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A HANDBOOK OF FORECASTING AND CONTROL MEASURE!

Agriculture Handbook No. 194

January 1961

FOREST SERVICE

U.S. DEPARTMENT OF AGRICULTURE
Ai^

AGRICULTURE HANDBOOK NO. 194 JANUARY 1961

SNOW AVALANCHESJ

A Handbook of Forecasting and Control Measures k

FSH2 2332.81

SNOW AVALANCHES

:}o

U^»TED STATES DEPARTMENT OF AGRICULTURE

FOREST SERVICE
SNOW AVALANCHES

FSH2 2332.81

Contents

2

2.1

2.2

2.3

2.4

2.5

2.6

3

3.1

3.11

3.12

3.2

3.3

3.4

4

4.1

4.2

4.3

4.4

o

5.1

5.11

5.2

5.3

5.4

INTRODUCTION

CHAPTER 1

AVALANCHE HAZARD AND PAST STUDIES

CHAPTER 2

PHYSICS OF THE SNOW COVER

The Solid Phase of the Hydrologie Cycle

Formation of Snow in the Atmosphere

Formation and Character of the Snow Cover

Mechanical Properties of Snow

Thermal Properties of the Snow Cover

Examples of Weather Influence on the Snow Cover

CHAPTER 3

AVALANCHE CHARACTERISTICS

Avalanche Classification

Loose Snow Avalanches

Slab Avalanches

Tyi)es

Size

Avalanche Triggers

CHAPTER 4

TERRAIN

Slope Angle

Slope Profile

Ground Cover and Vegetation

Slope Orientation

CHAPTER 5

AVALANCHE HAZARD FORECASTING

The Contributory Factors

Factor Analysis

Delayed-Action and Climax Avalanches

The Ram Profile

Wet Snow Avalanches

CHAPTER 6

STANDARD SNOW OBSERVATIONS AND

TERMINOLOGY

6.1 Snow Study Chart

6.2 Storm Plot and Storm Report Records

6.3 Snow Pit Studies

6.4 Time Profile

CHAPTER 7

7 SNOW STABILIZATION

7.1 Test and Protective Skiing

7.2 Explosives

7.21 Hand-Placed Charges

7.22 Artillery

CHAPTER 8

8 SAFETY

8.1 Safety Objectives

8.2 Safety Principles

8.3 Safety Regulations

8.31 Personnel

8.32 Avalanche Test and Protective Skiing

8.33 Avalanche Blasting

8.34 Exceptions to Safety Code

CHAPTER 9

9 AVALANCHE DEFENSES

9.1 Diversion Barriers

9.2 Stabilization Barriers

9.3 Barrier Design Factors

9.4 Reforestation

CHAPTER 10

10 AVALANCHE RESCUE

10.1 Rescue Equipment

10.2 Immediate Action

10.3 FoUowup Action

10.4 Avalanche Accident Report

10.5 Case Histories

CHAPTER 11

11 AREA SAFETY PLANNING

11.1 Terrain Analysis

11.2 Climate Analysis

11.3 Hazard Classification

11.4 Operations Plan

BIBLIOGRAPHY

For sale by the Superintendent of Documents, U.S. Government Printing Office,

t , Washington, 25, D.C. — Price 60 cents
2332.81 SNOW AVALANCHES

Introduction

The first publication by the U.S. Forest Service

dealing with snow avalanches, "Alta Avalanche

Studies," appeared in 1948. This booklet reported

the results of experiences and studies at Alta,

Utah, where the problem of snow safety for the

skiing public had confronted forest officers since

the area had been established 10 years previously.

The study program at Alta continvied, and experience was rapidly gained in methods of

avalanche control which made possible increasing

use of excellent alpine slopes without compromising public safety. Study stations similar to that

at Alta were also established at Stevens Pass in

Washington and at Berthoud Pass in Colorado,

where the investigations and tests of control

methods could be extended to encompass a wider

range of climatic conditions. In 1952 the experience of forest officers working in these areas was

compiled and published as the "Forest Service

Avalanche Handbook." This was designed to be a

manual of instraction for foresters confronted

with snow safety problems following the rapid

postwar expansion of skiing and development of

new ski areas on national forest lands. It also

served as a general text on the nature, characteristics, and formation of snow avalanches. The

extensive appendix included a report on avalanche

studies and research programs then in progress.

The "Forest Service Avalanche Handbook" was

the only text in English on this subject widely

available in the United States, and it was soon

accepted by the public for use far beyond that

for which it was designed. The training program

of the National Ski Patrol System alone generated

a heavy demand for it. Requirements of other

groups, such as ski instructor associations, plus

mterest of the general skiing and mountaineering

public, soon exhausted stocks at the Government

Printing Office.

Eight years have passed since the first "Forest

Service Avalanche Handbook" was published.

During this period much practical experience has

been gained, both through study programs and

through the ever-widening practical application

of snow safety measures. Many techniques proposed or described as innovations in 1952 are now

routine procedures used by Forest Service snow

rangers. For example, the use of artillery (fig. 1 ),

which was barely out of the experimental stage

in 1952, has now become a widely accepted and effective means of avalanche control in the Western

United States. On the other hand, the basic

principles of snow mechanics and avalanche formation, and the commonsense rules for safe conduct in avalanche terrain have not changed,

though an understanding of snow mechanics increases with passing time and longer experience.

This new handbook provides an opportunity to

bring forest officers up-to-date instructions on the

most modern methods of avalanche hazard forecasting and control, and to recast the general text

on avalaiiches in a more informative and concise

pattern. This handbook is intended as a textbook,

as a training aid, and as a field manual for the use

of forest officers and others whose duties involve

avalanche hazard.

Directly, or indirectly, many persons and organizations have contributed to this handbook.

We acknowledge a particular debt to our European

colleagues for material on the physics of snow and

avalanche defense structures.

h

à M '

F-495188

FIGURE 1.—105 mm. recoilless rifle mounted on a Forest

Service truck. Tliis is a highly mobile weapon for

avalanche control.
2332.81 SNOW AVALANCHES

Chapter 1

1 AVALANCHE HAZARD AND PAST

STUDIES. The remote mountain areas of

America are the scene of hundreds, perhaps thousands, of avahxnches each winter. No man witnesses them, and tliey have no measurable effect on

civilization. But in certain locations, which are

increasing in number, avalanches and mankind do

meet. This creates avalanche hazard—a threat to

life and property from the cascading snow (tig. 2).

The avalanche is a great natural force, not inferior in ix)wer to the tornado, the flood, and the

earthquake. It is not so well known as a destroyer

of life and property more because of timing and

location than lack of potential. In the first World

War, on the Austro-Italian front in December

1916, thousands of soldiers lost their lives in snowslides during a single 24-hour period. Total casualties from avalanches during the war on that

front were greater than from military action.

This was the greatest avalanche disaster in recorded history; it was brought about by an abnormal concentration of men stationed in the

mountains, abetted by an abnormal winter. It

demonstrates the difference between avalanches

themselves and avalanche hazard.

r-49B189

FIGURE 2.—A climax, hard slab avalanche ( HS-AE-5—meaning of abbreviations is given in figure 38). This slide

nearly buried a lift terminal, but lost its momentum just in time to prevent any damage. Such large slides have

not been observed on this slope since it has been stabilized by routine artillery fire during and after every storm
During the Gold Bush period, from about 1860

to 1910, the mountains in tlie Western States were

miicJi more heavily populated than they have been

at any time since then.

From the Sierras and the Cascades to the Rockies, miners swarmed into the high country and

built their camps wherever they found silver and

gold. Telluride and Aspen in the Colorado Rockies, Atlanta in the Sawtooths, Mineral King in the

Sierras, Alta and Brighton in the Wasatch—to

name a few—were fainous in their day for fabulous riches. They were known also for the sudden,

dramatic, and complete destruction by snowslides

that often obliterated both miners and their

camps.

The mining era was the first time that human

beings in large numbers invaded the alpine zones

of the United States. The consequences were

often disastrous. Verifiable records of avalanche

accidents in the vicinity of Alta, for instance, go

back as far as 1874, when the mining camp was

practically obliterated by avalanclies; more than

60 lives were lost. During the next 35 years, avalanches killed 67 more persons. Three slides in

three diiferent years wiped out more than 10 persons each.

Demonetization of silver plus exhaustion of the

richest claims ended the mining rush, and miners

vanished from their alpine camps almost as suddenly as they had appeared. Except for a few

trappers and prospectors, the mountains were

deserted again in the winter. The avalanches remained, but the avalanche hazard Avas sharply reduced because the number of potential victims was

reduced.

Other targets quickly developed after the mining camps were deserted—transcontinental railroads and highways. The railroads far excelled

the highway builders in establishing all-year traffic through the western mountains. Even so, it

was on a railroad that the greatest avalanche

disaster in United States history took place. In

March 1910, three trains were snowbound at Welhn^ton in the Cascade Mountains of Washington.

A smgle avalanche swept all three into the bottom

of a canyon at a cost of more than 100 fatalities

and a million dollars' property damage (fig. 3).

This tragedy forced the railroad underground.

Millions of dollars were spent on the tunnel that

bypassed the slide area.

The succeeding quarter century saw a reversal

ot the downward trend of avalanclie hazard in

nie United States. Industrial and agricultural

development of tlie West led to constniction of

v^. ;T" "^ I'lg^^^-ays and their maintenance the

DusLruii" 1 "'f reopened; logging operations

control nr'^T «^«^'^ti«"s ; reclamation 'and flood

telenwf ^!i^'''"g^^t the headwaters; power,

anjÄn i^P'P'^^'^^^ ^°"^P«ted with highways

tC monn^ ? ^' f' T'^ «f skiing attracted into

Alta, with its grim history of death and destruction at the hands of the "white death" was

reborn as a winter recreation area and the site of

the first avalanche observation and research center in the Western Hemisphere.

In 1960, avalanche hazard threatens more persons and more enterprises than ever before, and

the number is increasing. The spectacular increase in winter sports activities throughout the

United States in the decade ending 1960 has been

responsible for the greatest increase in hazard.

The number of skiers in the national forests of the

Intermountain Region has increased two and onehalf times since 1950. The number of persons exposed to possible danger continues to increase

yearly.

Witli the skier have come businesses, hotels, private cabins, and heavier highway traffic, all contributing to the economy and development of the

West. As new highways are planned and use

of the older ones increases, more and more nonskiers are also exposed to possible snow hazards

in the many mountain passes. The development

and increasing population in the Western States

is bringing more and more people in contact wúth

snow avalanches. Much of this contact occurs on

national forest lands, so the safety problem becomes an ever-increasing concern of the Forest

Service.

Large sections of Alaska are first-class avalanche country, comprising as they do some of

the world's most spectacular alpine terrain covering thousands of square miles. Alaska so far is

very sparsely populated and the hazard in most

places is not yet large, but the high mountains are

a dominant part of the landscape, and even now

the few roads and railroads connecting the scattered population centers are endangered by avalanches. If Alaska fulfills its promise as a new

frontier for rapid development, the avalanche

hazard will rise rapidly in the next few decades.

Much of the-accessible mountain terrain near the

seaeoast is within the national forests (fig. 4).

There is no possibility that exposure to avalanche hazard in the United States will again decline. The pressure of traffic, communications,

recreation, and industry is too great. Neither is

hazard the product of climactic winters only. Injury, death, and property damage are an annual

toll which merely rise to a peak in years of abnormal severity.

The oldtime answer to avalanche hazard—

•'either stay out of the mountains or take your

chances"—is no longer good enough and the winter

techniques developed by the Forest Service to cope

Avith this hazard are providing a new answer.

_ The study of avalanches is difficult and expensive. Necessity is the main reason for undertaking it. The Swiss, pioneers in snow research,

have lived with avalanche hazard for centuries,

and have avalanche defense works as old as the

towns they guard.
!»"AvVy. ^-'^VK.'_,,J, .;

FIGURE 3 —A.n example of avalanche hazard and protective measures. Here a series of slide paths descend through

timber and cross the old railroad grade leading over Stevens Pass, Washington. A long snowshed has been

constructed to protect the grade. The arrow points to the site of the Wellington slide disaster of March 1, 1910.

A major avalanche overwhelmed 3 trains at this point and carried over 100 persons to their deaths, T^V-" '•inroad has since been rerouted through a tunnel under the pass.

The railIn 1931 a national commission was established

in Switzerland to coordinate all of the work being done by various groups and organizations.

This resulted in the famous Avalanche Institute

with its snow laboratory and staff of technicians

and engineers. The Institute engages in all forms

of snow study from basic research in snow

mechanics to the development of avalanche

barriers.

Avalanche study in the United States began in

the winter of 1937-38 with the assignment of the

first snow ranger to Alta, Utah. Again the reason

577868 0—61 2

was necessity. This abandoned mining camp with

its history of death and destruction by avalanche

was being developed as the first true alpine winter

sports area in this country. Forest officers recognized an ideal combination of terrain and climate

near a large city of potential skiers within easy

driving distance. They further reasoned that safe

use by the public was possible only under the

supervision of a snow ranger armed with power

to close slopes under avalanche hazard conditions.

Actually it is surprising that the study of avalanches did not begin 30 years earlier, after the
F-495191

FIGURE 4.—A striking example of avalanche hazard. Here an important highway and railroad are threatened by a

series of slides which fall over unbroken sloi)es from a height of 3,000 feet. Turnagain Arm, on the Chugach

National Forest, near Anchorage, Alaska.

disastrous winter of 1909-10. Two official reports

were published. One, by a forester, noted the fact

that forest fires had recently denuded the slopes

above the railroad, creating avalanche paths

wJHcJi had previously been protected by timber

cover, i he other, by a meteoro!ofrist, described the

weather patterns whicli led to severe avalanche

conditions. While neither of the writers was a

trained avalanche observer, their analyses contain

some very acute observations. In fact, one statement by the meteorologist has become a motto in

the avalanche profession :

"It was not only the quantity of snow that

fell that caused so many avalanches but also

the manner in which it fell."
Lacking any means of combating them, the

miners endured avalanches as one more hazard

in a hazardous occupation. The railroads retired

into their tunnels and snowsheds and mitil recent

years, the law of averages has been a defender of

the highways. The odds against a moving car

and an avalanche arriving at the same point at the

same moment are good. But the odds are getting

poorer eveiy year with increasing winter travel by

bus, truck, and private automobile.

The Forest Service inherited tlie avalanche

hazard problem because so much of the desirable

alpine skiing terrain was on national forest land.

There was strong demand for the development of

these areas, but the cost in lives and damage the

gold minei-s were willing to pay was not acceptable for public recreation. Thus the Forest Service in the interest of public safety found itself in

the business of combating avalanches.

The Forest Service undertook to obtain maximum public use of alpine ski areas with adequate

public safety. It was obvious from the start that

snow and avalanche research in this country would

take a course different from that being carried on

in the Alps. There was neither the time, the

funds, nor the trained pei"sonnel to engage in basic

research. Efforts had to be directed toward the

rapid development of field techniques for the

recognition and reduction of hazard.

Geography was also responsible for a different

approach. Ski areas scattered throughout the

West were located in widely different climatic

zones and in varied types of terrain. In contrast

to the compact areas of the Swiss Alps, the mountainous zones of tlie Ignited States are spread out

over a large territory, exposed l)oth from north to

south and east to west to different weather patterns and storm tracks. It was obA-ious that

local methods would have to be developed in different areas, for no one location could be taken

as representative for all at any given time. The

N'orthwest mountains might be experiencing rain,

the Intermountain ranges snowfall, and the Colorado Rockies basking in sunshine.

Until 1950 the snow safety program was confined to winter sports areas under the jurisdiction

of the Forest Service. The principal objective

was the protection of human life.

It was soon learned that about 80 percent of the

slides large enough to endanger life and property

run either during or immediately after storms

(fig. 5). Therefore, it was decided that efforts

should be concentrated on analyzing the weather

factors which produce these direct-action avalanches. But the importance of structural changes

in the snowpack which cause delayed-action avalanches was still kept in mind. Delayed-action

avalanche hazard is an acute problem in the high

alpine zone. Studies of this hazard were initiated

Ko» ^ •

":>.i„T«__»u

P—495192

FIGURE 5.—The destructive force of a large avalanche.

This ear was carried several hundred feet off a highway and completely demolished by a large soft slab

avalanche (SS-N-4).

in 1950 with the establishment of the observation

station at Berthoud Pass, Colorado.

Publication of the "Alta Avalanche Studies"

l)rought about widespread interest in the avalanche

hazard problem in this country. This interest was

further stimulated by a series of severe winters

beginning with 1948-19, attended by numerous

avalanche fatalities and much property damage.

No losses occurred at ski areas protected by Forest

Service snow rangers. This led to requests for

assistance on avalanche hazard problems from

many sources. The development of numerous alpine-type ski areas following World War II led to

the establishment of observation stations covering

the full range of alpine climate in this country.

With the assistance of the U.S. Weather Bureau,

a number of weather instruments were installed—

Alta 1937, Berthoud Pass 1950, Stevens Pass 1951,

Squaw Valley 1956.

Alta is the central collecting station for avalanche data. Office space was added in 1955, providing accommodations for Forest Service avalanche files, an avalanche library, and modest research facilities. The recent study program at this

station has included instrumentation development,

compilation and review of records, tests of artillery and explosives for avalanche control, and literature review and translations. The U.S. Forest

Service Alta Avalanche School, initiated in 1949,

is held every other year. It was originally designed as a training course for forest officers, but

has since attracted students from a wide number

of private and Government agencies concerned

with snow safety problems.
2332.81 SNOW AVALANCHES

Chapter 2

2 PHYSICS OF THE SNOW COVER. The

physical properties of snow and tlie processes of

change which it nndergoes are complex subjects

being explored by research scientists. Tliis chapter can be no more than a short introduction to

the beliavior of snow. The reader is referred to

the bibliography at the end of this liandbook for

additional sources of information.

The serious student of snow behavior and avalanches is urged to give careful attention to these

fundamentals before proceeding to the practical

aspects of avalanche hazard forecasting and control. Though a purely empirical knowledge of

snow can serve in many instances, a thorough

understanding of the basic physical processes involved is essential to tlie intelligent interpretation

of observations. This is particularly true of the

more complex forecasting problems concerned

with delaved-action avalanche hazard.

2.1 The Solid Phase of the Hydrologie Cycle.

The conditions of geology, climate, and life on

this planet are immensely influenced by the existence of a temperature range wherein water, one

of the most common substances on the earth's surface, exists in all three states of matter—solid,

liquid, and gas. It is difficult to conceive what

the face of the earth would look like if the cycle

of evaporation, condensation, and runoff were

eliminated by temperatures high enough to keep

all of the water in the form of vapor, or low

enough to keep it in the form of ice. A complex

hydrologie cycle exists because the temperatures

that do prevail pennit water to pass readily from

one state to another. The liquid and vapor states

of water are the active participants in the hydrologic cycle of nature, while the solid state—snow

or ice—represents water that has been temporarily

withdrawn from the cycle and placed in a natural

storage reservoir. This withdrawal occurs mainly

in the form of precipitated snow (fig. 6). The

amount of water contained in this naUiral reservoir has varied during the history of the earth as

great climatic changes of unknown origin have

cUmS ^ succession of ice ages and tropical

\na^T^T "^ ^^^^ P^-''^'' i'^e ^^Ps i^"d continental

fitnrÜq'^!. Jiu'l^ ^*^"Ç ^<' . basis, and water so

cvolf/' 7•^^^^r^"'^ from the active hydrolosric

wîhhnïïi ""ï""'^' ^^ thousands of years. This

durW î^ ''* '' '*'' ^^«"^ the hydrologie cycle

oSf wTf' '"f '^^ ^^t^ during'clim-atic

the Si hflr°^^"'^^ ^ff^^t^ «" the climate of the earth, but these effects are measured on the

scale of geologic time, and their influence is so

slowly felt that they are obscured to human perception by short term climatic variations.

A similar withdrawal on a much more accelerated scale takes place with the deposit of a transient winter snow cover throughout the temperate

and arctic zones each year. This annual storage

exercises a strong effect on the yearly amount of

free water available to arid or semiarid regions in

these zones. In either case, the storage phenomenon owes its existence to the fact that vast quantities of energy ai'e required to return water to

liquid state once it has been deposited as a solid.

The amount of heat required to melt, without

change in temperature, a given weight of ice is

seven times that required to melt the same weight

of iron, and thirteen times that for the same weight

of lead.

2.2 Formation of Snow in the Atmosphere.

Wlienever atmospheric conditions favor precipitation of water vapor at temperatures below the

freezing point, precipitation occurs as snow.

Snow crystals are known to form around nuclei

of foreign matter in the air, such as microscopic

dust particles. The first step is the formation of

a small ice crystal around the nucleus. This crystal grows by the deposition of additional ice from

water vapor in the atmosphere. (The transfer of

water directly from the vapor to the solid state,

or vice versa, is known as sublimation.) Recent

investigations suggest that minute water droplets

(diameter around 1 micron) may also contribute

to the growth of snow crystals. These crystals in

general assume a hexagonal pattern, but, beyond

this, the variations in size, shape, and form are

almost infinite. A general classification according

to form is given in figure 7. The particular form

developed in the atmosphere depends on the air

temperature and the amount of water vapor available. Wlien a snow crystal falls through different

air masses with different temperature and vapor

conditions, the more complex or combined types

may develop. Crystals formed at the freezing

point, or falling through air whose temperature

is near it, stick together to become aggregates of

individual crystals or snowflakes.

When snow crystals fall through air which contains water droplets, these droplets freeze to the

crystals as a rime deposit. As the amount of rime

on a crystal increases, the original shape is obscured and a rounded ball results, giving rise to the

type of snow commonly known as pellet or tapioca.
FALLING SNOW

* O ^ / -i-^<

^

H20

'DROPLETS

NEW SNOW

G = .OI-.25

R-=l-IOKG

ATMOSPHERIC WATER VAPOR

HOAR

FROST

POWDER

SNOW

WIND-DRIFTED SNOW

6 = 0.2-0.4

D=0.5-l MM

R = IO-200 KG

H^

REGENERATED SNOW

G = 0.l-0.3

D=l.2 MM

R=5-23 KG

ayi/v">-

DEPTH HOAR

G=0.l-0.3

0 = 2-10 MM

R= 1-IOKG

OLD SNOW

6 = 0.1-0.4

D = 0.5-2MM

R = 5-I00KG

CONSTRUCTIVE METAMORPHISM

DESTRUCTIVE METAMORPHISM t I I I I—I—I I I—I—1>-

6LACIER ICE

6=0.8-0.9

D = 2MM-30+CM

FIGURE 6.—The solid phase of the hydrologie cycle, showing the various paths by which water vapor is deposited as a

solid on the earth's surface. The ice so deposited may at any time revert to the liquid phase by melting ; only

in cerUin favorable climatic conditions does it reach the final stage of glacier ice. This chart shows the terrestrial

part of the cycle only, sea ice having been omitted.

The technical term for these rimed crystals is

graupel (fig. 8).

A relationship has been noted between air

temperature and density of newly fallen snow.

In general, new snow density increases with air

temperatures. The relationship appears to be at

least partially independent of snow type. The

percentage of water in new-fallen snow may range

from 1 to as high as 25 percent, with the average

value around 10 percent. The lightest snow is

deposited under cold and very still conditions.

The highest new snow densities are associated

with graupel and needle crystals falling at temperatures near freezing.

2.3 Formation and Character of the Snow

Cover. Formation of the snow cover is in some

ways analogous to the geology of sedimentary

rocks. Solid precipitation from the atmosphere

(sedimentation) builds up the snow cover layer

by layer, with a resulting stratification that displays the history of the weather variations which

occurred during its buildup. With the passage

of time, structural and crystalline changes within

the snow cover (metamorphism) erase the original

stratigraphie differentiation and convert the snow

into new forms.

Graphing snow depths on a time (months and

days) base provides a general record of the snow

cover behavior throughout the course of a winter.

Periodic observations of density, snow type, hardness, thickness, and other characteristics of individual layers can be plotted as profiles on the same

time base. Pits dug from time to time provide

information about internal changes taking place,

and plotting meteorological data on the same

graph allows the effects of various weather elements to be studied. This technique is described

in detail in chapter 6, items 6.3 and 6.4.

Metamorphism within the snow cover is a continuous process which begins when the snow is

deposited and continues until it melts. The

normal path of metamorphism tends to destroy

the original crystalline forms of the deposited

snow crystals and gradually convert them into

rounded, isometric grains of ice (destructive

metamorphism). The physical process causing

these changes is not entirely clear, but transfer of

water vapor by sublimation from the points of

tlie crystal branches to the more central portions

of the crystal appears to play an important role.

The process is strongly influenced by temperature,

and proceeds at a rapid rate when the temperature
o

2 iv *ar

5

6

7 >r^ i¿ )tfH a»-^

8 Ü O ^^^ ^ ^^

9 A ©^

O A 5 .3

F-495193

FIGURE 7.—Types of solid precipitation (International

Snow Classification) :

1. Plates also combinations of plates with or

without very short connecting

columns.

2. Stellar also parallel stars with very short

crystals. connecting columns.

3. Columns and combinations of columns.

4. Needles and combinations of needles.

5. Spatial spatial combinations of feathery

dendrites. crystals.

6. Capped columns with plates on either (or

columns. one) side.

Î. Irregular irregular compounds of microscopic

particles. crystals.

8. Graupel pellet snow, or soft hail, with isometric shape. Central crystal

_ cannot be recognized.

». Ice pellets ice shell, inside mostly wet (sleet). 0. Hail

IS near the freezing point. At extremely low

temperatures metamorphisim is very slow, and

practically stops below -40°. As this proce.ss

take.s place with all types of snow crystals, they

tend to approach the uniform condition of rounded

ice grains, and the snow cover becomes more homogeneous. The rate of metamorphism is influF-495194

FIGURE 8.—Graupel, or pellet snow, which has been subjected to destructive metamorphism within the snow

cover. Magnified about 2.5X. The pellet characteristic has been retained, making layers of this snow

type identifiable for a long time after incorporation

in the snow cover. ( Note : Tliere is no figure 9. )

enced by pressure as well as temperature (pressure

metamorphism), and the weight of additional

snowfalls over a given snow layer causes its metamorphism to accelerate.

The destnictive metamorphism of the snow

crystals results in a reduction of the space a given

amount of ice occupies. This causes an increase

in density to accompany metamorphism, and the

snow cover to shrink, or settle. This settlement

is a continuous, visible evidence of metamorphism.

When the process is accelerated by rising temperatures, the settlement rate of the snow cover increases.

A process known as constructive metamorphism

may also take place in the snow cover. This occurs when water substance is transferred from one

part of the snow cover to another by the vertical

diffusion of water vapor. This vapor is deposited

around new centers of crystallization and forms

ice crystals with quite a different character from

those of the original snow. These crystals tend

to assume a scroll or cup shape, appear to be

layered, and often grow to several millimeters

in diameter (figs. 10^ and 11). They form a

very fragile mechanical structure which loses all

cohesion upon collapse and becomes very soft when

wet. The snow form produced by this type of

metamorphism is known as depth hoar, and is

popularly referred to as sugar snow. The conditions required for its formation are a rapid change

of temperature with depth (steep temperature

gradient) to cause diffusion of water vapor within

the snow cover, and a sufficiently high air permeability to permit this diffusion to take place.

These conditions are most common early in the

winter, when the snow cover is shallow and

unconsolidated.

' There Is no figure 9.

10
FIGURE 10.—Depth hoar crystals from the bottom layer

of a snow cover 65 cm. deep. This snow has undergone partial constructive metamorphism, and only

some of the crystals display the distinct layering and

cup-shaped form of mature depth hoar. Magnification

about 2.5X.

