Snow and Avalanche Mechanics

Avalanches and Snow Climate

Avalanches are falling masses of snow that can contain rocks, soil, or ice (McClung 1993).

Avalanches affect people and damage property, but these effects are fairly minimal when compared to ‘real’ severe weather.

While avalanches are interesting and their dynamics are still not well understood, there is little justification to fund any major research programs since their influence is limited in area and affect only tourism, recreation, and some transportation.

Avalanches and Snow Climate

Trend in avalanche fatalities in US is increasing.

This is due in part to increased recreation, i.e., backcountry skiing and snowmobile use.

Fatalities in Alps, on the other hand, are greater due to the larger populations in those areas.

Avalanches can have a larger impact on populations by destroying communication and power transmission lines.

(McClung 1993

Avalanches and Snow Climate

Avalanches are formed by a combination of snow layering and weather elements interacting with the snowpack.

The most destructive avalanche cycles are caused by direct loading of snowfall form synoptic-scale weather systems.

(McClung 1993

Avalanches and Snow Climate

Snow Climates:

Maritime Snow Climate- characterized by relatively heavy snowfall and relatively mild temperatures.

Snow cover (depth) is deep, rain may also fall anytime during winter.

Maritime snow covers are often very unstable with rapidly fluctuating instability.

Ex: Cascades-US, Coastal Mtns British Columbia, Western Norway. (McClung 1993

Avalanches and Snow Climate

Snow Climates:

Continental Snow Climate- characterized by low snowfall, cold temperatures, and locations inland from coastal areas.

Snow covers are typically shallow and often unstable due to persistent structural weaknesses.

Rocky Mtns-US, Brooks Range of AK, Pamirs of Asia.

Failures in old snow is a distinguishing feature of a continental snow climate.

Effects of wind on snow

The redistribution of snow by wind is a major feature of mountain snowpacks and it is essential for avalanche formation in some cases (McClung 1993).

Blowing snow is reserved to describe particles raised to a height of about 2 m or more.

Blowing snow often obscures visibility.

Drifting snow (90% of transported snow) is used to describe near-surface transport.

(McClung 1993

Effects of wind on snow

McClung 1993

Effects of wind on snow

The critical wind speed (threshold wind speed) at which snow is picked up from the surface by turbulent eddies of wind.

1. Threshold wind speed increases with increasing temperature and humidity.

2. If original deposition occurs with wind, particles will be broken into small pieces and will pack to a higher density to subsequently increase threshold speed.

3. Threshold speed will increase with time since deposition (due to bond formation between surface grains). Increase will slow with time and is slower at colder temps.

4. Threshold speed will be much lower if there is a source of particles such as new snowfall, a low strength layer at the surface or snow on trees.

Effects of wind on snow

For loosed unbonded snow, the typical threshold wind speed (at 10 m AGL) is 5 m/s. for a dense bonded snow cover, winds greater than 25 m/s are needed to produce blowing snow.

Blowing snow will occur with modest winds whenever snow is falling.

(McClung 1993c

Effects of wind on snow

Three modes of transport for wind-redistributed snow.

1. Rolling involves the creeplike motion of dry particles along the surface (depth 1 mm).

2. Saltation occurs as particles bounce along the surface in a layer about 10 cm deep, dislodging other particles as they impact the surface. Saltation is initiated with winds of 5 – 10 m/s over cold loose snow.

3. Suspension is caused by turbulent eddies lifting particles up to tens of meters above the surface.

(McClung 1993

(McClung 1993

Effects of wind on snow

Effects of wind on snow

In mountainous regions, snow redistribution is uneven because it is strongly influenced by local topography, including vegetation, rock outcrops, etc.

(McClung 1993

Lee Slope Deposition: Avalanche and Cornice Formation

On the lee side of alpine ridge crests, where a sharp change in slope angle occurs, cornices and avalanches deposits may form due to formation of eddies by flow separation.

The windward slope angle is thought to be critical in determining whether a cornice or snowdrift will form.

As snow is redistributed, the particles become broken and abraded as they impact snow surface. Upon deposition, become tightly packed and rapidly produce a slablike texture as they bond together.

Lee zones usually collect greater amounts of snow then nearby wind protected valleys locations. (McClung 1993

Lee Slope Deposition: Avalanche and Cornice Formation

Cornices usually form on ridge crests but they can form at any place where a sharp change in slope angle occurs.

Threshold wind speed for cornice formation and growth is about the same as for transport over loose cold snow (5 to 10 m/s).

For winds in excess of 25 m/s, studies have shown that cornices can decrease in size due to windward scouring of the root.

(McClung 1993

Lee Slope Deposition: Avalanche and Cornice Formation

Lee Slope Deposition: Avalanche and Cornice Formation

Cornices have three features important for avalanche safety:

1. An overhanging cornice provides a quick assessment of the prevailing wind direction in a mountain range from a distance

2. The steep, lee area below the face of a cornice is itself a prime area for unstable snow slabs to form.

3. The overhanging face of a cornice on a ridge crest can and often does collapse.

Avalanches are often triggered by cornices collapsing.

Cornices may form on both sides of a ridge.

(McClung 1993

Cornice Formation

Photographer unkown

Heat Exchange at the Snow Surface

The exchange of heat between the snow surface and the atmosphere is important for avalanche formation for both wet and dry snow.

Heat exchange can alter surface snow to produce weak snow there, which may fail or it may act as a future failure layer when subsequently buried.

Heat can enter or leave the snowpack surface by conduction, convection, or radiation.

Heat may be transferred to and from the snowpack by turbulent exchange-sensible heat.

If air is warmer than snowpack, heat is added to the snowpack.

Heat Exchange at the Snow Surface

If surface is warmer than air, heat is lost from the snowpack.

Warm air flowing over a snowpack can result in significant surface warming (foehn wind).

Heat may also flow to and from the snow surface by condensation resulting from diffusion of water vapor.

Direction of heat flow is from regions of high water vapor concentration to regions of low concentration.

Since saturated warm air can hole more H2O than saturated cold air, flux of heat (and H20) is from regions of high temperature to low temperature.

(McClung 1993)

Heat Exchange at the Snow Surface

Surface Hoar Formation

Surface hoar forms when relatively moist air over a cold snow surface becomes oversaturated with respect to the snow surface causing a flux of water vapor, which condenses on the surface.

The result is feathery crystals (ice/solid equivalent of dew) varying in thickness from 1 mm to several cm.

Once buried, by new snow, results in a weak layer.

Surface hoar forms at night when the snow surface generally cools and adjacent air becomes supersaturated.

Surface Hoar

Heat Exchange at the Snow Surface

Surface Hoar Formation: role of katabatic flows.

There has been a long research interest in determining the role of katabatic flows on surface hoar formation (Colbeck 1988; Hachikubo and Akitaya 1997, 1998).

Hachikubo and Akitaya 1997 investigated the role of wind speed on surface hoar formation.