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Method of measuring the significant characteristics of a snow cover
NATIONAL RESEARCH COUNCIL OF CANADA
ASSOCIATE COMMITTEE ON SOIL AND SNOW MECHANICS
TECHNICAL ｍｅｍｏｐｾｎｄｕｍ NOoS
METHOD OF MEASURING THE SIGNIFICANT CHARACTERISTICS OF A SNOW=COVER
ANALYZED
Go Jo Klein
A contribution from the Division of Mechanical Engineering NoR.Co, Ottawa, which is also distributed as
N.R.C. REPORT NOD ｍｍｾＱＹＲ
SUMMARY
Snow is a material the properties of which vary
consider-ably. The conditions and features of the snow which
deter-mine its properties are discussed and standardized methods and instruments for measuring these characteristics are
describedo The principal applications of the measurements
are: (a) snow-cover surveys for obtaining data on snow con-ditions in different localities and (b) performance trials of winter equipment in which it is desirable to define the
snow conditions that occur during the trialso The report
includes a snow nomenclature and general information on snow which may be helpful to observers.
.
.
.
TABLE OF CONTENTS
INTRODUCTION . . 0 0 • • • • • • • • • • •
PURPOSE OF THE TESTS . . . .
BASIS ON ｾｭｉｃｈ THE TESTS HAVE BEEN CHOSEN . . . . 0
INSTRUMENTS . • . . . e , • • • • • • • • • • • • •
GRAIN-FORM DENSI TY SCALE . 0 •
TEST PROCEDURE . . . .
GENERAL INSTRUCTIONS
GENERAL NOTES ON SNOW . . 0
NOMENCI.A TORE FOR SNOW CONCLUSION
APPENDIX I
FIGURES
1. Balance
2. Snow Sample Cutter (Soft ｓｮＰｾＩ
30 Snow Sample Cutter (Hard Snow)
4. Snow Hardness Gauge (N.R.C. Type)
5. Magnifying ｇＱ｡ｳｳｾ Graduated Cup and Spatula
6. Snow Penetrometer
7. Clinometer
8. Grain-Form Density Scale
9. Grain-Form Density Scale (cont'd)
10. Free ｾ｡ｴ･ｲ Content Nomograms
Page Noo 3 4 5 9 13 14 20 22 28 34 35
3
INTRODUCTION
10 In winter. snow presents a great number of problems
many of which are related to transportationo There are, for
example, the problems of snow clearance on highwaysJ railways
and airports, the performance of ｯｶ･ｲｾｳｮｯｷ motor vehicles,
aircraft skis, loggine: sleighs end many miscellaneous snow
problemso There is also the problem of avalanche control
which has received so much attentJon in Switzerlando Such
problems are not new and yet it ls only in the last twenty
years that snow mechanics research has been taken seriouslyo
Al though snow is often regarded as 8. nui sance9 the sueces se s
that have resulted from snow research amply demonstrate that many of the undesirable effects of snow can be controlled or lessened considerablyo
20 The proper tie s of snow vary wid eLy , New snow i s ､ｩｦｾ
ferent in many respects from old snow and the snow ｣ｯｾｾｯｮ to
temperate climates is again different from that usually found
in the Barrens, Experienced skiers are well aware of this
peculiarity of snow and have found that a fair number of ski waxes &re required to cope with the range of skiing qualities
of the many types of snow.. It is because snow properties vary
so widely that designers of winter equipment and observers of vehicle trials require a more complete description of a snow= cover than merely its total depth which is often the only information providedo
30 The solution for many snow problems may be found by
means of two separate lines of research which must be properly
correlated if they are to be effectiveo The first deals with
the properties of different types of snow and 'should be carried out in a well equipped laboratory where conditions are under
the control of the research workero The ｳ･｣ｯｮ､ｾ which is the
sUbject of this report. deals with the collection of snow=cover
data at a number of observation stations with a view to ･ｳｴ｡｢ｾ
lishing a reasonably reliable picture of the average and extreme
conditions which occur in different localitieso Over a period
of many years, the Meteorological Services of different count-ries have collected considerable data on the amount of snowfall
at a great many weather stationso In recent years5 hydrological
snow surveys have been conducted in many countries. usually in mountainous areas, for the purpose of predicting the amount of
spring run-off and its effect on irrigation and water-power
developmentso However. neither of these supply the kind of data
needed for those problems related to winter transportationo
40 The Associate Committee on Soil and Snow Mechanics
of the National Research Council of Canada have realized the value of collecting snow=cover data and it is due to their efforts that standardized instruments and methods have been
4,
developed in Canadac '].'L..o 11ls tr-c.ment S tile not new ｾＮＮ similar
instruments have been ｾｳＦ､ in various snow researches = but
they have been redesigned ao as to be as portable and as ｣ｯｮｾ
venient to use as possj.bleu Several sets of these instruments
were used during the winter of ＱＹＴＵｾＴＶ in connection with
trials of over-snow motor vehicles and they proved to be
suit-able for snow-cover survey plJrpO se s and for deflntng the Sl10W
eondi tions d1.JTing veld.GI.e tori a I.s c
50 . The Meteorologic:al Service of the Dep ar tment of
ｔｲ｡ｮｳｾｯｲｴ are also interested in snow-cover surveys and have
offered to conduct a trial survey at 30me of their prlncipal
Weather Stationso The Depart.me nt. of National Defence is also
cooperati.ng in the survey ｢ｾｔ ob t.ai nfng snow" cover data a.t
their Cold ]feather Test Stations"
60 This report, ｴｨ･ｲ･ｦｯｲ･ｾ deals
only
with the methodsof describing a snow=cover by means of IDe&suremertsand is not
concerned with mea.surements of snow pr-cpe r-ties . Its pu rpo se
is to prqvide:
(a) instructions for snow=cover measurements,
(b) a standardized snow nomenclature for observers and those who will read the reports of the tests, and
Cc) information on snow that w111 be helpful to
the ob servers0
PURPOSE OF 'IRE TESTS
70 The aim of the snow tests is to describe any ｰ｡ｲｴｩ｣ｵｾ
lar snow-vcover- by means of measurements (or photographs) in
order that the problems associated with it ｾ such as snow
per-formance of v en l.c Le s , skis, sno w-c Lear-an ce equipment, et c ,
-can be fully appreciated by the designer or user of such
equip-ment, The tests, ｴｨ･ｲ･ｦ｣ｲ･ｾ deal with the significant features
ot' tn e various layers of snow on the ground, a nd are coneez-ned
with falling snow only because of its relation to the snow in
the surface Layer-, In certain winter problems the contours of
the snow surface may become important, for ･ｸ｡ｭｰｬ･ｾ the drifts
01! waves of hard wind=packed snow found in the Barrens present
problems to designers of over=snow ｶ･ｨｩ｣ｬ･ｳｾ and designers of
aircraft winter landing gearo For this reason the tests also
deal with the shape of the snow surface, The observer should
alwavs keep the a Lm of thetes.ts in mind and should be continually on the lookout for unusual conditions which may affect the design
or performance of winter equipmento "I1'111e snow in its various forms is an interesting material from a purely scientific point
of view, the tests are primarily intended to fulfil a practical
purposeo
BASIS ON WHICH THE TESTS HAVE BEEN CHOSEN
8. An outline of the reasoning which determined the cI-lOiGS
of tests should gi ve the ob ser-ver- a bet t.ez- appr-e efat.Lon of their
value. Snow testsj unless well ｰＱ｡ｮｮ･､ｾ can consume a great
deal of t1IT"te; therefore only the as sen tial te st s 11ave been ch o sen