Snow is an essentially plastic material. Because

of its plastic nature, any snow cover situated on a

sloping surface tends to flow downhill under the

influence! of gravity. This behavior is known as

creep, and is highly dependent on temperature,

since snow is very plastic near the freezing point

and becomes increasingly brittle at lower temperatures. If the ground surface of a slope is smooth

(grass, for instance), the snow cover will slide at

its base as well as flow plastically within itself.

The creep process is usually very slow, and goes

unnoticed by the casual observer; but it can exert

tremendous pressure against solid objects located

within the snow cover. Even on flat ground, settlement alone can exert great force. These forces

can be computed, and must be taken into consideration when constructing such installations as

avalanche barriers, lift towers, and snowsheds.

The tensile and shear stresses produced by uneven

creep of snow are an important factor in avalanche

formation.

Variations in the strength characteristics of

snow are among the widest found in nature. The

hardness of wind-packed old snow or frozen firn

may be as much as 50,000 times that of light, fluffy

powder snow. Strength characteristics continually change as a result of metamorphism, and are

also dependent on temperature for a given snow

type. Wlien snow is disturbed mechanically, then

allowed to set, it undergoes a process known as

age-hardening. This process reSults in a gradual

hardening of the snow for several hours after it

is disturbed. The process is more pronounced

at lower temperature. The greatest source of

mechanical disturbance in nature is the wind, and

an increase in hardness is always associated with

wind-drifted snow. Age-hardening by the artificial disturbance of snow slopes such as skiing or

FIGURE 11.—Surface hoar crystals, formed by sublimation of water vapor from the atmosphere onto a cold

snow surface. These display the same characteristic

layering and crystal form as depth hoar, but usually

are flat instead of cup-shaped. Magnification about

2.5X.

foot packing is an important factor in avalanche

prevention.

2.4 Mechanical Properties of Snow. Snow belongs to that class of solid materials which are

termed "visco-elastic." That is, it exhibits the

properties of both viscous fluids and elastic solids

in varying combinations which depend on density,

crystal type, and very profoundly on temperature.

When a force is applied to snow it may deform

elastically, flow as a viscous liquid, or react as a

combination of the two (fig. 12). The rate and

duration of force application will also determine

the relative importance of the elastic behavior.

Snow which is brittle is able to remain in a stressed

state for long periods, resulting in persistent avalanche danger. Snow which behaves more nearly

as a viscous liquid is able to relieve stresses rapidly

by plastic flow, and will more quickly approach a

stable condition.

The degree to which snow behaves as a viscous

liquid is described by the quantity known as the

coefficient of viscosity ; the higher the value of this

coefficient the stifi^er and less liquidlike is the snow.

Figure 13 displays in generalized form the dependence of this coefficient on temperature and

density. The very great sensitivity of viscosity

to temperature in snows of higher density is an

important factor in slab avalanche formation.

The viscosity of snow also increases with increasing grain size.

In contrast, liowever, the elastic properties of

snow, described by a quantity known as the elastic

modulus (usually the shear modulus) are only

slightly dependent on temperature. The elastic

modulus does increase exponentially with density

over the range of densities found in winter snow

covers.

11
FIGURE 12.—The plastic character of snow has permitted

it to flow off this roof and develop a large overhang

without breaking.

The strength properties of snow, already

mentioned, are also determined by density, temperature, and grain type. The magnitude of such

properties as shear or tensile strength determines

the ability of a given snow mass to remain

anchored on a mountainside in opposition to the

forces of gravity whicli are trying to pull it down.

The changes of the strength properties with time

are important in avalanche formation, and, taken

together with time changes of stresses, determine

when the moment of critical instability may be

reached. A generalized picture of the dependence

of tensile strength on the three principal variables

IS given in figure 1-i. Note that the strength

reaches a maximum for fine-grained old snow, and

then decreases as the grain size increases. This is

one of the dangerous effects of constructive metamorphism and depth hoar formation already

mentioned.

These mechanical properties of snow will be reierred to agam m the discussion of slab avalanche

lormation.

2.5 Thermal Properties of the Snow Cover.

I tie common characteristic of the various properties of the snow cover just outlined is their de-

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and temperature. As density increases, changes in

viscosity become very sensitive to temperature.

pendence on temperature. Accordingly, the manner in which temperature of the snow cover is

controlled by heat loss or gain is of primary importance to its whole behavior. In addition, the

melting of the snow cover each spring is controlled by the supply of heat. An understanding of the relative importance of the various

meteorological factors which affect the heat supply

is thus important to forecasting snow behavior and

avalanches.

There are three basic characteristics of snow

which strongly influence its thermal properties.

First is the high heat of fusion of ice already

mentioned. For each gram of ice existing at the

freezing point, approximately 80 calories are required to convert it to liquid water without any

change in temperature. The second is the very

low heat conductivity of snow which ranges from

about 0.0001 to 0.0007 calorie/cm.-sec./°C., or up

to 10,000 times as great as that of copper. The

third characteristic is the presence of water in the

vapor and/or liquid phases as well as the solid

phase. Vapor and liquid water play an important

part in heat transfer within the snow cover.

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FIGURE 14.—Dependence of snow tensile strength on temperature, density, and grain type (generalized relations). The decrease of tensile strength with the

formation of coarse crystals (constructive metamorphism) is an important source of instability.

Tlie heat supply at the bottom of the snowpack

Í8 relatively limited. A certain amount of heat

is stored in the surface layers of earth each summer, and this heat will melt some of the snow deposited during the winter. If cold weather precedes the first snowfall, however, some of this heat

may already be dissipated by frost. The internal

heat of the earth also provides a small but steady

heat supply at the surface, and is suiRcient to melt

about 1 cm. of ice each year. In any case, an appreciable snow cover serves to insulate the ground

surface from external heat exchanges; and the

internal and stored heat, though it may be relatively small, is sufficient to melt small quantities

of snow throughout the winter. The occurrence

of melting means that the temperature must be

kept at freezing point. For this reason, the snowearth interface is almost always at a temperature

of 0° C. or 32° F. in snow covers throughout the

temperate zones. This is not the case m higher

latitudes where very low temperatures, shallow

snow cover, and permafrost are the rule. The temperature may also remain below freezing for a

short time at the bottom of temperate zone snowpacks early in fall, when there is frost in the

ground and the snow is shallow.

Heat is both lost and gained in much larger

quantities at the snow surface, and the temperature undergoes wide fluctuations below the freezing point. The temperature of the snow surface,

which is composed of particles of solid ice, can

never be higher than 32°F. because water cannot

exist in a solid state above that temperature. A

thermometer placed on the snow surface will often

give a misleading value higher than 32° because

of solar radiation. The principal media of heat

transfer at the snow surface are turbulence of the

air, long and short wave radiation, condensationevaporation-sublimation, rainfall, and internal

conduction. The air, radiation, and vapor exchanges, and internal conduction may either add

or subtract heat, while rainfall can only add it,

since liquid water must always be at a temperature equal to or higher than that of ice. These

media are shown diagrammatically in figure 15.

The medium of heat exchange with the largest

potential is the air. Heat may be conveyed to or

removed from the snow surface in large quantities

by the process shown as eddy conduction, or the

turbulent transfer of heat. This process depends

on the motion of air over the snow surface, and

does not occur with completely calm air. Under

this latter condition heat is transferred within air

only by molecular conduction and in very small

quantities. When the air is warmer than the snow

surface, heat is transferred to the snow, and as the

difference in temperature increases, so does the

amount of heat transferred. With air colder than

the snow surface, heat flow is in the opposite direction. The laws governing the turbulent transfer of heat are not y^i clearly formulated, but it

is known that the quantity of heat transferred

depends on difi'erence between air and surface

temperatures, the wind velocity, and roughness of

the surface. The amount of snow that can be

melted by a strong, warm wind, such as a Chinook

wind, is much greater than can be melted by any

other natural source, including solar radiation.

Large amounts of energy arrive at the earth's

surface in the form of short wave (visible) radiation from the sun, but only a small portion is avail577868 O—61- 13
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FIGURE 15.—The principal media of energy exchange at the snow surface.

able to snow because of its high reflectivity. Up to

90 percent of the short wave radiation arriving at

the surface of freshly fallen snow is turned back

by reflection. This figure drops to about 60 percent for wet spring snow, and may be even lower if

there is appreciable dirt on the snow surface.

In contrast to its behavior under short wave

radiation, snow reacts to long wave (infrared)

radiation as though it were black. In fact, among

all substances in nature, snow approaches the

nearest to an ideal black body for long wave radiation. This means, for one thing, that snow is an

excellent radiator of infrared, and that it can lose

heat through this medium as well as gain it.

When skies are clear, snow loses heat to space by

long wave radiation. The amount of heat lost

depends on the quantity of water vapor and carbon dioxide in the air, both of which strongly affect long wave radiation. Under overcast conditions, snow will also lose heat by long wave radiation to clouds if the cloud temperature is lower

than that of the snow surface. Clouds of any appreciable thickness also behave as black bodies to

mfrared, and if temperatures of cloud and snow

surfaces are known, the amount of heat exchanged

can be calculated. Wlien the clouds are warmer

than the snow surface, the process is reversed and

the snow gams heat by long wave radiation. This

condition occurs most often in spring or summer

when the air and clouds are warm and the snow

remains at 32°.

The eíFects of long and short wave radiation add

aJgebraically to produce the net radiation balance

of the snow surface. The net balance in winter

is usually positive (snow gains heat) in the day

and negative at night. Occasionally under conditions of clear skies at high altitudes in winter,

when the sun angle is low, the outgoing long wave

radiation may exceed the short wave radiation

absorbed by the snow even at midday, and the

snow will actually be cooled while the sun is shining. Maximum positive radiation balance is

achieved, not on clear days, but under conditions

of thin fog or low broken clouds in spring or summer, when both long and short wave radiation

contribute heat strongly to the snow surface.

The behavior of water vapor as a medium of

heat exchange at the snow surface is determined

by the high heat of vajwrization of water. The

conversion of 1 gram of water from liquid to vapor without change in temperature requires 600

calories of heat. If water is converted directly

from solid to vapor (sublimation), the heat

fusion must be added to this figure to give a heat

requirement of 680 calories per gram. It is thus

seen that, even though the heat of fusion is large,

the heat of vaporization or sublimation is very

much larger. Accordingly, addition or subtraction of material at the snow surface through the

medium of water vapor must be accompanied by

the exchange of large quantities of heat. This

means, first, that evaporation can take place only

if enough heat is available to supply the heat of

vaporization. Heat in such quantities is not ordinarily available to the snow cover, and evaporation losses are generally quite small. Signifi14
cant amounts of evaporation can occur only when

extremely dry air with temperature well above

that of the snow surface is accompanied by high

winds, provided tliat the condition for evaporation described below is satisfied. A second

corollary is that condensation of water vapor on

a snow surface can be a large source of heat, for

when condensation occurs, the heat of vaporization is given up to the snow. The transport of

M-ater vapor to or from a snow surface is governed

by laws similar to those governing the tui'bulent

transfer of heat, and increases witli increasing

wind velocity.

"\'Maether the condition for evaporation or condensation prevails depends on a simple physical

law. "\^nien the dew point of tlie atmosphere is

lower than the snow surface temperature, condensation can take place. If the air and snow surface temperatures are below freezing, the exchange takes place as sublimation. Conditions

most frequently favor condensation in spring or

summer when air temperatures (and dew points)

become higher, wliile the snow temperature remains at 32° F. Conditions favoring evaporation

are more common in midwinter, but very little

takes place because the available quantity of heat

is small.

Rainfall ordinarily can deliver only a limited

quantity of heat to the snow surface. The specific heat of liquid water is 1.0; that is, 1 calorie

of heat is involved in changing the temperature

of 1 gram of water 1°C. Thus, each gram of rain

which falls on a snow surface and is cooled to the

freezing point gives up only 1 calorie for each

degree centigrade it is cooled. It is seen that

large quantities of rain are required to melt any

significant amount of snow (ice) requiring a heat

of fusion of 80 calories per gram. Heavy snowmelts sometimes associated with rainstorms can

usually be attributed to the warm winds and condensation which accomi^any such storms. Very

warm rain in large quantities can cause appreciable melting, but such conditions are rare in the

high mountains, and mountain snow covers seldom experince much ablation (melting) due to

rainfall.

The flow of heat within tlie snow cover is a complex phenomenon due to the presence of all three

phases of water ; part of the heat is transferred by

simple molecular conduction and part is transferred by circulation of air in the spaces between

the snow grains. The air itself carries some heat,

but the water vapor with it carries more. It is

estimated that up to a third of the heat flow in

snow is due to sublimation and diffusion of water

vapor. Because of the high heat of sublimation

of water, only small quantities of water vapor are

required to transport appreciable heat in this

manner. In any case, the total amount of heat

transferred within the snow cover is quite small,

due to its overall low conductivity. This situation is altered when appreciable amounts of liquid

water are present. The downward percolation of

liquid water through a snow cover transports heat

very rapidly, and a sudden thaw or rainfall which

produces free water at the surface v/ill quickly

warm the whole snow cover up to the freezing

point. This process would take a long time by

conduction alone. Wlien water percolates into

snow below the freezing point, the water refreezes

and gives up its heat of fusion. Heat is thus transported through the snow by the physical penetration of water, circumventing the otherwise low

conductivity of snow.

2.6 Examples of Weather Influence on the

Snow Cover. A discussion of actual effects of

weather on snow as obtained from the records will

serve to illustrate some of the foregoing principles.

1. Radiation. The maximum amount of heat

available to the snow from solar radiation can be

calculated. The following tabulation gives the

amount of heat available to a level snow surface

exposed to the sun from sunrise to sunset at approximately the latitude of Alta, Utah.

Clear day in JANUARY cal/cm.' available

(reflectivity of snow 0.9) to snoiB

Sunrise to sunset 29

1000-1400 18

1100-1300 9. 4

Clear day in APRIL

(reflectivity of snow 0.7)

Sunrise to sunset 200

1000-1400 93

1100-1300 48

Tliese figures actually will be reduced somewhat by various factors in nature, and the heat

losses due to long wave radiation on clear days

will reduce them further. In January these reductions leave very little heat available to the

snow. In April the increased sun elevation provides much more heat from solar radiation, and

this source becomes important to snowmelt.

A good example of the effects of long wave

radiation loss occurred at Alta in February 1954.

A weeklong period of dry, clear, and calm weather

brought air temperatures as high as 50° F. during the day and no lower than 30° at night. The

snow remained light, crystalline, and fluffy during this warm spell, and even on south slopes there

was only very limited melting and metamorphism

at the surface. Snow temperature 2 centimeters

below the surface was 25° even at the end of this

period. Very strong long wave radiation losses

almost completely neutralized heat supply to the

snow even at midday, and cooled it strongly at

night.

2. Tem perafme Gradient. The month of January 1955 at Alta was marked by many large avalanches. Some of these removed the snow cover

on certain slopes so that only a thin icy layer remained next to the ground. Late in January a

fall of more than 40 inches of powder snow occurred, and in many places avalanched again off

this icy layer. One place that it stuck to the icy

layer was on the west face of Greeley Mountain.

AVliere this heavy snowfall was deposited on a

deeper snow base which had not previously ava15
]nnched it quickly settled into a very hard and cohesive layer- This settlement also occurred on

Greeley Mountain, but because the snow layer

rested practically on the ground, it was subjected

here to a much more severe temperature gradient

by cold weather early in February than was the

same layer which rested over a deep snow cover.

This gradient was high enough to initiate constructive metamorphism, which gradually weakened the mechanical structure of the snow. As a

result, a large and dangerous avalanche came down

the west face of Greeley Mountain more than a

week after the storm had ended. Given the same

temperature conditions at top and bottom of the

snow cover, and the same snow layer in question,

the shallow snow cover became dangerously weak.

Avhile the deeper one became stable. The only differences were the magnitude of the temperature

gradient and the character of the bond to the underlayer.

3. Heat Transsjer Within the Snow Cover. The

effects of liquid water on heat transfer within the

snow cover were clearly demonstrated at Alta in

December 1955. Cold and early storms had built

up a fairly heavy snow cover that was at temperatures well below the freezing point. On the morning of December 22, the snow cover was 157 cm.

deep. Internal temperatures were —1.2° C. at 60

cm., -2.4° C. at 92 cni., and -3.3° C. at 122 cm.

Three-fourths of an inch of rain fell during the

day, and by the morning of December 23, these

temperatures had started to rise. Then during the

day and evening of December 23, 3I/2 inches of rain

fell, and by next morning the entire snow cover

was isothermal at the freezing point. Temperatures of such points deep in a snow cover ordinarily change only very slowly, but the percolation

of rainwater through the cover caused large

changes within a few hours. This rain fell at

temperatures close to freezing, and actually melted

an insignificant amount of SIIOAV. In fact, later

water-content measurements of the snow cover

showed that more water had been retained by

freezing of the rain in the cold snow than had

been lost by melting. The effects of this rainstorm

on snow temperatures are displayed graphically

in figure 16.

4. Turbulent Transfer of Heat From the Air.

As might be expected, heavy melting of snow by

warm air often occurs over glacier surfaces in the

summer. A clear example of such melting was observed in July 1955, on the Lemon Creek Glacier

in Alaska. The ablation (melting) season commenced early in June, and wastage of the snow

surface amounted to an average of about 5 cm.

of snow each day through June and most of July.

However, on July 21-22 a period of clear weather

arrived, accompanied by a Chinook or foehn wind.

Air temperature was near 50° F., higher than the

daily means up to this time, and high winds to

40 mph. were noted. During this 2-day period,

45 cm. (11/2 ft.) of snow were melted away. Density of the snow was about 0.5 gm./cc., so this

represented the melting of about 22 cm. of water,

requiring 1,760 calories supplied to each square

centimeter of the snow surface. As soon as the

foehn blew itself out, the ablation returned to

about 5 cm. of snow each day. Similar high ablation rates have also been observed in this area

when storm conditions are accompanied by high

winds well above the freezing point. Warm winds

such as these in the mountains are often the cause

of extensive spring wet-snow avalanche cycles.

6LTA SNOW STUDY PLOT

DISTRIBUTION OF ISOTHERMS FROM

II DECEMBER 1955 TO 31 MARCH 1956

DEGREES CENTIGRADE

SNO» SURFÜCe

FIGURE lb.—Temperature changes in tue snow cover throughout the winter. The gradual temperature changes üue

to conduction and the rapid changes due to melt-water percolation are both shown. The rainstorm of late December IS discussed in the text. Slopes of the lines marked A and B indicate the rate of melt-water i>ercolation in

the spring. The lower layers of this snow cover had been made very hard and dense by refreezing of water from

the December rain, and offered greater resistance to the downward flow of spring melt water.

16
2332.81 SNOW AVALANCHES

Chapter 3

3 AVALANCHE CHARACTERISTICS. Avalanches which fall over long distances and varied

terrain may involve more than one type of snow,

or more than one method of motion. To permit

a miiform means of description, the primary classification of any given avalanche is always based

on conditions prevailing at the point of origin.

The description may be modified if necessary to

clarify the character of the avalanche over the

length of its path.

3.1 Avalanche Classification. Two principal

types of avalanches are recognized. These are

loose snow and slab avalanches. The distinction

is based upon the mechanical character of the snow

at the point of origin. Loose snow has relatively

little internal cohesion and tends to move as a

formless mass. Loose snow slides start at a point

or over a small area. They grow in size and the

quantity of snow involved increases as they descend. A clearly distinguishable sliding layer is

not always present, and the line of demarcation

between the sliding snow and that which has remained in place is often indefinite. Slab avalanches, on the other hand, are characterized by

internal cohesion of the snow involved. This cohesion may vaiy from only a very slight tendency

of the snow crystals to stick together up to extremely hard snow which is broken apart only

with great difficulty. Eegardless of the degree

of hardness of the snow slab, the common characteristic is that a large area of snow begins to slide

at once, instead of starting as a small point as

does the loose snow avalanche. A well-defined

fracture line where the moving snow blanket

breaks away from the stable snow is the positive

identifying characteristic. This fracture line

forms an irregular and often arc-shaped line

across the top of the avalanche path with the face

of fracture perpendicular to the slope. A definite

sliding layer is usually distinguishable (figs. 17,

18,19).

17
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FIGURE 17.—Skier-released small dry slab avalanche (HS-AS-2). The arrow points to the tracks where the skier

entered.

18
FIGURE 18.—Medium-sized dry soft slab avalanche (SS-N-3) released naturally by a cornice fall. The broken cornice

is visible at the apex of the fracture line. The slide originated as a shallow slab which triggered a deeper layer

of snow some 25 or 30 feet below the cornice ( arrow ).

19
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FIGURE 19.—Large, dry, hard slab avalanche (HS-?^). Manner of release unknown. Fracture line extends under

the lee side of a ridge, where heavy wind depo-sition has in places created an exceptionally thick slab (upper

center of picture). Presence of angular blocks in deposition zone identifies this slide as a hard slab. The ridge

running across center of the picture, the upper edge of the stable snow which was overridden by the falling slab,

is a characteristic feature of the slab avalanche.

Slab avalanches are subdivided into soft and

hard slabs. Soft slabs slide away with the typical

fracture line, but have the general appearance of a

loose snow avalanche until they actually slide and

reveal the soft slab characteristics. The snow

debris deposited by a soft slab avalanche is

usually a formless mass like that from a loose

snow avalanche. A hard slab is characterized by

snow of sufficient cohesion to retain its form in

motion. The principal feature identifying the

hard slab avalanche is tlie presence of angular

blocks or chunks of snow in the debris (figs. 20

and21).

3.11 Loose Snow Avalanches. T^oose snow avalanches arise when snow accumulates on slopes of

steeper angle than its natural angle of repose.

The snow on a given slope is placed in a state of

unstable equilibrium by: (1) deposition of light,

fluffy snow under conditions of little wind; (2)

reduction of internal cohesion among the snow

crystals by metamorphic changes; or (3) lubrication from percolating melt water.

Any small disturbance is propagated downward

from crystal to crystal, causing the snow to slump

downhill towards its natural angle of repose, or,

if any momentum is attained, to slide onto slopes

of lesser angle. Disturbance of the unstable

equilibrium proceeds in the nature of a chain reaction, confined to the downward direction by

gravity and laterally by the character of the snow

and the terrain. More and more snow is collected in the sliding mass until sufficiently gentle

slopes are readied for the moving snow to lose its

momentum and come to rest.

20
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FIGURE 20.—Typical debris deposition from a hard slab avalanche (HS-AE-3 or 4). Artificial release by cornice

blasting. The snow has sufficient internal cohesion to presers-e the large, angular blocks of the original slab.

The quantity of snow involved in a loose snowslide depends on the depth of unstable snow, the

degree of instability, and the length and inclination of the slope. The size ranges all the way

from tlie veiy common loose or powder snow sluff

during or after a snowstorm to the rare, very

large loose slide accompanied by destructive windblast. In terms of number, loose slides are probably the most common and generally are minor in

size and significance. As will be noted in the

discussion of avalanche hazard forecasting, these

small, loose snow sluffs are actually a stabilizing

factor which inhibit development of the more

dangerous slabs. Dry loose snowslides (wild

snow) of large size are very rare; most of those

which might be so classified actually originate as

soft slabs.

The most dangerous loose snow avalanches are

those which involve wet snow in the sprino- or

summer. Originating often high on the mounFiGURE 21.—Debris blocks from a large hard slab avalanche (HS-AE^).

577868 O—61- 21
tain as a tnie loose snow avalanche starting at a

point, tliese may gatlier large quantities of snow

by tlie time they reach the valley floor. The

weight of the wet snow gives them large destructive power even though the velocity may be low.

3.12 Slab Avalanches. Tlie slab avalanche is

of major concern because it forms tlie most important source of winter hazard in the mountains.

In fact, a review of avalanche accidents recorded

both in this counti-y and in Europe shows that

the dry, loose snowslide may for all practical purposes be dismissed as a source of hazard. Slab

avalanches, in most cases triggered by the victims

themselves, are responsible for the overwhelming

majority of accidents.

Slab avalanches may originate in snow types

ranging from soft, fluffy new snow through compacted, hard old snow to the hea^'y wet snow of

spring. Snow crystals may include almost every

stage in the metamorphic processes from newly

deposited stars or dendrites to completely metamorphosed old snow, as well as the forms of constructive metamorphism. It must therefore be

clearly emphasized that slab avalanches are not

exclusively associated with any particular snow

type, and especially cannot be correlated with some

particular visible characteristic of snow. True,

certain types of fresh-fallen snow, noticeably those

of higher density, tend to favor slab formation,

but this is as much a property of their water content as of their crystallography. Wind drifted

snow very commonly develops unstable slabs, and

wind is one of the predominant natural factors

in slab formation. But wind alone is neither a

necessary nor a sufficient factor to cause these

dangerous slides. Unstable slabs may develop

through processes of internal change alone. This

is especially true of climax avalanche development involving several layers of snow, any one

of wliich may be stable when considered by itself,

but which taken together may fall as a large

slab avalanche.

1. Mechanical State. It is therefore apparent

that we must regard the slab avalanche condition

as a mechanical state of the snow cover. The

tendency of a mow layer to slide is dependent not

only on the character of that layer itself, hut on

its relation to adjacent layers and on the size and

shape of the slope. The primary requirement for

slab formation is snow which has sufficient internal

cohesion among the crystals to stick together as a

united mass which can be set in motion as a

blanket over some definite area. "\'\nien the internal cohesion is very low, the slide behaves like a

loose snow avalanche but the dividing line is not

always clear. When the internal cohesion is high,

the snow layer in question cannot be broken loose

and stability prevails. It thus might be said that

a certain amount of strength in the snow bears

with it the seeds of hazard. In practice, the very

unstable snow types of low mechanical strength

actually offer less source of danger, for they slide

off the mountain long before avalanches of dangerous size can form.

Assuming that a given snow layer has sufficient

internal cohesion to behave as a slab, the question

of whether or not it will slide is answered by the

mechanical character of its attachment to the

mountainside. This character is determined by

the so-called static relations.

2. Static Relations. The primary anchorage of

a snow layer on the mountainside is its bond to the

underlayer. Strength of this bond depends on

the strength properties of the two layers, the nature of the underlayer surface—smooth, windblown, etc.—the temperature at the interface, and

the age of the bond. Gravity acting on the slab

layer which lies on an inclined plane develops a

component of force parallel to the undersurface.

This shear stress tends to break the bond between

the slab and underlayer. Opposed is the shear

strength of the bond, which ordinarily increases

with time in the course of destructive metamorphism and decreases with rising temperature.

The primary condition of instability is reached

when the shear stress exceeds the shear strength.

This may be achieved either by a rise in the stress

(increased snow load) or decrease in strength

(e.g., constructive metamorphism or crust disintegration). This stress can be calculated rather

simply if the slab thickness, its mean density, and

the slope angle are known (fig. 22). The shear

strength, on the other hand, cannot be readily calculated, and is difficult to measure in the field.

The general relation for primary instability can

be formulated, but a quantitative determination

cannot be readily made. The natural law which

governs can be recognized, but numerical values

can be assigned only with difficulty.

22
T = nig SIM -©.

WHEdEm.MASS or UNIT

VOLUME OP SNOW

in load or decrease in snow strength causes one

of the anchorages to rupture ; suddenly increased

stresses are then thrown onto the other supports

of the slab and they too break.

- y COMft-t SStON

FIGURE 22.—The primary condition for slab avalanche

release is achieved when weight of the slab exerts a

component of force parallel to the sliding surface

(shear stress) greater than the shear strength of the

bond.

A shear stress which exceeds the shear strength

of the bond is a necessary but not sufficient condition for a slab to slide. The snow layer is also

anchored to the slope at the top, bottom, and sides

as well as at the undersurface (fig. 23). The

strength of these anchorages must also be less than

the stress applied to them if a slide is to be released. As with -the bond between layers, the critical point may be reached either by a rise in stress

or a decrease in strength. Most important of the

anchorages is that at the top of tlie slope where

the snowslab is subjected to tensile (stretching)

stress, for this is where a rupture is most likely

to be initiated, either naturally or artificially.

