On contributory factors in avalanche hazard evaluation1
R. I. PERLA
United States Forest Service, Alta Avalanche Study Center, Alta, Utah
Received February 11, 1970
(**We apologize to the author and readers of this paper for the image
Using 20 years of storm and ramsonde profile data, the probability of avalanche hazard
to the Alta Highway and Village is plotted as a function of ten contributory factors.
Results indicate that the hazard probability is related strongly to precipitation and wind
direction. Certain factors previously thought to be important (wind speed, settlement, and
ramsonde profiles) do not seem important for the south facing slopes and large avalanches
tested in this study.
Atwater (1954) proposed ten contributory factors for avalanche hazard
evaluation. Table 1, constructed from early investigations, shows the
original contributory factors with corresponding quantitative and qualitative criteria
favorable for avalanching at Alta, Utah, and Loveland-Berthod, Colorado. There are two
important reasons for further exploration of contributory factors:
(1) They are conceptually simple. This is important in a field keyed to subjective
interpretation of data which contain relatively large amounts of scatter.
(2) They can be studied by means of nearly 20 years of data from Alta and other areas.
Pedagogy would suffer if we excluded this backlog of information.
Analysis of Storm Reports and Snow Profiles
In order to test and draw conclusions con concerning a set of contributory factors we
have analyzed 107 storm reports and 87 ram profiles prepared by the Alta Snow Rangers
during the period l950 to 1969. The seasonal interval of investigation has been selected
as 15 December to 31 March, which represents a compromise of narrowing the interval
without seriously depleting the data. Atwater suggested that solar radiation be considered
as an additional factor. There have been no continual records made of this factor at Alta
so we have removed the predominance of radiation effects by cutting all April storms from
this investigation . Available data did not permit analysis of all the original
contributory factors. Instead we have analyzed the following set which is supported by
data and hence reflects those factors considered important by personnel stationed at Alta:
(1) Total precipitation
(2) Average precipitation intensity
(3) Maximum precipitation intensity.
(4) Gross amount of new snow.
(5) Settlement of new snow.
(6) Wind speed.
(7) Wind direction.
(8) Maximum temperature change during any storm interval.
(9) The precipitation intensity factor devised by Atwater which we designate the
(10) A "ram ratio" (snow load/ram strength)
Factors 1 through 9 are meteorological observations for an arbitrary and not clearly
defined period of precipitation, loosely referred to as a "storm" by avalanche
personnel. Factor 10 is the structural observation of the snowpack for the same
First, scatter diagrams were prepared of the actual hazard that developed on slopes of
common aspect versus each of the factors. Figure 1 shows the
scatter diagram for the factor, total precipitation. We have elected to work with
slopes that pose a serious problem to the Alta Highway and Village, the analysis could be
repeated for slopes that threaten the ski area. Because of their southern aspect
these slopes do not form extensive layers of depth hoar which tends to dominate the
evaluation problem. A numerical value of the hazard is obtained by summing the magnitudes
(see U.S. Forest Service vice Handbook 1961) of slab avalanches that ran during and
immediately after the storm period. All slab avalanches, natural and artificial , of
magnitudes 3, 4, and 5 are summed. The inclusion of artificial releases can be thought of
as a margin of safety. Magnitudes 1 and 2 are omitted since these do not reach the highway
Next, hazard probability diagrams were prepared for each factor (Figs.
2 to 11). Each scatter diagram was divided into 4 or 5 approximately equal intervals
on the abscissa. Two probabilities were calculated for each interval: the probability of
any hazard of magnitude 3 or greater, P3, plotted as solid lines;
and the probability of a hazard of 10 or greater, P10, plotted as
dashed lines. More specifically, from an interval on a scatter diagram
From Figs. 2 to 11, we note that the shape of P3 can fall into one
of two groups: either P3 can show a general trend to increase as the
factor increases, or P3 can oscillate about 0.5. We designate the
former shape as "first order" and the latter as "second order". The
same distinction can be made for P10 except the oscillation is
about 0.2 for "second order".
Thus, we observe the following:
(1) With respect to P3, the factors with first order effects are 1,
2, 3, 4, 7, and 9; and with second order effects 5, 6, 8, and 10.