and they have been made as simple and convenient as possibleo
90 In the following Itsnow" refers to 8. WA,-SS or volume 0 f
ｾｮｯｷ rather than an individual crystal or grain,
100 Snow is very porous and the r-ef'or e con ta.lns a
corLsider'-able volume of air. At times it may be wet so that water in the
liquid state may also be present. In general thenj we can consi=
der anew to be a mixture of 1ce9 water and airo The relative
amou-...t::i and the physical properties of each consti tuent will
therefore affect the physical properties of the snowo The only
one of the constituents whose properties vary appreciably with
temperature is ice, since water can only be present at one ｴ･ｭｾ
ｰ･ｲ｡ｴｵｲ･ｾｬｯ･ｯｾ the melting point of ice, and the main temperature
effect on air is that it determines its moisture contento Near
the melting point the properties of ice vary considerably with
temperature0
110 The shape and size of the ice crystals also influence
the physical characteristics of snow; long slender forms are more fragile than more compact forms and cause the snow mass to
be structurally weako In new snow there are structural effects
produced by the interlocking of the crystal branches and these
too depend on the size and shape of the crystals0 31.2;6 and
shape of the grains have another important effect = the cohesion
of wet snow is due largely to surface tension forces which¥ as
explained in paragraphs 114 to 117j are most effective ｾｬ･ｮ the
snow texture is very fineo
12 0 Changes which take place during ageing or "flrnlfica=
tion" of the snow of ten cause the grains to grow together to
form a miniature ice networko This ｢ｯｮ､ｩｮｧｾ which is usually
present to some degree in all forms of dry snow¥ can add greatly
to the strength of the snowo
130 From the above di scussion it follows triat the
f'unda-mental features upon which the properties of snow depend and which distinguish one form of snow from another are:
(a (b (c s;r.l.(';v/ ｾＺ crnp6Y:'Gt, ＼ｾＭＺＧ＼ｾＳ re:la ｾＬｾｯ ｐＺｏｐｾｾｬｾｾ shape and ::dZS< of cry st G.I s ; and degr'ee of ｢ｏｄ､ｾＬｴＩｧ
is·)f 1 Ce]l ｾｶ aｴｾ e r 811d airｾ
ＡｾＶ 2 lvidunl grains or
hetween the grains,
140 The methods ｦｯｾ ob"cajnirlg (a) and (b) are obvious.
if (b) is expressed :i:n. terms of sp e ci ri.c gr·avl.ty and percentage
free water contenr 3."cw 6TDj:')Y'D tur e eiS,n.be me asur-e d vd. th a.
t.hez-momet er-j ::."peeifl. c gra v;
t:v
can b6 f by weigh rng a knownvolume of snow • And. 1:'105.' ｾ｜［､ＮｴＶｲ content ce..n be dot8rmined byｾ a
simple ca.Lor-Lraetri i: ｭＺＺＺ［ｴｾＮｾｬﾫＺＧ dE'sc r :thed J.L l;i:l.Fii:..graphs
e,6
to 70>150 'There are 8.1tel'fi.H,t,1.ve methods fo:r obtaining (cl ,
These ar-es (1) meft8Ul'ements uT'a:l.rfloW' permeability as a.dopted
by the Ins titut fur Scr.ineec c uno. Law:ineT.lf'orschung, Swi tzerland9
f2) macrophotography of Ｓｦｦｩｾｬ･Ｘ of snow grainsJ and (3)
obser-vations with the aid of a ms.grdfylng glass using a ruled b
ack-ground and an arbitrary but st&ndardized Bcale of "grain=form
density". The first of these math8Gb, while excellent for
laboratory tests, requires apparatus that cannot easily be
made portabLe , The second is also a good method but is not as
simple as the thirdo 'I'ne third method is very convenient and
} ..ＺＺＧ､Ｇｾ ); found to give sufficiently accurate results for most
practical purposeso Therefore the third method has been
adopted in this reporto
160 The moat direct measure of the degree of bonding
between grains, (d) above, is the tensile strength of the snow,
Although tensile strengt.h tests have been carried out in the
Labor-a t.or-y , they present a number of d.i,fficulties for routine
use in the fl$31do The next best metho d , although somewhat
Indirectj is a test of snow hardness or firmness ｾ a form of
compression test of snow in situo
170 'I'wo types of instruments have been used for snow
hardness tests" (l) the Conic Ram (Kegelaonde) developed by
the Institut fur ｓ｣ｬｭ･･Ｂｾ und Lawt nent'or acnung, and (2) the
NoR"C o Snow Hardness Gauge.
180 The Conical Ram was designed 1'01> a particular purpose
- that of routine tests in the Swiss Alps with a view to
esti-]':JRti.ng the avalanche tend enc I as of' deep snow on steep s lcp eS0
Its outstanding feature is that it ::an measure the firmness of
any layer down to depths a.s great as three or more meters
with-out the necessity of digging a test trench in the snowo Although
i t 1s not a d:i.rBct indicating instrument, only a ver:ysimple
oa.1':111 ation ls required to ob t.ai n the
r rrmn
e s e of the snow0In SWitzerland, where avalanche research is considered essential to safety in the mountains, the Coni cal Ram has been used ext.en-sively and is regarded as a 5tandard snow testing instrumento
190 'Ihe N.H.C. Snow Hardness Gl=iuge messur-e s essentially
the same qualit.y of the snow 8.3 does the Conical Ram although
the numerical results are not identicalo It is si nn.Lar- to an
instrument developed in Switzerland and referred to by Seligman
in "Snow Structure and Ski Fields". Its main features are
that it is simple ｾｾ､ compactJ it indicates snow hardness ､ｬｾ
rectly, and it covers a very wide range. It does, howeverj
require the digging of a test trench, but this is only a oi
e-advantage when snow ｽＺﾷｊＮｦｾｲ､ｮ･ｳｳ measurements alone are taken , In
most snow research -" avalanche research bei ng an exception ,"'
the trench is requ,ired for other- t e st s , in many cases "tt need
not be dug right dOVID to the ground0
200 The shape of a snow surface is not easily described
by simple field measurements a10neo In most cases a word des=
cription will suffi c e , but when the surface is such that it
has a definite bearing on the problem under consideration, its shape may be most conveniently described by means of a
photo-gr-aph ,
210' A snow- cover can therefore be completely de scrib ed
by the methods outlined aboveo The various tests are given
in the table belowo Air temperature, although not essential,
is alao included since it is often used as a convenient referenceo
220 The various tests tabulated in this table are not
without a few interrelations. Snow of very lpw specific
gravity will have crystals of slender dendritic form and will
generally contain no free watero Dry snow of ｭｯ､ｾｲ｡ｴ･ｬｹ high
specific gravity will be rather hard and its grains will be
of compact forma Very wet snow will have compact grains and
very high specific gravityo ｾｦｵ･ｮ the snow temperature is
equal to the melting point of ice, free water will be present, but at all other temperatures there will be no free watero
230 When snow' measurements are made in connection wi th
tests of skis, vehicles, etco, two additional instruments are
very usefulc These are: (1) a clinometer for measuring the
slope of the surface on Which the tests are made, and (2) a penetrometer for measuring the variation of penetration with
unit pressureo The clinometer and penetrometer are only ｩｮｾ
eluded in tp.e sets of instruments that are to be used :'r\ con,
TEST
Air Temperature Snow Temperature
Specific Gra vity
. of Snow
Free Water Content
Grain Shape and Size
Snow Hardness
Form of Snow Surface
TABLE I
Ai'PAHA I'TJS
'I'her-momet er- (B) Balance Snow Cutters Snow ｋｲ､Ｎｬｾ･ Balance Thermometer (\! Magnifying Glass Graduated Cup Spatula Grain-Form Density Scale,
Snow Hardness Gllige
or
Coni,cal Ram,
Any Photographic Camera.