The applied tensile stress can be estimated from

measurable properties of snow and slope, though

not quite so simply as the shear stress mentioned

above. The tensile strength cannot be readily calculated, and must be measured in the field, again

with considerable difficulty.

If all the forces acting to release the slab exceed the strength of the anchorages, it obviously

will not remain in place. In practice only certain

anchorages may be dangerously stressed, or some

or all of them may be stressed close to but not exceeding the breaking point. Any outside triggering force, natural or artificial, which cuts or

breaks part of the slab then initiates a sudden redistribution of stresses which may break the anchorages and release an avalanche. The same

thing probably happens when a gradual increase

FIGURE 23.—The secondary condition for slab release is

reached when stresses at top, bottom, and sides of

the slab exceed strength properties of the snow.

If some of the anchorages are sufficiently strong,

they may sustain the slab even if other supports

are removed. Tliis behavior is clearly seen in the

common case of a snow slope which cracks but

does not slide. In this case the tensile strength

of the slab layer has been exceeded, and it breaks,

but bonds to the undersurface, or other supports at

sides or bottom, hold it in place.

3. Dynamic Relations. The discussion of the

static relations assumed that the snow slab was an

inert mass lying on the mountainside. In practice

this is not the case. The slab and the snow layers

beneath constantly experience the slow downhill

motion of snow creep, which introduces additional

complications to the problem of snow stability.

It is the additional combination of stresses created

by this creep motion that gives to the slab avalanche its dangerously unpredictable and unstable

character.

Under the force of gravity, snow on a slope is

continually trying to move toward the valley, both

by sliding on the ground and by internal deformation. On level ground the only expression of this

motion is the snow settlement. On a slope the

motion results from both creep parallel to the slope

and vertical settlement.

Zones of stress are formed when one part of a

snow slope is free to flow by creep while another

part is anchored and cannot move, or one part of a

slope creeps faster than another. The most dangerous of these zones are those where tensile

stresses occur, frequently found near the top of

the slope.

The visco-elastic character of snow has already

been described in chapter 2. The reaction of a

snow layer to applied creep stresses depends on

this visco-elastic character. As the viscosity of

23
snow increases, it becomes more and more brittle

(the elastic behavior becomes important only for

hifrli rates of stress application). Sucli snow

isîess able to relieve stresses by plastic (viscous)

deformation or flow, and may remain in a highly

stressed state for a long time. A slab so stressed

is said to be subjected to creep tension.

Wlien some triggering agent breaks a slab and

relieves creep tension, the reaction of the snow

may be very rapid. A whole slope may suddenly

shatter into blocks before the slab has a chance to

be set in motion. If the snow is veiy dense and

cold, such as in hard slab, the release of tension

may be signaled by a sharp cracking sound like a

rifle shot. The propagation of a fracture line in

snow which is subject to heavy creeji tension is

extremely rapid, and such fractures may run for

many hundreds of yards across a slope faster than

the eye can follow.

If enough time is available, the snow will

eventually relieve or redistribute these stresses

by internal deformation. The amount of time

required depends on the viscosity of the snow.

Snow of low viscosity (near the freezing point)

can flow quite readily, and dangerous stresses do

not persist for long periods. Thus direct action

slab danger from warm snowstorms (25-32° F.)

is usually of short duration and the snow quickly

stabilizes. As temperature falls, the viscosity increases rapidly and creep tension becomes more

persistent (ñg. 13, ch. 2). This explains the basis

for the general rule that the lower the temperature, the longer avalanche danger may persist.

A quantitative realization of this behavior may

be obtained by referring to the concept of relaxation time introduced for the general class of viscoelastic materials by the famous English physicist

James Clerk Maxwell. If a fixed initial stress

is applied to such a substance, the time required

for internal deformation to reduce this stress to

approximately 38 percent (1/e) of the initial value

is the relaxation time. For the range of densities

and temperatures commonly found in nature, the

relaxation time for snow may vary from less than

1 second to nearly 2 weeks. Thus it is seen that,

on a seasonal basis, creep action will eventually

cause the snow cover to deform in any case, but it

is during the few days or hours when the snow is

at first unable to accommodate this creep that the

slab avalanche danger may be at its highest.

An illustration of the temperature influence on

slabs is the Ijehavior of hard slabs often found

at high altit udes in a continental climate. On such

slopes a slab may api:)ear to be quite stable during a sunny day, but will become very sensitive to

triggering as soon as it falls into shadow in the

late afternoon. The very marked drop in snow

temperature due to radiation cooling when the

sun goes down is presumed to increase the brittleness of the surface snow layer. This is then easily

fractured, and the break propagates throughout

the entire slab. A fatal accident occurred on just

such a slope that had been used all day by skiers.

4. Layering. The formation of unstable slab

layers is higldy dependent on discontinuities in

the snow cover. A completely homogeneous snow

cover with an even gradation of density, crystal

types, and other properties from top to bottom

would not likely fonn a slab avalanche. A possible exception is where the very strong discontinuity represented by the ground surface causes

the entire snow cover to slide. Each interruption

of snowfall, each advent of a new storm with different snow conditions, causes a change in snow

characteristics from one layer to the next. The

interface between two layers usually will have less

strength tlian that which exists internally within

a layer. Tliis creates an opportunity for instability. It is therefore a general rule that an inliomogeneous snow cover with shaqi discontinuities in stratigraphy is more likely to be the source

of slab avalanche clanger than a homogeneous one.

The formation of discontinuities in layering is

quite sensitive to even minor variations in snow

and weather conditions. Frequently a short lull

or cessation of snowfall in the middle of a storm

will result in tlie snow of that storm being divided

into two distinguishable layers. Though the

snow in each of these layers may be for all purposes identical, they will have a weakened bond

at the interface which marked the lull. In such

cases the upper, more recent layer sometimes can

be dislodgecl as a shallow slab while the lower

layer remains stable.

The major discontinuities from one snowfall to

the next are of course much more strongly developed. The exposed snow surface in the interim between stomis can readily be altered and

aged by the various meteorological factors.

Many slab avalanches, and especially direct action slides that occur during a storm, avalanche on

such a surface.

The surface of discontinuity from which a slab

layer may be dislodged is called the sliding surface. It is the top of tlie older, stable snow which

remains in place. Hard crusts, such as those

formed by sun melt or rain, are particularly good

sliding surfaces which may be the source of repeated avalanche formation. Tliere often exists

between the sliding surface and the slab layer

proper a shallow, weak layer deposited by a light

snowfall or surface hoar sublimation. " Such a

layer, which provides a region of low shear

strength, is called the lubricating layer.

3.2 Types. Both loose snow and slab avalanches

are further classified as dry, damp, or wet, according to the amount of liquid water present in

the snow involved. Knowledge of snow conditions at tlie time the slide occurred will help determine this, as will a study of the snow debris

and pattern of flow of the 'slide. Damp or wet

slides usually deposit snow in rounded balls or

lumps as distinguished from the angular blocks

of the hard slab avalanches. The wetter slides

tend to flow in grooves and channels, often leaving

snow ridges between the grooves and displaying

24
the feature of "wheeling," or curving off in different directions, when they reach flatter ground

(fig. 24).

Another feature of classification, and also a general basis of distinguishing wet from dry snow

avalanches, is the form of motion of the slide.

Three general types of motion are recognized;

motion mostly in the air (dust cloud), motion

purely on the ground, and mixed motion where

both forms are present. Motion through the air

alone is rather rare, and usually occurs when dry

snow flows over clift's or very steep terrain. Mixed

motion—some on the ground and some in the accompanying dust cloud^—is the most common form

encountered with the dry avalanche of midwinter,

both loose snow and slab (fig. 25). Motion mostly

on the ground, with little or no accompanying

dust cloud, is peculiar to the damp or wet avalanche of spring and summer. Wind blast, the

strong wind winch rushes ahead of the sliding

snow, is caused by the dust cloud accompanying

air motion.

Avalanches are also classified according to

whether they slide upon an underlying snow layer

(surface avalanche) or on the ground (gromad

avalanche).

Figure 26 illustrates the more important features of various types of avalanches.

3.3 Size. The dimensions of a slide would seem

to be the logical method of determining its size.

But to be of any value, the measurements should

be accurate and these are often difficult, dangerous, even impossible to obtam. The shape of an

avalanche is irregular. Wind action, sun action,

and snowfall quickly disguise it. Dimensions do

not always indicate the momentum, one of the most

important features of a slide.

'-%2r--.

.^0t»^^^

FIGURE 24.—The path of a large wet snow avalanche

(WS-N^), showing the grooving and channeling of

the path and the snow boulders in the deposition zone,

both characteristic of this slide type.

Classification of avalanches as to size is based

on the observer's estimate of threat to life and

property :

1. Large or major—highly dangerous to human

beings and property. A person caught would be

killed or severely injured. Slides in this class

destroy buildings, trees, or structural facilities.

2. Äledium—dangerous to human beings but not

likely to cause property damage.

3. Small—harmless to humans or property.

Abbreviations for the convenient description of

avalanches in reports of snow and weather records

are listed in chapter 6. (These abbreviations are

used throughout this handbook whenever a specific avalanche is described, especially in the photo

captions.)

3.4 Avalanche Triggers. Direct action avalanches occur during or immediately after a storm

as the result of a new fall of snow. Delayed

action avalanches do not result directly from a

fresh snowfall, but are caused by wind action,

temperature changes, melting, or internal structural changes in the snow. Climax avalanches

are caused by the accumulation of snow from a

number of storms until an unstable state is reached.

This latter type ordinarily involves a large part

of the snow cover, and occurs infrequently (fig.

27). Their occurrence depends on the character

of weather and snow cover development. The

conditions leading to a climax avalanclie situation

on a given mountain may occur only once in 5 or

more years.

When snow reaches a critical state of instability,

there often is some definite triggering force which

actually sets it in motion as an avalanche. There

are many unstable slabs in the mountains each year

which never fall as avalanches because no trigger

was present to cause their release. Such snow

eventually stabilizes and the hazard disappears.

In some instances, one period of critical instability

may pass without release, only to be followed by

another from a different cause—such as thaw in the

spring—wliich starts a slide.

External triggers are those involving some disruptive force exterior to the slab or loose snowlayer. Common natural triggers in this class are

falling cornices, rolling snowballs, lumps of snow

falling from trees, icicles, rockfall, or small slides.

Small avalanches or loose snow sluffs which might

otherwise be harmless frequently apply enough

dynamic stress on the layers over which they slide

to dislodge a larger slab avalanche. Large avalanches and massive cornice falls are very effective

triggering agents, for they usually develop enough

force to carry with them any snow layers which

have even the remotest inclination to instability.

The involuntary artificial release of avalanches

is a common cause of accidents. Most avalanche

accidents involve snow slabs released by the artificial external trigger furnished by the victims

themselves. Many such avalanches no doubt never

25
FIGURE 25.—Dust cloud accompanying a dry avalanche (HS-AA-4) released by artillery firing in midwinter,

cloud indicates mixed motion that distinguishes this from a wet avalanche.

F-496204

The

would fall if they had not been artificially triggered. Artificial external triggers are often deliberately supplied by skiing, artillery fire, or explosives for the purpose of avalanche control.

Changes in the stress-strength relationship of a

slab layer, such as temperature changes, overloading by snowfall, or constructive metamorphism,

are regarded as internal triggers whicli carry some

part of the mechanical relationship of the slab

beyond the breaking point and so effect release.

The operation of these agents is less obvious than

the external triggers, but they nevertheless are responsible for the natural release of many slides.

The most common one is overloading by fresh

snowfall. In this connection it may be noted that

the rate of overloading or deposition of a slab

layer also appears to play an important part in

avalanche formation and release. This rate is

expressed practically by the measurable meteorological factor or precipitation intensity. The importance of this factor to forecasting techniques

is discussed in chapter 5, item 5.1.

Tradition has long regarded the sound of a human voice as an avalanche trigger. However, it

has not been clearly demonstrated that such a weak

force as the sound waves from voices can cause

an avalanche. The possibility cannot be entirely

dismissed because stronger sound waves, such as

the concussion from explosions, have been observed

to dislodge slides. It is therefore conceivable that

much weaker sounds could trigger a slide when

extreme instability exists. Evidence, on the other

hand, seems to indicate that many cases where the

fall of an avalanche has been attributed to sound

may actually have occurred because the collapse or

fracture of tlie snow cover was propagated over

long distances. It has been observed that such a

collapse can travel far, even over level ground.

Local mechanical disturbance of unstable snow,

instead of sound waves in the air, may thus at

times be responsible for distant avalanche releases.

26
TYPE.

OF

3N0¥V

SLA/i LOOSE 5NOW

TYFe.

OF

Afor/oâi

IN THE A/J^.

L :^

ON THE oeO(/Â/D /f/X£D

Fe.B£

UVATES

CQ/VT£HT

DH.y

MO fM.ee WATCH.

DAMP

TitAce o^ f»ee wATee.

tV£T

Fx.ee WATet I'/sibLv nttsettr

(SUZFAC£

AVALANCHB)

SLIDING

sa/zFAce

(GROUND

AVALANO/B)

i/nOB/LLYIN«

SNOW LAyem.'x

Tt4£ <:B.Oí/NP

FIGURE 26.—Avalanche classification chart.

27
mL

^«tr^d:j -'.

,-\^r

-. •*- 4äa:: ..'v

• V^:^^ A-:.'

r * ^

> -i .^^ "'•*» 'i •

P-4B5205

FIGURE 27.—Major rliniax slab avalanche which overran a highway located in the bottom of a canyon (HS-N-5).

Origin of the slide was far above the slo¡>e shown in this picture. Smaller slides on this path are normally arrested on the bench seen near the top of the picture. Argenta Slide, Big Cottonwood Canyon, Utah.

28
2332.81 SNOW AVALANCHES

Chapter 4

4 TERRAIN. There are two fundamental requirements for snow avalanche fonnation—snow

and a mountain for it to slide on. Terrain is the

fixed factor, important but more amenable to

description and classification, for it remains unchanged in any given locality. Avalanche barriers

are no more than the artificial modification of

terrain.

Steep gullies and steep open slopes are natural

avalanche paths. Ridges, outcrops, and terraces

are natural avalanche barriers. Ridges parallel

to the fall line set definite boundaries to an avalanche path, while those running across the fall

line halt, slow down, or divert the moving snow.

As a practical matter, ridges are always the safest

route to travel in avalanche terrain. On the other

hand they modify the orientation of a slope so

that one side may be stabilized by sun and wind

while the other is an accumulation zone for slab.

Rock outcrops make effective diversion barriers

or islands of safety whicli attract the eye of the

area phmner as likely locations for structures.

Terraces provide a zone of transition where the

slope angle changes rapidly. They slow an avalanche clown and give it a chance to spread out.

Many major avalanche paths include one or more

terraces which retain most of the annual slides and

only avalanche to the full length of the slope under unusual conditions. The snow ranger takes

full advantage of terrain features in preparing

and carrying out his area safety plan.

Terrain variations are as wide as nature itself.

The following are tlie most important.

4.1 Slope Angle. Critical slope angles for avalanclie formation have been the subject of many

and varied pronouncements in handbooks and

manuals. Several sources have set the arbitrary

figure of 22° as the critical angle above which

slides are apt to occur. Avalanches in fact have

been observed on slopes as moderate as 15°, though

this is an unusual condition. The proljability of

avalanching increases with slope angle up to a

certain steepness, and then decreases as slopes

get steeper and approach the vertical. Tlie reason

for tliis decrease is the failure of snow to adhere

in large quantities to extremely steep slopes; it

simply runs off in small, harmless sluffs before any

great depth can be accumulated. Occasionally,

certain snow conditions will permit the formation of avalanches on very steep slopes, but this

is not the general rule. It does not seem reasonable to draw any sharp dividing lines, but rather

to recognize that there is a zone of slope angles

within which dangerous avalanching is most common. Figure 28 has been drawn with this in mind.

FIGURE 28.—The relation of slope angle in the release

zone to probability of avalanche release.

Avalanches of large size may originate on slopes

steeper or gentler than 2,5° to 60°, but the probability gets smaller as the slope angle approaches

0° and 90°. Slope angles at the fracture lines of

major avalanche paths within the Alta ski area

range from .35° to 40°.

Practical field experience by the snow ranger

leads to a certain intuitive "feel" for the steepness of slope above which slides are commonly released in the course of test and protective skiing.

Actual check of this intuition against an Abney

level suggests that the common danger zone lies

above .30-33°.

4.2 Slope Profile. Slopes whose profiles are

convex in tlie vertical plane definitely favor slab

avalanche formation (fig. 29). The reason for

this can easily be understood in terms of slab

mechanics discussed in chapter 3, item 3.12 for

tlie increase of angle downslope on a convex surface results ill an increase in creep velocity downslope. The tensile stress from differential velocity

is thus added to that between creeping and anchored snow. On concave slopes, differential

velocity tends to form compressive stresses which

577868 O—61- 29
oppose the basic tensile stress near the top of the

path This should not, however, be construed to

mean that avalanches are unlikely on a concave

slope, for many large avalanche paths are concave in vertical profile. Actually, the concavity

(rate of change of slope angle) near the top of

natural concave slopes is frequently low. Since

such slopes do not differ greatly from a plane

surface, the stress modifications due to slope curvature are small. Creep tension is apt to be more

strongly developed on convex slopes. This may

also be true of slopes which are convex in horizontal profile.

4.3 Ground Cover and Vegetation. Koughness

of the ground surface has two important effects.

One is to detennine how much snow is needed to

fill in the terrain irregularities and present a

smooth sliding surface; the other is to control the

amount of creep motion of the snow over the

ground and the ability of the snow to avalanche

on the ground itself.

FIGURE 29.—The influence of slope profile on stress distributions within the snow cover.

Smooth, grassy slopes favor avalanche formation, for very little snow is required to smooth

over the irregularities. Wet grass also provides

an excellent sliding surface which encourages the

release of ground avalanches. Snow creep is particularly rapid over a grass surface.

The opposite extreme in ground cover is a field

of large bouldere which may take 6 feet or more

of snow cover before a smooth sliding surface is

presented. Creep is low on such a slope, and

ground avalanches practically unknown.

Certain types of buslies—willows, for example—

provide a stabilizing influence early in the winter.

They effectively retain the snow while it is still

shallow. Such light vegetation, however, is

quickly flattened by snow creep and settlement.

By midwinter, or earlier, it is usually covered

and offers no further stabilizing influence for surface avalanclies. . . Heavy stands of timber are usually positive assurance of stabilized snow. Avalanches do not

ordinarily originate in a thick stand of trees high

enough to remain well above the maximum snow

depth, though loose snow slides here are possible

under the rare condition of extremely unstable

wild snow deposition. Scattered lar^e trees are

not proof against avalanche formation. Open,

alpine parklands, even where many large trees are

present, are the sites of numerous avalanche paths.

An avalanche fatality in Colorado in 1960 occurred in just such terrain which, though the

many trees made it appear safe to the casual observer, was long known as an active avalanche

path.

Heavy timber inhibits the release of avalanches,

but is not necessarily a protection against slides

falling from open slopes above. Well-developed

avalanche paths originating above timberline

usually have obvious paths cut through timbered

slopes below, where forest growth has been prevented by repeated slides year after year. Occasionally an abnormal situation will bring about

large climax slides which break new paths through

the heaviest timber. Traces of destruction from

such slides can be found in most mountain areas

of the Western United States. The catastrophic

avalanches in Switzerland during January 1951

furnished several examples of unusual climax

slides which fell through heavy timber where

none had been observed for hundreds of years.

The presence of trees on a slope also has an

important influence on wind currents and the deposition of snow during storms. This influence

may either inhibit or encourage avalanching, depending on relation of the trees to a given slope

and prevailing winds. Artificial control of snow

deposition is possible either by cutting the trees or

duplicating their effect with snow fences.

4.4 Slope Orientation. Orientation of a slope

in respect to the sun and prevailing storm winds

has a prime influence on avalanche development.

The amount of heat the snow surface receives from

the sun is directly dependent on both orientation

and slope angle. The importance of heat transfer

to snow behavior has already been emphasized in

chapter 2. Slopes w^hich face the northern quarter receive very little energy from the sim during

the winter, and those above a certain angle, depending on latitude, receive no sunlight at all during part or most of the winter. Such snow surfaces receive maximum cooling by radiation losses,

thus remaining colder and in a less metamorphosed

state than snow that can be warmed by the sun.

North exposures are the most favored locations

for depth hoar formation early in the winter. In

general, avalanche danger from the dry slides of

winter can be expected to be most frequent and

persistent on north exposures, other factors such

as snow deposition being equal.

30
South exposures, on the other hand, receive the

full benefit of solar radiation. Surface of the

steeper slopes lies nearly at right angles to the

rays from the smi when it is low in the winter

sky, so that maximum concentration of radiant

energy per unit area is obtained. Destructive

nietamorphism and settlement are thus accelerated by higher snow tem^ieratures, and snow stability is more easily achieved in the winter snow

cover. As the sun rises higher in the sky with

the approach of spring, heat supply increases to

the point where substantial melting occurs, and

then south-facing slopes become the site of instability and wet snow avalanching (fig. 30).

The lee slopes, those which receive the heaviest

deposition of snow by winter storms, are frequently the site of avalanche paths. In such locations the snow lies deeper, accumulates rapidly

during storms, and is most apt to be deposited as

unstable slabs. In addition to this combination

of favorable factors of avalanche formation, lee

slopes are often overhung with cornices which

may fall and provide effective triggers. Those

parts of a mountainside which lie to the lee of

prevailing winds are thus very likely to be the

most dangerous avalanche slopes. This applies

to the individual lee aspects of gullies, ridges, and

other terrain features subordinate to the principal

mountain faces. Such lee slopes may receive snow

deposits even during fair weather by wind

drifting.

Windward slopes ordinarily are much less likely

to avalanche, for they receive less snow deposition,

and this snow is apt to be more firmly compacted

by the wind. Wind pressure no doubt plays a part

in this compaction, but the cementing of snow

crystals by rime during storms is also considered

an important factor in stabilizing windward

slopes. The windward exposures should not be

considered as completely avalanche proof, because

under certain conditions of heavy snowfall, slides

may occur without respect to slope orientation.

'j^^-

F-495208

FIGURE 30.—A small, wet, loose snow avalanche (WLSN-2). Released on a south exposure by natural sunball activity.

31
2332.81 SNOW AVALANCHES

Chapter 5

5 AVALANCHE HAZARD FORECASTING.

The "art" of forecasting is not sufficiently advanced to predict the exact time a slide will release on a given a^-alanclle path. It is possible to

estimate the probability that slides will occur on a

slope. Such an estimate is a hazard evaluation.

When the effects of snow and weather developments are projected into the future, the evaluation

is a hazard forecast.

Under many conditions, especially those for direct action slab avalanches discussed below, the

period of high hazard (high probability of slide

occurrence in an area) can be evaluated with reasonable accuracy. These periods can also be forecast with good accuracy if the weatlier trends—

particularly siorm behavior—can ])e foreseen.

The practicing avalanche forecaster will soon

learn that this "if" is the biggest stumbling block.

The application of fundamental knowledge of

snow and avalanche behavior, qualified by local experience, often makes it possible for the forecaster to say with confidence, "If certain weather

conditions persist, large avalanches will almost

certainly fall": or "If a tliaw does not set in within the next 24 liours, the snow may be considered

stable." Xeither he nor the professional meteorologist can say with an equal degree of assurance,

"Precipitation intensity will be 0.15 inch per hour

tonight," or "Heat balance of the snow cover will

not exceed a given number of calories per square

centimeter per hour this afternoon." Unfortunately the art of mountain weather forecasting,

especially wliere precipitation quantities are concerned, lags behind that af avalanclie forecasting.

The degree to which a snow ranger is able to forecast avalanche hazard largely depends on his

shrewdness in estimating mountain weather. In

actuality, his forecast is more often an evaluation

of present conditions. Mindful of the adage that

"only fools and amateurs ti-y to predict mountain

weather," the avalanche forecaster is in an uncomfortable position. Nevertheless, "hazard forecasting" will continue to be referred to in this chapter

wliile recognizing the limitations of this

expression.

Understanding of the various contributory factors used to forecast direct action dry slab avalanches has improved. Experience has demonstrated that direct action dry slabs represent the

greatest source of hazard in ski areas. Tliis is the

type of avalanche which is encountered throughout the winter, storm after storm. Delayed action

and wet slides may be just as dangerous when

they fall, but they occur less frequently.

There is absolutely no substitute for field experience as training in avalanche hazard forecasting. A thorough practical knowledge of snow behavior and an understanding of the basic principles outlined in this handbook are essential to successful hazard evaluation. Moreover, each region

and locality has its own peculiarities of terrain and

climate which must be learned by actual observation. The principles of snow behavior are the same

everywhere, but the individual variations are

endless.

5.1 The Contributory Factors. The analysis

of contributory factors is a method of estimating

the development of avalanche hazard. This

method applies primarily to direct action dry slab

avalanches which fall during or immediately after

a snowstorm. Delayed action climax avalanches

and the wet slides of spring are not readily predictable from a contributory factor analysis;

danger from these can be foreseen only by the

careful study of structural, crystalline, and temperature conditions within the snow cover.

In the Western United States, weather and

climate produce three alpine zones, each with its

dominant type of avalanche :

The High Alpine Zone, such as the Colorado

Rockies, is characterized by comparatively low

temperatures, light and very dry snowfall, very

strong wind action, and hard slab avalanches

associated with depth hoar.

The Middle Alpine Zone, such as the Wasatch

Range in Utah, is characterized by fairly cold

temperatures, hea\'7 and dry snowfall, strong

wind action, and dry, soft slab avalanches.

The Coastal Alpine Zone, such as the Cascades

in Oregon and Washington, is characterized by

moderate temperatures, heavy and damp snowfall, strong wmd action, and damp soft slab avalanches.

The zones overlap and may produce slides having similar characteristics, but the dominating

types of avalanches are distinct enough to warrant

identification of these zones.

The forecasting factors which follow were developed primarily in the Middle and Coastal Alpine Zones.

Experience at the Berthoud Pass avalanche

station in the High Alpine Zone has shown that

the critical values of some of the factors listed below have to be modified for the conditions prevail32
ing at high ahitudes and low temperatures.

Storms in this zone generally bring higher winds

and lighter snowfalls than those in the other zones.

The result is a tendency for localized formation

of hard slabs instead of the more widespread soft

slab development encountered when snowfalls are

heavier and the areas in question are nearer to or

below timberline.

The ten factors which are used in this method

of hazard forecasting have been selected empirically on the basis of experience and snow, storm,

and avalanche records over the years. There may

well be other factors, or aspects of the present

ones, which have not yet been clearly recognized.

For instance, the exact type and percentages of the

different kinds of snow crystals in a fresh snowfall are believed to have some bearing on the formation of snow slabs, but this relationship has not

yet been exactly determined. At present the practice is to give all ten factors somewhat equal

weight in the analysis. Actually, they do not all

have equal effect, "but the relative importance of

the different factors has not been measured.

Experience has shown that this method of hazard forecasting is reasonably accurate. A general

direct action avalanche cycle almost always coincides with a predominant number of factors favoring avalanche formation; and, conversely, when

most are unfavorable, a general slide cycle seldom

is observed. In practice, the observer's analysis

should always be checked by actually testing the

snow on steep slopes, either by explosives or by

skiing, to see if unstable conditions actually are

developing. This is particularly true in borderline situations Avhen the contributory factor analysis does not give a clear indication one way or

another.

