As man enlarges his activities in mountain regions these natural
juggernauts pose more of a problem. Ten phenomena that give rise
to them have been found, suggesting methods for their prediction
by Montgomery M. Atwater
SLAB-SNOW AVALANCHE occurred at Alta, Utah. Slab is a type of
wind-packed snow which
commonly forms on lee slopes. This slab fractured to a depth of 10 feet, starting a
slide with the
cloud of snow which belies the power of this type of avalanche.
LOOSE-SNOW AVALANCHE was photographed at Murren, Switzerland.
The billowing cloud
of snow is characteristic of this type of slide. Piling up on the snow in the
valley below, the
avalanche left a mass which had to be broken up with pickaxes.
The snow avalanche is one of the great natural
forces, not inferior in destructiveness to the tornado, earthquake or flood. The only
reason it is not so well recognized as a killer and destroyer is that it takes place in
winter in remote mountain fastnesses where it less often hits human targets. But during
World War I a series of snow slides on the Austro-Italian front killed 10,000 soldiers
within a single day-one of the worst natural disasters in recorded his history. During the
Gold Rush snow avalanches buried many of the prospectors who swarmed into the mountains of
the Northwest. In 1910 a snow slide in the Cascade Mountains of Washington swept away
three stormbound trains, with the loss of 108 lives and more than a million dollars in
Today the avalanche problem is greater than ever, because we are providing the tidal waves
of snow with ever more numerous and valuable targets: railroads and all-year highways,
pipe lines and power lines, reclamation projects, logging operations and mines. Within the
last decade skiing has lured into the mountains a greater horde of people than Gold Rush
days ever saw.
Avalanche research is therefore a matter of more than purely academic interest. Perhaps
the most striking recent illustration of its practical importance is the fact that
transcontinental television did not become a reality until a way was found to solve an
avalanche problem at the site of a microwave relay station in Nevada. The safety of
increasing numbers of people and enterprises depends upon avalanche studies.
Either as a research subject or as a problem in the field, avalanches are elusive. The
forces at work are numerous and difficult to measure. Avalanches can not readily be
subdivided, reconstructed or educed to laboratory scale. They are best observed in their
native habitat, and this is an occupation something like trailing a wounded African
There are several different types of avalanche. Although that circumstance is of minor
significance to a person caught in one, it is the starting point for investigating the
forces that produce slides. Analysis of these forces has led to the art--it would be
presumptuous at this point to call it a science--of forecasting avalanche hazards.
To begin with, we can divide all avalanches into
two general classifications: loose snow and packed snow. The two classes are basically
different in the way they start and develop and in the kind of hazard they pose.
An avalanche of loose snow always starts on the surface from a point or a narrow
sector. From the starting point it grows fanwise, expanding both in width and depth. The
speed and nature of its development depends on whether the snow is dry, damp or wet. If
the snow is dry, its particles are quickly pulverized and form a cloud of frigid snow
dust, so that the avalanche travels as much in the air as on the ground. It moves at high
speed. Unless it is very large, this kind of avalanche is not particularly destructive,
but a person caught in one can die of suffocation.
An avalanche of loose snow which is damp or wet stays on the ground and moves more
slowly. Its mass is many times greater than that of a dry avalanche and it is much more
destructive. The slow-moving but enormously heavy wet avalanches of spring are noted
destroyers of property. I have seen one, not unusually large, completely dismember a
modern reinforced-concrete bridge.
On the other hand, a loose-snow avalanche, wet or dry, spends itself quickly and the
hazard is soon over. When the slide has run its course, the snow stabilizes in place.
Avalanches of packed snow behave in an altogether different manner. This kind of slide
is released suddenly as a great, cohesive slab of snow. It may originate either at the
surface or through the collapse of a stratum deep within the snow pack. It starts on a
wide front with penetration in depth. The place where the slab has broken away from the
snow pack is always marked by an angular fracture line roughly following the mountainside
contour. I have seen one such fracture edge that was more than a mile long and up to eight
In a packed-snow avalanche the main body of the slide reaches its maximum speed within
seconds. Thus it exerts its full destructive power from the place where it starts,
whereas a loose-snow avalanche does not attain its greatest momentum until near the end of
its run. Moreover, slab snow does not necessarily slide immediately after weather
condition have made it unstable. The slab may lie in an unstable condition for days, weeks
or even months, during which it may be triggered at any time into an avalanche. For these
reasons the slab avalanche is the most dangerous of all types. A series of slab avalanches
may stabilize conditions only locally, leaving the slab on an adjacent slope as lethal as
an unexploded shell.
There are also, of course, combination avalanches composed of both loose and packed
snow. Naturally they are more difficult to analyze, and they lead to argument of the
which-came-first variety. A slab which might otherwise have remained in place can be
released by the moving load of a loose-snow avalanche; on the other hand, a slab avalanche
may carry with it a volume of stable loose snow even greater than that of the slab itself.
Usually it is possible to determine which part of the combination acted as the trigger and
which as the main charge of the avalanche.