REMARKS
'1';1k'3n one foot above snow surface
At least 3 or 4 read= lngs reqid to establish
t ernperat.ur-e gradient.
(See paragraph 58),
Taken in each distinct
layer for first 12 or
1811 and most layers
below this depth.
(See paragraph 71). Only req'd when snow
tempo
=
OOC. Taken inat least 2 or 3 soft layers near surfaceo
(See paragraph 65)0
Obser-vat i.on s in each
distinct layer through=
out depth
(See paragraphs 62
and 63) >
Taken in each distinct layer throughout deptho (See ｾ｡ｲ｡ｧｲ｡ｰｨ 59)0
(See paragraphs 75
and '76).
Thermometer ｾａＩ has 110° ｴｯｾｬｏｯｃＮ r-ange
90 INSTRUMENTS
Choice of Instruments
240 The instruments which have been ｣ｨｯｳ･ｾ are simple,
compact and reasonably durable and no special skill is required
to use them, All, except the penetrometer, are graduated in
metric units to facilitate comparison with the comprehensive
Swiss researches in snow mechanics0 The penetrometer, which 1s
used mainly in conjunction with vehicle tests, has been based on British units since they are used in vehicle tests in this countryo
Care and Maintenance
250 All components of the instruments are rust proof
with the exception of the balance knife edges Which are made of
hardened steel. Should the knife edges show signs of rustingl
some very light oil may be applied and then wiped ｯｦｦｾ but oil
should not be ｵｳ･ｾ on any other part of the instruments 0 If the
snow cutters or snow knife begin to rust they may be protected
with a thin coating of wax0
260 The interior of the carrying case should be kept as
free of snow and water as possible; any snow or water adhering to the instruments should be removed before the instruments are put back in the casao
27. All instruments should be kept free of dirt or grit
and may ｾｾ oiled or waxed before they are stored for the summer.
However, if oil is used it should be removed before they are put into service againo
28. It is advisable to keep the instrument s in their
carrying case in an unheated shelter so that the instruments, at the time they are used, will be at approximately the same temperature as the snow.
290 If the liquid column of a thermometer becomes ｳ･ｰ｡ｾ
rated, as ｭｾｹ happen during shipment, it may be reunited by one
of the following methods:
(1) ｍ･ｾ｣ｵｲｹ ｴｨ･ｲｭｯｾｾｴ･ｲ with separation near bulbo ｉｭｭ･ｲｳｾ
the bulb in a freezing mixture* until all the mercury has
been drawn into the bulbo Tap the thermometer to dislodge
gas bubbles in the bulb.
*Freezing mixture (parts by weight)
ｾ parts snow at OOCo added to 1 part NaCl . . 000 .•...• 0.0 ｾＲＰＰｃｯ
1 part snow at OOCo added to 1 part alcohol ....••••• 0 -300Co
10,
(2)
( 3)
Me_ _ _".rｃｬＲＺｲｾ｜ｲ ＮｾＮＭｩＮＺ __ ..｟Ｎｾ th er-mome__Ｎ｟ｾ _ ..._ ｾ｟Ｎ ter'ｾｾＮ ｾ...'vv:i.ｾ-ｾＭＧ .." _ttl." ' > ; -ＬＬＢＭｾＮＮＮＮＮＮＮ 8ＶＭＰｂｾｉ［Ｚ ｟［ｾｾ •.._ ','5.t l ＮｾｃｊｾＩＬ, _ - - . "• • ._ne._.,,0- _,,'"arＬｾＮＬＮＮＭＬ t.cn -: HoldＮｾ the
thermome ter 8.t B. con31:':'erabl.o 11e.ｾ ght over a low flame
and by mova ng it bad.: and forth Ql)ply teat gent Ly until
the brokan place in the thread has moved upward into the
reserve,
CAUTION: TIlls method is more Lf.keLy to r-esu I t in breakage
and extreme ca r-e should be-used"
ｒ＿ＮＮＮＹＮ［Ｍ｢ＮＱＮｓｌｕＮＺｾＮｾ｟ｾｾﾷＺＺｾ［［ｪｾＬＬＺＺＺｾｾＩ＿Ａ｝ＺｾＬｴ･Ｚｲ 2'6CL.I'e1jir f'E,\:;en one end of a
strong cord fibc'u'; :3 f':; 10ng to 1;he rrcometer- b7l bi ndLng
it tighely vri+,b ＧｾＧｅｬｾｾｵｬ ();36 t!ll'O &.101'1,'1; the 1 length of
the th er-rnomet er the thermometer hula bet 0PPOSlte the
free end of the cord, Securely hold the free Bnd of the
cord and r-api dLy whi r L tne thermometer In ,9, horizontal
plane over head,
Red liquid thermometer USB a method similar to (2) but
Ｇ｡ｰｰＭｩｹＭＭｴｨ･ＧＧＧｨ･｡ＧｴＭ｢ｹｾＭｩｲｬｬｩＭｮ･ｲｳＺｬＮｮｧ the bulb in water being heated
on 8. stove, Stir the water cont i.nuaDy and do not let
the ｴｨ･ｲｭｯｭｂｾ･ｲ come in contact with the vessel containing
the water.
The nt.OVA methods have all been used suc c e s sf'u lLy , If column
"'j(";'fJt1T:'t.; n oc cur-s frequently i t may be advts abt.e to remove the
thermometers from the kit and stor-e them in a heated bud LdLng ,
30. If reasonable care is takenj damage to the instruments
or loss of any of their parts can be avoidedG
Balance
310 The beam type balance shown in Pig" 1 is ideal for
snow measurementsc It is more ｡ｾ｣ｵｲ｡ｴ･ than a spring type
balance and its auxiliary rider C simplifies measurements of
free water contentn
320 'Ihe balance frame A car-ri es two rlders Band C of
equal weightc Moving either rider B or rider C from extreme
Le.rt to extreme right i s eCJ:l..d vaLen t to 500 gr-ams so t.hac
measurements up to 1 000 grams oan be made by utilizing both
riderso Rider B carries a vernier which reads directly to
2 gr-ams0 Measur-emen t s to the neareat gram can be made wi tn-C,.J.t much diffie-ul tyo
330 Rider C contains a spring clronp which prevents
｡｣｣ｩ､ｾｮｴ｡ｬ movement of the rider when the frame is at an ex=
t r-eme angle0 The clamp must be released by pr-e sai.rig on button
D wnerie ver- i t is necessary to mo ve rider C"
340 Hook E is for hanging the balance on any convenient
supporto A balance support, which springs on to the edge of
the carrying c a ae , is provided with each set of Lne t rument.e ,
350 The balance has been adjusted to read zero when the empty bucket is in place on hook F and the rider C is in its extreme left positiono
360 The senat ti vi ty of' the balance can be altered wiｴｨｾ
out disturbing the zero adjustment by changing the vertical
posi tion of the counterweight G. If the balance has to be
used exposed to the wind, it is advisable to denrease
sensi-tivity. Generally, the balance should be as sensitive as
condi tions will per-mi t . Frequent adjustment of sensi tivi ty
should be avoided To change balance sensitivity loosen the
clamping screws H5 move the counterweiept G (downward to
decrease, upward to increase sensitivity) and securely tighten both screws H.