1. Old Snow Dejyth. The primary contribution of the old snow, without reference to its character, is to fill in the terrain irregularities and

provide a smooth, even surface upon which an

avalanche may start. The amount of snow it takes

to cover up the irregularities—rocks, stumps,

bushes, etc.—obviously will depend oh the nature

of the terrain, so the exact value of old snow depth

which begins to contribute toward avalanche formation will depend on the locality. On smooth,

open grass slopes as little as 6 inches might be a

favorable factor, while in a field of large boulders

it may take 6 feet before everything is covered

and a smooth snow surface is present. In the

usual mountain terrain a depth of 2 to 3 feet might

be taken as an average value. The general rule in

evaluating this factor is that the deeper the old

snow, the more likely avalanches are to form.

However, given the proper type of old snow, even

a few inches might be a dangerous indication.

2. Condition of the Base. The nature of the old

snow surface determines in part whether a new

fall of snow will form a good bond. A crust on

the old snow may provide a smooth sliding surface and favor avalanches. A crust covered with

a few inches of unconsolidated light powder is

more likely to contribute to avalanche formation

when fresh snow falls, because the intervening

layer of loose snow provides a lubricant between

the new snow and the crust. New snow may sometimes make a firm bond to even a very hard crust ;

whether or not this liappens is controlled primarily by new and old snow temperature. If the

new snow falls at temperatures near the freezing

point, or if the old snow surface layers are warm,

then a good bond is possible. A surface of very

loose or fresh snow may sometimes form a poor

bond, while firm or settled old snow is more apt to

offer a good bonding surface. Extremely rough

windblown snow (sklaver) offers sufficient irregularities to provide bond for the new snowfall,

so that slide formation is hindered.

The character of the old snow also determines

the amount of support it can give to subsequent

fresh layers, and whether or not it will itself become involved in slides originating in the new

snow. Unstable slabs in the old snow may in

themselves be the primary source of an avalanche,

for the weight of the new snow may provide a

trigger for their release, even if the new snow is not

inclined to slide. Weak or ¡looi'ly bonded layers,

old crusts, or air spaces within the old snow may

all be considered favorable to avalanche formation. A particular type of weak layer which is

especially disposed to cause avalanching is depth

hoar (chapter 2, item 2.3). This type of crystal

forms a very weak and fragile mechanical structure and offers little support to successive layers

of snow. Even a very shallow layer of old snow

which consists mainly of depth hoar may induce

avalanching.

3. Neio Snoiö Depth. The single most important cause of direct action avalanches is a large

quantity of new snow. Deep falls may not necessarily produce avalanching, and even small quantities of new snow can cause large slides under

the right conditions; but as a general rule, deep

new snow is a warning of possible hazard. The

deeper the ne^y snow, the more likely that unstable

conditions exist. New snow of a foot or more

may cause avalanches of significant size. New

snow of less than a foot does not usually build

up a general hazard, except in the High Alpine

Zone where 4 or .5 inches of new snow with high

winds may produce a general hazard. A few

inches accompanied by high winds may produce

localized danger from slabs on lee exposures in

the other zones.

-t. New Snoio Type. The type of snow crystals

which make up new snow considerably influence

the type of avalanche which may develop or

whether any develop at all (chapter 6). Pellet

snow, or graupel, for instance, is peculiarly disposed to form slab avalanches, both because it is

usually denser than normal snow types and because It easily forms a stiff and slablike layer A

snowfall composed entirely of needles also leads

to a dense slab formation, while granular snow

(fine, rime-coated crystals) is another type asso33
ciated with this kind of nvalanche. The fluffy,

feltlike snow built up primarilj' from stai'crystals

and spatial dendrites does not usually leací to a

slab formation unless subjected to heavy wind

driftinij. However, there appears to be rehationship between this snow type and soft slab formation when there is some rime coated on the crystals.

Stellar or dendritic snow, if deposited in appre<?iable depth under cold, calm conditions favors loose

snow avalanches.

5. New iSnoui Densiity. Avalanche occurrence

is definitely associated with new snow densities

which depart widely from normal. Careful

measurement of the density of new fallen snow is

therefore one of the standard obsei-vations for

hazard forecasting. What constitutes the limits

of normal density is open to some interjirelation.

Annual average new snow densities computed

from several years of data may range from 0.07

to 0.10 gm./cc. (7 to 10 percent water content) or

higher, depending upon climatic conditions.

Wide departures from this range are sometimes

obser\^ed; and the greater the departure, the more

likely is the snow to be prone to avalanche. The

higher densities, which may range up to 0.25 or

0.30, are associated with slab avalanches. Low

densities, as low as 0.01 when snow falls under

very cold and calm conditions, generally lead to

loose snow avalanches. New snow densities must

be measured in the surface layers within a short

time after the snow is deposited; otherwise settlement will have altered the original density, especially at warmer temperatures. "

6. Snowfall Intensity. Snowfall intensity is

the rate at which snow is deposited during a

storm, expressed in inches of snow per hour. Two

feet of new snow deposited over a period of 2 or

3 days may cause little trouble, but the same

amount falling in a period of 10 or 12 hours can

precipitate widespread avalanching. The influence of snowfall intensity appears to be related

to the ability of the snow to stabilize itself by settlement. High rates of snowfall create instability

faster than settlement can reduce it. Observations indicate that for most conditions a snowfall

intensity of 1 inch per hour is about the critical

rate above which heavy avalanching often occurs,

especially if this rate of fall continues for several

hours. In the High Alpine Zone a snowfall intensity of one-half inch per hour is favorable for

avalanching.

7. Precipitation Intensity. Precipitation intensity is the rate at which water, in the form of

snow, is deposited by snowfall or wind drift. It is

thus a measure of the rate at Tidiich mass is being

added to a snow slope and is probably a more basic

factor for avalanche formation than is snowfall

intensity, though the latter is easier to measure in

the field. High precipitation intensity is associated more definitely than snowfall intensity with

avalanche occurrence and notably with the formation of slab avalanches. Experience has shown

that a rate of water deposition greater than 0.10

inch per hour, accompanied by winds above the

critical level for an extended period, has a high

correlation with avalanche occurrence. Short periods of high precipitation intensity do not necessarily lead to avalanches, but the deposition of 1

inch or more of water (as snow) at a high rate is

a favorable factor. Extensive direct action slab

avalanches are almost always associated with high

precipitation intensity coincident with high wind.

The effects of rate of mass deposition may be

expressed in tenns of the precipitation intensity

factor. This factor has a numerical value equal

to the amount of water, expressed in inches, which

has been deposited for a continuous period at a

rate of 0.10 inch per hour or greater, while winds

are above the critical level. Thus a sustained precipitation rate of 0.15 inch per hour for a period

of 10 hours while the wind is at high levels would

yield a precipitation intensity factor of 1.50. This

factor expresses the coordinate influence of wind

and precipitation. Values in excess of 1.00 in the

Middle and Coastal Alpine Zones and 0.50 in the

High Alpine Zone are taken as warning of possible avalanche development. Extensive and destructive avalanche cycles are often associated with

a P.I. factor of 3.00 to 4.00 or greater. The catastrophic avalanches in Switzerland in January 1951

coincided with development of a P.I. factor

greater than 4.80.

8. Settlement. Settlement is a process of stabilization in the snow cover, the rate of which is

more or less proportional to temperature. Snow

which has undergone strong settlement is less

likely to slide than snow which has not. Of especial importance is the settlement that takes place

in new snow during and immediately after a

storm. Even very deep snowfalls may create little

avalanche activity if the new snow settles very

rapidly while it is still being deposited. Settlement in the old snow during a storm indicates to

what degree it is stabilizing and compacting under

the weight of the new snow. This settlement is

also significant in hazard forecasting. The actual

numerical values of settlement which are significant for avalanche formation are widely variable

and it is difficult to give precise figures. In general, new snow settlement in excess of 15 to 20 percent indicates a trend toward stabilization.

There is one situation in which settlement is not

a stabilizing factor. An imderlying snow layer

may settle away from a Stifter layer above, even

to the point of leaving an air gap "when the latter

is strong enough to be self-supporting.

9. Wind. Wind is the primary agent in formation of slab avalanches. The presence of wind

during a storm, or at any time when strong

enough to cause snow transport, is a warning sign

of avalanche danger. Below a certain critical

velocity, apparently no extensive slabs are formed.

Study of many storms, snow, and avalanche records suggests that the average figure for this critical value is in the neighborhood of 15 miles per

hour, although the figure may vary considerably.

34
Some light wind can occur without development

of hazard. Prolonged periods of high wind cause

dangerous conditions. When average wind velocities are 25 or 30 miles per hour or moi-e for any

length of time, some sort of slab formation is almost certain. The magnitude and extent of slab

formation, however, is highly variable and some

of the factors aft'ecting it are not clearly understood. At one time wind action may develop only

liighly localized slab avalanches in certain lee exposures; while at other times, especially during

heavy snowfalls, slabs may form everywhere

regardless of wind direction. The only safe rule

is to beware of avalanche danger whenever high

winds blow.

There also appears to be an upper critical wind

level, not clearly defined, above \\hich snow tends

to form windpack rather than slab. Tliis phenomenon has been observed but not in sufficient detail

to be of use in hazard forecasting. Scanty data

suggest that winds above 60-70 m.p.h. may fall in

this category.

10. Temperature. Temperature has more ramifications in avalanche formation than any other

single factor. Both air temj^erature and the temperatures within tlie snow cover must be considered in estimating hazard development. The deposition of snow, the changes it undergoes on the

ground, and consequently the formation and persistence of avalanche conditions, are all highly

dependent on temperature. As a broad rule, avalanche hazard varies inversely with temperature

during tlie winter when snow and air temperatures

are generally below freezing. Under warm conditions, near the freezing point, the period of instability in snow is quickly reached and passed.

Either slides follow directly from storms or wind

action, or else the snow quickly stabilizes and offers little further source of hazard. Avalanche

danger thus can be high when temperature is near

the freezing point, but the period of instability

is usually short. As temperatures decrease snow

tends to persist in an unstable state for longer

periods of time, and at very low temperatures

(below 0° F.) avalanche conditions may last for

many days or even weeks. Low temperature

coupled with high wind is particularly favorable

for formation of the unpredictable hard slabs.

Such slabs may remain dangerous for long periods

if the temperatures remain low. Slab formation

may also be slower to develop in very cold weather,

and the danger may not develop for several days

or weeks after the snow layers involved are deposited. Low temperatures may also create snow

instability by forming depth hoar.

Snow and air temperatures are also important

keys to wet snowslides in the spring. During

most of the winter, snow temperatures remain below freezing; but with the coming of warm winds

and strong solar radiation in tlie spring, melt

water forms at the snow surface, percolates downward, and rapidly warms the snow to the melting

point. Excess free water provides a lubricating

action causing wet slides, but this cannot occur

until after the snow has been warmed. Snow

temperature is thus an important clue to wet slide

occurrence. Wet slides will not fall while the

snow is still cold. It is often observed in early

spring that the snow remains stable on clear, sunny

clays, but that wet slides of considerable size fall as

soon as a hazy or cloudy day occurs, particularly if

accompanied by a warm wind. In clear weather

the heat supply from solar radiation is counteracted to some extent by long wave radiation loss

from the surface, especially at high altitude; the

snow thus gains little heat during the day and

freezes hard at night. Warm winds can melt the

snow both day and night, and clouds reduce the

radiation loss; under these conditions much

greater melting can take place and provide large

amounts of melt water to trigger wet slides.

The trend of temperature during a snowstorm

afl^ects avalanche formation. Snowstorms which

start with damp, sticky snow near the freezing

point, then gradually become colder and end Avith

lighter snow, generally leave a stable deposit of

new snow. The warm, damp snow at the start

often bonds well to the underlying layer, and the

lighter, drier snow coming later does not add a

heavy burden. Storms starting with low temperatures and liglit, dry snow, followed by rising tem-

¡Deratures and wetter siiow, are more likely to lead

to avalanching. Tlie dry snow at the start forms

a poor bond with the older snow surface and often

has insufficient mechanical strength to support the

heavier snow deposited later in the storm.

5.11 Factor Analysis, It must be remembered

that these contributory factors are not isolated

elements of avalanche formation. Many of them

are interrelated, and, though they may be treated

one at a time in the analysis, it is their effects as

a whole that produce avalanche action. In the

method of factor analysis described below, the

forecaster can exercise a certain amount of judgment concerning these interrelationships and express them in his hazard estimate. Even so. the

result of analyzing the contributory factors is no

more than an approximation of a complex natural

phenomenon that is far from being completely

understood.

In order to sum up the results of the contributory factor analysis for a given storm, one

possible method is to assign to each factor a plus,

minus, or zero value, indicating that the factor is

favorable, unfavorable, or neutral to avalanche

formation. A preponderance of pluses among th^

factors will then indicate possible hazard development. If most of tlie factors are found to be

minus, the snow is likely to be stable. Another

method, which gives the obsei-ver wider latitude in

expressing his judgment and experience, is to

assign to each factor a number between 0 and 10

with 0 indicating minimum contribution to avalanche formation and 10 indicating a strono- disposition toward avalanching. By this method

more exact degrees of influence are expressed for

35
each factor than by the simple plus and minus

method. Tlie numbers so chosen are then summed

to o-ive a single iiumber, between 0 and 100, to express the degree of hazard development expected

in a given situation.

The liiffher the "liazard factor," the stronger the

indication is tliat avalanches may be expected.

A hazard factor falling in the midrange;—40 to

60—would indicate a borderline situation, or

limited slide activity ; while a high number—75 or

more—would be a definite warning of impending

hazard.

The analysis of contributory factors is a useful

aid in estimating avalanche danger; but it does

not give the whole answer nor a positive one.

Tliere is no substitute for actual testing of snow

conditions in the field; and decisions involving

public safety should, whenever possible, be based

on such tests as well as on general hazard forecasting procedures.

5.2 Delayed Action and Climax Avalanches.

These are the avalanches which fall as the result

of cumulative factors working over longer time

intervals than those associated with single storms.

The most common type of climax avalanche probably is that which occurs because successive snow

accumulations finally overcome the support of

some weak layer within the snow cover (fig. 31).

Successful assessment of approaching danger from

such slides depends on the recognition of mistable

configurations of snow stratigraphy and the processes wliich weaken the snow, especially constructive metamorphism.

Dangerous stratigraphie weaknesses cannot be

recognized by casual inspection. Excavation of a

pit is one means of gaining insight into the snow

structure, but to be most useful, such a pit should

be dug near the release zones on the avalanche

paths. The ram penetrometer discussed in chapter

6, item 6.3, can be used to obtain part of the necessary structural information without digging a pit,

but evidence gathered by this method is far from

conclusive. The day-to-day record of snow,

weather, and avalanche conditions throughout the

winter in a given area is another means of determining whether an unstable condition may be

forming. The most satisfactory information

comes from a combination of all thi-ee methods,

estimates from the daily observations being

checked by periodic pit excavations and by ram

profiles on the actual avalanche paths. This ideal

is achieved only in certain areas where trained observers are on the groimd all winter to observe and

record snow and weather developments. Elsewhere, climax avalanche danger can only be

crudely estimated, especially if the history of snow

and weather conditions is missing.

A knowledge of early winter weather conditions

is essential to anticipate many climax slides. The

character of the lowest layers of the snow cover

and their subsequent evolution largely determine

the stability of the snow cover as a whole. The

most frequent cause of instability is a layer of

i'% 4>¿:: 4^mS^

'>.

0-

, i.

F-495207

FIGURE 31.—Major climax avalanche (HS-N-5) which

crossed a small canyon, covered a highway, and ascended 300 feet up the opposite slope. Channeling in

the deposition cone indicates damp snow was involved

in this slide. Scale is given by the tractor in the

foreground, working directly over the highway. Coalpit Slide in Little Cottonwood Canyon, Utah.

depth hoar formed by periods of clear weather

early in the winter when the snow is shallow and

easily subjected to steep temperature gradients.

This condition is most common on northerly exposures which receive little sunlight. The mechanically weak layer of depth hoar near the ground

is unable to support a very large overburden, so

that the first heavy snowfall is likely to trigger

widespread avalanching. If the depth hoar is

only partially formed (incomplete constructive

metamorphism), the layer may not fail mechanically until several storms have deposited their

loads. Once an avalanche occurs in such a condition, exposure of the remaining depth hoar next

to the ground sometimes results in a hard crust

formation which provides an excellent sliding

surface for subsequent avalanches.

Formation of a smooth crust or ice layer next

to the ground early in the winter by rain or thaw

is another frequent source of climax avalanches.

This layer provides a good sliding surface, but

unlike depth hoar, has high strength. The critical factor then becomes the character of the bond

between the ice layer and subsequent snowfall.

Such a bond, though weak, is generally stronger

than the support offered by depth hoar alone, and

accumulation of much snow may be required be36
fore critical instability is reached. An ice crust

exposed by avalanching may also serve as a sliding

layer once again as more snow accumulates. Like

depth hoar, it can be the cause of repeated avalanche action, either direct or climax in character.

By late winter or early spring rising temperatures

eventually cause a bond to form between new snow

and such layers, so that the hazard from this

source diminishes as the season progresses.

The existence of marked inhomogeneities, especially those caused by weak layei-s or crusts, is apt

to introduce the possibility of climax avalanches

at almost any stage of snow cover evolution.

Tliose of early winter have been emphasized because they are most frequently associated with the

large and destructive avalanches.

A reduction of tensile or shear strength within

certain snow layers can lead to eventual avalanching of layei-s which may at one time have been

entirely stable. The most common form of this

weakening is that resulting from melt water intrusion, details of which are discussed under item

5.4. At this point we are concerned with the more

subtle influences of constructive metamorphism

and crust disintegration.

The internal diffusion of water vapor and recrj'stallization which characterizes constructive

metamorphism leads to reduction in strength

properties of snow, whether or not the process is

carried through to the end product of fully developed depth hoar. A reduction in tensile or

shear strength so effected can create dangerous instability in a snow slab; hence this weakening

process must always be watched as a possible

source of hazard. As pointed out in chapter 2, a

steep temperature gradient is the prime cause of

constructive metamorphism, so the temperature

influences on the snow cover must be accurately

estimated if hazard development is to be foreseen.

Long periods of cold weather without precipitation should be a warning sign, as should periods

of clear weather which permit strong radiation

cooling on shaded slopes. If the snow cover is

deep, strong surface cooling will usually affect

only the surface layers. These might either become dangerous in themselves, or else serve as a

weak layer, inadequate to support a subsequent

snowfall. When the snow cover is shallow, steep

temperature gradients throughout the snow cover

are possible. Even when winter is well advanced,

the latter conditions may be found on avalanche

paths whicli have lost part of their snow cover by

sliding. The surrounding undisturbed snow may

be deep enough to forestall development of strong

gradients, but this will not be the case on the

partially denuded avalanche path (chapter 2, item

2.6, example 2).

Icy crusts tend to disintegrate under these same

influences. The individual crystals lose their

strong cohesion to one another and become loose

grains which may provide a lubricating layer for

a slab resting on them.

A series of light snowfalls, especially at low temperatures, will form a much more unstable stratigraphy than will heavy snowfalls. In the former

case, the bond between snowfalls is often poor,

and tlie individual layers never develop very high

mechanical strength unless subjected to reworking by wind drift. Very deep falls of dry, loose

powder snow usually attain high mechanical

strength once they have undergone settlement and

incor]5oration within the snow cover.

5.3 The Ram Profile. Snow and weather observations, snowpit techniques, and operation of

the ram penetrometer—all necessary for the estimation of climax avalanche hazard—are described

in detail in chapter 6. The following paragraphs

illustrate some of the principles used in interpreting ram profiles in terms of potential avalanche

danger. These interpretations are based on work

conducted at Berthoud Pass.

At Berthoud Pass the studies have been focused

on identifying the characteristics which indicate

a snow slab that will fracture and become an avalanche. The principal test area is "The Koll," a

slope prone to the formation of slab in snow transported from the surface by wind.

During one observed 36-hour period, wind averaging 27 m.p.h. deposited 6 feet of snow on "The

Roll." This high rate of deposition indicates that

a delayed action slab avalanche condition can develop in a very short time in areas similar to the

test area. In fact, the development is so rapid

that stabilization by skiing could not be effective

unless done day and night. Incidentally, the test

area is an area of concentrated public use.

Tlie following profiles and inteq)rotations illustrate the methods used to reach certain preliminary conclusions regarding the stability of slab

as revealed by the penetrometer. The profiles

were all taken directly in the slide path either

just before or just after the occurrence of an avalanche. In two cases the avalanches were triggered by explosives. In one case blasting produced extensive fracturing with stabilization in

place. In two cases avalanches were produced by

protective skiing.

1. Interpretation : Chart No. 3 {fig. 32). Profile was taken in the Berthoud Pass test area. The

sections of particular interest are marked J., 5, C,

and D. A was slab deposited during a storm when

there was wind action on snowfall and on snow

transported from the surface. B was a weak layer

of older snow. 0 was an old comparatively thin

slab of high resistance. D was a very weak layer

of cup crystals.

Under explosives, the low-resistance slab layer

A fractured. Tlie break penetrated weak layer B

Significantly, high-resistance layer C withstood

the shock of tlie explosion and supported the vibration and weight of the avalanclie until it had

attamed full momentum. Layer C eventually collapsed and the fracture penetrated weak layer D

to the ground.

577868 O—61- 37
CHART NO S

PENETROMETER TEST

ON "ROLL" ABOVE SUOE OF DEC 15, 1951, BERTHOUO PASS

UECEMBER l6,1951

RESISTANCE TO PENETRATION (KILOGRAMS)

TOP OF CLIFF

CHART NO 5

PENETROMETER TEST

- 200 YDS NORTH OF TOWER NO 3, BERTHOUO PASS

JANUARY 5,1952

FIGURE 32.—Chart No. 3, penetrometer test.

This profile illustrates several important points.

The snowpack contained two slab layers, each

lying above a zone of minimum support. The

upper slab layer had low resistance, the other high

resistance. The low-resistance layer was 33 cm.

thick, the high-resistance layer only 13 cm. thick.

Layer C was subjected to a blast and a moving

load which amounted to 108 pounds per square

foot, deadweight. Nevertheless it was able to

maintain its position on a 40-degree slope until the

avalanche from layer ^4 attained full speed.

2. Interpretation: Chart No. 5 (fig. 33). This

profile was taken in an area similar to the test area,

and located near it. To the observer it indicated

a situation comparable to that on Chart No. 3—

a slab layer of low resistance over a zone of little

support. Under explosives, the area avalanched

to the ground.

3. Interpretation: Charts Nos. 7 and 8 {ßgs. 34

and 35). These profiles illustrate similar conditions in two areas of similar aspect but a number

of miles apart. Again we have two slab layers,

each lying over a zone of weakness.

In both cases an avalanche was triggered by

protective skiing. Chart No. 7 shows that the

fracture penetrated both slab layers but only the

0 1 C

RESISTANCE TO PENETRATION (KILOGRAMS)

20 30 40 50 60

0

i:^ 1

20

10"

1 1

Snow deplh = 76 inches

Woter equivalent = 24

Averoqe deosily 3 1

Steeper portions slid lo ground

40

w<lh (Hosting 1-3-52

'zo"

"L,

Í

^ 30"

1

1

-40"

1 1

i

|I20 , ^

VI

-50"

cL H

I6Ü

-60- s

1

-TO'

j

f^ »round

Í Measurements bf Sttllman, Cram

1 Borlona

s^SVMIIIII'y'/V<<\\\MllllfoV/X'yVWSJ Mil Y///^/K

FIGURE 33.—Chart No. 5, penetrometer test.

upper layer avalanched. The observer concludes

that the lower layer stabilized in place due to its

strong anchorage at the toe of the slope. It was

buttressed by the debris from several earlier

slides.

Chart No. 8 indicates that only the upper layer

avalanched at first. Eventually the lower, highresistance layer did break and carry with it the

remainder of the snowpack.

5.4 Wet Snow Avalanches. Of the various

types of slides, those originating with wet snow

sometimes may be predicted with the greatest precision as to time and place, because their formation

depends on measurable formation of melt water

and its penetration into the snow cover. The

absence of wet slide hazard can be rather firmly

established by the observation of snow temperatures below the freezing point. The presence of

liquid water in snow means that its temperature

has been raised to 32°F. Cold (subfreezing) snow

will not contain water in the liquid phase and does

not participate in the origin of wet slides. Snow

temperature readings provide a useful guide to

wet snow avalanche hazard, but the measurements

should be made at the fracture zone to be effective.

The positive heat balance of the snow cover which

38
SNOW SURFACE

CHART NO 7

PENETROMETER TEST

UPPER SLOPE OF ROLL, 8ERTH0U0 PASS

MARCH I7,|9SI

RESISTANCE TO PENETRATION (KILOGRAMS)

20 40 60 80 100 120

1

L_ ,

-10-

c

i=

1

/

/^ Snow slid to here caused by observer

digging out sno« sloke (Slide occurred Oder )5 minutes of digging! -30- ^1

.J

-L"1

"1

-60'

-TO'

r

L 1

-80' L —1

'90*

|00'

J

[unabi

II »a

to defer

about t5

mm« SnoM

feet }

Dap h e OCtly.

Povio Ck, Bronsc

1.

Is t>y

", BorlOfitJ

FIGURE 34.—Chart No. 7, penetrometer test.

leads to melting may differ greatly between valley

floor and ridge top, between north exposure and

south. Moreover, penetration of percolating melt

water can alter snow temperatures very rapidly.

A slope which appears safe because of cold snow

one hour may be ready to avalanche the next.

Wet snow avalanches are directly associated

with a source of liquid water at the snow surface

which can wann up the snow and then lubricate it

to reduce internal cohesion, or the adhesion between layers. An obvious source of liquid water is

rain. Rainstorms in winter or spring should always be regarded with suspicion until the .snow

has eitherslid or proved itself stable. Warm rains

in midwinter immediately following a heavy fall

of dry powder snow are a common cause of major

avalanche cycles in the Coastal Alpine Zone of the

Cascades and Sierras. Heavy melt at the surface

is the other common source of liquid water which

may precipitate slide action. A review of the

discussion on thermal properties of the snow cover

in chapter 2 is recommended at this point. The

major sources of heat which might cause melt

CHART NO e

PENETROMETER TEST

LOVELANO PASS, SEVEN SISTERS,- 7rh SISTER AT TIMBERLINE

MARCH 7, 1951

RESISTANCE To PENETRATION (KILOGRAMS)

-0 40 80 120 160

Í Uj .^

£'

?

^

^,^Shd

Tills portion Gild March 7,19^2

Caused by skier

J='

t

Ueosurements by

Taylor, Weich, Borland

g"" I ——/ 1

FIGURE 35.—Chart No. 8, penetrometer test.

should be noted. In particular, observe that the

heat balance at the snow surface may be more

strongly positive on a hazy day than on a clear

day, and that warm winds are an especially eflFective source of lieat. These two conditions sometimes combine to form very marked thaw Aveather

which may precipitate slide action. Remember

that wet snow action on south exposures is often

absent during clear, cold weather, even though

sunlight is intense, but a slide cycle can follow

when the weather turns warm, windy, and cloudy.

South exposures receive the full benefit of direct

sunlight and are the ones most commonly associated with wet slide activity. North slopes are

not immune, because a warm Avind or rainstorm

will affect all slopes alike without regard to orientation. Solar radiation has one additional property which makes it especially effective in warming and melting snow. The sun's rays can penetrate the snow surface to a depth of 4 to 8 inches

and cause subsurface melting, even at times when

the snow surface may be maintained at subfreezing temperatures by cold air.

