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 fall.During winter1994-95we did alotofexperi-
mentswithburiedloadcells,inordertoincreasethe
numberofexperiments.Asnowblockwascutout,the
load cell placed,and then the snow blockcarefully reset
onto the load cell. No significantdifferencescouldbe found
between the two placement methods forthe snow condi-
tions found (Schweizer et al.,1995b).
Foreachexperiment the load procedure by the skier fol-
lows partly the procedure of the rutschblock test: (1) stand-
ing atop, (2) weightingseveral times (four or five), (3) jump-
ing several times. Usually a single loadcell was loadedcen-
trally andthedifferent loadsteps wereappliedsuccessively.
For eachloading the recording time was 20 s.
Before each experiment the snow thickness on the load
cellandduring theexperiments theskipenetrationfor
each load step was measured, so that the depth of the load
cell relative to the skieris wellknown.The difference in^{Fig. 2. Skier and load cell after load step jumping.The load cell was}
partly dug out to show the measurement configuration. Notice the
penetrationdepth betweentwoloadingstepsmayyield^{concave snow surface due to compaction. For scale: the load cell's}
informationabouttheenergyabsorbedforcompaction.^{dimension is 50 cm.}
Each set of experiments was completed with a snow cover
profile,includingsnowdensity,grainshape,grainsize,^{(28March 1996)there was a crustnearthe surface and}
snow temperature, snow hardness index (hand hardness)^{below soft snow with thin crusts in between (mean den-}
and liquid water content.^{sity:210kg/m3and hand hardness index of crust:pen-}
cil).Theinitialdepth wasforbothexperiments35cm.
RESULTS^{The final depth was 11 cmforthe softsnow conditions}
An example of an experiment is shown in Fig.1 and de-^{and 18 cm for the hard snow conditions.The decrease of}
scribedindetailinthe following.On21February1995^{the measured additional normal forces in hard snow for}
the load cell (#2) was put onto the snow surface. Twodays^{the load steps standing atop and weighting is large, 60%}
laterthe cell was covered with a few centimetres of new^{and 80%,respectively.There are two reasons forthe de-}
snow. During the following snowfall period 59 cm of snow^{crease.First,the forcetransmission,hard layershave a}
were accumulated on the load cell; the cell was loaded on^{bridging effect. The impact is spread out within the hard}
27 February 1995.Forthe load step standing atop the av-^{layer, and effectinto depth is reduced. However, hard lay-}
erage additionalnormal force wasabout90N.The pen-^{ers spread the impact overa larger area than soft layers.}
etration depth was 27 cm, so that the distance between the^{Second, thereis less ski penetration in hard snow than in}
load cell and the skis was reduced from59 to 32cm.For^{softsnow.The largereffective depth means smallerim-}
the load step weighting the measured additional force was^{pact forces atthe weaklayer(load cell)depth.Because}
about 220 N (mean of five peaks) and the mean peak width^{the crusts were broken after the first jump, the decrease is}
0.20s.Theadditional penetrationdepth wasonly3cm^{less important forjumping (less then 20%) and is prima-}
and the distance between skisand load cell decreased to^{rily due to the different effective depth.}
29 cm. Between eachweighting the value of the additional
force of the load step standing atop is not reached, because
weightingcausedasnowcompaction concentratedjust
below the ski binding,so that the snow surface below the
skisgotconcave and the contact between skis and snow
got worse during standing (Fig. 2). The same happenedfor
theloadstepjumping,butthiseffectisnotvisible,be-
cause jumping increased the ski penetration additionally
(from 29 to 21 cm), so that the load for standing increased
as well. The maximal additional load was about 380 N for
the first jump and increased to about 920 N(fifth jump).
This increase is not only the result of the depth decrease,
butalsoofthebetterforcetransmissionduetosnow
compaction.Theimpactoccurredinaveryshorttime
(mean peak width 0.05 s).Typical for jumping is the dou-
ble peak,shownalso in Fig.1.The first,smallerpeak is
due to the weighting done just before the jump.
Fig.3 shows the impact fordifferent snow conditions.
Fortheexperimentperformedon20February1995the^{density: 180 kg/m 3and mean handhardness index: fist or 4 fingers)}
snow layerabove the load cell wascharacterised assoft^{was performed 20 February 1995 and the initial depth was 33 cm.}
snow(partlynewsnow/decomposingparticles/rounded^{The one withhard snow (crust above soft snow, mean density: 210}
kg/m ³and hand hardnessindex of crust: pencil) performed on 28
grains),mean density:180kg/m³andmeanhand hard-^{March 1996 and the initial depth was 35 cm.}
ness index:fistto4fingers).Forthe second experiment

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