A loose-snow avalanche usually occurs during or immediately after a storm or other weather
situation that creates instability. A slab avalanche may come as a delayed action. In any
case, every avalanche must have a trigger. The idea that it may simply materialize at
random is not acceptable. There has to be some final nudge, some force or combination of
forces, to account for the release of these masses of snow at a particular time and place.
The engine of your car may be rated at 200 horsepower, but it cannot deliver an erg of
energy until you press the starter.
SLAB FRACTURE and a subsequent avalanche may be triggered by one of
Such a fracture usually occurs at the sharpest part of a convex slope. The plane of
cleavage of the
fracture is characteristically perpendicular to the snow surface.
We recognize four avalanche triggers: overloading,
shearing, temperature and vibration.
How overloading operates to trigger an avalanche is fairly obvious. Weight simply piles
up until it overcomes cohesion-a final straw breaks the camel's back. Shearing can be
applied in various ways: a skier cutting across a slope, a wad of snow falling out of a
tree or over a cliff, a small slide of snow exerting a shearing effect on the layers
beneath. Temperature plays its part by its effect on the cohesion of snow: a rise in
temperature weakens the bonds, while a fall in temperature retards settlement of the snow
mass and increases the brittleness and tension of a slab. Vibration is related to
shearing, but it is treated separately because, unlike the other triggers, it can operate
at long range. Avalanches have been released by thunder, by explosions and by other loud
or sharp sounds--vibrations transmitted through the air. They may also be started by
vibrations transmitted through the earth and snow from heavy machinery or blasts.
Avalanche control work with ex plosives frequently starts secondary avalanches at some
distance from the detonation point.
The most common triggers are overloading and temperature. There is nothing, of course,
to prevent any of them from working in combination. And one may argue that there is really
only one trigger: shearing. But at this stage that view seems an oversimplification.
As in the case of other natural forces we know more about the overt behavior of
avalanches than we do about their basic nature. Theoretical snow mechanics is an abstruse
and controversial subject. Nonetheless we can arrive at some practical ideas about
avalanches, just as we get along with electricity with out fully understanding its nature.
Fundamentally we can reduce the causes of avalanches to two essentials: snow and a
grade for it to slide on. Most avalanches originate on slopes of 80 degrees or more, so we
can consider this factor a constant. The variables are the condition of the snow and the
weather. Forty-three years ago observers of the U. S. Weather Bureau, after a study of a
disastrous series of avalanches in the Northwest, made an acute observation which has
become the basis for modern techniques of avalanche hazard forecasting. In brief, the
observation is that the hazard depends not only on the quantity of snow but also on the
manner in which it fell.
Snow comes in a variety of forms, from the familiar crystal flakes to pellets. In size
the particles range from flakes as big as a silver half dollar to motes barely visible to
the naked eye. The moisture content varies from less than one twentieth to more than one
half of an inch of water per inch of snow. The consistency of snow ranges from a dry,
flour-like powder to a gluey slush. All of these variations affect its stability. And it
is constantly changing from one state to another.
AVALANCHE CONTROL may be accomplished in some cases by from
its anchorage by the blast.
Cornices are related to slab snow, the use of pre planted charges. This snow
cornice is lifted entirely
but build up along ridges at right angles to the wind direction.
Whether snow can maintain its position on a slope is determined by its cohesion versus the
force of gravity. That force is calculated in terms of the weight of the snow (W) and the
angle of the slope (X); stated as a formula, gravity equals WsinX. If this quantity is
equal to or greater than the cohesion of the snow, the snow is theoretically unstable.
Unfortunately the prediction problem is not quite so simple as this formula. Cohesion
varies with time and place, and so does the weight of snow, which is blown and drifted by
the wind. Thus cohesion and weight measurements made on one slope do not necessarily apply
to a neighboring one. Moreover, the cohesion factor itself is loaded with variables. We
have to consider not only the cohesion between particles within a layer of snow but also
the cohesion between layers, which may slide over one another if the interlayer cohesion
is low or may be locked in place if it is high. Finally, the formula does not take trigger
action into account. Thus it is impossible to predict exactly where and when avalanches
will occur, and even the forecasting of hazardous situations is a difficult art.
Of course snow changes after it has lain on the ground, and this too complicates
matters. For example, the wet avalanches of spring are the product of destructive
metamorphosis due to deep thawing and will occur regardless of the original nature of the
snow. Again, the action of wind on snow picked up from the surface is a large factor in
producing delayed-action slab avalanches.
A decade of observation in the U. S. has
identified 10 factors which contribute to the avalanche hazard. First, there is the depth
of old snow on the slope. If there are two feet or more, that is generally sufficient to
cover ground obstructions so that it becomes easier for new snow to slide over them;
further more, the deeper the snow, the more ammunition it supplies to the avalanche.
Secondly, the character of the old snow surface plays its conflicting part: a loose
snow surface promotes good cohesion with a fresh fall but allows deeper penetration of any
avalanche that starts while a crusted or wind-packed surface means poor cohesion with the
new snow but restricts the avalanche to the new layer.
The third factor is the depth of the new fall: 12 inches is regarded as the minimum
generally necessary to produce by itself an avalanche of dangerous proportions.