39
Dangerous wet slide activity occurs most commonly when a sudden thaw or rainstorm affects

a cold, unsettled snow cover. The very rapid

metamorphic changes and melt water penetration

in dry loose snow create serious instability. The

same amount of rain or thaw that would generate

large slides in soft, new snow would be less likely

to cause trouble in settled, old snow which had

already been warmed to the melting point by previous thaws. The first heavy thaw of late winter

or early spring is likely to be the most dangerous

one. Tlie same conditions might be repeated later

in the season after a fresh fall of new snow. Wet

surface slides are likely to occur when a late spring

snowstorm is followed by a bright, sunny day,

even if new snow falls on settled, stable old snow.

Some of the largest and most destructive wet

avalanches are the gromid avalanches which result from prolonged thaw or rain. The entire

snow cover is thawed and permeated with melt

water, which lubricates the ground that serves

as a sliding surface. The total depth of the winter snow cover is then released as a wet slab avalanche. Such slides are most common on smooth,

grassy slopes or smooth rock which provide a

good sliding surface when properly lubricated.

In some situations a heavy ice layer located deep

in the snow cover may serve the same function as

a smooth gromid surface.

40
2332.81 SNOW AVALANCHES

Chapter 6

6 STANDARD SNOW OBSERVATIONS

AND TERMINOLOGY. The importance of

standardized, systematic observations of snow and

weather phenomena cannot be too strongly emphasized. Estimates and forecasts of snow and

avalanche conditions are based on the total experience of the observer and of others wlio have preceded him. To permit comparison of present conditions with this accumulation of knowledge and

past experience and drawing valid conclusions,

observations and records must all be expressed in

the same terms and the same units of measurement, and to the extent feasible, measured and recorded in the same manner. One observer's experience and knowledge must be recorded in a

manner that permits the next man to compare the

results with his own observations if the information is to be useful to future workers in the field.

Systematic terminology and methods of observing and recording snow data liave gradually been

evolved by Forest Service snow rangers working

every day in the field. The records they are accumulating are gradually being brought to a

single standard. It is the purpose of this chapter

to instruct Forest Service officers in these uniform

methods of observation.

6.1 Snow Studies Chart (fig. 36). The fundamental record is that of the daily observations of

weather, snow, and avalanche conditions entered

in the Snow Studies Chart ( fig. 36 ). The methods

and terms for each item on this chart are discussed below.

1. Temperature. Accurate measurement of free

air temperature, which requires that the thermometer be unaffected by radiation or local air variations, is extremely difficult. The best approximation is achieved with standard thermometers

mounted in a properly designed and installed

meteorological shelter. A proper shelter is the

most important item of the temperature equipment ; much more accurate and consistent records

can be obtained witli a dime store thermometer

correctly mounted than with the finest instrument

casually himg on tlie wall of a building. Whenever possible, a standard Weather Bureau meteorological shelter should be used, located, and

mounted according to Weather Bureau instructions. Standard maximum and minimum thermometers, used according to instructions, are preferred. Where a station cannot be attended

regularly every day, a thermograph mounted in a

meteorological shelter may be used. Tliis instrument does not give the accuracy of thermometers.

Maximum and minimum temperature readings

are taken once daily and at the same time every

day. The recommended time is in the late afternoon, but early morning readings are acceptable.

The maximum and minimum temperatures are entered in degrees Fahrenlieit each day on the Snow

Studies Chart.

2. Wind. The preferred information is the

number of miles of wind past the station each 24

hours, to be entered in the 24-hour total column.

Either a dial anemometer which displays miles of

wind directly, or a continually operated wind recorder is required to obtain this information. The

dial is the easiest to operate but requires that the

observer climb up the anemometer mast to get the

reading. A remote recorder can be read inside of

a building but is more complicated and requires

continuous power.

When an indicating or recording anemometer is

not available, the observer must enter his estimates

for the different periods of the day under the 6-

hour average velocity column, and indicate the

direction. Velocities are given in miles per hour

and estimates may encompass a range of velocities,

such as "3-5 mph." or "10-15 mph."

"\ATienever terrain, building location, and other

considerations permit, the wmd record should be

obtained from an anemometer mounted on a high,

exposed ridge or peak. Tlie wind behavior at

higher elevations is of primary concern in avalanche hazard forecasting.

Snow movement refers to the amount of snow

observed drifting each day. Drifting snow can

develop avalanche conditions on lee slopes just as

a storm can. An estimate of wind-transported

snow is an essential part of hazard forecasting.

A^Tiere the drifting is not directly observed, evidence such as fresh deposits in lee areas can serve

as the basis for estimating what may have happened behind clouds or during hours of darkness.

Widely different conditions between ridges and the

valley floor may be noticed. A separate record

should be kept for each area.

3. Precipitation. Regular, accurate, daily

measurements of snow depths are the backbone of

these records. In addition, more frequent measurements of new snowfall—as often as once every

hour in some cases—are an important part of

storm records and hazard forecasting.

41
42
A minimum of four snow stakes are customarily

used for standard observations. One is a fixed

stake used to measure total snow depth on the

ground. It is usually a length of clear 2x2 inch

lumber, painted white, with zero at the ground

level and each inch of height clearly marked with

dark paint. This stake is bolted to an angle iron

driven deep enough into the ground to provide

permanent support, and is long enough to record

total snow depth in the area. This stake should

be labeled with total inches from bottom to top,

rather than in feet and divisions thereof. The

othei-s are short, portable stakes made of a yardstick firmly bolted in an upright position to the

center of a 2 x 2 foot platform of i/4-inch plywood.

The plywood bases must be painted white, both

to protect the wood and to reduce radiation melting of the snow.

The snow observation site where the stakes are

to be located must be as free as possible from wind

drifting and snow creep. Tlie best location is in

a level opening in heavy timber. "VAHiere such a

site cannot be found, a compromise must be made,

but the stakes should be kept as far as possible

from buildings and other large objects which influence the deposition pattern of snow. The site

should not be located so far away from the observer's dwelling that daily observations are apt

to be neglected because of inconvenience.

Snow observations should always be made in the

early morning. Erroneous values may be obtained

if snow depths are read in the late afternoon or

evening, because the new snow depth will be reduced by settlement, or possibly melted away if

the sun has come out during the day following a

snowfall the previous night.

The first of the three short stakes is known as

tlie storm stake. This is set out on the surface of

the old snow and is left undisturbed for the duration of any one storm period. "Wlien the storm is

over it is dug up and reset on the surface to be

ready for the next storm. (The total depth of

snow deposited on the ground from a single storm

lasting more than 24 hours cannot be measured

without the storm stake. Summing the readings

of the 24-hour stake gives a different value because

of settlement.) The second yardstick is used as

the 24-hour stake. At each morning's observation

the depth of new snow which has fallen in the past

24 hours is read from this stake ; it is tlien cleared

of snow and reset on the surface. The third stake

serves as the interval stake. It is used to obtain

snowfall measurements over intervals shorter than

24 hours or during a storm, being reset on the

surface after each observation. The use of observations with the interA'al stake is discussed in more

detail under item 6.2.

Water content of the new snow which has fallen

during the past 24 hours is measured at each

morning's observation. The ovei-flow can of the

standard 8-inch Weather Bureau rain gage is used

to collect a snow sample from the 24-hour stake

platform. The can is held upside down and vertically over the platform adjacent to the yardstick

and pushed down through the snow until it rests

against the platform. Both can and platform,

carefully held together, are turned over. The

platform may then be removed and the core of

snow collected allowed to slide down into the 8-

inch can. The can is carried inside a building and

the snow melted. This may be done by pouring

onto the snow a measured quantity of hot water,

which is later subtracted from the measured precipitation ; or the can may be set over a low source

of heat until the snow is melted. Holding the can

in a sink so hot water runs over the outside is a

quick way to melt the snow. The water obtained

in the can is then poured into the narrow collecting tube of the rain gage and the amount of water

measured with the reguhir 10:1 rain gage dipstick.

pjxactly the same procedure is used to collect

and measure water content samples from the interval stake for use in computing precipitation

intensity (see under item 6.2). Instead of collecting the 24-hour core from the 24-hour stake platform, a separate plywood sheet may be put out on

the snow surface and reset each day like the 24-

hour stake. Both new snow depth and new snow

water content are entered each day in the appropriate columns on the snow studies chart.

The specific gravity of the new snow measured is obtained by dividing the water content,

in inches, by the snow depth, also expres.sed in

inches. Tliis figure is also known as the water ratio, which may be expressed as a decimal figure the

same as specific gravity, or as percent of water

(multiply specific gravity by 100). For instance, a

1.5-inch fall of new snow which contained 1.20

inches of water would have a specific gravity of

1.20/15 = 0.08, which is the same as a density

of 0.08 gm./cc. or a water content of 8 percent.

Though it is actually specific gravity that is determined when snow depth and water equivalent are

measured, this figure is usually referred to as the

density for the sake of convenience. The correct

symbol for density is the Greek letter Rho, p.

The form, or type of snow, in a fresh snowfall

is entered each day on the snow studies chart. A

general description such as "powder," "pellet," or

"granular" is given. In addition, the exact type

or types of snow crystals should be recorded according to the system of the International Snow

Classification, Types of Solid Precipitation (fig.

7, ch. 2). The number or numbers corresponding

to the type of crystals observed are noted immediately following the written general description.

Special care should be taken to note whether or

not the individual crystals show a coat of rime.

The presence of any rime should be noted on the

chart. Most crystals can easily be identified with

the naked eye, but occasionally a low-power magnifying glass may be required.

43
The following explanations of crystal types associated with the broad snow classifications will

aid the observer.

Powder snow. Fluffy, feltlike new snow consisting mostly of stars and spatial dendrites, with

occasional admixture of some plates or needles.

Gramdar snow. A collection of small crystals

made up of stars or plates which are partly coated

with rmie. It may also consist of irregular

crystalline aggregates. Snow that is almost all

needle crystals will likewise appear granular.

Pellet snow. Also known as graupel, or soft

hail, is made up of crystals (most often stars)

which have become completely and heavily coated

with rime so that they form individual round

balls.

Snow ßahes. Aggi'egates of individual crystals

which stick together at warmer temperatures.

Examine carefully to see what kinds of crystals

are actually present.

Windblown snow. Either during or after a

snowfall, may often consist of fragments of such

crystalline forms as stars or spatial dendrites.

The original crystal type is difficult to identify.

The settlement column under "Pi^ecipitation"

on the chart refers to old snow settlement. Old

snow in this context means snow which as been

on the ground for more than 24 liours. This

settlement is computed for each day according to

the following expression :

K Total snow \

depth of pre- I

vious day /

-I- {

new snow in

V past 24 hours)]-[total depth "1

at time of I

observation J

4. iSfate of Weather. Report the daily weather

conditions concisely and accurately. The term

"clear" should be reserved for days with cloudless

skies; a sunny day with higli scattered clouds

should be reported as such. To give more accurate

information, report cloud cover as scattered,

broken, or overcast. Scattered clouds cover less

than half of the sky, broken clouds cover more

than lialf but not all, while overcast means that no

blue sky is visible. ^^Hien changes in weather

occur during the course of a day, note them.

Record times and duration of precipitation on the

storm records (see item 6.2). Wlien Weather

Bureau forecasts are available, they should be

entered in the column provided.

5. Snoio Surface. This record is important both

for evaluating avalanclie conditions and for collecting data on skiing conditions. Follow the

terminology of this liandbook and be careful to

distinguish between the different conditions on

nortli and south exposures. This record sliould be

the source of accurate information to the public

on the actual state of skiing conditions (fig. 37).

6. Avalanche Conditions. The degrees of hazard should be entered in tlie appropriate column.

The "1700 actual" column is the record of avalanche conditions that actually occurred, to be

entered at the end of the day. The "0800 estimated" column is for entering the anticipated hazard in the morning. There may often be a wide

difference between the two entries for the same

day, resulting from changes in snow and weather

conditions or forecasting errors. Every effort

should be made to keep the "1700 actual" record

as accurate as possible.

The time and location of each avalanche observed in the area are entered in the appropriate

column. A\niere a large avalanche cycle has occurred and there is not enough space on the chart

for all the slides, enter a reference to the storm report where the avalanches are described in detail.

I)escriptions of the avalanches are entered according to the "Avalanche Abbreviations for Snow

Study Cliart" ( fig. 38 ). When these abbreviations

are used, an avalanclie is assumed to originate in

dry snow unless the lettei'S for damp or wet conditions are included in the description.

Avalanche types and classification are discussed

in detail in chapter 3.

It is important to record all snow movement.

This includes minor activities such as sunballs on

south exposures and small loose snow sluffs.

These are indications of changes taking place in

the snow and are significant in evaluating future

avalanche hazard.

6.2 Storm Plot and Storm Report Records.

The methods of observing and recording snow conditions described above apply in many cases to the

storm plot and storm report forms (figs. 39 and

40). These more detailecl records of snowfall will

be kept whenever avalanche liazard is a problem

and forecasting activity must be undertaken.

Certain refinements of observations are required

when reporting on storms. They are discussed as

follows :

1. Neto Snow Density. Snow density determinations have already been described in connection

with the daily snow observations. Density values

determined from the 24-hour records, however, are

an average for the total amount of snow that has

fallen in that period. In the course of 24 hours

snow type and density can change widely. Even

if they do not, settlement will make the average

value for the total quantity of snow different from

that of the snow most recently deposited on the

surface. For tliis reason, accurate new-snow densities must be measured at the surface almost as

soon as the snow falls. One way to get an undisturbed sample is to put a shallow container such

as a 1-pound coffee can on the snow surface and

allow it to fill full of snow. Tlie snow is then

carefully scraped smooth over the top of the can

and weighed. If the weight and volume of the

can have been previously determined, the snow

density can be obtained by dividing the volume of

the snow so collected into its weight. For example, a can with a volume of 1,000 cc. and a

weight of 100 gm. which, when filled with snow,

Aveighed 170 gm., would show that the snow density was:

170-100 ^_

^^ööö-=0.07gm./cc.

44
Snow Conditions Reports

Where reports of snow conditions are made to the public, particularly

for winter recreational purposes, only major classifications would be

used.

The following items are of interest to the general public:

1. Total depth of snow.

2. Depth of snow fallen in previous 2k hours.

3. Type of new snow: Dry, damp, or wet.

k. Condition of the surface; new snow, old snow,

ski-packed, crushed, etc.

5- Condition of the undersurface as it affects

skiing conditions.

6. Weather conditions and forecast.

7- Avalanche hazards (if any). Closures.

8. Condition of highways.

9' Classification of skiing conditions, which may vary

on different parts of the area.

Skiing conditions are classified as follows:

I. Excellent

1. New powder

2. Old powder

3. Skipack dry or damp

h. Com snow

II. Good

1. Damp new snow

2. Damp old snow

3. Skipack wet

h. Unbreakable crusts

5. Windpack dry or damp

III. Fair

1. Variable crusts k. Wet new snow

2. Icy crust unbreakable 5. Wet old snow

3. Windpack wet

IV. Poor

1. Breakable crusts 2. Slush

FIGURE 37.—Snow conditions reports.

45
Avalanche Abbreviations for Snow Study Chart

HS —- hard slat N -

SS —- soft slab A -

WS --- wet slab AA -

L - — loose snow AE -

l'JL --- wet loose snow AS -

D --- damp (prefixed to 0 -

HS, SS, or L)

G -

B - — sunballs

J -

natural

artificial

artificial, artillery

artificial, explosives

artificial, ski

surface avalanche

ground avalanche

windblast

1 -

2 -

3 -

1+ -

5 -

- sluffs

• - small

- medium

•- large

-- major

T| temperature rising

T I temperature falling

Example : A large, dry, soft slab avalanche released by artillery

fire would be entered on the Snow Study Chart as SS-AE-^l-.

FIGURE 38.—Avalanche abbreviations for snow study chart.

If available, a standard 500-cc. snow sample tube

may be used to collect a core of snow from near

the surface and the density determined by weighing as above. The tube should be chilled and very

carefully inserted into the snow to get an accurate

sample of light, fluffy new snow.

2. Gross and Net Snoivfall and New Snoio

Settlement. The amount of snow measured on

the storm stake is the net snowfall for the storm.

This is less than the amount of snow which actually fell because the new snow is continuously

settling. The amount which actually fell, without settlement, is called the gross snowfall, a quantity determined with the aid of the interval stake.

When only 2 or 3 inches of snow are allowed to

build up on the interval stake each time before it is

reset, there is little chance for any settlement to

take place. If the interval stake is reset frequently

during a snowfall, a number of small increments

of accumulation will be obtained. The ffross

snowfall is the sum of these increments. The

more frequently the interval stake is reset, the

more nearly will the sum of the increments approach the true Avalué of the amount of snow

which actually fell, free of settlement.

The settlement in the new snow during the time

of its deposition is the difference between the gross

and net snowfall. This difference divided by the

gross snowfall and multiplied by 100 gives the

percent settlement of the new snow during deposition. Once snowfall has ended, further settlement of the new snow can be observed directly by

watching the drop of the surface level on the storm

stake. (An example of these computations is

given in figure 40, sheet 1.)

3. Snowfall Intensity. Snowfall intensity

(S.I.) is the rate at which snow is deposited, expressed in inches per hour. Divide the amount of

snow which has fallen in a given interval of time

by the length of that interval. To be correct, this

46
storm Plot No. Date 12/25/59 Ai-ea AItu

Time Snow Depth New Snow Old P. I. S. I. WinA

rotal ätorm

2k

Hr fi #2 Type

Den

sity Sett

Snow

Sett T.B. Gatie Core Stake Instr. Speed

31-

rec. Temp. Notes

2^ Decerabe r

0800 17 6 6/c 6/ Qnp. 0.12 0.10 1.1 15 W 27°

1215 __ 12 5 5/' 1 Qnp. 0.10 0 0.12 1.2 15-20 W 2S°

__ a sha rp tei pel at ire drop aroui d 12G 0, wir d aji L stom inte nsity pickin ! up.

1520 28 16 9 V' ) 0 0 1-3 9 w

26 Decembi r

ü6oo 37 25 19 ^^

It.

pdx, 0.0Í 0 0 0.6

It. &

variai le 8"

Density from 2l4--hour

core, clouds now

breaking, snowfall

light.

FIGURE 39.—Storm plot form.

iunouiit of snowfall should be the gross snowfall

during the interval. Measurements sliould be

made over sliort intervals whenever possible so

that variations in the snowfall intensity during a

storm can be recorded. If this cannot be done,

then the average S.I. for each day can be obtained

by dividing new snow depth of the 24-hour stake

by 24.

4. Precipitation Intensity. Precipitation intensity (P.I.) is the rate at which water (which may

be in the fomi of snow) is deposited, and is expressed in inches per hour. Divide the water

equivalent of the snow that has fallen in a given

time interval by the length of that interv^al to get

precipitation intensity. The customary method of

obtaining the water content of the snow that falls

over short intervals is by collecting sam^jles from

the interval stake platform with the 8-inch rain

gage can, as previously described. A recording

precipitation gage may also be used. The slope of

the precipitation curve on the chart is the P.I.

Such a gage should always be equipped with a

scoop for obtaining accurate sampling of snowfall ; core samples with the rain gage must also be

collected to check the catch in the gage.

The average value of P.I. for a 24-hour period

can be obtained from the regular morning snow

observations, by dividing the total amount of

water collected from the 24-hour stake platform

by 24. Variations in P.I. during a storm, howevei', are highly significant for avalanche formation. P.I. values for shorter intervals should be

determined whenever possible.

5. Precipitation Intensity Factor. The Precipitation Intensity Factor is an index of possible

hazard due to direct action soft slab avalanches.

The first criterion is that winds must be above the

critical level of velocity. When this is so, the

P.I. Factor is the amount of solid precipitation,

(expressed in inches of water) which fell during

any given interval when the precipitation intensity was continuously above 0.1 inch of water per

hour. Two inches of precipitation at a rate of

0.0.') inch per hour obviously would not fit this

definition, and for such a situation the factor

would be nonexistent and inapplicable. If 2 inches

of i)recipitation fell continuously at 0.15 inch per

hour while winds were high, this factor would

exist as an index of hazard, and the "P.I. Factor'"

would be 2.00.

6. Hno\i^ Settlement. Settlement of old snow

during all or parts of a storm period may be determined just as it is for 24-hour periods.

6.3 Snowpit Studies. Periodic study and recording of suljsurface snow conditions to show

snow cover evolution through the winter has become standard procedure at Forest Service avalanche stations. This section gives instructions for

collecting, plotting, and interpreting snow pit

data.

47
Storm Report Instructions

Storm No.: Number the storms of each season consecutively.

1, Record the hour of beginning and ending of snowfall as closely as possible.

Record the duration, or durations, of the intense storm period in hours.

4. The snowfall according to measurements taken at the shortest intervals in

use at your station. Record this interval: 2 hrs., 4 hrs., 24 hrs., or

whatever.

5. The amount of snowfall at the end of the storm. Should be taken from a

platform stake set on the old snow surface and not reset during the storm.

7. Average S.I. for the major storm period. Maximum S.I.: Record periods of S.I. at l"/hr. or more.

8. Amount of water per inch of snow. Computed from the snowfall of each

24 hr. period.

9. Average P.I.: for the major storm period. Maximum P. I.: Record all

periods when P.I. was . 10"/hr. or more. The gage record must be reasonably close to the precipitation according to the snow core method. If

a recording gage is not in use, P.I. can be computed directly from the

snow core, but the snow should be sampled not less often than every 4 hrs.

10. Record any period when P.I. was continuously at or near . 10"/hr. with

strong wind action. Example; 6 hrs. at . 18: P.I. Factor 1.08.

11. Average wind speed for major storm period. Maximum: Record periods

when wind speed averaged 15 m.p.h. or more.

12. Sample computation assuming that you have a total snow depth stake, a

storm stake, and an interval stake:

Old snow depth- ------ 12" (total stake)

Gross snowfall ______ 12" (from interval stake summation)

Net snowfall- - ______ 10" (storm stake)

Total depth at end of storm - 21" (total stake)

New snow settlement was 2 inches and old snow settlement was 1 inch.

13. Percent of settlement is computed by dividing gross snowfall into the

amount of settlement.

14. Record general trend and any marked changes.

15. Record time, type, size, and location. Give best estimate of the trigger.

16. As above; also record method of release: Ski, explosives, artillery fire,

etc. Record stabilization in place if it occurs.

It is important for the observer to record his personal opinions, also any

unusual or unfamiliar developments.

17. Sufficient copies of the Storm Report should be made so that one is available to forward to the avalanche study center at Alta, Utah. Reports

should be made promptly following the storm.

FiGUKE 40.—.Storni rei>ort (sheet 1).

48
STORM REPORT

Havens and

Storm No. 1 Date 12/2^/59 Area Alta Observer LaChapelle

TECHNICAL DATA

1. Began 0100 2$ December 19$9 Ended 0600 26 December 19$9

Duration Ivlajor Storm Period 29 hrs.

2. Old Snow Depth 8"

3. Old Snow Surface depth hoar on N. slo-pes. bare ground on S. slopes

k. Gross Snow Fall 2¿]

5. Net Snowfall g^

6. New Snow Type 13" damp new snow foUnwPd by 12" dry powder

7. Snowfall Intensity

Average 0.9 in/hr.

Maximum 1.3 in/hr. ~

8. Water Ratio 0.09 Total Precipitation

9. Precipitation Intensity

Average 0.07$ in/hr.

Maximum 0.1$ in/hr.

10. P.I. Factor never reached 1.0

2-1¿^"

11. Wind

Speed Average 10 mph Max. 18-20 moh (short i^erindRl

Direction W & SW

12. Old Snow Settlement negligible

13. New Snow Settlement Inches

14. Temperature began 30°, falling to 10° at end of storm

15. Avalanche Occurrence - Natxiral

General cycle of small, medium and large soft slabs of northerly exposures.

16. Avalanche Occurrence - Artificial

Extensive ski and explosive releases--see gun report and analysis.

Observer's Analysis

Trouble had been expected on north exposures with the first storm to deposit

much snow over the existing shallow depth hoar. The danger was further heightened

by the existence of a crust layer in the depth hoar resulting from a November

salt storm. The early part of this storm brought damp, heavy snow which favored

slab formation, and avalanche activity began early, when little more than 6 to fi"

of snow had fallen

FIGURE 40.—Storm reiwrt (sheet 2).

49
Test skiing on the morning of the 25th revealed highly unstable

conditions, and the skiers were restricted to minimum hazard areas

on the Collins lift for the rest of the day. A test release in

one of the chutes of the Peruvian Ridge resulted in release of a

soft slab extending all the way from Peruvian Bowl to Westward Ho.

Very extensive cracking of the snow was observed even early in this

storm, and persisted for a day or two afterwards.

Artillery fire on the morning of the 26th brought down several

large slides, the most extensive being those on Baldy Shoulder

and Simspot-Race-Course. Many small patches of slab existed

throughout the ski area, and test skiing and blasting in the

Peruvian Ridge-V/ildcat areas proved these to be sufficiently

dangerous that they were kept closed throughout the day. A small

part of the ridge around the top of the Wildcat Lift was left

open to permit use of this lift, but even here the snow was unstable, and two small slabs were dislodged by skiers, one of them

burying another skier who had the misfort\ine to fall just as the

slab patch was released. The latter was quickly dug out and was

uninjiored. A heavy influx of skiers out to take advantage of the

first good skiing of the winter made the area control problem

difficult, and there was considerable complaining about the

restrictions until it became obvious to even the most careless

skiers that a serious hazard situation existed.

It is interesting to note that the slopes of Stonecrusher and lone

Pine, which had been previously foot-packed, did not slide, though

adjacent slopes slide vigorously following artillery release.

Increasing evidence confirms that foot-packing is an effective

countermeasure against the hazards of depth hoar. Peruvian Bowl

also did not slide, for it had been thoroughly side-stepped with

skis several days prior to the storm.

Blasting and artillery fire continued for two days after the storm,

and danger in the outlying ski runs was eventually reduced. A

serious condition of instability continues on north slopes, however,

and more trouble may be expected from future storms. At the date

of this writing (3 January), occasional patches of unstable slab

have continued to be uncovered in the ski area, and a general

hazard condition is presiimed to exist in the back country where no

control measures have been taken.

FIGURE 40.—Storm report (sheet 3).

50
1. Preliminary Requirements. The prerequisites

of a site for snow profile investigations are few,

but important. For practical use, the study plot

must be accessible to the observer throughout the

winter. It must be an area free from avalanche

danger, disturbance by unnatural deposition of

snow (such as the discharge from rotary snowplows), and foot or ski traffic. If disturbance by

passing skiers is likely, the area must be roped

off. The study plot should be reasonably level

and free from exposure to snow creep pressure

from adjacent slopes. It should be located away

from ridges, cliffs, or other terrain irregularities

that would tend to modify the normal conditions

of snow deposition and metamorphism, and should

be protected from the scouring action of high

winds. The ideal location is a level opening in a

stand of heavy timber. It should be such size that

the distance from the edge of the study plot to the

nearest trees is at least as great as the height of

the trees. Normally, the study plot should be

located close to the total snow stake where accumulation and snow depth records are obtained.

If tliis is not feasible, a separate total depth stake

must be provided at the study plot. A number of

pits will be required in the course of a winter;

therefore the test area must be staked out and the

sites of future pits planned in advance. Each pit

is refilled with snow after the studies have been

made, to keep the snow cover in the test area as

homogeneous as possible. The limits of the area

which has been dug up for study must be clearly

marked, so that fi-esh undisturbed snow can be

excavated from each succeeding pit. Excavated

snow must be thrown away from any area which

may subsequently be sîtudied.

In sliallow snow covers, a square pit dug through

to the ground is usually the quickest and most convenient. For deep snow, a long narrow pit increasing in depth by steps from one end to the

other provides easier access ; or a vertical shaft can

be dug with the snow hoisted up from the lower

levels by bucket. A D-handle shovel with a square

blade is the best for all-around use. A short

shovel with a pointed blade may be needed to

break up very hard snow.

