perhaps 1,000,000 times per year, world-wide. Snow on an
incline adjusts to the pull of gravity. The vast majority of
these slides are not a problem, because an avalanche, in
and of itself, is not a hazard. A person (or a person's stuff)
has to get involved in order for there to be a problem.
The fact is, that avalanches don't drop from the peaks onto
the heads of unsuspecting innocents with the
unpredictability of a plummeting meteorite. 95% of people
who are caught in avalanches are caught by a slide that
was triggered by themselves or a member of their party. If
it is our behavior that is creating the hazard, then we can
change our behavior to avoid problems.
A rapid flow of snow down a slope, from either natural triggers or human activity.
Occurring in mountainous terrain, an avalanche can mix air and water with the descending snow.
Entrain ice, rocks, trees, and other material on the slope
Initiated in snow, are primarily composed of flowing snow, and are distinct from mudslides , rock slides , rock avalanches , and serac collapses from an icefall .
In mountainous terrain avalanches are among the most serious objective hazards to life and property
Snow is deposited in successive layers as the winter
progresses. These layers may have dissimilar physical
properties and an avalanche occurs when one layer slides
on another (Surface Avalanche), or the whole snow cover
slides on the ground (Full-Depth). An avalanche may be
Dry or Wet,according to whether free water is present in
the snow. It may be of Loose Snow, when the avalanche
starts at a single point or a Slab Avalanche which occurs
when an area of more cohesive snow separates from the
surrounding snow and slides out. In practice, any snow slide
big enough to carry a person down is important.
Terrain affects avalanche occurrence and development
through three factors:
1. Terrain affects the evolution of the snow pack by determining the meteorological exposure of the snow pack.
2. Terrain affects the stability of the snow pack, through the geometry and ground composition of the slope.
3. The down slope features of the terrain affects the path and consequences of a flowing avalanche.
Avalanches are classified by their morphological(structure)
characteristics, and are rated by either their destructive
potential, or the mass of the downward flowing snow. Some
of the morphological characteristics used to classify
the type of snow involved,
the nature of the failure,
the sliding surface,
the propagation mechanism of the failure,
the trigger of the avalanche,
the slope angle,
Avalanche size, mass, and destructive potential are
rated on a logarithmic scale , typically made up of 4 to 7
categories, with the precise definition of the categories
depending on the observation system or forecast region.
All avalanches have common elements as :
A trigger which causes the avalanche,
A start zone from which the avalanche originates,
A slide path along which the avalanche flows,
A run out where the avalanche comes to rest, and
A debris deposit which is the accumulated mass of the avalanched snow once it has come to rest.
A failure layer that propagates the failure and
The bed surface along which the snow initially slides, in most avalanches the failure layer and the bed surface are the same.
The nature of the failure of the snow pack is used to
morphologically classify the avalanche.
Slab avalanches - are generated when an additional load causes a brittle failure of a slab that is bridging a weak snow layer; this failure is propagated through fracture formation in the bridging slab
Loose snow, are generated when a
stress causes a shear
Point release failure in a weak interface,
either within the
Isothermal avalanches snow pack, or at the base
When the failure occurs at the base they are known as full depth avalanches.
Spin drift avalanches occur when wind lifted snow is funneled into a steep drainage from above the drainage.
Crown fracture at the top of the start zone,
Flank fractures on the sides of the start zones, and
Shallow staunch fracture at the bottom of the start zone.
The crown and flank fractures are vertical walls in the snow
delineating the snow that was entrained in the avalanche
from the snow that remained on the slope. Slab avalanches
account for around 90% of avalanche-related fatalities, and
occur when there is a strong, cohesive layer of snow known
as a slab. These are usually formed when falling snow is
deposited by the wind on a lee slope, or when loose ground
snow is transported elsewhere. When there is a failure in a
weak layer, a fracture very rapidly propagates so that a
large area, that can be hundreds of meters in extent and
several meters thick, starts moving almost instantaneously.
2.LOOSE SNOW AVALANCHES
Occur in freshly fallen snow that has a lower density and
are most common on steeper terrain.
In fresh, loose snow the release is usually at a point and
the avalanche then gradually widens down the slope as
more snow is entrained, usually forming a teardrop
ISOTHERMAL AVALANCHE- (wet snow avalanche),
Occurs when the snow pack becomes saturated by water. These tend to also start and spread out from a point. When the percentage of water is very high they are known as slush flows and they can move on very shallow slopes.
Powder snow avalanches
Is a powder cloud that forms when an avalanche
accelerates over an abrupt change in slope, such as a cliff
band, causing the snow to mix with air. This turbulent
suspension of snow particles then flows as a gravity current
the largest and most powerful of among avalanches –
exceed speeds of 300 km/h, and masses of 10,000,000 tonnes;
travels long distances along flat valley bottoms and even up hill for short distances.
The information provided on temperature, wind speed and
direction often enables useful predictions to be made before
leaving home.For instance, if a SW wind of 25mph is
indicated with freezing temperatures and snow known to be
lying, then it may be assumed that some avalanche hazard
will be building on NE - facing slopes
1. Explosives – Ski Resorts
4. Earth Mounds
5. Snow Nets
Terrain Management - Practice safe route finding skills,Distinctions between geographic areas, elevations, slope aspects and slope angles are approximate and transition zones between dangers exist,be aware of changing conditions.
Shovels - digging snow pits as part of evaluating the snow pack
Avalung - consists of a mouth piece, a flap valve, an exhaust pipe, and an air collector
Other Equipments - Emergency Position-Indicating Radio Beacon (EPIRB) or Personal Locating Beacons (PLBs) containing the Global Positioning System (GPS).
Avalanches are formed by a combination of 3 things that together are known as the " Avalanche Triangle". These 3 ingredients may be present in one location but absent 10 feet away. The 3 legs of the triangle are Snowpack, Terrain and Weather .
Avalanche victims are almost exclusively backcountry recreationists--snowmobilers, climbers, snowboarders, snowshoers, skiers and hikers. Snowmobilers lead the list with twice the number of fatalities as any other activity.