370 The condi tion of' balance is indi ca ted when the pointer
J, which forms part of frame A, swings an equal amount on each
side of the knife edge plates Kc
Snow Sample _Cutters
380 Each set of instruments contains two snow sample
cutters which are used to cut snow samples of known volume for
speciflc gravity measurementso The internal volume of each
cutter is 250 cCo
390 The cutter shown in Figo 2 is for soft snow conditionso
Its cylinder A has a thin cutting edge and its handle is bent
so that the axis of ｾｨ･ handle coincides with that of ｾｨ･
cylin-dero The removable back cover plate C has hooks which engage
pins D on the cylinder0
400 The soft snow cutter is always used horizontallyo A
trench must first be dug in the snow covero During the cutting
of the samplej the cutter is rotated about its handle axis in
order to avoid compressing the snow sample. A kni£e is used to
trim the sample level with the forward and rear edges of the cylindero
41. The back cover plate may be used to prevent very
slippery samples from sliding out of the cutter before they can
be placed in the balance bucket. It is also useful when ､･ｴ･ｲｾ
m:l.ning the specific gravity of loose grains such as hail.
420 The snow sample cutter shown in Fig. 3 is for hard
snow conditions. Its cylinder A has a saw tooth cutting edge C
and a simple handle Bo The internal volume between the planes
ｘｾｘ and Y-Y is 250 cCo This cutter may be used either
hori-zontally or vertically. To determine the specific gravity of
a thin crust, cut sufficient discs of the crust to fill the
length of the ｣ｹｬｩｾ､･ｲｯ The method of use should be fairly
Snow Har-drie e s Ga'2.ge (N.1":.C e, 'r,'.i1'''j'
----,...-
-
...ｾｾＭ-430 The sr:ow h ardne s s gai)ge is 8. push type spring
balance on whi.ch anyone of severat pr e s sur-e discs can be
moun tedJ Tne gauge is shown r.n Pig 4:
44c The end of the pusn rod E 13 arranged so that the
two smallest discs3re permanently s tt.s ohed to "the push rod.
The area of the smai Lest Cli::>;; A ｾｳ 0 }::Y£12; and t' are a of
the. , annU.81 ,_'1.,.-, ehou Ldu .... ,...ｶｾer- .r.,p ....'t-.c; \ . , 1 , , 'nOn',;:?'ｾＮｲＮＬ t) ..•.",', ',"'!''-,>.,,7. ""'0Ｇｾｬｬｾ d.re,.' ' ' ' , ' ' ' 0 ｬＬｾｾｦＧ ../J'1, ".ne.q' 'j
B + '""aKe ...1, t·· "','lJL:..,C,,,J...1. ...r,t'h .:>Y',J..Sｾ '\ＮＢＮＢＬｾＧＧＭ［ r, em,:? ,"! '. ," " - " , , . 0> r1. Ｂｾ '" -, <:), d I' ( D
if .. ｾ｜ＺＧ｜ｾＬＮＮ ［ｊｺＧＢＧＡｵＨＮｾＬｲＬ .." f -•., ｾ｜ ..ＧＺＧｾＧＮｾｾ ". ':J:.t ) \j",
i S not ｓＱＮ｜ＮＮ［ｾＧｖｄＬ 1 '"ｾＭＺ I ｾ..LrJ., ｊＺＧｬＮｌｴＮｬＧｾr:u ｾ ,FL ')ｾＬ ·h. ..-, , ｾＢＮ ｯＮｾ 0' , , ' " ＢＬｾＮ , ,,' ｾ 'I.'" ')
A- ...G,J6 ｾＮＢＢｊＮｾ｣｣ＧＮｊ v1. ｾ ....I arL1 --l.o._J ＡＺＺｲｲＡＮｾ ｬｾｾ［ｳＺｰ･｣Ｍ
t1 veLy , Dl sc C (or d1 sc D) '1',:\ hel d in pla,ce on the end of
the push rod by the frict.'Lon of' E rJ:bber washe r which 1,3
cemented to the disc. rthenevor a disc is fitted to, or ｲ･ｾ
moved from the push rod, t.hf:' d::J" i:l1)()'c:ld be rotated to avoid
pu'l Li ng the r-ubb er- ws she r off' the b ack of the disc,
45i: Th e cppo s ; te end of the pusn rod c ar-r r es a scale
gradua ted from C to 10 _ Since 10 on the scale corresponds
to a push rod force of 1,000 grams, one d:"vision of the sca.le
corresponds to 100 grwJs, Thus, when the largest disc D is
1...8«1 cL.9 push rod scale indicates the pressure directly in
r':, IJ':Jr cm2 0 'Therefore this disc has engraved on i t "X 1"
to show that the snow hardness number i,s the scale reading
times one. Similarly, disc C is marked
"x.
1011 to indicatethat the hardness number is the scale reading times 10. The
factors by which the scale reading must be multiplied in
order to obtain the hardness number) are given in the
follow-ing table0 TABLE II
-_
.•.._•..｟ＭＭｾＮ｟ＧＭＢＭＭＧＢＢＢＮＭｾ ..• Disc Area (cm2 )--_._---_._
.._
-fA Bc
D .1LO
10 100 1,000 100 10 1460 A few snow kits have been provided with an
addi-tional snow hardness gauge ｾｾｬ｣ｨ has a ten times stiffer
springo Since the sca.le of these gauges is graduated from 0
to 100 (instead of
a
to 10 as in the case of the standardgauge) the factors given in the table above apply to both the
13.
Graduated Cup " ｍ｟ｾｮｩｦＱＺＱｮｧ G1:.13-8｟ｾ｡ＮＮｮｾ
...
Spa tula470 These are shown in Fig, 5 and are used for observing
the size and shape of the individual grains or crystals,
48. The cup B has a flat bottom on which are engraved a
number of concentric circles whose radii vary by one millimeter
stepso These circles form a scale for measuring grain sizeo
49. The spatula C is used to place and arrange the ｧｲ｡ｾｵｬ･ｳ
in the cup. Aggregates may be broken up with the flat end of
the spa,tula0
ｔｨ･ｲｭｯｭ･ｴ･ｲｾ I
50. The thermometers are ｳｾｰｰｬｩ･､ in two ranges: (A) +110
to -lOoe. and (B) +50 to =lOOoC. Thermometer A is used to
meaSure water temperatures in free water content testso It may
also be used for air and snow temperatures that come within its
range. Thermometer B is intended for measuring temperatures of
air and snow onlyo
Penetrometer (Not supplied with- , ｳｮｯｷｾ｣ｯｶ･ｲ survey kits)
510 The penetrometer is shown in ｆｩｧＮｾＮ It consists of
three parts: A which has a disc area of 5 in and ｷ･ｩｧｨｾ 5 ｉ｢ｯｾ
and weights B and C which weigh 5 and 10 Ibo respectively. Thus
the various combinations give disc pressures of ｬｾ 2;1 3 and
4 Ib/in2, but in practice only the combinations that give 1, 2,
and 4 lb/in2 need be used. The handle of part A and the weights
B and C are graduated in inches penetrationo
Clinometer (Not supplied with ｳｮｯｷｾ｣ｯｶ･ｲ survey kits)
520 The clinometer shown in Fig. 7 is a pendulum type
graduated in percent grade. Each graduation represents 2 percent
grade but the scale can be easily read to the nearest one percent.
