winds, and chinook winds.
  • Outflow Winds: Cold, dense, arctic air that pools in the interior of BC and the Yukon, eventually spilling into coastal valleys. These winds can reach 100km/h, usually confined to the valley bottom.
  • Katabatic Winds: They can result in unusual and unexpected wind loading patterns
  • Chinook Winds: Strong, warm air on lee sides that can promote instability in recently fallen snow, wind slabs, and loose snow avalanches at lower elevations.

• The Vertical Component. The precipitation rate is approximately proportional to the vertical component of wind velocity. Positive wind velocities results in cloud formation and precipitation, whereas negative wind velocities result in dissipated clouds and clearing skies. Also, because a warm parcel of air can potentially hold more water vapour prior to condensation than a cold parcel of air, the amount of moisture wind contains and its initial temperature are crucial factors for determining the amount 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.

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.

• Precipitation Patterns. Wind loading occurs most effectively at wind speeds of 30-50km/h, and at these speeds snow accumulates on the leeward side of terrain features; primarily in open and exposed terrain. Under ideal loading, snow can accumulate via wind-loading 10 times faster than snow falling from the sky. At higher speeds, wind-blown snow may sublimate into the atmosphere; or be redeposited below avalanche starting zones. Windblown snow is denser, stiffer, and may feel hollow underfoot.

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.

• Snowfall Intensity. The addition of load through precipitation is the primary factor in avalanche formation and release. Avalanches will occur once a critical rate of load greater than the weakest layer strength is achieved.
• Snow-Water Equivalent (SWE). Snow Water Equivalent is a measurement of the equivalent water content in a certain volume of snow. It represents the actual load added to the snowpack. New snow in drier climates tend to be less than 10% to water, and in maritime climates often greater than 10%. If the SWE of 30cm of new snow is 30mm, it is around 10% water. Measurements of SWE requires a precipitation gauge.
• Snow Crystal Size and Type. The addition of large crystals like stellar dendrites on a snowpack can result in immediate loose-snow avalanches. These low-density layers can then become a future weak-layer once buried. In contrast, wind-blown smaller crystals with their tips rounded-off may quickly form into denser stiff layers where a slab may form. Determining the density of new snow is a useful measurement of the likelihood of a slab formation, with the snow density for slab formation in the range of 100-400kg/m³. Snow of less than 60kg/m³ is considered low-density snow.

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 0^oC.  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.

• Heat can enter or leave the snowpack surface via conduction, convection, or radiation. Heat-flow by conduction is negligible with respect to the other mechanisms, as the thermal conductivity of air is extremely low.
• Heat may be transferred to and from the snowpack by turbulent exchange (called sensible heat) due to wind eddies. Warm and moist air flowing over the snowpack can result in significant surface warming by this mechanism (a chinook wind, for example).
• Heat may also flow to and from the snowpack by condensation, resulting from the diffusion of water vapour. In this case, the direction of heat flow is from regions of high water vapour concentrations to regions of low water vapor concentrations. Since saturated warm air can hold more water vapour than cold air, the flux of heat and water vapour is from regions of high temperatures to low temperatures.

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.

• Short-wave Radiation. When short-wave radiation strikes the snow, up to 90% of it is reflected back into space when the snow surface is dry. This percentage decreases to at least 80% when the snow surface is wet. The intensity of short-wave radiation also decreases exponentially with snow-depth. For dry alpine snow with the density typically of that found in slab avalanches (100km/m³), it is estimated that less than 10% of solar radiation remains after a distance of 10cm. For fresh, finely-grained snow, this distance decreases to a few centimetres. For wet, coarsely-grained snow, this distance can be 10 cm or more.
• Long-wave Radiation. While solar radiation provides heat input to the snow-surface, long-wave radiation from the Earth and clouds can either heat or cool the snow surface. The porousness of a snow-surface closely approximates a black-body radiator to long-wave radiation with an estimated 50% of long-wave radiation absorbed right at the surface, therefore it does not penetrate more than about 1 cm. The snow surface also continuously gives off long-wave radiation, thus on a clear day in mid-winter, it is expected that the Southern-facing snow-slopes may be warming (short-wave radiation exceeding long-wave radiation), whereas the Northern-facing snow-slopes may be cooling (long-wave radiation exceeding short-wave radiation). During a clear night, almost all long-wave radiation leaving the snow-surface can be expected to escape into space, and it is common to find the snow-surface 5^oC to 20^oC cooler than the air above.

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.

Penetration of Heat into Alpine Snow

Once heat is applied or subtracted to the snow-surface, it is transferred within the snowpack primarily by two mechanisms:

1. Conduction, through the network of ice grains and bonds; and
2. Vapor diffusion, through the air spaces between the ice grains and bonds.

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.

Influences of Meteorological Factors on Snowpack Instability

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.