(2) With respect to P10, all factors with the exception of 5 and 6
have first order effects.
(3) Both P3 and P10 are more sensitive to maximum
precipitation intensity than to average precipitation intensity.
(4) Wind direction is a better factor than wind speed.
(5) For P3, the WNW maximum may reflect competition between lee
effects and orographic effects. The Wasatch Range is oriented north-south.
(6) For P10, a rising temperature during the storm indicates an
increase in the hazard.
The "Atwater Number" (Fig. 10) has a numerical value equal to the amount of
water, expressed in cm, which has been deposited for a continuous period at a rate of 0.25
cm h-1. For P3, the Atwater Number has too flat
characteristic to be especially useful. However for P10, a transition
from 3.8 cm to 6.3 cm seems to be a meaningful indicator of high hazard. It may be
possible to base the Atwater Number on wind direction rather than wind speed. Also, if the
lower limit on precipitation intensity is reduced to 0.125 cm h-1, then the
Atwater Number may have more sensitivity for southern slopes at Alta.
Figure 11 is based on ram profiles taken at the Alta Study Plot2 during the
period 1954 to 1969. For each profile on record, a maximum value of the "ram
ratio" was determined by trial and error. The ram ratio corresponding to a
particular storm is found by linear interpolation of the profiles taken before and after
the storm. For P10 this factor has only a small first order effect when applied
to slopes and profiles of southern aspect. The situation may be quite different on slopes
of northern aspect.
The results of this study on large avalanches which release on the south facing slopes
above the road and village at Alta, Utah, indicate that the probability of an avalanche
hazard varies considerably with precipitation and wind direction, only slightly with
temperature change, and seems to have no definite relationship to wind speed and snow
settlement. The strong dependence of the hazard probability on the maximum precipitation
intensity of the storm suggests that the alpine snowpack is sensitive to the rate of
stress application. It appears that the Atwater Number, which expresses the combined
effects of precipitation intensity, quantity of precipitation, and wind, should be
modified to account for wind direction instead of wind speed. The hazard probability and
the ratio, snow load/rarn strength, have only a small relationship.
It would be interesting to see how the foregoing compares with results from other areas,
not so much as to specific values, which we would expect to differ, but with regard to
shapes of curves and whether or not a factor is important.
1 Presented at the Journal, 7, 414 (1970)
2 The Alta Study Plot, located on a flat portion of the valley (elevation m)
only partially describes conditions in the fracture zone of southern exposed slopes
(elevation 3700 m).
This study was undertaken by the A1ta Avalanche Study Center, Wasatch National Forest, as
part of the administrative studies program for avalanche forecasting and control. Guidance
given to the author by Dr. E. R. LaChapelle is gratefully acknowledged. The author
is also indebted to Snow Ranger Will Bassett for this thorough collection of storm records
during the period 1967 to 1969.
ATWATER, M. M. 1954. Snow avalanches. Scientific American, 190, No. 1, pp.
BORLAND, W. M. 1954. Investigation of snow conditions causing avalanches.
Interim Report No. 3, U.S. Army Corps of Engineers, Denver, Colorado.
U.S. FOREST SERVICE 1953. Avalanche handbook. United States Department of
Agriculture, Washington, D.C. Out of print.
_________ 1961. Snow avalanches: A handbook of forecasting and control measures, United
States Department of Agriculture, Agriculture Handbook No. 194, Washington, D.C.
Fig 1. Avalanche hazard to the Alta, Utah, road and
village versus total precipitation of storm period
Fig 2. Hazard probability versus total
precipitation. P3, solid line. P10, dashed line.
Fig. 3. Hazard probability versus average precipitation
Fig. 4. Hazard probability versus maximum precipitation
Fig. 5. Hazard probability versus gross new snow.
Fig. 6. Hazard probability versus new snow settlement.
Fig. 7. Hazard probability versus average wind speed.
Fig. 8. Hazard probability versus average wind
Fig. 9. Hazard probability versus maximum temperature
Fig. 10. Hazard probability versus the Atwater Number.
Fig. 11. Hazard probability versus the ram ratio.