S n o w

C o v e r

S t a b i l i t y,

A v a l a n c h e

I n i t ia t i o n

a n d

F o r e c a s t i n g

Now if,in addition,warmertemperatures laterreach the
weaklayer,failuretoughnessandstrengtharereduced
implying reducedstability since less work is required dur-
ing deformation to achieve failure and the failure strength
is reduced. All three effects (reduced stiffness in the slab
and weaklayer stiffness and reducedfailuretoughnessand
strength) canwork together to reduce stability in this case.

Delayed effects

Warmingtheslab initiallywill increasethecreepratewhich
will slowly increase the slab density and hardness. By the
logic above, stability will slowly increase.In addition,meta-
morphismrateswillincreasecausingbondformation,
hardness andstrength increases.Also,surfacewarming
usually reducesthe overall snowpacktemperaturegradient
to slow or stop formation of facets or depth hoar to increase
stability.

SKIER TRIGGERING AND SNOW TEMPERATURES

Consider now the case of skier triggering with the immedi-
ate effects of snow temperatures on stiffness, failure tough-
ness and strength. Whena skier moves over a snowpack a
dynamic loadis impartedto thesnow cover andthestresses
penetratethroughtheentiredepthof the snowcover (Föhn,
1987).In orderto failsnow in a weak layer,a skierwill
have to impart significant deformation to that weak layer.
It is not possible to fail snow (or any other material) without
significant deformation even if stresses are very high.
Measurementsof snow deformation imparted by skiers
(Schweizeret al.1995a,b)showthatthe mostimportant

variable with respect to the penetration of significant de-
formation is the snow hardness. Harder layers permit less
deformation at depth thansofter layers therebymaking the
chanceof failure less. Furthermore, by theargumentabove,
snow hardness is strongly temperature dependent. Figure
2showsschematicsofslabhardness variationsandthe
relationtoskiertriggeringderivedfromtheresultsof
Schweizer et al. (1995a).
Combining the results above, increasing snow tempera-
tures can immediately decrease snow slab stability impor-
tantly in two ways when a skieris present: 1.by decreas-
ingstiffnessofsurface(slab)layerssignificantdeforma-
tion may penetrate deeper in the snow covertoincrease
weaklayerdeformationandalloweasierfailure(more
deformation) and propagation and 2. if warming tempera-
tureslaterreachtheweaklayer,failuretoughnessand
strength are reduced allowing easier failure.Of these two
effects,weconsiderthefirsttobethemostimportant,
particularlysincea persononskisisdirectly incontact
withthesurfacelayers.Furthermore,surfacelayersare
subject to great variations in temperature and snow hard-
ness is affected immediately. Therefore, one canexpectthe
stability to vary greatly when conditions are right depend-
ingonthehardnessofthesurfacelayersandtheir
fluctuating temperatures. For example, on a cold morning
or northerly aspect when surfacelayers are cold and hard,
deformation underskis will not penetrate as deep and as
effectively as later on or at other placesif surface layers are
warmer. Furthermore, the effect on stability is immediate:
thereisnorequirementfordelayedeffectssuchas
metamorphism orcreep (settlement) effects to take place.

STABILITY TESTS

The results above have important implications aboutin-
terpretations of stability tests in particular theRutschblock
test. Stability tests provide some of the most important data
elementswithrespect toevaluationofinstabilityinthe
snowcover(McClung and Schaerer,1993).Theyshould
beusedasmuchaspossibletocollectinformationto
evaluate potential sources of instability.Below we briefly
discuss some ofthe testsand the related effects ofsnow
temperatures and hardness.

Rutschblock test

The Rutschblock test is often favoured because it results
in loading the snowpack by a skierand the sample size is
large. The testincludes the effects of the slab properties
and is directly related to snow stability.It implicitlytests
the surface layer hardnessand its propensity for deforma-
tion penetration.However, theextrapolationis actuallyvery
complex.Todotheextrapolationitisofparticular
importance to considerthe slab properties,including the
hardness structureofthe slab,which can varyconsider-
ablynotonlyspatiallybutalsorapidlyintimeaslayer
temperatures change.For example, it is possible to go from
a condition indicating stability to one of instability as sur-
face layers warm to allow deeper penetration of deforma-
tion and henceeasier failure and propagation. For the ex-
trapolationitisthereforeimportant tocompletethetest
with a snow profile and follow and quickly assess the slab
properties (hardness of surfacelayers) at different locations
and overtime.

Figure 2: Schematic of influence(not to scale) of deformation
imparted by a skier (a) for soft, warm slab properties (b) for hard, cold
slab properties. In (a) deformation penetrates deeper whereasin (b)
deformation does not penetrate as deep.

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