GRAIN-FORM DENSITY SCALE
530 It has been pointed out in paragraph 11 that the
structural properties and cohesion of a snow layer depend to a
large extent on the shape of its individual grains. Grain shape
also has a marked effect on both the static and sliding resis-tances of skis, and on the internal friction of snow (snow
sliding on snow). These-effects are so pronounced that skiers
540 ｶｾｬｬＱｩｊＮＮ･ there 13 an in.fln1.te variety of snow grain
shapeSF the important fea tur-e is the degree' of' solidi ty (or
a.Lt e rria tiveLy the dagr:ee of 1 aoi ne38) of the snow gr-at.ns or
cr-yst e Ls ｾｬＧＧＧＧ t.hi s ＢＧｔＭＢＢＩｾＢＢＫ the +-eYC1'l ｉｦｧＭｉＧＧＧＧＧｬＧＧｮｾＧｦｯｲｾＧＧＧ ｾＧｬＢＢｲＢＧｬﾷｴＢＧｔＢ is
v , l l . , ;. .Ｇ［ＬＴ［ＬＭｊｾＮＡＮＮＮＮｬｩ c ".J,A ,.';.I!...,;,..;- ,-1· l:fJ ....ｑｾ｟ .., '.... ,,,.... "" 'i." ｾ..ｾ t. . i;r"""-L - ｊ｟ｾＴＮ ｵＧｾＧｾＬＮＬＺＮＮｊ ..,.·, J
u sed tc ､Ｎ･ｾＺＺｇｲＺﾷｩﾷｴＩｦＳ 2· ｊﾷ･ｬＹＮﾷｇ｜ＢＱＺｲｾｾﾷ tria snow ｧｲＧ｡Ｎｩｲｬ｛ｾＬＨ［ ffhe ｃｬｾｩｴｾ
rion of ｧｲＸＮｩｲＺＧｾＬｦ｣ＡＮＢｭ deri 0l.ty t"illY be taken as th,:;l roe} a t1v e
structuraIstr'flhgth cf' trre gr8J,ns O!' crystals due to shape
alone"
550 A cbart of' grainuFOi'm den s:i.ty i.5 giv en in Figs 8
and 9" The c.Las sirtcat.Lon i 9 mor-e or 10138 arbitrary but th.ia
does not in a.ny way detr-act rr-om 1ts usefulness, ｾｊ［ＬＧＱＮｩｬ･ the
c La sse a may net be uniformly spa.ced 8J1. s.tt empt has been made
to arrange them in proper ordeX'c E&ch cla.ss refers to many
different shapes of snow grains but all ahap e s in snv one
class have appr-oxLmately the same .influence on the properties
of a. mass of snowo The classes in the ｧｲ｡ｬｮｾｦｯＡＢｭ denslty
scale are described in Table IlIa
TEST PROCEDURE
560 A1"!3hongh the t eet s shou'ld be made Wlthreasonable
care, a ｨｩｾｬ degree of accuracy 1s not requiredo Even in flat
open c ountr-y the snow-cover- Ls not enti.r-eLy 'U,n:H'orm and tests
made in one place will not yield exactly the same results as
similEH' tests made. r.n another onLy ten or twt:3nty ｹ｡ｲ､ｾ awayo '
In general the observer ｾｨｯｵｬ､ try to obtain as Dear average
r-e au l t s I:L'? possible and this applies not only to the differ'ent
tests but also to the location of the test trench which should
be chosen w5.th a view to ob ta:in1.ng 8 '::f'sasonably typical sec tion
through the snow- cover0
Air Ｇ｝Ｑ･ｭｮ･ｸＢｾＧｬＮ
__,..ｾ.._ '_ ...ＺＺｊｩＺＮＮＮＮＮＺＮＭＮＬＮＢＮ｟Ｂ｟ｾ ..ｾＮｾ ... _ _
A:'Lr t ampere. t.ur-e should be taken at about OTit3 foot
a oov, ［ＢｌｾＮ［ snow surfaceｾ The thermometer mu:s'tbe she Ltared
from the sun's rays and should be given sufficient time to
reaoh a constant readingu When snow tests are made within a
short distance of a weather station (as in the snow-cover survey) air temperatures as measured at the weather station may be used
even though ｴｨ･ｾｬ sr-e usually taken at a somewhat greater height
TAi3LE III Class DescrlEtion CA) fB) (C)
CD)
(E) (F) (G)*eH)
(J)ａｬｾ crystals of very slender proportions and open
pattern" These include very slender needles and plane
crystals with six very slender rays without ｢ｲ｡ｾ｣ｨ･ｳ
or with not more than three pairs of delicate branches
per ray , This class .'ls limi.ted to crystals of new snow,
Plane crys tals wi th In8..J.'1Y de Lt.ca t e brancheSo Al.l very
feathery new snow crystals and similar hoar crystals belong to this class"
Plane crystals and needles of somewhat more substantial form than those above, and settling snow grains of
similar proportionso
Plane crystals and needles of still more substantial
form than class ｃｾ and settling snow grains of similar
p r-op ortLoris ,
Hexagonal plate crystals of snow or hoar and hexagonal
cup crystals of hoaro The plate crystals may have
either straight sides9 various forms of small notches
in their sides or small extensions at their corners, Columns having a length to diameter ratio of four or less and settling snow grains of similar proportions. Graupeln (see Nomenclature) of approximately spherical,
conicalJ or thick irregular forme
Old snow and settled snow grains With crystal facets and roughly spherical proportions
Old snow and settled snow grains with no crystal ｦ｡｣･ｴｳｾ
and h a lL,
*
The thickness of rime coatings on snowflakes varyconsiderablyo Only thickly coated flakes belong to
class Go Light to medium coated flakes should be
classified as the class preceding G in which its shape including the rime deposit would place ito
58" "- For t.hf.s and rncst of tbef'ollowing tests a trench
must be d1-\g in trw snow-cover- to dLs c Lo ae the different snow
layers. The temperature measurements should be carried out
immediately after the trench has been dug in order to minimize
errors due to exposurra to air and suno They need not be taken
in eV6r:y Laye.r = uS1.,j,:llly there are no di.scontlnuities in the
snow t.emperat.ur-e gJ'adi ent and o:t.ly suf'fioi ent readings to
reasonably establish the ｧｾ｡､Ｑ･ｮｴ are necessary. At least 3
or 4 r-e ad'i.ng a wi 11 be r-e qutr-ed arid each shou.Ld ref0r' to a
par-ticular layer in order to facilitate recording the results.
The thermometer bulb should 'be inserted about 4H into the snow
near the m.ld-sdepth of the ],ayer'" Ihesofter snow layers should
be chosen S1X;;C8 the thermometer can be more readily inserted
into them, Tb.B thermometer should be gi.v en auf'f'LcLent time to
',reach a constant r-aad i.ng , Snow Hardness Tests
59, Snow hardness readings 'should be taken in each
dis-ti:nct 2now layer. 'Ine hardness gauge is used horizontally
L , t the wall of the t r-encn , Car-e should be taken in
cut-tlng the f'a c e on which the test is to be made so as to obtain
as flat a surface as possible and to avoid disturbing the ,snow.
