M o u n t a i n

We a t h e r

a n d

S n o w p a c k

TABLE 2.Snow climate averages

of these sites during the short period of record available.
Although the database is not long at any site,there exists
alarge difference inlength ofrecord betweenthe sites.
This makes comparisons between the areas and snow cli-
mates difficult.
Measurements at the snowstudy plots cannot always
be transferred to starting zones because of elevation dif-
ferences and wind effects.Elevation differences may be
accounted for if local data on precipitation variation with
elevationexist.Wind oftentransportslargeamountsof
snow into starting zones,therefore the slabthicknessat
study plots probably are not representative of thickness at
the crown in some avalanche starting zones. This mis-rep-
resentationisprobablysmallerinlargeavalanche start-
ing zones where wind erosion and deposition effects tend
to cancel one anotherthan in small starting zones.
The following conclusions probably can be drawnfrom
the data analyzed in this paper:

• Dry "storms," as defined in thispaper,do tend tobe
  longerand of lesserprecipitation intensity in the con-
  tinental areas;
• Although the maritime storms have much larger aver-
  age precipitation intensities(forfewerdays),they do
  not appearto result in significantlythickerordenser
  dry slabs;
• The "design, 100-year" dry-slab avalanches may there-
  fore be similarin thickness in the various climate ar-
  eas;
• Slabdensities,appeartobelessthan200kg/m ³ ,on
  average, in the majorstorms;this issignificantlyless
  thanthe300kg/m ³assumedasa"default"valuein
  standard Swiss avalanche-dynamics calculations.

THE FEBRUARY, 1986 STORMAT MAMMOTH,
CALIFORNIA

This storm probably had a much longer return period than
theotherstormsanalyzed. Thisstormproduced a larger
numberofextremely large,long-return-period avalanches
in theeasternSierraNevada(from Mammoth Ski Area north
to Alpine Meadows Ski Area) thanany other storm to have
beendocumentedthiscentury.Many oftheavalanches,judg-
ing from forest destruction andthe agesof trees, had return
periodsofmore than 100years.The return period ofthe
stormisnotknown, butmayalso beon the order ofone
century.Theeight-day stormproduced 695mmofwater
equivalent(87mm/dayaverage), resultedin asnowpack

depth increase of 2.82m with an average density of 247kg/
m³ . No other storm listed in the Westwidedataapproached
the magnitudeof the Mammoth storm of 1986.
The relativelyhigh elevation(2,700m) of the snow study
plotatMammothensuredthattemperaturesremained
belowfreezingthroughoutthestormand allofthe pre-
cipitation was snow. Other sites in the Sierra, such as Al-
pine Meadows at 2,100m,recorded large amounts of rain
with the stormtherefore did not produce highly mobile,
dry-slab avalanchesin spite of high precipitation amounts.

REFERENCES

de Quervain, M., 1975, Avalanche formation,inU.S. Forest Serv-
ice,1975,AvalancheprotectioninSwitzerland:U.S.Depart-
ment of Agriculture, Forest Service, Rocky Mountain Forest and
Range Experiment Station General Technical Report RM-9, p.
6-18.

McClung,D.M.,1990, Amodel for scaling avalanchespeeds:
Journal of Glaciology, v. 36, no. 123, p. 107-119.

McClung, D.M., 1984, Statistical avalanche zoning, Proceeding
of the International Snow Science Workshop, p. 95-98.

Mears,A. I.,1992, Snow avalanchehazardanalysis for land-
use planning and engineering, Colorado Geological Survey Bul-
letin 49, 55 p.

Salm,B., Burkard,A., andGubler, H.,1990, Berechnungvon
FliesslawineneineAnleitungFurPraktikermitBeispielen:
MitteilungendesEidgenossischenInstitutsfurSchnee-und
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