In practice the ram profile is taken first, and the

penetrometer left in place to guide the pit excavation and to serve as a scale of reference. The subsequent tests are then made in more or less the

order given below.

2. The Ram Penetrometer. The ram penetrometer (Haefeli cone penetrometer), or "rammsonde,"' was developed in Switzerland over 20

years ago. It has Ijecome a very useful tool in

various kinds of snow investigations. This instrument measures the resistance snow offers to penetration of a cone-shaped point, and indicates the

relative degrees of hardness and cohesion in various snow layers. The method of use provides a

graph of ram resistance against depth without

digging a pit. The stratification of the snow is

readily apparent from the graph and provides the

snow investigator with clues to the various conditions existing in a given snow cover.

The penetrometer consists of an aluminum alloy

tube approximately '?> cm. in diameter, 1 m. long,

and weighing 1 kg. The 4-cm. diameter point

is a 60° cone tipped with steel for wearing quality.

A scale in centimetei-s is engraved on the tube,

reading upward from the point. Depths over 1 m.

may be probed by the addition of successive 1 m.

lock-connected lengths of tube.

A sketch of the ram penetrometer, together with

tlie fornnila used to compute ram resistance, appears in figure 41.

The force of penetration is developed by the fall

of a weight along a guide rod supported on the

upper end of the penetrometer tube. The force

may be varied by varying the height of fall and

size of weight. The calculated value of ram resistance, given in kilograms, is an arbitrary figure

depending on tlie dimensions of the penetrometer.

Headings taken under different conditions and by

different observers can usually be compared. Operation of the ram penetrometer is as follows :

a. Hold the penetrometer tube vertically with

point at the snow surface a)ul allow it to sink

by its own weight into the snow. Read and record penetration.

b. The 1 kg. ram weight and guide rod are then

carefully set on the tube and any further penetration due to the increased weight is recorded.

c. Raise the ram weight and let it fall from a

height sufficient to produce at least 1 cm. penetration. Note scale reading at the snow surface

and the fall height of the weight. As the process is continued and firmer snow layers are

reached, tlie distance of fall for the weight will

have to l)e increased. "When maximum possible

fall height is insufficient to produce at least 1

cm. penetration, then two or more falls must be

used between successive readings. A greater

sensitivity to small variations in ram resistance

and to thin snow layers is usually obtained if

short fall heights and a number of falls between

penetration readings are used as regular practice. If heavier ram weights are available, they

may be substituted for use in very hard snow.

d. When the first tube has penetrated to approximately 80 cm. a second tube is added, as is

a third tube when the depth has reached about

180 cm., and so forth. Addition of the extra

tubes must be noted in the record. Changing

the total weight of the penetrometer changes the

calculated value of ram resistance for given

conditions. When the point has penetrated

through the snow cover and reached the ground,

a sudden increase in ram resistance is usually

observed, and the ram weight makes a duller

sound when it falls.

e. From the quantities recorded (fall height,

ram weight, penetrations, etc.) the ram resistance for each snow layer can be calculated with

the formula given in figure 41.

51
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FIGURE 41.—The ram penetrometer.

o. Tem-perature. The simplest way to determine temperature profile is to insert thermometers

at regularly spaced intervals in the wall of a

freshly dug pit. Eeliable temperature records

cannot be obtained in this manner after more than

an hour or so of exposure of the pit wall. A usable curve of temperature against depth can be

obtained if a few precautions are taken. The best

method is to measure the temperature from the

pit wall as the pit is being cíug. This insures

minimum disturbance of the natural temperature

gradient. Where more accurate records are required, resistance thermometers, thermistors, or

thermocouples must be allowed to accumulate with

the snow cover during tlie winter so that they are

surrounded by undisturbed snow.

The thermometers may be inserted in the pit

wall at 20-cm. intervals, unless more detailed

studies of snow temperature changes near the surface are being made. If glass thermometers are

used, a metal punch should be kept handy to make

holes for the thermometers in all but the softest

snow. Tlie Weston dial thermometer is more

rugged, and is i-ecommended where great precision

is not required. A punch should also be used for

it in very hard snow. Care must be taken to avoid

parallax when reading the dial thermometer.

After the thermometer is inserted in the snow, it

sliould be allowed to remain there for a few minutes to approach equilibrium. Then it should be

shifted to a fresh spot of snow at the same level

and once more allowed to approach equilibrium

before being read. This is necessary because the

tliermometer initially inserted in the snow is usually at a different temperature than the snow, and

the first equilibrium temperature reached will be

appreciably different than the surrounding snow

temperature.

52
An accurate temperature of the snow surface

I is very difficult to obtain. An approximation may

be obtained by sliding the thermometer just under

the snow surface, and shielding it from direct solar

radiation.

4. St7'atigraphy. The stratigraphie, or layer,

profile ig derived from detailed observations of

grain size, grain type, layer boundaries, hardness,

and other special features in the snow cover being

investigated. The results as plotted show the

various layei-s of snow and their characteristics.

See the pit profile plot in figure 42.

The snow crystals observed are classified according to the outline given in the Types of Deposited Snow chart, and the appropriate graphic

symbols are used for plotting the results.

A few general remarks atjout visual observations will aid the field worker. One of the quickest ways to locate the various layei-s, where large

differences in hardness are present, is to draw the

finger or a pencil tip down the pit wall at an even

rate from top to bottom. The variations in resistance offered by the difièrent layers will be quite

apparent. Wliere the dift'erence is slight, careful

horizontal stroking of the pit wall with a soft

brush will model out the layers. AVhen applied

to new snow near tlie surface, this teclmique will

even reveal the variations in wind velocity which

occurred during deposition. Care should be taken,

however, to make a close visual inspection for the

layer boundaries, because a change in grain type

or size will sometimes occur with very little change

in hardness.

In the absence of a hardness gage or penetrometer, a rougli estimation of hardness may be obtained by notjng the size of common articles which

may be easily pushed into the snow. Tlie following table will serve as an example :

Approx, ram

Hardness resistance

Mittened fist (sidewise mo- very soft Ü-3 kg.

tion possible).

Flat hand pushed endwise._ soft 3-1.5 kg.

Single outstretched finger. _ med.-hard 15-50 kg.

Sharpened pencil hard _. 50-150 kg.

Knife blade very hard over 150 kg.

Special features such as sun or rain crusts, thin

layers of surface hoar crystals, the presence of

depth hoar, ice laminations, and ice lenses, should

Ije noted.

Individual snow layers may be marked by

stretcliing colored thread on the surface after each

snowfall when profiles are to be taken throughout

the winter at a single site for the purpose of plotting a time profile of snow cover evolution (see

item 6.4). The threads become incorporated in

the snow cover and plainly mark layer boundaries

wliich might otherwise be difficult to distinguish.

5. Dem'ify. Snow density is determined by the

familiar method of weighing a sample of known

vohune. The snow specimen is collected by inserting a standard 5()0-cc. sampling tube horizontally

in the i)it wall. Tlie snow is cut away around the

tube, and the snow at the ends of the tube is carefully cut or scraped flush with the tube, and it is

weighed. The tube weight is deducted from the

total weight to get the snow weight, and the density is computed from this known weight and

volume. If a large number of sam^ile tubes are

available, the specimens from an entire profile may

be collected at once and weighed at a convenient

indoor location. If the tubes will be exposed to

melting temperatures before being weighed, they

must be capped with iiibber caps to prevent loss

of any meltwater formed.

Density samples should be collected from each

separate snow layer in the profile large enough to

admit the sample tubes. Several samples should

be obtained from the thicker layers. The height

of each sample collected in the snow cover must

be recorded. The density values are plotted

against snow depth to give the snow density curve

for the profile, as illustrated in figure 42.

B. Plotfing the ProfJex. The general technique

in plotting a complete snow profile from pit data is

suggested in the accompanying example (fig. 42).

The direction of the axis on which any given characteristic is plotted is merely a matter of convention: the example shows the method of plotting

foiuul convenient after considerable use. Millimeter graph paper used with a scale of 1: 10 (one

millimeter for each centimeter of snow depth)

gives a convenient and easily manageable profile.

6.4 Time Profile. The end product of snow profile determinations is the time profile, from which

the investigator may observe the overall character

of snow coAer evolution throughout the winter.

This time profile is prepared by plotting (on a

time base) a series of pit profiles obtained from a

sequence of pits dug at the same study plot

throughout the winter. Daily snow depths, settlement data, and meteorological records are also

incorporated in the time profile. If care is taken

to identify individual layers in each pit profile,

tlie behavior of these layers can be followed

throughout the winter. The desirable interval for

collecting pit data is every 2 weeks.

53
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FIGURE 42.—Typical plot of pit profile data, showing symbols for various deix>sited snow types.

54
2332.81 SNOW AVALANCHES

Chapter 7

7 SNOW STABILIZATION. Snow may be

stabilized and avalanche hazard consequently reduced or eliminated by the following methods.

These represent measures of avalanche control and

prevention as applied to the snow itself, in contrast to such defense measures as retention barriers which permanently alter the nature of the

terrain.

Mechanical disturbance of the snow with one

exception results in a trend toward greater stability. This works in two ways. One is the process of age-hardening already mentioned in chapter 2, item 2:2. "VMienever snow is disturbed and

then allowed to set, its ñnal hardness is greater

than if left undisturbed. This eft'ect is accentuated

at lower temperatures. The other method of

stabilization is by relief of creep tension or inhibition of its development. Existing creep tension

is relieved if the homogeneous layer of a snow

slab is broken. Once the slab is broken into segments, there is much less likelihood that such tension will again develop. Slabs which have been

broken without an avalanche release tend to become a well-attached part of the snow cover.

The one exception to the stabilizing effect on

snow of any mechanical disturbance is wind, for

it transports the snow from place to place as well

as disturbs it. It is a potent natural stabilizer

when it forms windpack and windcrust. When it

constructs a slab or transports an overload of

snow from one part of the area to another, the

reverse is true.

Skiing is the most obvious means of mechanical

disturbance available in a ski area. It is a wellestablished general observation that slopes which

receive heavy ski use are much less likely to avalanche than those which do not. Light skiing—

a few tracks criss-crossing a hill—also has a stabilizing influence. First, it may relieve creep tension in a soft slab. Second, each ski track and

each area of snow displaced in a turn, represents

an island of harder snow which will impart

strength to the upper snow layers. In short, a

little skiing is better than none, and a lot of skiing

is even better. Breaking up the snow will improve its stability; compacting it will assure a

solid, well-anchored layer.

When snow reaches a certain degree of hardness,

caused by high winds and low temperatures, the

ski is no longer able to penetrate the surface. In

such cases more violent means of breaking the

snow are required, such as foot-packing or the use

of vehicles.

High explosives represent another elïective

means of inti'oducing mechanical disturbance in

the snow. P^xplosives are safer when large avalanche paths are involved, because they may be

delivered from a distance either by hand-throwing

or by an artillery shell. The shock of an explosion is extremely effective in relieving creep tension, even in the hardest slab. Intense age-hardening is also initiated in the immediate vicinity

of the blast crater, but this effect is conñned to

the snow whicli is physically displaced by the

explosion. î^or this reason it is not efficient to

depend on explosives to create widespread ageliardening over a slope. Rather, they are used

primarily to initiate avalanche release and/or to

relieve creep tension. The use of explosives is

discussed in detail in item 7.2.

An avalanche release also stabilizes the snow.

An avalanche in motion represents a major mechanical disturbance of both the sliding snow and

tliat nearby which falls under its influence. The

consequent age-hardening tlius eliminates large

quantities of snow from the probability of being

responsible for a later release. Instead of stabilizing snow in place on an avalanche path, a slide

removes a large quantity of unstable snow and

carries it to more gentle slopes below where it can

cause no harm. The immediate hazard on a given

path is eliminated, and the future hazard is reduced, for there is less snow available for subsequent slides. Avalanching is frequently a less desirable protective measure than stabilization in

place. In a ski area good skiing snow is removed

from attractive slopes, and on a highway a slide

on the roadbed means extra labor and cost of

clearing. Nevertheless, avalanches are frequently

precipitated to insure public safety, and the beneñcial effects should be recognized.

7.1 Test and Protective Skiing. Protective skimg is the deliberate, day-to-day disturbance of

snow on avalanche slopes in order to encourao-e

stabilization as described above. Test skiino- is an

attempt to artificially release avalanches on

selected small slopes by skiing. It serves as a field

test of snow stability to check the conclusions of

hazard forecasting. The two functions of test and

protective skiing are so often combined that they

are treated here together. The safety precautions

for both are identical.

The reaction of snow under the snow ranger's

skis constantly provides him with information on

the physical processes which are taking place in

the snow cover. This technique provides warn55
inff of snow which is tending to form slabs and

create an inipendinc; hazard, particuhxrly during

storms. An important key to successful test skiing

is the selection of test slopes which accurately

represent the snow conditions found on the larger

and more dangerous avalanche paths. In most

areas it is possible to locate some of these small,

short avalanche patlis where slides may safely be

dislodged by skiing. It is very desirable to find

such paths which have the same orientation and

approximately the same slope angle as the larger

paths. Configuration, relation to timber, and altitude must also be considered. Safe routes of

approach, access, and exits are essential.

Cracks which form in the snow with the

passage of skis are the best indicators of developing slab avalanche danger. Breaks in the snow

immediately adjacent to the skies are not necessarily a danger sign ; the weight of the skier may

simply be cracking the snow locally. Breaks or

cracks wiiich run ahead of the skies are more

clearly a warning of hazard. A crack which may

appear for just a few feet ahead suggests that slab

conditions are forming and caution is indicated.

Cracks which run far ahead of the «ki (e.g., 50

feet or more) indicate serious instability. Under

these conditions avalanches on steep open slopes

are almost certain. Occasionally conditions will

be found in which cracks appear everywhere in

the snow at the slightest disturbance—on gentle

slopes and among trees—as well as on obvious

avalanche paths. Snow in such a condition is very

"touchy"' and avalanches are often released unexpectedly on slopes where they normally do not

occur.

If snow actually avalanches on a test slope following the passage of a skier, the character of the

slide will furnish additional information on the

degree of instability. Considerable external force

may be required to initiate an avalanche, if only

the snow dislodged by the skier slides away. If

the fracture line propagates for some distance, and

the slide gains mass and momentum as it descends,

the triggering of large slides ol)viously will l)e

much easier. "\^nien relating these observations to

larger avalanche paths, remember that differences

in location, slope, angle, etc., will cause ditferent

paths to behave in different fashions. The test

skiing procedure offers an additional check on

snow conditions, but it does not funiish all the

answers. When done by an experienced observer,

however, test skiing does often furnish accurate

evidence on the approach of dangerous conditions.

The experienced snow ranger will enlist the aid

of the skiing j^ublic in his program of snow stabilization and avalanche prevention. Once a slope

has been adequately tested (usually with explosives) and proved safe, skiers should be encouraged to use it and get the snow broken up and

packed into a solid layer. Closure of ski runs is

not a satisfactory avalanche defense measure.

Neither is it an acceptable substitute for approved

control measures whicli can be used to safeguard a

slope and open it to use. Closures are a temporary

measure intended to protect the public during periods of high avalanche danger, especially durnig

storms. They should be lifted as soon as possible

after the critical period passes, so that full benefit

of stabilization by use may be achieved. It should

be the objective of a snow safety program in any

ski area to keep all the slopes open just as much as

possible without exposing the public to hazard.

Safety precautions to be observed by men engaged

in test and protective skiing are outlined in chapter 8, item 8.3.

Depth hoar requires special attention for proper

stabilization. When a shallow, early-winter snow

cover has been converted to depth hoar, the base

for subsequent snowfalls is dangerously unstable.

I^nlike fi-eshly fallen snow, depth hoar is not readily stabilized by skiing. It is not easily compacted

into a hard layer, and tends to reform if steep

temperature gradients persist. Repeated sidestepping with skis tends to compact it sufficiently

to harden the layer, but breaking down the structure by skiing alone is not sufficient. The same

slope nnist be tramped over several times by a

number of skiers to insure adequate stabilization.

Foot packing is a more reliable method. The

higher specific pressure of a boot appears to provide satisfactory compaction and hardening. Experience has demonstrated that foot packing will

satisfactorily stabilize early season depth hoar,

while adjacent slopes not so treated persist as a

source of avalanche hazard. Increasect safety and

ease of control warrant the work involved to foot

pack slide paths which threaten a busy ski area.

The initial formation of depth hoar is inhibited

by thoroughly packing each fresh fall of snow as

it occurs. This will not prevent depth hoar from

developing, Ijut a reduction in the porosity and

increase in density will restrain free circulation

of water vapor within the snow cover and reduce

the adverse effects of steep temperature gradients.

7^ Explosives. The use of explosives to stabilize snow—either hand placed or as projectiles—

has been routine in the United States for a number

of years. Techniques and equipment have steadily

improved.

Before the explosives techniques were developed, the only avalanche defense the snow ranger

liad was the closed area sign. The signs did nothing to deter avalanches and very little to deter the

people.

Explosives have numerous advantages as a

meaiis of avalanche hazard control. They are a

logical extension of the test and protective skiing

techniques which seek a definite answer to the

question "Is this slope stable?'' A heavier shock

can be delivered to the snow, and it can be done

quickly and safely. Explosives are the only means

of getting a definite answer where hai-d slab conditions prevail.

Where projectiles are available, large areas can

1)6 controlled from a few firing positions. The

savings in time and labor are notable. At Squaw

56
Valley, during the 1960 Winter Olympic Games,

30 different and Avidely scattered avalanche paths

were controlled as a matter of routine in less than

'2 houi-s. Where a highway crosses the area, the

rifle can be hauled along it and deliver projectiles

to avalanche patlis otherwise inaccessible.

Contrary to the belief of the general public,

the objective of using explosives in avalanche

liazard control is not to start avalanches. The

objective is to determine if the snow is stable or

unstable and eliminate hazard. The latter does

not necessarily require that an avalanche fall.

Education of the public is important because

it leads to the cooperation and acceptance by the

public, without wliich no snow safety program

can be successful. Accordingly, avalanche control opex-ations are often performed in full view

of spectators, and all personnel engaged in avalanche control work are encouraged to explain the

objectives.

When an explosive charge is detonated in an

avalanche area, one of three things happens:

1. An avalanche occui-s. This proves that the

snow is unstable and further control action is

needed on other slopes.

2. The snow cracks and either slides only a short

distance or stabilizes in place. This indicates a

moderate degree of instability. It calls for further control action, since some other slope may be

ready to slide. Stabilization in or nearly in place

is a better outcome than an avalanche, for there

is no debris to roughen a ski slope or fill a highway.

?). Nothing happens except the formation of a

crater in the snow. This suggests stability. If

confirmed by several more shots on key avalanche

paths, it may be taken as evidence that the area

as a whole is stal)le. This is the best answer of all.

Any one of tlie three answers is satisfactory,

for it is definite. If there was a hazard, it has

been removed; if there was no hazard, this has

been demonstrated.

Continuity of avalanche liazard control operations is also important. Tlie routine and frequent

stabilization of avalanche paths by all methods—

from protective skiing to explosives to skiing

use—is the best guarantee tliat no deepseated,

climax-type hazard can develop. In the educational aspect, people become accustomed to the

operation, miderstand what it is all about, and

even identify tliemselves witli the program if they

are allowed to help out in small ways.

7.21 Hand-Placed Charges. For slope avalanches, exi)losives with a liigh rate of detonation

such as military demolitions and seismograph

powder are the most effective. For coniice removal, the slower explosives such as 40 percent

gelatin dynamite are moi'e effective. The faster

explosives deliver a maximum blow to the surface

and also propagate a shock wave which counters

the marked absorptive powers of snow. Cornice

removal, on the other hand, is like a quarrying

opei-ation which i-equires a strong push rather than

a sharp blow (fig. 43).

F-462503

FIGURE 43.—Removal of cornice by a heavy charge of

tetrytol.

For a soft slab, a 2y2-pound block of teti-ytol

(military demolition) is generally a sufficient

charge. If the slope is wide it is better to fire

a nudtiple charge strung out along the fracture

line than to increase the size of a single shot. If

the slide path is unusually large it may be necessary to blast at several different trigger points.

For the hard slab-depth hoar combination, the size

of the shot may be increased to 5 or even 10 pounds.

The ignition type detonator (blasting cap) saves

time and is approved for avalanche blasting. (For

winter work such as this, the fuse is most easily lit

with a pullwire igniter.) With this type of

detonator slide paths under a lift line have been

successfully blasted by dropping the charge from

a moving cliair.

For cornice removal, the standard procedure is

to drill shotholes along the expected sheer plane

at 6- to 8-foot intervals and to a depth that will

])lace the explosives in tiie center of the snowmass.

The holes are loaded with 3 or 4 sticks of dynamite

and well tamped with snow. The charges are

linked together with primacord and fired as a

multiple blast (figs. 44 and 45). The shotholes

can be dug with a snow sampler. A handier tool

is a sectional avalanche probe with a casehardened

cone-shaped head, the same diameter as a dynamite stick, which can be screwed on to the bottom

section of the probe.

57
FIGURE 44.—Dynamite releaise of massive cornice at Twin Laiies Pass, Alta, Utah. This cornice is about 75 feet long

and 2G feet high at the face. Ten shot-holes were used, spaced 8 feet apart along a straight line and drilled 6

feet deep. Each hole was loaded with two sticks of 40 percent gelatin.

A cornice wliich forms every year or several

times durino; the winter may be préchargée!. Explosives are placed on tlie ground as near as possible to the shear plane, or ¡placed after the cornice

is partially formed. Tlie charge can then be fired

whenever the overliang becomes dangerous and

the j)rocedure rei)eated as often as necessary. The

precliarging technique has advantages: it effects

more complete removal and permits placing the

charge in favorable weather. Moistureproof and

frostproof explosives must be used. Military

demolitions, seismograph powder, and bangalore

torpedoes are suitable. Such cliarges must be very

securely anchored against snow creep, and should

be armed with 2)rimacord—never with blasting

caps. Where tlie cornice is inaccessible, projectiles

have been used effectively. The technique is to

get "on target" by tlie laclder method with pointdetonation fuse; tlien to fire delay-fuse rounds

into the cornice to get the burst inside the snowmass.

7.22 Artillery. Fundamentally, artillery represents an extension of the snow ranger's throwing

arm. A projectile launcher must be accurate,

mobile, reliable, simple to operate and maintain,

and have adequate range.

Of tlie numerous weapons listed, the 75 and

105 mm. recoilless rifles and the 75 mm. pack

howitzer have proved to be best adapted for avalanche control operations. Weapons larger than

105 mm. ai'e not I'ecommended.

The 75 mm. recoillesx rife is especially suitable.

It weighs l(í7 pounds, is readily mobile, and has

a range of 7,000 yards with pinpoint accuracy up

to 3,000 yards. This rifle is easily operated and

maintained and can be fired from any platform

whicli will support its weight (fig. 46). Its only

drawback is the "backblast" common to all recoilless rifles. Gun crews should use air-valve type

ear plugs because of its sharp concussion.

The 75 mm. pack honntzer is similar to the recoilless rifle of the same calibre in range, reliabil58
FiGUKE 4').- -Sefiuel to figure 44, after coruice has fallen. Arrows point to location of shot-holes,

smooth surface of an older cornice underneath the one removed.

Note the hard,

ity, iuid shock power. However, it is consideriibly

more complicated. It weighs approximately 500

pounds and is thei'efore much less mol)ile. Unlike

the recoilless which hres fixed ammunition, the

pack howitzer lias a variable powder cliarge. Unless fired with Charge No. 4 (full propellant

charge) there may he a i^roblem with duds or deep

bursts in soft snow. Since there is no backblast,

it can be fired from restricted areas such as the

doorway of a shelter. It is well suited for use in

a ski area where it can be fired from a fixed position, or along a highway where it can be towed.

The 105 mm. recoiUe^K rife (fig. 47) is similar

to the 75 mm. in accuracy and reliability. It has

greater range and shock power. It weighs 700

pounds and is not mobile over snow, but it can be

mounted in and fired from a wheeled vehicle.

Concussion and backblast are heavier than with

the 75 mm. recoilless but not so sharp. liar plugs

are required. The 105 mm. recoilless is recommended particularly for use on liighways where

ranges may be extreme.

Firing ponifion. The selection of firing positions requires careful study. They must be accessible under all conditions of weather and hazard

and protected from avalanclies. The safety of

habitations, parking areas, and other facilities

must be taken into accoiuit. With all these limitations in mind, the snow ranger chooses firing positions from which the maximum number of taro-ets

can be reached. They may be strung out along a

highway or concentrated in the vicinity of the

ranger station, or located on a ridge if a lift provides access. With proper care, rifles and other

weapons can be left in the open all winter.

In a ski area, gun platforms raised above the

maxinnmi snow level are essential. They also

make it possible to fire on short, notice without

delay. They provide the fixed position for blind

firing, "\\niere the rifles are to move along a hio-hway, fixed positions can be provided by markîng

the wheel positions of a mounted vehicle or a

towed weapon.

59
F-495210

FiGuiíE 46.—Typical fixed position for a 75 mm. recoilless

rifle. Here the rifle is on a pedestal mount, which is

interchangeable with the normal tripod mount. An

aiming stake for blind firing is visible in front of the

platform. The elevated position gives ample clearance

for the backblast with all snow depths.

Targeting. The estimated range should l^e determined from a topograpliical map when firing

at target for the first time. Tlie first shot sliould

be visually aimed so that it will land well down

on the slope ; then proceed upward, ladderwise, until the burst is on target. At all times, be sure that

there is enough dead space behind the target so

tliat an "over" will not endanger life or property.

A range caid should be prepared for each gun

position showing both the range in yards to each

target as determined by visual firing, and the elevation of the barrel as measured by the gunner's

quadrant.

Firing projectiles into or over areas occupied by

people is not permitted. For this reason most firing in ski areas is done in the early morning. On

highways a blockade system is essential.

Projectiles should not be used where the target

is close to structures, because of fragmentation.

Shrapnel dispersion from tlie lOf) mm. shell is particularly dangerous.

BVtnd ßring often starts a premature avalanche

cycle or stabilizes the snow in place. Whether in

a ski area, along a highway, or for any other purF-495211

FIGURE 47.—10.5 mm. recoilless rifle permanently mounted

on a tower for avalanche control in a major ski area.

A winter's supply of ammunition is stored in the base

of the tower. With such an installation, accurate

blind firing data can be acquired so that control measures can be taken during storms. Alta, Utah. Note

lightning grounds as safety measure.

pose, it is extremely etfective in shortening closure

time, preventing the development of major avalanche conditions, and reducing the volume of

snow and amount of avalanche debris in a ski area

or piled upon a highway. Blind firing can be

done safely during blizzards when it would be unsafe to use hand-placed explosives. With proper

blind firing data, the gunner can hit any target he

can see during clear weather.

At each fixed gun position, the snow ranger sets

up aiming stakes which determine the line guntarget. Elevation is set by the gunner's quadrant

as taken from the range card.

Training. Personnel assigned to use artillery

or hand-placed explosives must receive special

training. (See Forest Service Safety Regulations, chapter 8, item 8.3.)

60
2332.81 SNOW AVALANCHES

Chapter 8

8 SAFETY. By now it should be obvious to the

reader of this handbook that avahvnches are a

dangerous and sometimes unpredictable force of

nature. It cannot be emphasized too strongly that

the artiñcial release of this natural force involves

considerable danger unless carried out in strict

conformance with established safety procedures.

8.1 Safety Objectives. The three principal objectives of avalanche control safety regulations

are to :

1. Carry out avalanche control without endangering forest officers or others participating in the

control operation.

2. Assure that control operations do not endanger skiei-s or others who may be in the vicinity.

3. Prevent damage to lifts, buildings, automobiles, powerlines, and other facilities.

To attain these objectives, the Avalanche Control Safety Regidations in this chapter have been

promulgated. These have the official status of

a supplement to the Forest Service Safety Code,

and should be so respected.