The reading is obtained by slowly pressing the gauge squarely
against the snow and noting the ｟ｶ｡ｾｵ･ on the Beale at which
the disc begins to enter the snow, Wl-J.ile the manner in which
the disc enters the snow varies somewhat with snow conditions,
it is usually fairly suddeno Very slight compression of the
anew should be disregardedo Readings to the nearest half
division are SUfficiently accurate.
60a 'Ina gauge may be used verti cally to measure the
hardness of a thin layer at the surface, When used vertically,
a cor-r e ctd.ori for the weight of the moving parts musf be added
to the observed reading. The correction can be found by reading
the scale when the gauge is held vertically with tqe disc end
uppermost0 It is approximately 1/3 of a division for disc C
and 3/ 4 ｩＧｯｾ［ d1 so Do
If the snow kit contains both the st.and ar-d ",,(JC
B):>:(:1a1 snow har-dneas gauges it is suggested that rae.dings be
obtained with both for comparison purposes, particularly when
nrre snow har-dne s s is above 1,90000 The r-e ad Lnge may be
diffe-rentiated by underlining all those obtained with the special
gauge0 Nor-maLly , the standard gauge cover-s t.he snow hardness
Average Grain Si ze ｡ｮＨｾｌＮﾧｾｾｅ･
620 Observations of grain size and shape should be made
in each distinct snow layero Grain size is taken as the ｭ｡ｸｩｾ
mum dimension
ex
Figo 8) of a single grain or crystalo In alayer formed by a fall of compound snowflakes, only the sepa= rate crystals which make up the flakes should be consideredo A fair number of grains should be examined and an estimate made
of their average size. Estimates to the nearest 00 2 romo are
sufficiently accurate. When the grains are bonded together the
aggregate must be broken up before the measurement can be made,
but grain fragments must not be mistaken for ind1.vidual grains.
In cases
or
extreme bonding or in very wet snow, grain size andshape begin to lose their meaning and need not be r-ecorded ,
63. ｾｲ｡ｩｮ shape should be recorded as the class in the
grain-form density scale which most nearly corresponds to the
average shape of the individual grains0 In new snow and in
par-tially settled snow several distinct types are often present, and in this case an estimate of the average shape should be used. When this is difficult the two or three predominant shapes may be recorded.
Frea Water Content (F.W.C.)
640 Free water content need only be measured When the snow
is wet, ioeo, when the snow temperature is 000 .
650 FoW.O. measurements are most important in the softer
layers near the surface, and since these measurements are ｳｯｭ･ｾ
what inconvenient - mainly because they require a supply of hot water - measurements in at least 2 or 3 of the softer layers
near the surface will suffice. The observer will have to use
his own judgment in deciding the number of measurements and the layers in which they should be takenc Snow layers that are ob-viously wet, but for which no F.W.C, measurements are made,should
be described in the test records as IIsligh tly wet "; "moderately
wet", e t c , and the apparently dry layers as "dry".
66. To determine F.W,C. put about 300 grams of hot water
(between 500 and 800C) in the balance bucket and measure its
weight. Measure the water temperature using the +110 to ｾＧＱＰＰｃ｣
range thermometer. Do not use the low range thermometer since
it will break if heated above its upper limit. Add sufficient
snow to bring the temperature to between 5 and 15 0C0 Stir the
mixture to make sure that the snow is ｾｯｭｐｬｾｴ･ｬｾ meltedo Measure
the final temperature, and determine the weight of snow that has
been added. The tare slider (0,' Fig. 1) has been provided to
s1.mp.li.fy this operation. The method of calculating the free
18
For example assume
the values below
Let W -S -Tl -T2
=
wei gnt of water wei.ght of snow:Lnitial temperature of' water
final temperature of mixture
350 grams
175 grams
50°C,
120C 0
'r1'len, since i t takes 30 c,11or183 to trBX1.8f'oI1YI 1 gram of ice at
ooe.
into water at OOG) EL"I'ld since the temo er-abur e of wet snow 1sa'l.way s ooe., the wei.ght of i cs!ｾ I j In ｴＬｨ･ｾ wet snow .i8 ｾ
I
(3 )
130
and the weLght of fr'ee water is:
s.. r.
Since F.W.Co is expressedas a percentage by
weight,-Ci T
P.W.C, - 100 ( ws.L·)
%
(8)Combining e qua t.Lon s (1) and (2) we obtain:
F W C Ira 5(T m ,W _5T2 • ' " • 0
=
U = 4" 1=.L2 'S 4 ｾ Let X = 100 = -, .Ll5(m ｾ T) W2 - + K and 4 'S Y .- -T2=·K54 . then F.W.Co=
X + Y.
670 Nomograms A and B, Fig. 10 may be used to evaluate
equation (4), The value of X for a particular set of values of
Sj VI/:, and Ｈｔｬ｣ｾｔＲＩ can be found by means of Nomogram A and the
a.dd.l tion of' Z and Y can be performed by using Nomogram Bo 'The
constant K, whLch has been Lntr-oduc e d to eliminate negative values
of' X, has been given a value of 25 in the construction of Fig. 10.
68. Assuming the measured values of S, W, T1 and T2 to be
s) w, tl and t2, proceed as follows. First calculate (tl=t2).
U::dne; Nomogr-am A; drs. was traight line through sand wand find
its lnt61'Section r with the r-ef e r-enc e line; draw a straight line
'through ＨｴｬＧｾ t2) and r arid find x , Transfer the value cf x t.o th e
X ucaJe of Nomogram B and draw a straight line through x and t2
and ftrld Co The free wat.e'r content is then e%_
690 ThB dotted L i . n es :i.n . F ' L g, 10 ill:ustrate t he method as
applIed to the numerical example ｧｩｖＧｾｮ in paragraph 66.
'/0" The nomograms can only b e used ror temperatures me
a-sured in oC. A reasonably true straight edge should be used to
Specifl,c Gravity:.Me aSl.l:P ements
710 Measurements of sp e c i.ri.c gr-ava ty should be made in
each distlnct snow layer for the first 12 or 18" below the
8ur-f'ace,» Below this depth the measur-emerrts need not be quite as
complete, thin hard crusts may be omitted provided their
hard-ness has been measuredj but measurements should be made in the
Layer-s directly above and below each hard ｾｬＧｕＮｂｴＬＩ
720 'Irie measurements are made by weighing a known volume
of snow. '!Jhen the snow is mode r-at.eLy dense, two Damples cut
with one of the snow ｳ｡ｮｾｬ･ cutters should be weighed at a time,
when it is light, four samples may be weighed0 The specific
gravity is then simply the weLght divided by the volume, Volumes
of 500, lOOOj or 2000 CCo ar-e the most c onverri.ent since the
cal.cul.a tiona teen become extremely ｳｾＮｭｰｬ･ 0
730 Soft snow samples should be very carefully cut to
avoid compressing the snow and any snow samples which do'not
completely fill the internal ｶｯｬｾｭ･ of the cutter should be dls=
cardedo
74. In cutting samples, it 1s advisable to first cut, them
from the top layer and then. the next layer down arid so·onuntil the lowest layer from which samples are to be cut is reachedo Frequently a considerable amount of digging is !"equiredto ob-tain the snow samples and the procedure suggested above tends
to minimize the amount of ､ｩｾｧｩｮｧｯ
Form of Snow Surface,
750 When the shape of the snow surface is of secondary
importance in winter transportation problems, it ma'y be de acr-ibed
by ｷｯｾｱｓ ｯｮｬｹｾ such as: fairly flat with wind ripples, moderately
wind eroded; rolling with patches of wind alab, etc" If it is
felt that more· detail is requ:lred, a few simple measurements =
such as average heigh t and spacing at' drifts Ｎｾ may be add ed ,
76. Whenever the shape of the surface is such that it will
affect the performance of o ve re sncw vehicles, skis, etc , J it
should be described by means of a photograph which should include
some object of known sizeo It is often desirable to add a few
simple dimensions to the photographic de s cr-t p t.Lon , 'wVi1.sn phot
c-graphy is used, consideration should be given to the effects of shadows as these can often be employed to good advantage since they emphasize the irregularities of the snow 8urtacec
Penetrometer Tests
770 Penetrometer tests are only required in connection
with tests of ｶ･ｨｩ｣ｬ･ｳｾ skis or similar testso
780 The method of using the penetrometer is that adopted
by the U.S. Army Air Force A.D.T. Branch who developed the
instrument0 Two type s of tests are made: (a) "Static Tests" in
which the penetrometer is held just in contact with the snow surface and then suddenly released, and (b) "Dynamic Tests" in which the penetrometer is released from a height of 12 inches
above the surface (see Fig. 6). Upon release, the observer
guides the penetrometer by letting the handle slide freely
through his hand. Both static and dynamic tests should be made
for ｬｾ 2, and 4 Ib/in20 Penetration should be measured to the
nearest 1/8 inches. The penetration of the flat surface of the
vehicle track (disregard grousers) should also be recorded.