The fourth factor is the type of the new snow, especially its free moisture content.
Free moisture acts as a cement and improves cohesion, within limits. But good cohesion may
be dangerous as well as helpful, for it may enable the wind to pile up greater masses of
snow for release in an avalanche. The amount of free moisture in snow can be gauged by a
simple test: squeezing a handful in the gloved hand. Dry snow will not pack; damp snow
packs readily; wet snow be comes waterlogged and slippery.
The fifth factor, not quite the same thing as the preceding, is the total water content
of the snow, i.e., the ratio of water to snow. Here the most significant circumstance is a
departure from the norm; for example, dry snow types nor normally average 5 to 8 percent
water, but when the proportion of water in such snow exceeds 10 percent, we have a clear
warning that its weight may be in creasing faster than its cohesion. We have recorded one
storm in which the ratio was 28 percent. Other factors being favorable, the outcome was an
avalanche cycle of extraordinary violence.
MULTIPLICITY OF FACTORS which produced a series of unusually
violent avalanches at Alta,
Utah, were plotted for a storm on January 17 to 19, 1953. The solid blue line
indicates the accumulated
snow depth over the 42 hours recorded. The solid gray line represents precipitation
measured mechanically from the snow core, which was unsatisfactory according to the
intensity record made by tipping bucket gauge as shown by broken blue line. The
decrease in old snow
surface is indicated by the solid black line. Wind and temperature records are
shown on separate
horizontal coordinates below. The occurrence of individual avalanches on the
specific peaks names is
marked by the blue arrows drawn through all the factors effecting them.
Sixth, there is the intensity of the snowfall, measured in inches of snow per hour.
When the snow piles up at the rate of an inch or more per hour, the pack is growing faster
than the stabilizing forces, such as settlement, can take care of it. Moreover, this
sudden increase in load may fracture a slab beneath, just as a quick blow will snap a
brittle stick which could resist the same amount of pressure if it were applied gradually.
The seventh factor is called precipitation intensity: it is the actual amount of water,
measured in inches per hour, being deposited as snow. This measurement gives a combined
image of the type, water ratio, quantity and intensity of the snowfall, plus some
indication of the temperature and wind action. It is the most promising single guide to
avalanche hazard yet discovered. The techniques for observing it are new. To date we have
made precipitation intensity studies on some 30 mountain storms, about half of which
resulted in avalanches of dangerous proportions. On the basis of these observations, we
have concluded that with a continuous pre precipitation intensity of one tenth of an inch
of water or more per hour and wind action at effective levels the avalanche hazard becomes
critical when the total water precipitation reaches one inch.
The eighth factor is wind action, and it
is the most important and versatile of all. It overloads certain slopes at the expense of
others; it grinds snow crystals to simpler and less cohesive forms; it con constructs
stable crust and fragile slab, often side by side. Warm wind-the chinook of North America
and the foehn of Europe-is as effective a thawing agent as rain, more effective than
sunlight. By sudden changes in direction and velocity wind can act as a shearing trigger
on a layer of snow it has just deposited. Finally, it is essential to the formation of
slab in ways which we do not yet completely under stand. An average velocity of about 15
miles per hour ( in the mountains air currents are so erratic that they can only be
sampled) is the minimum effective level for wind's action in building avalanche hazards.
Ninth, there is temperature, which directly influences the snow type. Dry snows
normally fall at 25 degrees Fahrenheit and below. Temperatures above 28 degrees promote
rapid settlement and metamorphosis of the snow-sometimes too rapid. A sudden rise of
temperature causes a loss of cohesion fast enough to trigger an avalanche. A sudden drop
increases the tension, particularly in slab. The gradual warming of the temperature in the
spring leads to cumulative deterioration of the snow and to heavy, wet avalanches.
The tenth factor is settlement of the snow, which goes on continuously. With one
exception it is always a stabilizing factor. The exception is the shrinkage of a loose
snow layer away from a slab above, thus robbing the slab of support. In new snow a
settlement ratio less than 15 percent indicates that little consolidation is taking place;
above 80 percent, stabilization is proceeding rapidly. Over a long period ordinary snow
layers shrink up to 90 percent, but slab layers may shrink no more than 60 per percent.
Thus abnormally low shrinkage in a layer in indicates that a slab is forming.
These contributory avalanche factors are all variables, and their relationships are
complex. The closest we can come to a categorical hazard formula is the precipitation
intensity factor. This does not tell us the exact moment when the trigger will be pulled,
but it tells us some thing even more important: whether the gun is loaded.
In short, snow avalanches obey mechanical laws; we can identify and evaluate the forces
at work. But avalanche re- searchers are in the same situation as those people who fly
into the eye of a hurricane or take the pulse of a volcano. We know the information we
would like to have. Getting it and bringing it back intact can be difficult.
SPECIFIC INSTRUMENT developed for avalanche research is
the penetrometer. Simply a graduated rod driven into snow by a
weight, it yields data for estimating snow cohesion.
BATTERY OF INSTRUMENTS used by U. S. Forest Service for
avalanche studies includes two different precipitation intensity
gauges (rear) and snowfall intensity gauge (front).