82 Safety Principles. The following general

safety principles are based on accumulated

experience.

1. Avalanches nnist never be artificially released

unless it is absolutely certain there are no humans

on tlie slopes below. If these slopes are not visible

to the man effecting the release, a system of guards

and signals must be arranged. Normally, release

should not be attempted during poor visibility

(fog, snowstorm, etc.) except when blind firing

with artillery and then only if rigorous safety

precautions are taken.

Merely walking or skiing along a ridge or near

an avalanche path may effect release during conditions of exceptional instability. With such

conditions, artillery fire or blasting may cause

the sympathetic release of large slides at great

distances from the actual target point. Fracture

lines or the collapse of an unstable snow structure

may be propagated for a very long distance. In

one recorded case an artillery shellburst started

sympathetic release of one adjacent slide path

after another until a large avalanche fell on the

opposite side of the mountain from the target

point. When such conditions are suspected, great

care must be exercised to see that thei'e are no

people in possible danger zones.

2. Avalanche accident statistics illuminate one

important point which should be a constant source

of caution to men engaged in control work and

to anyone traveling in avalanche terrain. Almost

all accidents involve slides which are started hy

the victims themselves. Accidents which result

from naturally released slides falling on an unsuspecting victim are rare. The time it takes for even

a large slide to fall is measured usually in seconds.

The chance that this time will coincide with the

passage of a traveling party across an avalanche

slope is very small, unless an artificial trigger is

supplied. The moral of this lesson is very plain:

don't cross avalanche slopes and provide the trigger for a slide whicli may overtake you. If necessary, go below the slide path—this is safer than

being on it. Better yet, go alrove the slide, even

if this means a long detoui".

o. It should become a matter of habit among

men engaged in avalanche control work to avoid

using wrist loops on ski poles. Once a man gets

caught in a slide, his chances of survival, or escaping injury, will be greatly enhanced if his hands

are free to protect his face, excavate breathing

space, or dig himself out. He may be in trouble

if his arms are pinned by buried ski poles.

4. Small avalanclies, which ordinarily would be

harmless to a man caught in them, can be dangerous if they fall over cliffs or into gullies or ravines.

Burial and suffocation are usually thought to be

the common fate of those caught in avalanches,

but practical experience has shown that mechanical injury from striking trees, stumps, or rocks

are as responsible for death or injury as burial.

Early winter slides running on a depth hoar layer

next to the ground are particulai'ly dangerous in

this respect. Their paths are often studded with

I'ocks exposed by the sliding snow.

5. Safety rules for handling explosives are

treated in detail in the Forest Service Safety Code

and handl)ooks prepared by explosive manufacturers. Strict observance of these blasting regulations must be enforced in avalanche control work

the same as in any other blasting operation. Modem explosives and blasting equipment are manufactured to a high standard, and misfires due to

defective manufacture are very rare. Most misfires can be traced to improper handling, storage,

or preparation of the charge. Since there is more

than normal chance for these to occur under the

often adverse field conditions of avalanche blasting, special care must be taken. Do not allow

frozen toes, cold fingers, or an unpleasant blizzard

61
to encourao-e short cuts or Cixreless preparation of a

(•liar<Te. A few extra minutes spent in doing the

job rTfiiit is always easier and safer in the long run.

6. Never work alone. The minimum group for

any avalanche control work is two men. They

must conduct themselves so that never more than

one is exposed to possible danger. The primary

purpose is to liave someone around to aid you if

you get caught in a slide, or to go for help if

necessary. This amounts to applying the general

rule of never traveling alone in the mountains, and

exposing only one man at a time to possible avalanche danger. It is a very elementary precaution

tliat must always be observed.

7. The administration of an area where avalanche dangers can threaten should never become

a matter of habit or routine. Snow conditions

exist in many forms and are subject to change.

Their prediction cannot be reduced to a fixed set

of rules. The most important safety rule of all

is always to THINK.

T rain for the job.

H ave good equipment.

I nsist on correct ¡procedure.

N ever get in a rush.

K eep on thinking.

8.3 Safety Regulations. Avalanche control

operations by means of protective skiing and explosi\'es are highly specialized. The following

safety instructions will go\ern all forest employees

whose duties include such work.

8.31 Personnel,

1. Qualifications. Any Forest Service employee who conducts avalanche control

operations must :

a. Be a competent ski mountaineer.

1). Have received special training,

c. Hold an avalanche blaster's card, if he conducts operations with explosives. Requirements:

(1) General training in use, transportation, and storage of explosives.

(2) Special training in avalanche blasting

techniques.

8.32 Avalanche Test and Protective Skiing.

1. Test Skiing:

a. Is a field test to determine actual stability

of snow.

b. Is done on very steep slopes, short enough

so that a slide will not endanger the

tester.

c. Is carried out by not less than two men,

working as a team, one testing, one observing.

d. Precedes protective skiing.

2. Protective Skiing:

a. Is carried out to :

(1) Hasten stabilization.

(2) Remove avalanches from the slope.

b. Must be repeated often so that :

(1) No unstable layer becomes hidden in

the snowpack.

(2) Only the top layer can avalanche.

c. Snow safety leader decides whether or not

to employ from:

(1) Hazard forecast.

(2) Results of test skiing.

(3) His familiarity wñth the area.

(4) Size and steepness of the slope.

(5) Amount of snow that may avalanche.

(6) Observation of natural avalanche

activity.

d. May follow use of explosives to :

(1) Clean out small pockets of avalanche

snow.

(2) Complete stabilization of adjacent

slopes.

(3) Encourage stabilization by use.

e. May be employed during a storm to prevent

major hazard from developing on key

slopes.

f. Is effective if slope is thoroughly cut up,

though not ski-packed (exception : depth

hoar and hard slab).

g. Must be repeated for eveiy radical change

in snow conditions,

h. Protective skiing team :

(1) Not less than two.

(2) Only one man on the slope at a time.

(3) Begin work at top of slidepath and

cross from anchor point to anchor

])oint.

(4) On cornices or very steep slopes, tester

will be belayed,

i. Is not to be employed when hazard is higher

than Condition 2 (moderate),

j. Once the safety of a slope is established,

stabilization of the surface is improved

use by the skiing public.

8.33 Avalanche Blasting.

1. Authorized explosives:

a. For slope avalanches :

(1) Military demolitions tetrytol, C-3,

TNT blocks.

(2) Commercial seismograph powder.

(3) Military projectiles, not over 105 mm.,

HE, superquick fuse.

(4) In an emergency and in absence of the

above, commercial dynamite.

(5) For experimental and test purposes,

other explosives may be used if authorized by forest supervisor.

b. For cornice removal :

(1) Commercial gelatin dynamite 40 percent, nonfreezing.

(2) Military demolitions or seismograph

powder

c. For preplanting cornices :

(1) Military demolitions.

62
cl. Detonators:

(1) Type: electrical, ignition, or primacord.

(2) Size: for military demolitions—No. 8

or larger.

For commercial explosives—as recommended by manufacturer.

For primacord—No. 8 or two No. 6 taped

together. When used as a detonator,

primacord will be attached to the explosive as prescribed in Dupont

Blaster's Handbook or Army Field

Manual FM5-25.

2. Storage.

a. ^Magazines for explosives and detonators

will meet Forest Service Safety Code

standards. They may be raised above

ground to normal snow depth.

b. Field caches : during operations season not

more than 50 pounds of explosives—not

detonators—may be stored in the field

convenient to the blasting area. Requirements :

(1) In a substantial, locked box.

(2) Plainly marked.

(3) Isolated location.

c. Primacord is classed as an explosive and

will be so stored. Distinguish carefully

from safety fuse.

d. Ignition type detonators require special

protection from moisture. Use waterproof packaging in storage and ring or

cutthroat type crimping tool in the field.

Taping joint between cap and fuse is

recommended.

3. Transportation.

a. According to Safety Code except over

snow.

b. Over snow, vehicle or backpack, requirements :

(1) Redflagging.

(2) Physical separation of detonators and

explosive.

(3) Detonators in box individually

wrapped or padded.

(4) Projectiles in individual containers.

c. Via lifts and tows :

(1) Normally at times when not in use by

the public.

(2) lOO-foot minimum separation between

explosives and persons on the lift

other than blasting party.

4. Party Management.

a. Blasting party consists of the blaster and

at least one assistant.

b. Blaster organizes and conducts all operations:

(1) Brief assistants and notify authorities.

(2) Assemble, check equipment and materials.

(3) Choose the targets.

(4) Transport the detonators.

(5) Pei-sonally arm, place, and fire the

charges; aim and fire the gun.

(6) Provide for party safety: routes, protection from blast, secondary avalanches.

(7) Provide for area safety: closures,

guards, connnunications, and signals.

(8) Prepare report on each operation as

required by forest supervisor or other

autliority: i.e.. Army Gun Book.

c. For training purposes, the blaster may

supervise while an assistant performs any

of the above.

d. Assistants must :

(1) Be competent ski mountaineers.

(2) Have proper equipment as directed by

blaster.

(3) Obey blaster's instructions.

e. Spectators. It is recognized that avalanche blasting operations have educational value. Spectators are permitted.

They must be :

(1) Briefed on safety requirements.

(2) Controlled, by extra guards if necessary.

f. Publicity:

(1) Will be avoided.

(2) Scheduled demonstrations may be held

for press and public with prior approval of forest supervisor.

. Technique.

a. Preparing cliarges:

(1) Use standard procedures outlined in

Blaster's Handbook or Army Field

Manual.

(2) Make sure all materials and equipment

are in working order.

(3) Premature detonation of electrical caps

by snow static is possible. Ignition

caps are recommended for avalanche

blasting.

(4) When using electrical detonators,

check firing circuit with galvanometer at each stage. Keep circuits

shorted until ready to fire. If blowhig snow is present, prepare complete

firing circuit before inserting detonator in explosive.

(5) When using ignition detonators, test

at least 3 feet of each new lot of

safety fuse for burning time. Fuse

may be cut and cap crimped on before proceeding to the field. Cut

fuse for minimum 3-minute delay,

longer if necessaiy for protection of

blaster.

Pull-wire fuse igniters are recommended.

Note the time when fuse is ignited.

Be sure fuse is burning before leavinothe charge.

Replace igniter if fuse fails to ignite.

63
If blaster lenves the charge and it fails

to explode, it must be treated as a

misfire, even if he thinks the cause

is the failure of the fuse to ignite.

(6) Primacord:

Is recommended for detonating multiple blasts.

Is required for 6-shot blasts or over.

Attach ¡irimacord pigtails to main

line with 2 clove hitches, pulled tight.

An ignition cap crimped on the end of

each primacord pigtail and inserted

in the explosive is recommended.

Primacord mainlines over 50 feet long

should be doubled.

(7) Amount of explosive to use :

Determined by blaster.

Four pounds demolition or seismograph powder generally sufficient for

slope avalanche. For wider slopes

use multiple shots at 100-foot intervals. Do not use less than 2 pounds

per shot.

Where hard slab is prevalent, with or

without depth hoar, increase size of

charges, up to 10 pounds per shot.

Three to four sticks of dynamite per

hole is generally sufficient for cornice

removal.

Use too mucli explosi\e rather than too

little. Get the answer.

b. Placing slope avalanche charges :

(1) May be lowei-ed or tossed into position.

The following precautions should be

taken :

Detonator securely taped or fastened.

Charge securely packaged.

No strain on lead-in wires of electrical

caps. Lower by blasting wire or

separate line.

Belay charge if snow is hard and it

might slide.

If blaster goes onto avalanche slope to

])lace the charge, he will be belayed.

Belay j^ersonnel working on cornices.

(2) May be di'opped from ski lift, but lift

must carry no other passengei"S besides the blaster.

Pull wire igniters required. Throw the

charge to one side and behind the

blaster.

c. Firing :

( 1 ) Use a])proved warning signals.

(2) Establish a minimum party clearance

of .50 feet plus substantial terrain

barrier.

(3) Party location must be protected from

secondary avalanches.

(4) Blaster must have visual observation

of the slide path and adjacent danger areas, or fire upon signal from a

guard.

d. Preparing cornice shotholes :

(1) Use coring tool or drill such as snow

sampler.

(2) Place holes so as to split off overhang.

(3) Stem the holes with snow.

(4) Holes should be 6 to 8 inches apart,

depth so that charge is in center of

snowmass.

(5) Man working on the cornice must be

belayed.

e. Preparing preplanted cornice charges :

(1) Primacord firing circuit required.

(2) Charges should be securely anchored

againt snowcreep.

(0) Protect firing circuit from snowcreep.

(4) Fire the blast well before cornice might

fall naturally.

f. Misfires:

(1) Electrical detonator: check the circuit

and attempt to fire; rearm with a

new detonator and fire.

(2) Ignition detonator: wait 30 minutes;

without disturbing the charge, place

a new charge alongside and fire.

(3) If unexploded powder goes down the

slope in an avalanche, treat as a misfire.

g. Projectiles :

(1) Follow Field Manual instructions.

(2) Avoid firing over buildings, other

sti'uctures.

(3) If necessary to fire over a structure,

the distance gun-to-structure must

be not more than two-thirds the distance, gun-to-target.

(4) If necessary to fire alongside a lift,

the line of fire must diverge from

the lift line.

(5) Area gun-to-target, to be clear of all

personnel.

Exception—Personnel may be inside

a fragmentation-proof structure, not

less than 300 yards from the target

area, not within the avalanche path,

and not on the gun-to-target line.

(6) Safety limit, gun crew to target:

600 yards.

(7) Beware of damage from dispersion

of fragments—especially with 105

mm. weapons.

(8) Provide traffic control if firing on or

near a highway.

(9) Vehicle clearance from gun position,

other than gun carriage, should be

100 yards for 75 mm. weapons and

200 yards for 105 mm.

(10) Prepare one round at a time. Keep

other ammunition in container's.

(11) Conventional cannon : Provide clearance for muzzle blast. Keep personnel behind and to either side of

the breech.

64
(12) Eecoilless cannon: Provide clearance for back and muzzle blast.

Keep personnel on a line at right

angles to the barrel.

(13) Visual firing: Check sighting equipment by boresighting before each

operation. Do not fii-e wliere an

"over" endangers life or property.

If range is unknown, use ladder

method of getting on target. Place

first sighting shot below target or

on an adjacent, higher slope. Make

a range chart.

( 14) Blind firing : Target data must be predetermined by visual firing. Double

check the lay of the giui. Provide a

fixed gun position and aiming

stakes. Recheck target data frequently by visual firing or aiming.

( 15) Firing on slope avalanches : Use point

detonation fuse. Exception—If

an unstable layer within the snowpack is suspected, fire first with

point detonation fuse, then with

0.05-second delay fuse.

(16) "When firing on cornices: Point target weapons such as the recoilless

rifle may be used effectively against

cornices. Do not employ unless

there is a backslope or sufficient

dead space behind the target to take

care of an "over." Use the ladder

and burst-on-target method of getting the exact range, with superquick fuse. "WHien a direct hit can

be obtained on the cornice, use delay

fuse.

(17) Misfires: Proceed as instructed in

Army Field Manual. Dispose of

defective ammunition by demolition.

(18) Duds: Note point of impact. Make

search for dud as soon as possible.

When found, dispose of in place by

demolition. Be extremely careful

not to disturb the dud. Secure

military assistance if necessaiy.

(19) Care and maintenance: i'ollow Army

Field Manual instructions. If

rifles are left in the open remove the

breechblocks wlien not in use. Inspect the rifle before using for defective parts, especially sight mount

and obstructions in the bore. Check

all moving parts for malfmictions

before loading first round.

8.34 Exceptions to Safety Code. Due to the

special conditions of avalanche blasting—absence

of flying debris, danger from snow static, for

example—certain exceptions to tlie Forest Service

Safety Code are authorized; summarized as

follows:

1. Hard hats are not required.

2. ]\Ietal cutters are permitted for opening

boxes with steel strapping.

.'5. Ignition type detonators are authorized and

recommended,

i. For electrical detonation. Army combat wire

or eciuivalent is authorized as lead wire.

Maximum length is 200 feet. Damaged

wire should not be repaired or spliced.

65
2332.81 SNOW AVALANCHES

Chapter 9

9 AVALANCHE DEFENSES. Avalanche barriers of all types are much moi'e extensively used

in Europe tlian in the U.S.A. In the Alps, towns,

lines of supply and communication, farms, and

factories are exposed to avalanche hazards (fig.

48).

The need for avalanche defense structures in the

United States is increasing rapidly. Motor traffic

on mountain highways is growing heavier and

more passes are being kept open each winter. Dam

builders, loggers, miners, and winter sports areas

are moving farther and higher into the deep snow

country.

In addition to some snowsheds and tunnels in

the Western States, only a few barriers have been

constructed by highway departments and lift

operators in recent years. For the latest developments in this field a pei-son must turn to Austria

and Switzerland. Both countries maintain research establishments to develop and test new

desigjis. Significant progress has been made

recently.

The modei-n trend is away from avalanche walls

and towards fencelike retaining structures which

do the same job as well or better at less cost. The

trend is also away from solid or continuous retaining structures.

9.1 Diversion Barriers. Diversion barriers give

protection by diverting avalanches—over highways or railroads, away from villages, around

electric transmission pylons (fig. 49). Walls, tunnels, snowsheds, and bulkheads are examples of

diversion-type barriers. Although they have the

effect of slowing down or halting an avalanche,

they do nothing to prevent it from starting and

little to limit either its size or penetration. Sometimes they are the only solution. The Flexenstrasse, access highway to tlie famous ski centers

of Zurs and Lech in Austria, would be blocked or

hazardous most of the winter without its elaborate

system of snowsheds and tunnels. Stabilization

of these slidepaths would be too expensive (fig.

9.2 Stabilization Barriers. Barriers of this

type attempt to control the avalanche hazard by

holding the snow in place or reducing its movement to minor proportions. To be effective, they

must go to the top of the avalanche path. In

former years, design was limited to walls and

terraces. Such structures are difficult and costly

to build where the avalanche release point may

be several thousand feet above the valley floor.

Although long lasting, masonry deteriorates from

the effects of frost and Avater seepage, and repairs

are a major project in themselves. Walls and

similar continuous barriers have other disadvantages. They are very costly and frequently increase the load on a slidepath or cause the development of cornices.

European engineers have studied the forces exerted by both avalanches and snowcreep. This

has led to the development of fence-type structures which are cheaper to install and maintain,

and are more efficient than masonry.

1. ArJherg Fence. This is the prototype for

all desigiis, so-named because it was invented to

protect the railroad in the Arlberg Pass (fig. 51).

A typical installation utilizes railroad steel posts

set in concrete 8 to 10 feet apart. Cables which

support vertical poles spaced 4 to 6 inches apart

are fastened to the posts, which are braced uphill

with guy wires. The height seldom exceeds 12

feet.

This makes a very rugged structure that has

proved itself over the years. Modern engineering

has added many innovations in designs, materials,

and layout. One of the most important advances

has been to determine the optimum angle of inclination of the fence with respect to the slope.

Originally, avalanche fences were built perpendicular, regardless of the grade. Modern practice

is to set the fence at an angle slightly more than

90 degrees from the slope on the uphill side. This

arrangement utilizes the full height of the fence

and snow pressure drives it against, not away

from its anchorage. It is also possible to brace

the fence from the downhill side so that avalanche and snowcreep do not affect the supports

(fig. 52). ...

Avalanche engineers are experimenting with

treated and untreated wood, steel, aluminum, nylon, prefabricated concrete, and combinations of

these. They are also testing various fence designs,

including the spacing and arrangement of the bars

(horizontal, verticalj lattice, or net type). The

objective is to obtain maximum efficiency with

a minimum amount of material. Kegardless of

how the bars are arranged, it has been found that

if airgaps in the fence exceed a width of 6 inches,

very dry and very wet snows flow through them.

The design of interrupted fence patterns coming into use takes into account that it is seldom

possible to keep avalanches from starting under

all conditions. The interrupted pattern requires

66
.^*!*^:'*'-

t '

0"

'uK

j^ I.

,t

if-

'Mgyr

r^

V-- >

•••.•V-.* ''/>.>-''

-^ -^

-« j»*^'^*

F—49'»212

FIGURE 48.—A century of avalanche defense at Davos, Switzerland. Ancient masonry walls and terraces with Arlbere

fences added later, and flanked by modern fences. The six dots near the center of the picture are wind hiffli^

c-ontroUing an adjacent slide path. ocimei.

less fence to cover a given area and is less likely to

act as a snow catcher (fig. 53).

2. Nets., Hedgehoga, Trianghs. In their search

for effectiveness at least cost in labor and material,

European avalanche engineers have developed the

avalanclie net. In the Austrian version the net

is hung on two posts of ingenious design, like

very large ski poles. Eight to ten feet apart, these

posts are driven into the ground and guyed above

and below at an angle to the fall line (fig. ,54).

The net cables, located on each side and in the

middle, are hung with considerable slack and

anchored upliill. Among the fillers under test

are cable mesh, wire mesh, and horizontal wooden

poles. Results to date favor the wood poles

spaced 4 inches apart. The poles counteract the

nefs tendency to fold together when struck by an

avalanche. The virtues of this net are simplicity,

ease of construction, and flexibility. It can roll

witli the puncli of an avalanche. An installation

using nylon nets has recently been made in France.

67
FlGIMíE 4!l.—('(Hiérete

transmissiiHi iiyhm.

houses to right, Arlberp fences above

at ui>i>er left. Stuben, Austria.

F-40Ô213

l)ulliliead guarding an electric

Note diversion wall protecting

and snowshed

FlOUKE 51.—Old style Arlberg avalanche fence. Installed

for protection of pipeline in foreground.

HRLREHG FEhlCE

iO 'HISH 70% EF^tCTIVt

MODBaN FENCE

10'HIGH /ooyo EffecnvE

F—I

FiGUKE 50.—The Flexeustrasse snowsheds.

FIGURE 52.—The original Arlberg vs. modern avalanche

fence, .showing angle of inclination and effect of snow

l)ressure (broken arrow).

68
STEEP

NCT CAbLt

ANCUOR

y TRANSITION

5TEEP

FIGURE 53.—Avalanche fence layout, interrupted pattern.

3. Windbaßes, Drift Fences. The windbaffle

(Kolktafel) is a panel set at right angles to the

prevailing wind in a slab area. It was based on

the supposition that a system of these panels would

break up the current of windborne snow and

create islands of strength and discontinuity in any

slab that did form and, perhaps, inhibit the formation of depth hoar. Baffles do some good but are

not an absolute control. In Europe they are used

to reinforce or supplement other defense measures

(fig. 55).

Drift fences have long been used along highways to alter the fallout pattern of windborne

snow. Strategically located fences can reduc« the

amount of snow deposited on an avalanche path by

causing premature fallout. In a similar manner,

a fence can modify the windflow over a ridge in

such a way as to reduce or prevent the formation

of overhanging comices (fig. 56).

Conventional drift fences, made of wooden slats

woven together with wire, are not strong enough

for the winds that blow over the peaks. At the

present time, the location, height, and spacing

are a matter of trial and error on the ground.

Every installation must be a tailormade job depending upon terrain, prevailing wind direction

and force, amount of snowfall, vegetation, and

nature of the hazard.

GUY^,

QbLIQU^

FIGURE 34.—Austrian avalanche net.

Emergency airfield mats have proved successful

for fence material. They come in perforated steel

panels 8 feet long and 18 inches wide, convenient

to transport and erect. U-bolt fasteners make it

possible to slide the panels up and down the posts

for adjustments in height and airgap. With

proper bracing they are strong enough to use as

defense stiiictures (fig. 57).

4. Avalanche Mounds. Momids are a type of

braking or diversion barrier. They are effective

when installed on a slidepath that includes a transition zone. The crosscurrents, internal friction,

and age-hardening process set up by the mounds in

a falling avalanche reinforce the natural slowing

down effect of the transition.

Avalanche mounds originated in Europe, where

they have proved their value (figs. 58 and 59).

A system of mounds has been constructed by the

highway department on tlie Willow Swamp slidepath in southern Colorado that has proved to be

successful. Mounds have effectively protected two

lift towers and two lift terminals in Squaw Valley. In Alaska mounds have been installed to

protect highways near Anchorage. These have

proved very successful, including those constructed

on a slope above Turnagain Arm where the

69
inouiitíiiiiside falls steeply for about 3,000 feet

witliout any transition slope. They have been

])articnlarly eflfei-tive in arresting wet slides on

these steep slopes (ñg. 60).

FIGURE 55.—Recently designed Austrian windbaffle, omnidirectional, tripod suspension, nose of baffle anchored

to ground. The material is aluminum strips woven

together with wire.

\^:^

l'-405217

FiGUUE 56.—Arrow ¡wints to cornice on Squaw Peak,

controlled by drift fence. Compare absence of overhang with adjacent, uncontrolled cornices.

^\ X f<

F-495218

FIGURE 57.—Snow fences made of steel landing mats.

These are used to prevent cornice formation near the

summit of Squaw Peak, California (see fig. 56).

F-495219

FIGURE 58.—Arzler Aim and the original system of avalanche breakers. Arrow indicated the mound of waste

material which was the model for present-day design.

Note natural reforestation behind the breakers.

Where terrain favor's the construction of

mounds, they can be Iniilt economically and

quickly witli bulldozers out of the material at the

site. Fill should always be taken from the uphill side of the mound. Spacing should be as close

together as possible, leaving enough room between

mounds so tliat equipment can work and get sufficient material. To be most effective they should

be alternated as shown in figure 61, although

exact layouts are not essential. The practical

70
.^s!^:

^

w^

.it:

~i t

- y-.^-

FIGURE 59.—Modern avalanche mound system, near Innsbruck, Austria.

height of mounds is generally 12 to 15 feet. However, 20 feet may be necessary in areas of exceptionally heavy snowfall.

Mounds have been successfully installed on

grades up to 2.5 degrees and experimentally up to

35 degrees. The steeper the grade the longer the

transition zone must be for an effective mound

system.

9.3 Barrier Design Factors. Due to variations

of terrain and climate, the following data must

be taken as rules of tliumb rather than exact

formulas. Used in this manner, they are helpful

guides in designing avalanche barriers.

1. The pressure exerted by snowcreep against a

barrier, or any obstruction, can be estimated by

using the formula in figure 62.

Experiments with pressure gages have revealed

that the force exerted at the ends of a barrier is

from two to two and a half times greater than at

the center. These effects are due to the plastic

nature of snow.

2. To determine the spacing of avalanche

barriers, construct a right triangle using the slope

as the hypotenuse (fig. 63). The perpendicular

leg is a contant 45 feet. Project the horizontal

leg until it intersects the line of the slope. The

lengtli of the hypotenuse determines the spacing

of the barriers, under average conditions, for the

particular grade. Once the profile of the slidepath is known it is possible to plot the barrier system on a map or diagram.

3. The following rules of thumb apply to the

construction of barrier fences (fig. 64).

a. Distance between posts—8 feet to 10 feet

b. Distance between bars—4 inches to 6 inches

c. Fence angle—105 degrees on the uphill face

d. Support—35 to 40 degrees attached twothirds height of fence

4. Under average conditions, the wind-shadow

of a fence extends for a distance 11 times its

height.

71
F-495221

FIGURE 60.—Earth mounds used as avalanche breakers near Girdwood, Alaska. Though a transition zone is absent

on this slope, these mounds have proved an effective defense against wet slides.

9.4 Reforestation. From the long-range viewpoint all autlioriries agree that a forest is tlie best

and most economical avalanche barrier. A century ago, according to Austrian engineers, there

was no avalanche Itazard at Innsbruck because of

its screen of timber. The hazard has been created

by forest fires and logging.

To be effective, the forest cover must be dense

and extend to the avalanche release points.