79. An approximate value of the maximum pressure exerted
by the vehicle track may be obtained by carrying out static
tests on1.z in the rut formed in the snowc This test should be
made soon after the rut has been formed since snow continues to
har-den rapidly after it has been compacted ,
Clinometer Tests
i
800 In performance trials of vehicles, skis, etco j the
slope of the snow surface should be measured even when it appears to be reasonably horizontal since slopes are often deceptlveo The readings should be marked "plus" to indicate an up-grade and
"minus" to indicate a down-grade. vrhen the ｷ･ｩｾｨｴ of the vehicle
and the slope of the snow surface are known, the performance of the vehicle can be reduced to its performance on the levelo This is desirable for comparing different trials of the same vehicle or for comparing trials of different vehicleso
GENERAL INSTRUCTIONS
81. These tests will be conducted at a number of observation
stations and may extend over a period of several winters.
Tnere-fore it is highly desirable that a uniform procedure be adopted. Uniformity of methods will also simplify the analysis of the many
observations0
ＸＲｾ The amount of shelter from wind and sun has an important
bearing on the form of the ｳｮｯｷｾ｣ｯｶ･ｲｯ It is rather difficult to
210 areas that provide either one of the two extremes of shelter.
ioeo flat unprotected areas or hea ly wooded areas, are the
most suitable from the point of view of Ｇｊｮｬｦｯｲｭｾｬｴｹｯ However.
most of the observation stations will be located at airports
where the. surrounding country 1.9 t'Lat and open, therefore flat
unprotec ted areas free from d1. s trur-nsn ce s due to tree s bUl1dlngs-.
roads, etc , have bean chosen as t?e principal test ar-ea s ,
830 An area of at least 500 sqeftcis r-equa r-ed for the test
trenches. If possible it should be located on a slight rise 'to
avoid flooding with water in spring A snow-cover depth gauge
in the form of a post marked off in inches should be set up at one edge of the test area in a convenient location,
840 The trenches should be no larger than necessary for
carrying out the measurements, 2!6" x 316 11 should be ampleo One
trench is needed for each set of measurementsQ The first trench
should be dug a t the down-wane edge of the test ＸＮｾ･｡ and as new
trenches are required, they snould be located so.as not to dis-turb the normal drifting of snow on the unused part of the area ,
On completion of a set of measurements. the trench should be
filled and the snow surface roughly leveled 01'1'0 :
850 A complete set of measurements as de scz-Lb ed in p
ar-a-graphs 57 to 76 and including the total depth of the- snow=cover
should be made re:gularly once a waeko In most aaae s7 the
measu-rements need on;1.1'-1;>e made in the various layers q:own to a depth
of two r eet , but: at least once each month the measurements
should be extended right down to the groundo '
860 The test trench should have one wall that 1s fairly
vertical and clean cut. After careful examination of the aec t.Lon ,
the boundaries of the more pronounced snow layers should be·,
marked by any convenient methode Often there are many sublayers
within a snow layer but these need not be consideredo Air spaces,
however. should not be disregardedo .
87 0 Snow temperature tests should be made f'Lr.st , and while·
the thermometer is reaching a constant. reading, the depths to
the boundaries, of the various layers may be recordedo It is
ad-visable to leave the cutting of samples for specific gravity measurements to the last as this saves a considerable amount of diggingo
880 In ｡ｾ､ｩｴｩｯｮ td the weekly testsj daily observations
should be made
of
the total depth of the snow-cover and of thesize and shape ｾｦ the snow grains in the surfaee layer onlyo
890 \'1eekl:r and da.ily tests should be carried out either in
the middle of the morning or in the middle of the afternoon when
900 A few additional tests may be made in heavily wooded
areas if such conditions occur within easy reacho Howeveri
these should not interfere with the tests made on unprotected areaso
910 Suggested forms for recording the results of weekly
and daily snow=cover survey tests are given in Appendix 10
Snow Measurements Connected wi th Vehicle ('
2E_.9!il.2
ｔｲｪＮ｟ｾｬｳ92. In fairly deep snow9 a vehlcle does not usually
com-press the snow in the layers near the ground0 The lower layersj
then, have no bearing on performance and therefore there 1s no need to take measurements in the snow layers that are
undisturb-ed by the vehicleo
930 The test trench should be dug across a rut formed by
the vehicle tracko The layers disturbed by the vehicle can
easily be determined by observing the discontinuities in the
various snow stratao The layer directly below the lowest broken
stratum should be considered a disturbed layerc 940
should depth
total
The measurements described in paragraphs 57 to 76 be made in an undisturbed part of each layer down to the
affected by the vehlcleo Penetrometeri clinometer and
depth of ｳｮｯｷｾ｣ｯｶ･ｲ measurements should' also be carried out.
950 Whenever any part of ｴｨ･ｾ｡｣ｫ mechanism tends to ice
up, the free water contentj specific gravity and grain size and
shape for the surface layer should be determined even though no other measurements are made due to the lack of sufficient time,
960 A suggested form for recording the measurements is
given in Appendix 10
GENERAL. NOTES ON SNOW
970 The fact that water becomes solid either by freezing
or by sublimation is of fundamental significance in all snow
studies. Sublimation is the transi tion from vapour to solid (and
from solid to vapour) without passing through the liquid state.
