Avalanchas

CAUTION – AVALANCHES!

Edited by:
Swiss Federal Institute for Snow and Avalanche Research SLF, Davos • Federal
Office of Sports Magglingen (BASPO) • Swiss Alpine Club SAC • Swiss Army
(Cen exce A mtn tng) • Swiss Ski

With support of:
MeteoSwiss • Friends of Nature Switzerland (FNS) • Swiss Council for Accident Prevention bfu • Swiss Commission for the Prevention of Accidents onSnowsport Runs SKUS • Swiss Air Rescue (Rega) • Swiss Foundation for
Alpine Research (SFAR) • Association of Swiss Mountain Guides (ASMG) •
Swiss Cableways (SCW) • Swiss Snowboard Association (SSBA) • Swiss
Snowsports • Swiss Association of Mountaineering Schools (SAMS) • Rescue
Organisation of the Canton of Valais (KWRO/OCVS)
To be obtained from: the Editors

Authors: Jürg Schweizer (SLF), StephanHarvey (SLF), Bruno Hasler (SAC),
Martin Hepting (Cen exce A mtn tng), Walter Josi (BASPO), Hansueli Rhyner
(Swiss Ski)
Concept: faktorplus.ch Graphics: Barbara Regli

30.80.1050e

Fifth edition: 2005

Avalanche formation
Types of avalanches (dry or wet)

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Snow slab avalanche

Terrain
critical steepness:
(steepest part of
starting zone)

Loose snowavalanche

30° for dry snow slab avalanches
(less than 30° for wet snow avalanches)

Snow slab avalanches
Cohesive snow on top of a weak layer: The slab breaks away as a unit. Dry
snow slabs are the most dangerous avalanches for winter recreationists. They
are also possible when the surface layers seem to consist of cohesionless
powder snow.
Avalanche danger prevails above all with:
- newsnow combined with wind
- rapidly and distinctly rising temperatures
- weak layers within the snow cover

Beware of runout zones!

Avalanche release (spontaneously or artificially) by either an
increase in stress (e.g. by a skier) and/or a decrease in strength
(e.g. by warming or rain).

The first sunny day after a snowfall period is
particularly dangerous.

Wet snow avalanchesTowards spring the snow cover looses its strength due to gradual soaking:
increasing danger of wet snow avalanches.
If the snow surface is distinctly refrozen, following a clear night, mostly
favourable conditions prevail before midday. Pay attention after midday and
generally at any time when the sky is overcast.
Pay attention to daily evolution!

Avalanche hazard evaluation: «3x3»-methodConditions: Weather and Snow
1. Trip planning
Trip with alternatives and
time-schedule

Terrain

Human factors

• Avalanche bulletin
• Weather forecast
• Additional information: from hut
warden etc.

• Map
• Guide book
• Photographs
• Key passages
• Measure steepness (map 1:25'000)
• Alternatives
• Time-schedule

• Who will participate?
• Group size
• Technique and fitness
•Equipment
• Responsibility

• Precipitation, critical new snow
depth
• Alarm signals
• Wind, wind-driven snow
• Air temperature
• Visibility
• Clouds
• Weather trend
• Snow cover conditions

• Is my perception correct?
• Steepness
• Aspect
• What is above/below?
• Near ridge?
• Topography/relief
• Forest

• Who is in my group?
• Check of equipment and
transceivers
• Who elseis travelling?
• Frequently check the physical
condition of members and the
time-schedule

• Critical new snow depth
• Snow quality
• Recent accumulations of winddriven snow
• Radiation, temperature
• Visibility
• Frequency and extent of previous
skiing activity

• Steepness, shape, location (e.g.
distance to ridge), size of slope
• Rocky outcrops
• Elevation and aspect
• Dangerof fall and of burial
• Detour possibilities

• Physical and mental fitness
• Technical ability
• Group size
• Discipline
• Leadership

«Which tour is possible?»

2. Assessing the local hazard
Route selection and
alternatives
«Is anything unexpected?»

3. Assessing specific slopes,
key passage
Trail selection
Safety measures, turning
back
«To go or not to go?»

Avalanche...