Where forest cover lias been destroyed by fire,

logging, or climactic avalanche cycles, the problem

is to reestablish tlie cover. Some great slidepaths

are dormant for decades at a time and reforest

themselves. Others are active every winter.

Snowereep often prevents the establishment of

young trees. Reforestation on slidepaths, whether

natural or by man, needs protection. Any avalanche defense project should include reforestation if feasible.

The practice in the Alps is to reforest immediately where mature timber has been destroyed

by avalanche action and install barriers to protect

the seedlings.

The Austrians and Swiss are experimenting

with methods of protecting seedlings. Posts set

on the uphill side give good results. Other protective devices include tripods and wickets. In

many cases, trees are planted in the shadow of

complete avalanche defense systems. The Austrians favor the "island method'' of reforesting a

denuded area or pushing timberline to higher

elevations. They have learned from experience

that the mortality rate is lower where seedling

are planted near established growth, even if it is

no more than a bush.

72
A V A LANCM6

PLAN

AVALANCME

> 4-5 FT.

TWNSITION

SLOPE

PR,Of-iL-E;

FIGURE 61.—Avalanche breakers. > -fJ'A/'

FIGURE 63.—Avalanche fence layout. The separation depends on the slope angle, the fences being located at

4.j-foot contour intervals.

^j or HE/sur

A-L-W- SIN X

' AttA Of ftWCt OR OBSTRUCTION

( HEIGHT TIMES WIDTH)

: tfEtCTIVt LtNGTU OF SNOW MA55

ACTING ON ObSTaUCTION,

(TUII££ TIMEÏ DEPTH OP i,NOw)

DEkJSITV OF SNOW MASS

(r. Of WAT Eli IM THE SNOW

TIMES 62 4-THe WT OE A

CUfT Oi^ WAT£li )

' ANGLE Of THE SLOPE

FIGURE 62.—The calculation of snowcreep pressure.

FIGURE 64.—Avalanche fence design, showing optimum

angle of inclination and bracing arrangement.

73
2332.81 SNOW AVALANCHES

Chapter 10

10 AVALANCHE RESCUE. Prompt and organized rescue operations are the only hope of

getting the victim out alive if a human being is

buried in an avalanclie. There are records of persons who lived as long as 72 hours while buried.

Ordinarily they are either killed instantly, or die

within a short period from suffocation, bodily injury, or shock.

Investigations of a number of avalanche accidents, fatal and nonfatal, lead to the conclusion

that 2 hours is the average survival limit. Snow

is porous and ordinarily contains enough air to

support life, thougli not consciousness. It appears

that in about 2 hours an ice mask, from condensation of the victim's breath, forms an airproof seal

around his face. Rescue operations are therefore

designed to get the victim out witliin the 2-hour

limit. Due to special circumstances which prolong the life of the victim—he may be in an air

pocket—rescue oi)erations nuist not be abandoned

for at least 24 hours.

Successful avalanclie rescue opei'ations depend

upon special equipment, trained leadership, and

manpower. Darkness and storm are normal ;

parties setting out to participate in a rescue action must be prepared for the worst that weather

and snow conditions have to offer. The rescue

leader must disi)atch only groups of men who are

properly clothed, equipped, and in good physical

condition to face the rigors of exhausting work

in deep snow at high altitudes.

Obviously the accident would not have happened if avalanche hazard did not exist. This

same hazard condition is a source of danger to

the rescue party. Therefore, the highest degree

of caution is required both during the march to

the accident scene and while rescue work is in

progress. In an avalanche rescue operation in

19.58, two independent and uncoordinated parties

set out for the scene. One of them, following

high ground, dislodged a slide whicli buried a

member of the other party below,.resulting in a

second fatality. The strictest observance of all

precautions for traveling in avalanche terrain

must be enforced to prevent such accidents.

Rescue opei-ations of all kinds can easily become

disorganized uidess tliey are directed with a firm

hand. The most effective rescue is one which is

carried out with military precision and rigid

discipline.

10.1 Rescue Equipment, Rescue equipment is

needed imediately when an accident occurs. Rescue caches must be properly stocked and easily

accessible, and their location and contents known

to all who might have occasion to use them.

Caches must be kept sealed until actually needed

for a rescue operation. The establishment of

properly stocked rescue caches must be one of the

first steps in developing a ski area where avalanche danger may exist.

1. The minimum equipment that should be

available at all times in a readily accessible location is as follows:

12 probes, sectional preferred; otherwise

metal rods, i/^-inch diameter, minimum 12

feet long. Aluminum thick-wall conduit

is recommended.

12 snow sliovels

12 electric headlamps with carton of spare

batteries

100-foot climbing rope

Five yards of reel flagging cloth

200 feet of i/4-incli nylon cord

First aid kit (standard ski patrolman's kit

satisfactory)

Toboggan with blankets. Collapsible toboggan preferred, light enough to be backpacked

2. The following additional equipment is

recommended :

Emergency rations

Chemical hot pads for toboggan

Ski climbers in assorted lengths

Additional probes

Gasoline stove

Camp cook kit

Mountain tent

Gasoline lanterns

Portable radio

3. Highly trained German shepherd dogs have

come into widespread use in the Alps. Time and

again they have located buried avalanche victims

rapidly and efficiently. Avalanche case histories

show that buried victims have sometimes been

located by untrained dogs who happened to be on

the scene and instinctively joined in the search.

Therefore, whenever possible a dog should be taken

on rescue operations, whether he has been trained

or not.

Proposals to locate victims with mine detectors

or other unusual means have not as yet proved

practical.

74
10.2 Immediate Action.

1. Snow ranger will take charge of rescue operations and sound general alarm. He sliould not

hesitate to requisition any needed equipment or

call for volunteers. All ski patrolmen and experienced skiers will be ordered to report to a

central location.

2. The informant or eyewitness should be questioned on exact location of accident. Even if in

poor physical condition, eyewitnesses should return to the accident location with the first party to

point out where the victim was last seen. This

is extremely important.

3. Dispatch the fii-st party. A snow ranger

does not necessarily lead this party. It must be

in command of a reliable ski mountaineer and

first aid man, preferably trained in avalanche

rescue. It consists of not less than three persons.

Five is a desirable number, large enough to do the

work, small enough to travel fast. Tliis party

should be dispatched within 15 minutes after

receiving the alarm.

i. The first party travels light witli whatever

equipment can be picked up at once. Speed is

the first consideration.

5. Upon reaching the location of the accident,

the leader posts an avalanche guard, unless he

decides this is not necessary. Leaders are responsible for the safety of their parties. Avalanche

guards must be kept on duty if there is the slightest danger of another slide in the same vicinity.

Rescuers must know where to go in case of an

alarm. If the danger becomes critical, leaders

must not hesitate to call oft' operations.

6. Locate the spot where the victim was last

observed and mark the "last seen"' point so that

it cannot be obliterated by wind or snowfall.

From this point downhill, the first party makes

a hasty search of the slide surface for the victim

or any of his equipment.

7. If any indication of the victim is found, begin

probing in the vicinity. If no indication is found,

begin to probe in likely locations. These places

are obstructions in the slidepath such as trees,

boulders, or transitions, also the tip and edges of

the slide. A human ]>ody is bulky and all other

things being equal, is likely to be thrown toward

the surface or the sides.

8. If the victim is found, give first aid. ITnless

danger of further avalanches is acute, the firet

party should not attempt to move tlie victim out

of the area. Regardless of physical injuries, the

victim must first be treated for suffocation

and shock. Get the snow out of nose and mouth.

Apply artificial respiration if breathing has

stopped. Get the victim waim, applying body

heat if nothing else.

10.3 Followup Action.

1. In a well-organized ski area, the followup

party can usually start in half an ho)ir. It must

start within an hour. It goes completely equipi)ed

for extended rescue operations with everything

except food. It may be dispatched in groups with

at least probes and shovels.

2. The snow ranger will serve as leader after

notifying the county sheriff or designated disaster

authority. People not physically qualified for the

exliausting work of rescue should not be taken.

They can help by preparing and transporting

supplies.

3. In the 30 to 60 minutes at his disposal, the

snow ranger will organize and equip the followup

party, notify autliorities outside the area of the

accident and of possible need for further help. He

will obtain, if possible, the correct name or names

of victims and appoint a leader to take charge of

any further reinforcements and equipment.

4. Before beginning to jirobe, the followup

party must make a tliorough search of the slide.

They should foi-m a single line, shoulder to

shoulder, and march back and forth across the

slide debris kicking and scufling at the snow to uncover any additional clues and possibly the victim

or his e([uipment, which might lie somewhere just

under the surface. Systematic probing of the slide

begins at the tip and works up tlie fall line toward

the last-seen point (fig. fî5). Rescuers are spaced

shoulder to shoulder and probe eveiy square foot

(fig. 66 and 67). If the victim is not located on

the fall line below the last-seen point, or the latter

is not known, the entire debris cone nuist be systematically probed. It is very important to mark

the probed areas as the search is completed, so that

no effort is wasted by duplication (fig. 68). Flags

or other nuirkers foi- this purpose are an essential

part of the rescue equipment.

5. The first party may be relieved and sent out

of the area. lixhausted rescuers will become

casualties themselves if allowed to remain. Thej'^

can cai'ry messages giving an estimate of additional equipment and assistance needed.

6. Shovel crews will accompany the probers, relieving them at intervals and digging in any likely

spots. They should be prevented from digging

hai)hazardly and wasting energy.

7. Small slides may be systematically probed in

a few hours. Large slides may take much longer,

especially if the snow is deep and hard. If the

probing action is unsuccessful, the slide must be

trenched. This work, where possible, should be

under the supervision of the county sheriff.

8. Trenches are dug parallel to the contour down

to ground level or undisturbed snow at intervals

of 6 feet. If sectional probes are available the interval can be increased to 10 feet. Digging begins

at the tip of the slide and proceeds uphill. It is

best to space the shovel crews along one trench

with frequent reliefs. In this way snow from one

trench can be thrown into the one just duo-.

9. The operation ceases to be of an emergency

type if trenching is nece,ssary. A constant system

of relief crews must be organized.

75
FIGURE 65.—Arrangement of a probing line on an avalanche path. It i.s much easier for the probers to

keei) in a straight line if two men hold a string to

mark l(K*ation for the probes, and move this marker

forward each time the line advances.

10.4 Avalanche Accident Report.

1. Date, time, and location of accident.

2. Names of victim and other members of party,

with any information on their mountain

experience.

3. Summary of events leading up to accident;

departure point, route, and objective of

party.

4. Eyewitness account of accident, if available.

Otherwise, observer's deductions based on

tracks and any other evidence. Important

points are: location of skier in relation to

release point of slide; how was slide

released ?

5. Summary of rescue operations. Times and

names are imjwrtant.

Time of accident.

Time report of accident received and from

whom.

Time finst party dispatched. leader. Number in party.

Time first party arrived at scene of accident.

Time followup party dispatched. Leader.

Number in party.

F-495222

FIGURE 66.—Close-interval probing requires insertion of

the avalanche probe by the right foot, between the

feet, and then by the left foot. The prober then advances one step and repeats the sequence. Such close

probing is esx)ecially important in deep avalanche

debris (see fig. 67).

Time followup party arrived at scene of

accident.

Procedure at slide.

Time and location of victim when found, injuries sustained.

Cause of death or injury.

Time operation was concluded.

6. Weather and avalanche hazard background;

wind, temperature, and snow data; restrictions in force, if any; type of slide and

extent.

7. Terrain data, including maps and diagrams.

8. Recommendations and conclusions.

10,5 Case Histories.

1. Case History No. 1 (fig. 69).

a. Memher.'< of party: A and B, guests at

, excellent skiers; enthusiastic but inexperienced ski mountaineers. C and D, ski instructor and

ski patrolman, respectively, excellent

skiers and experienced mountaineers.

Events leading up to accident: C and

D were going to test the Run,

known to be dangerous at that time. A

and B went along at their own request.

76
If / -;>jí£íA

dtipiu buried victim

deflection of proof;

witfi deptfi

FIGURE 67.—Why a close probing interval is necessary.

FIGURE 68.—String marker method used to guide probing

operations. The string shown tied to the ski pole is

moved one foot ahead after the complete line has been

probed. The other string stays in place as a permanent marker for the area probed.

77
\

/y

4/KIER/ENTERED AREA HERE

AT HEAD OF DRAU/.

- /'i /;r-

/ I W

Poji+ion of Aierr //.• TA. ^K

i-^ ///-.. ''% ;,/ ^\ ^V L^ '///,..

'/,

^ .sPoritiort^of 'B'

when sV^t'Q. /tjr+«J-

^. ''--/^

/

>/

/• >iï5Uteîji'4"" ^^^"^ Ledge , 10' hiqh

\^;;V^>=.

///

Tip of ifTx\\àe buried

v^ bij 2nd j-lidc-"

'A'found h€re,af+€r+renchinq.

. • •• Trenche/.

?^jfc;- Area probed.

^--/ir.

V,

^'^(^''"Ony

%

%

FIGURE 69.—Avalanche accident. Case 1.

78
b. The accident: C skied onto the slope, followed immediately by A and B. Slab,

plus dry snow, avalanche fractured

while all three were on the slope. C

made an experienced mountaineer's typical test run of a doubtful slope: a short

fast dip in and out of the danger zone.

Only tlie edge of the slide touched him.

B and A went farther out onto the slope.

B, nearest to the release point, managed

to ski out to one side. A attempted to

outrun it straight down the gully. He

went over a rock ledge, apparently fell,

and was picked up there by the slide.

B, looking for A, went out onto another

avalanche slope and started a second

slide which buried the tip of the first one.

c. Rescxie operation.^: Rescue operations were

prompt, considering the distance from

help, and were well organized. Accident

happened at 2 p.m. First party reached

the scene at 3:80 p.m. Followup party

reached the scene at 9:45 p.m. Victim

was found at 5 a.m. the following morning. Elapsed time: 15 hours. Cause of

death : suft'ocation. Immediate action

was unsuccessful. Systematic probing

was unsuccessful. Body was finally located by trenching.

d. Weather and hazard data: Successive

storms followed by prolonged cold.

Snow had not settled and was unstable.

e. ConrJusion><: The following errors were the

direct cause of accident :

(1) A and B went onto the slope at once

instead of waiting for C to complete

his test nm.

(2) A and B went too far onto the slope.

(3) A tried to outrun the avalanche.

(4) The second slide started by B complicated rescue operations. Under the

conditions set, the only possible means

of locating A quickly would have been

a trained dog.

2. Case History No. £.

a. Location: An alpine highway.

b. Members of party : A, highway crew foreman and B, in a pickup truck; C and D,

plow operators in a rotary snowplow;

E, supply truckdriver; 1, 2, 3, skiers

traveling the highway on foot.

c. Events leading up to the accident : At high

elevations a prolonged and severe storm

was in progress. Because of the hazard

the highway was closed to public travel.

and it was blocked by snowslides. The

weather was cloudy at the elevation

where the pickup truck and the snowplows were operating. The plow was

working in a slide; the pickup was

parked a short distance behind. An

avalanche observer had earlier warned

the highway crew of the hazard and

recommended no traffic of any kind.

d. The accident: An avalanche on the same

track as the one that had already blocked

the road buried both the plow and the

i:)ickup. The rotary plow, being higher

and heavier than the pickup and close

to the iiphill bank, withstood the force

of the slide. C and D were trajjped in

the cab but not injured. The pickup,

parked near the doAvnhill bank, took the

full force of the slide. All glass was

broken. A and B were packed in the

cab as if in concrete. The avalanche

was probably a temperature-released

slab.

e. Rescue operations: E, the driver of the

supply truck, arrived on the scene shortly

after the accident. He saw the exhaust

stack of tlie rotary, which projected out

of the snow, and freed (• and D. The

tliree then departed to get help without

attempting to locate the pickup. On the

way out of the canyon they encountered

the three skiers traveling on foot, and

told them what had liappened. The

skiers proceeded to the avalanche, founcl

the pickup by means of its radio mast,

and dug out A and B with their skis

and hands. A recovered quickly, but B

was dead.

f. Weather and hazard: See above.

g. Terrain data: The accident happened

where a known major avalanche path

crosses a highway.

h. Conclusions: The accident was caused by

failure to heed a definite hazard warning

and failure to recognize the possibility

of a repeater avalanche. It was faulty

technique to have both vehicles and all

members of the party exposed to the

same slide. If C and D had remained

behind to look for their companions

while E went for help, there would probably have been no fatality.

i. Recommendation : All men working in hazardous areas should receive avalanche

rescue training.

79
2332.81 SNOW AVALANCHES

Chapter 11

11 AREA SAFETY PLANNING. Use of an

area exposed to avalanche hazard requires a systematic snow safety phin. This phm is tailormade

to fit tlie area. A safety plan for an isolated

structure like a mine building could scarcely be

applied to a highway extending for many miles.

Neither plan would have much value for a complex development such as a winter sports area.

All safety plans, regardless of the type of use

or character of the area, have the same objective—

maximum use with adequate safety. Knowing

the type of use contemplated or already developed,

the observer proceeds according to the following:

1. Terrain analysis

2. Climate and weather analysis

3. Hazard classification

4. Operations plan.

11.1 Terrain Analysis. Terrain analysis should

be made on the ground. It is desirable for the

observer to see the area in both winter and summer (fig. 70). Photomaps of excellent quality

are almost a necessity for this work. Topographical maps are of limited value for analysis, since

they lack sufficient detail. They can be used to

record the results after an analysis has been made.

The low oblique photomap is the most desirable,

followed by ground level panoramas and verticals.

Vertical photomaps have a special value in locating cross-country routes. Generally it is possible

to i)ick out the major slidepaths at once and make

a tentative route selection.

The purpose of the terrain analysis is to identify

the slidepaths and the zones of safety, to estimate the effect of natural barriers, and to determine the hazard pattern of the area. This pattern

will govern the location of improvements. Buildings are placed to take advantage of the natural

barriers. If this is impossible for any reason, the

fact must be noted and recommendations made for

protection by other means. It is seldom possible,

for instance, to survey an alpine highway on an

entirely protected route (fig. 71). However, a

hazard reconnaissance prior to construction will

suggest ways of reducing the number of avalanche

paths which must be crossed.

For a winter sports development, the layout of

the ski runs is as important as the location of

buildings (figs. 72 and 73). Minimum hazard

areas must be identified. Lift sites are chosen so

that they not only open up desirable ski slopes

but also make it possible to protect the people

using them. The lift line itself requires careful

study so that terminals and intermediate towers

will not exposed to avalanche danger as in figure

LkLk

F-495224

¡•iGVKE7().~Terrai>t analysis: (1) Major slide paths. (2) Lesser slide paths. (3) Narrow gully. (4) Cornice forniation. (o) Convex .slope. (6) Concave slope. (7) Safe area, no avalanche danger. (8) Cliffs and steep

r^Lf'*'r'^ (fractiire zone). (!)) Gentle, open slopes—heginner's .ski area. (10) Pos-sible lift location. (11) Arrow

Ro • »..^"""r;!"? ^^^ tenninal just beyond pass. Almost all sloi)es shown are accessible from this lift. Albion üa.sin. Alta, L tah.

80
FIGURE 71.—An example of avalanche hazard. A major

avalanche path, with vertical relief of nearly 4,000

feet, overhangs the access highway leading to a major

ski resort. Protective measures used are road closures during high hazard, and artificial release of

avalanches by artillery fire.

The trend in lift design is changing as a result

of damage to lift lines caused by avalanches. It

used to be thought necessary to install a lift on

many towers so that the cable would closely follow

the land profile. Numerous towers multiply the

exposure to avalanche hazard—and snowcreep,

which can be just as destructive. Modem design

utilizes a minimum number of towers and long

spans extending over the avalanche paths.

Protection of the lift line and structures from

avalanche hazard and snowcreep should always

be fully considered.

11.2 Climate Analysis. It is more difficult to

make a climate and weatlier analysis than a terrain

analysis. In a new area, the data are usually incomplete. The kind of information an avalanche

observer wants generally is not recorded. Some

information is always obtainable by studying the

available records, by questioning inhabitants or

people familiar with the area, and by studying the

conditions on the ground. Persistence and ingenuity are required. In one case, a snow ranger

recommended minimum lift clearances on the basis

of snow course records at a station 20 miles away

but in a similar location. The recommendation

was generally thought to be excessive, but it was

accepted and proved to be correct the next winter.

Ordinarily, an original climate and weather

analysis must be based on superficial data: some

temperature, maximum snow depth, and total precipitation records ; some word-of-mouth history of

avalanche occurrence. To these, the inspector

adds his own observations in the field : prevailing

wind directions, snow types, and storm characteristics. This information, even though fragmentary, should permit him to form some opinion

on the predominating types of hazard.

11.3 Hazard Classification. After analysis of

the terrain, climate, and weather, the next step is a

hazard classification of the area (fig. 75). Identifications must be accurate and should be plotted

on a map. All factors must be considered : grade,

contour, and dimensions of the various slopes;

natural barriers; exposure to wind and sun; type

of use ; the avalanche history of the location, and

the protective measures that will be available.

Four degrees of liazard are recognized :

1. Minimum hazard. This classification indicates practical absence of hazard. Examples : A building fully protected by natural

or artificial barriers; a slope not likely to

avalanche.

2. Low intermittent hazard. This indicates

occasional exposure to avalanches of

dangerous size. Examples: A slope not

steep enough to avalanche in dangerous

volume except under extreme conditions ; a

structure close enough to a slidepath to be

damaged under climax avalanche conditions.

3. High intermittent hazard. This indicates

areas frequently subject to avalanches of

dangerous size. Example: A high-angle

slope of sufficient dimensions so that hazard

is likely to exist with every major storm

or under delayed-action avalanche conditions. (The distinction between low and

high intermittent hazard is frequency.)

4. High intermittent hazard, not controlled.

Indicates highly hazardous areas which

are not feasible to control.

The classification of an area may be changed if

protective measures are applied. Thus an area

would take a lower classification if it were put

under stabilization by use, explosives, or barriers.

The reverse situation is also possible.

11.4 Operations Plan. The final step in snow

safety planning is to formulate a plan of operations. Where a number of different activities are

involved, such as a ski area and a highway, with

several cooperating agencies, the operations plan

can be complex. Ordinarily it is a brief and

simple outline of procedure. A snow ranger, well

trained and thoroughly familiar with his 'area

seldom needs to look at it. He knows it by heart.'

The same is not true for his successor or a temporary replacement. Either could easily get into

trouble without a systematic plan of action. The

following is a plan outline :

81
Snow Safety Operations Plan

I. Objective

II. Ivesponsibility

1. Leadership and control

2. Finance

;3. Personnel

4. Equipment and facilities.

III. Table of Organization

1. Snow Safety Leader

2. Assistants

;'). Keinforcenients.

IV. Table of liquipment

Rifles

2. Explosives

;5. Snow-Weather-Avalanche observation

equipment

4. Communications

5. Rescue equipment

(i. Transportation, wheeled and oversnow

7. Siffns.

V. Table of Facilities

1. Explosives magazine and field caches

2. Headquarters

;i Gun towers

4. Shelters

5. Barriers

VI. Procedures

1. Area hazard classification

2. Protective measures

3. Opening and closing procedures

4. Restrictions procedures: signs

5. Sequence of avalanche control operations

(>. Search and rescue

Where national-forest land is involved, a snow

safety plan approved by the forest supervisor is

customarily a permit requirement. It covers ski

patrol functions and plans for handling other

.iiJijL

F-+nr,226

KiGUKE 72.—A new .ski slope in tlie process of development. (1) Lift line partially cleared. (2) Only observed

avalanches on the mountain originate in this area of scattered timber. (.3) A natural clearing. (4) An avalanche Kully flankins the new area. (5) Area of heavy timber, very steep but completely stable. (6) Construction road. Switchbacks .screened behind timber. Arrow at right, prevailing wind direction. The sloi>e faces north.

82
hazards related to ice, breakable crust, insufficient

snow cover, high wind, low visibility, and lift

operations.

Where possible a snow safety reconnaissance

should be made in advance of development. This

woukl insure a sound basic safety plan takingadvantage of natural barriers for the protection

of major installations, and reducing the need for

expensive artificial protection measures to a

minimum.

F-49D227

FIGURE 73.—The slope shown in figure 72 after development. ( 1 ) The liftline, held to minimum width for

wind protection. All defective trees removed. (2)

A ski trail cut around the mountain. (3) Ski trail

partially cleared around the other side of the mountain. Timber between trails left for avalanche protection. Tree clumi>s below are al.so left so that no

avalanche path the full length of the slope is created.

(4, 5, 6) The.se isolated trees and part of the clump

were later removed. (7) Jumping hill. Tree screen

retained for wind and avalanche protection and to act

as a natural barrier between skiers and the avalanche

gully. FIGURE 74.-

F-495228

-Avalanche destruction of a lift tower.

tfA

^4»\^

.V .^ ' • ^- "^Ài--- Vd^^f t'^^i^ A^ 'A^w^

ill Liiii ' *>)'l

,, F-49.5220

i'lGURE 75.—Hazard classification of a major ski area. 1, Minimum hazard. 2, Low intermittent hazard 3 Hieh

intermittent hazard. 4, High intermittent hazard not controlled (not illustrated here).' '

83
Bibliography

The followinjí list encompasses some technical

references on snow and avalanches available in the

English language. There exists a large body of

literature in otlier languages—principally German, French, Italian, Russian, and Japanese—in

tlie form of texts and articles or paper-s in periodicals. A more complete bibliography of this literature is available on request from the Forest

Supervisor, Wasatch National Forest, Salt Lake

City, Utah.

The classic work in Englisli has long been

"Snow Structure and Ski Fields" by Gerald

Seligman. This well-illustrated book has been out

of print for a long time, and accessible copies

are rare.

Some of tlie more important Swiss texts have

be«n made available in English by the U.S. Army

Corps of Engineei-s' Snow, Ice and Permafrost

Research Establishment (referred to below as

SIPRE). Most important of these is ''Snow and

its Metamorphism," a standard reference on snow

mechanics published in Switzerland in 1939.

Bader, H., R. Haefeli, E. Bûcher, et al.

January 1954. Snow and its Metamorphism, SIPRE

Translation No. 14.

Bueher, E.

February 19.")6. Contributions to the Theoretical

Foundations of Avalanche Defense Construction,

SIPRE Translation No. 18.

Fuchs, Alfred

July 1955. Avalanche Conditions and Avalanche Research in the United States with Recommendations

for Future Work, SIPRE Technical Report No. 29.

Fuchs, Alfred

July 1957. Effects of Explosives on Snow. SIPRE

Special Report No. 23.

Nakaya, U.

1954. Snow Crystals. Cambridge: Harvard University Press.

Rikhter, G.

August 1954. Snow Cover, its Formation and Properties. SIPRE Translation No. 6.

Roch, Andre

April 1956. Mechanism of Avalanche Release. SIPRE

Translation No. 52.

Seligman, G.

1936. Snow Structure and Ski Fields; With an Appendix on Alpine Weather by C. K. M. Douglas.

London : MacMillan and Co. Ltd., 555 p.

Yosida, Z.

1955 and 19.56. Physical Studies on Deposited Snow, I.

Thermal Properties II. Mechanical Properties. Contribution to the Institute of Low Temperature

Science. Sappro, Japan: Hokkaido University No. 7

and No. 9.

Various authors

1951. Review of the Properties of Snow and lee.

SIPRE Report No. 4.

For additional papers and texts on the general

subject of snow and ice, see List of Publications,

U.S. Army; Snow, Ice and Permafrost Research

Establishment ; Corps of Engineers, July 1959.

The technical journal which carries papers on

various aspects of snow physics and occasionally

on avalanches is the Journal of Glaciology, published twice a year by the British Glaciological

Society.

84 U.S. GOVERNMENT PRINTING OFFICE: 1961 O—577868