|
 freeze layering andstrengtheningof the snowpack may
occur.Such layering reduces the abilityof the snowpack
to quickly fluidize as a dry-flowing avalanche and reach
high velocity.
The precipitation periods called "storms" in this paper
were not necessarily storms in the meteorological sense.
If a continuous precipitation period satisfying the condi-
tions listed above occurred, it was consideredto be a storm
evenifthatperiodconsistedofmorethanonedistinct
source foratmospheric moisture.The emphasis here was
to view storms in terms ofdeveloping new, dry slabson
the snowpack.
In some cases the storms analyzed here are known to
haveproduce majoravalanchesInothercases itisnot
knownifmajoravalanchesoccurredorwerereported.
Avalanchesmaynothaveoccurredduringsomeofthe
stormperiods analyzed because the new snow layerwas
strongly bonded to the old snow. I have not attempted to
correlatethe stormswithactual avalanche occurrences.
The objective was todetermine what slab thickness and
density each climate areahas producedduringa dry, cold,
precipitation period and to compare each area.
ANALYSIS OF THE DATA
Two parameters were analyzed in each area: (1) HW = the
total waterequivalent of the new snow, and (2) DH = the
height increase of the snowpack during the storm(depth
after the storm minus depth before the storm). From these
data the meanslab density,rwas computed from the rela-
tionshipr= HW/DH.This relationship tends tosystem-
atically overestimate mean density and underestimate the
thickness of the new snow layer. The weight of new snow
tendstopushthe oldsnow/new snowsurfacecloserto
theground.Thissystematicerror,becauseitappliesto
|
datacollected fromall 11sitesand18storms,does not
affect the validity of the comparison between areas.
Table 1 lists the 18 storms analyzed in each of the 11
areas.Column 1 - 3 list the location,month of the storm
and the duration of the storm in days. Column 4 provides
the average temperature from the beginning to the end of
thestormperiod.Column 5isthetotalsnowfall,meas-
ured as the[!]24-hourtotals.Column 6 is the total snow
waterequivalent (HW) and column 7 is the average pre-
cipitationrateperdayofthe storm.Column8issnow-
pack depth increase (DH) which is taken as the slab depth,
and column 9 is the average slab densityr=HW/DH.
Surprisinglylittlevariationinaverage slabthickness
and density occurred between the snow climate areas, as
indicatedinTable2wherethesevenmaritimestorms,
eight continental storms, and three intermountain storms
arecompared.The majordifferences betweenthe snow
climates was found to be in stormduration and average
daily precipitation intensity.The majordry storms were
of much shorter duration and of higher precipitation rates
during the big maritime vs.continental storms.
STUDYLIMITATIONSANDDISCUSSION
This study is limited by the short data base used. The pe-
riod of record rangedfrom only 3 snow seasons at Elkton,
Colorado to 46 snow seasons at Alta, Utah with an overall
averageof25years.Thefourcontinentalsiteshadthe
shortest periods of record, with anaverageof only 13 years.
Applicationofencounter-probabilitycalculations
(LaChapelle,1966) indicates that a "100-year" return-pe-
riodeventhas onlya39%chance ofoccurring ina 50-
yearperiod and has only a 22% chance of occurring in a
25yearperiod.There existsahighprobabilitythatthe
major dry-slab formation would not have occurred atany
|
|
1
2
3
