C a s e

H i s t o r i e s

lanche:they stayedbetween about-18ºCand -4ºC.Cold
air moved into the areaon February 1 and the temperature
cooledto-20ºCbythemorningofFebruary2.Figure5
showsthetemperatureand relativehumidity trendsfor
the 145-hourperiod from January 1 to February 2.
Certainlythe strong windsand heavy January snows
played a significant factorin the magnitude of the Over
the Rainbow avalanche, but other weather factors earlier
in the season might also have made a significant contri-
bution.
FromDecember15-30,highpressuredominatedthe
Southern Rockies. At Loveland Basin the averagetempera-
ture during this period was -14ºC,and forthose 15days
windsaveraged only3.9m/s(s=±2.3).Only3.5cmof
snow fell during theperiod. Cold temperatures, lightwinds
and generally clearskies fueled surface hoar and kinetic-
snow metamorphismin theupperportion of thesnowcover.
Avalancheworkers throughoutColorado reported extensive
surfacehoardevelopmentonallaspects.Thelargest
crystals formed on north-facing slopes.

SNOWPACK CONDITIONS

SnowpitsduginearlyNovembernearthestartingzone
showedashallowandstrongsnowpack.Snowdepths
ranged from0.40to0.80m.Mostofthesnowpackcon-
sisted of small,rounded grains perched on a 3.5-cm-thick
rain crustthat had formed in late October. The only weak-
ness in the pack was in the 10 cm between the rain crust
and thegroundwhere cohesionlesskinetic grains (2-5 mm)
wereforming.Theaverage temperaturegradientofthe
snowpack was 16ºC/m.
Hastysnowpitsinthesameareasin Novemberand De-

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cembershowedthe snowpacktobelosingstrength.Air
temperaturescontinuedtheirseasonalcooling,butthe
snowdepth rarelyroseabove 1maswinds scoured the
area. The clear, calm and cold conditions in late December
causedtheentiresnowcover(about1m)tochangeto
faceted crystals.The weakest snow was at and just below
the surface where surface hoarcrystals and large faceted
crystals formed.
When71 cm of new snow fell over 2 days at the start of
January widespread natural and triggered avalanches oc-
curred. Smallavalanches ranon top of the old snow, while
largeandlong-runningavalanches,steppeddownand
releasednear the ground. On January 1, low in the track of
theOvertheRainbowpath,snowfellfromatreeand
releaseda small soft slab in thick conifers. Though small it
ranasurprisingdistancewithdebrisspillingontothe
Rainbow ski run. Thesnow slid on a thick (1.5 cm) layer of
surface hoar crystals.

TERRAIN

The Over theRainbow avalanchepathis situatedat treeline
at an elevationof 3603 m, immediately east of the Loveland
Basin Ski Area. The starting zone is a shallow depression
covered by baregroundandtalus. The pathis stair-stepped
in the starting zone and track, creating several very small
potential starting zones within the path. Historically ava-
lanches had always stopped before,orjust into,the thick
conifers on the bench at about 3,414 m feet. At about 3,353
mtheslopesteepens and becomestheRainbowskirun
that ends at the south edge of the ski area's parking lot.