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

Snow Temperature Effects According to Time Required and Relation to Stability _______________________________________________ Immediate Effects:No Time Required _______________________________________________ 1) StiffnessorHardness:Warming decreases stiffnesssignificantly;stabilitydecreased 2) Failure Toughness: Warming decreases failure toughness;stabilitydecreased 3) Strength:Warming decreases strength slightly;stability decreased _______________________________________________ Delayed Effects:Time Required _______________________________________________ 1) Bond Formation (Metamorphism): Warming increases bond formation rate and strength; stability increased 2) Creep:Warming increases creep rates,causing settlement, densification,increased strength and hardness; stability increased 3) Temperature Gradient: Warming usually causes decreased temperature gradient with crystal formchanges and increased strength; stability increased ________________________________________________ Summary:For warming, immediate effects promote instability;delayed effects promote stability.Underwarming, instability islikely tocome from immediate not delayed effects.Strength effects may be immediate (decrease) or delayed (time dependentwith increase) under warming with the greatest strength changes being delayed.

Table 1: Snow temperature effects accordingto time andstability

diatetemperaturedependence ofstrengthisnothighly
significant in shear testing.
What emerges is that, significant strengthincreases by
metamorphism require considerable time to take place for
a buried weak layerthat is warmed. Thus, except for new
snow instabilities, such strength changesare usually slow.
Similarly,exceptfornewsnowinstability(where
densificationisrapid), creep and itseffects onhardness
also require considerable time forsignificant changes.
On the otherhand, snow stiffness (hardness),and fail-
ure toughness are affectedimmediately. It is these proper-
ties which aremost affected when air temperatures change
to affect surface layers which overly weak layers. It is also
these properties that are of primary concern forskiertrig-
gering since they can rapidly decrease the snow stability
when buried weak layers are present. Furthermore, of the
three, snow hardness is the most significantly affected by
changingsnow temperatures (as opposed to failure tough-
ness orstrength).
Perhaps the most significant result from Table1 is that
the immediate effects promote instability and the delayed
effects promote stability under warming. Therefore, in the
followingsectionswe willemphasize theimmediateef-
fectssinceforskiertriggeringthe concernismostoften
with the present instabilityand its variations.

76

SNOW SLAB MECHANICS

The key to understanding slab avalancherelease involves
considering both the slab and the weak layer, not in isola-
tion, but as interactive, dependentelements. Furthermore,
it is much more useful to think about the problem in terms
of deformation instead of stresses (e.g. calculationof a sta-
bilityindex) particularly when skiertriggering is consid-
ered.

Immediate effects
Considerfirst,theslabreleaseprobleminrelationto
changes insnow temperatures,forexamplebywarming
airtemperatures.Initially,warmingwillaffectthe slab
only (itis assumed the weak layerhas not yet felt the ef-
fects) and the hardnessof the slab is reduced. For this sce-
nario,it has been shown (McClung,1996) that stabilityis
reducedas slab stiffness is decreasedeven thoughtheweak
layeris unaffected by temperature changes. The primary
reasonis that conditionfor fracturepropagationin theweak
layer contains the slab stiffness (or modulus) and reduced
stiffnessimpliesenergeticallyeasierpropagation.The
analysis(McClung,1996)showsthatsnow slabstability
canbeimportantlyreduced bywarmingsnowtempera-
tures in the slab without the weak layerbeing affected by
the warming. Sincesnow stiffness (hardness) is highlytem-
perature dependent,the effect is probably the most impor-
tant temperature effect forskier triggering.