M o u n t a i nWe a t h e ra n dS n o w p a c k

^{portant role in densification of the snow layers above and}
below the surface hoar.
The surface-to-volume ratio of snow isinverselypro-
portional to grain size (taken as the average dimension),
andreflectsthegeneralmetamorphicstateofasnow
layer(Colbeck,thisvolume).Thevaluedecreasesas
grainsgrow,butispartiallyoffsetbycompaction.The
value also decreases as grains sinter,but is not important
as grain growth in the early stages.Figure 3bshowsthe
surface-to-volume ratio of the three layers over time. Over
the first half of the test period,this variable decreased by
about30%forthelayerabovethesurfacehoarand by
Figure 1. Measured shear strength, rutschblock and grain sizes^{about25%forthelayerbelow,whilethesurfacehoar}
plotted as time series with calculatedrutschblock score for surface^{only changed a few percent (Figure 3b).The large size of}
hoar layer buried 7 January 1995. Indicator shows time before whichthe individual crystalsresulted inthe lowvalue forthe
avalanches were observed._{surfacehoar.Thecombinationofmorerapid}
The heli-sking operation that surrounds the study site re-^{densification and greater loss of surface area of the layers}
portednumerousnaturalandskier-triggeredslabava-^{adjacent in the profile to the surface hoar shows that the}
lanches sliding on the January 7th surface hoar from 9-19^{surface hoarcrystals were relatively unaffected by meta-}
Januaryfollowedbyansingle avalanche on21January.^{morphismduringtheperiod,whichwassupportedby}
Noavalancheswerereportedtostartontheparticular^{the snow pit observations (Figure 1).}
surface hoar layerafter 21 January.
As shown in Figure 1, no substantial change in the size
ofthedisaggregated crystalswasobservedduringthe7
week period even though the shear strength changedsig-
nificantly.
Measurements of thickness ofthe buried surface hoar
layer from section analysis show the layer settled by about
20%overthefirst10daysoftheperiodJanuary17to
March 8.Afterthat the rate of thinning reduced signifi-
cantly. Figure 2 shows this trend and alsoshows the range
ofthicknessmeasurementsforeach sample date.While
thestandarddeviationseemedlow,therangebetween
minimum and maximum thicknessoverlapped substan-
tiallyforall dates and increased with time (Figure 2).
Densities and surface areas from the buriedsurfacehoar
and layers immediately above and below show differences^{Figure 3. Density (a) and surface-to-volume ratio (b) of layer of}
in grain growth and densification across the three layers.^{surface hoar buried 7 January 1995 (V), and layers above (U) and}
below (L).Measurements were made from section cuts.
Figure 3a shows snow densities of the buried surface hoar
layerandthe underlyingandoverlyinglayerswithin2
cm of the hoar. All three layers started with similar densi-Surface hoarlayerburied 28December1995
ties,followingtheusualtrendofagradualincreasein_{We tested this layernine times between 5 January 1996}
densitywith depth (U,V then L -Figure 3a).The snow_{and 22March 1996.Specimens were collected and pre-}
layers above and below the surface hoar became dense at_{served for subsequent microphotography on each test day}
a higher rate during the first half of the test period.Most_{except9January.Thesamepersonoperatedtheshear}
ofthedensificationofthesurfacehoarresultedfrom_{frame on each test day except for6 February.}
compaction of the layer,ratherthan changes to the crys-_{Figure 4 shows the averageshear strengthfrom the shear}
tals.On the otherhand,grain growth played a more im-_{frame tests,plotted along with the 95% confidence inter-}

^{vals. (These intervals increase on 12 March and 22 March}
when the numberof shearframe tests was reduced from
12 to 7.)Following the initiallow strength on 5 January
and 9 January, the strengthincreasedto 3.7 kPa on 18 Janu-
ary and varied between 4 and 6 kPa from 27 January to 22
March. The strength decrease measuredon 19 February is
consistentwith the densitydecrease measured on plane
sectionsfromthe same date.Such a decrease in density
and strength may be due to metamorphic factors or to test-
ing a particularly weak spot in the surface hoar layer (spa-
tial variability).
The rutschblock score calculated from the shear frame
Figure 2. Thickness of surface hoar layer buried 7 January 1995,^{tests reached5.5 on 18 January, so all the slab avalanches}
measured from section cuts._{occurred when the calculated rutschblock score was less}

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