980 ｾｭ･ｮ water freezes the shape of the resulting ice is
approximately, that of the water prior to freezing, but when the solid is formed by sublimation its shape is that of a crystalo Fr-e ez Lng produces ice and rime while sublimation produces the
beautiful crystals of snow and hoaro Actually all solid forms of
water have a crystalltne structure but only those formed by sub-limation show evidence of this in their external shape"
23. Formation of Snowf'la,kes
990 Wnen air is coo Le d its r-e Lat i.ve humid:L'ty rLae s ,
The most Gammon way in whi ch air r s cooled t s by expansion
due to the air mass rising to 8 hei.ght where the atmospheric
pressure is reduced, but cooling may take place in several
other wayso If the cooling continues beyond the saturation
points the excess moisture will be deposited on any solid ob-ject such as dust po.rtlcles which are generally present in
the atmospherec At temreratures above freezing the moisture
will condense and form f'og , V\hlle at temperatures below freez=
ing the saturation point with respect to ice will occur before
the saturation point with respect to water and ｣ｯｮｳ･ｱｾ･ｮｴｬｹ
the moisture will sublime and form snowo
1000 'Ihe pattern of a snowfLa ke depends mainly on the
rate of growth of the flakeD Slow growth produces the more
solid forms such as hexagonal columns and thin hexagonal plates. while very rapid growth results in plane crystals of
very slender proportions and open pattern Generally the rate
of growth is not constant and as a ｲ･ｳｵｬｴｾ flakes of complex
patterns are produced, Sometimes the flakes have astonishing
forms; a syrmnetrical combinat.ion of a short column and two or
more plane crystals is not uncommono A twelve ray flake is
simply a very short column with six rays attached to each end and even some six ray flakes have their rays in two planes
separ-ated by a very short column0 Not all snowflakes are
ｰ･ｲｦ･｣ｴＬｾ various circumstances produce some surprising formso
101. All flakes which consist of a single crystal are
called "simple snowflakes" to distinguiSh them from "compound
snowflakesII which are compo sed of several partially mel ted
flakes that have become stuck together during their fall to
earthc
102. The rate of growth of a flake depends largely on
the temperatur-e at the a I ti tude at whi ch the flake is
formed,-the lower formed,-the temperature formed,-the less formed,-the amount of moisture
available for snowflake growth" At very low temperatures the
flakes are therefore small and of hexagonal plate or column
fOrnlj at moderate temperatures the flakes are larger and
mostly of complex patterns, while at temperatures above ｦｲ･･ｺｾ
ing they are large compound snowflakes or "sleet"o
1030 Frequently. snowflakes fall through a cloud layer of
water droplets and become coated with rt.me, The thickness of
the coating varies considerablyv ifuen the coating is fairly
thick the flakes become roughly spherical and are called
"graupelnrr or "soft hail", A layer of graupeln has a dull
chalky white ｡ｰｰ･｡ｲ｡ｮ｣･ｾ and is the only form of freshly fallen
24.
104. Snowflakes suspended in the a tmo sphere are often
carried to higher altitudes by rising air currents" Sometimes
a flake falls to a warm level where it is partially melted and is then carried upwards to a cold level where it freezes agaf.n ,
ｾｦｵ･ｮ flakes pass through a series of melting and ｦｲ･･ｺｩｮｾ stages
wi th the addi tion of mo i.s ture be tween the stage s , they become coated with successive layers of ice and are called "hail". Some
authori ties on snow 1-;38 a separate term "Lce pellets" to
des-cribe frozen rain drop sｾ but in snow- cover mea sur-emen ts ther;e is
no reason for making this distinction and both forms should be described as "hail".
Changes tha t Take Place in Fallen Snow
105. Immediately after the snow has fallen, changes begin
to take place in its structure. Tne points and slender branches
begin t6 evaporate an.d sublime again on the more solid parts of the crystals which gradually become small irregularly shaped
grains. As the crystals change shape they become more compact
and the snow settleso
106. The settling of snow usuallW takes place in two
f'eJ.rly ､ｾＳｴ［ｩｮ｣ｴ stages. 'Ihe principal difference between the
two is that the rate of settling is much greater in the first
than in the second stage, In the first stage the crystals lose
all their feathery structure and become small and granular, Snow is said to be settled when the end of' the first stage has
been reached, In the second stage the grains grow in size and
become bonded together. Sunj rain and thawing conditions
gene-rally playa part in the second stageo
Change3 due to lUnd Action
1070 Most of the bulk of fallen snow is ｡ｩｲＬｾ dry new
snow contains about 90%' air by vo l.ume , and hard crust about ＵＰｾｌ
A snow layer is therefore very porous and contains a great many
int0rconnected passages through which air may flow o A wind
blowing over the snow surface induces air flow within the snow
layer and in this way greatly accelerates the settling of snowo
108. 'rhe changes Ｑｾ snow structure due to wind action
depend upon the temperature j relative humidity, strength and
duration of the wind. Whenever the air temperature is below
freeZing, wind accelerates both evaporation and sublimation
with-out causing melting. When the wind is drY1 evaporation
predomi-nates and the snow settles with very little tendency for the
grains to become bonded together ｾ in fact, a very dry wind tends
to loosen grains that have previously been bonded. But when the
humidity of the wind ls high, sublimation predominates and Causes bonding of the snow grains, and thus toughens or hardens the snow.
The har-den i.rig of snow by wind action i:3 called "wi.nd packing"0
Pr-o Loriged wind packd.ng forms very haz-d "wind crust" in which
skis and even hob nailed boots make little or no impression
1090 When new snow Ls compressed its cr-yst.a Ls b ecome
broken and the fragment.s lie more cLos eLy together than
origi·-nally. Very little wind action is then required to bond the
grains and considerably harden the snow. Trlis pr-op e r t.y of snow
has been utilized at many e.Lr-port s when freshiy fa:1..10:: snow is
compac ted by rollingc Irrrrnedi ately a r t e r r-o11.J.ng, the snow is
still fairly soft but :it hardens SUfficiently in only a few
hours to permi t landings with wheel land.ing gear even when
there is only a s 1igll t w:'ind0
1100 V'/henever the wind veloci ty is great enough to cause
drifting of old snow, the gra.ins become rounded by i'rj,'::.tion
with the snow surface and thus lose all traces of their ｣ｲｹｾ
stalline facets, If the rounded grains of old snow are deposited
in an area of complete calm, a drift of loosely lying snow grains
will forme However, if they are deposited in an area of low
winq velocity and the humidity is high enough to cause wind
paCk-ing, Wfwind . lab" will forme The feature which distinguishes
wind slab from wind crust is that the anchorage of wipd slab to
the snow beneath is very weak, Settling of the underlying snow
often forms an air space below wind 'slab. Generally wind crust
forms on areas exposed to the wind and its 8urface,.is often
considerably eroded9 whereas wind slab forms in sheltered areas
and for this reason its surface is only slightly wind rippled, Wind slab is usually found in patches and can be easily recog=
nized by its dull whi.te appearance 0 1fll:1en wind slab is broken
by a vertical load, it breaks suddenly with long cracks
radiat-ing outward in all directionso This peCUliarity is due to the
bri ttleness of the slab and the presence of a sp ac e directly
below the slabo If part of a large area of wind slab on a steep
slope is broken by the weight of a skier or vehicle, the slab will in most cases break up into blocks and initiate a
dange-rous avalanche0
1110 Wind also cha.nges the shape of the snow surface by
forming erosion ridges, drifts and snow cornicese
Changes due to Melting ｡ｮ､Ｂｆｲｾ･Ｌｾｩｅﾣ
112. The changes in snow structure caused by melting and
subsequent freE: zing differ from those due to evaporation and
sublimation. Sublimation always produces forms that have ｣ｲｹｾ
stalline facets, while melting followed by freezing produces
forms devoid of facetsn The amount of melting determines the
extent of the change - if a small amount of melting occurs,only the sharp ends of the crystal branches will melt and become
rounded; if a greater amount of melting takes place j the
26.
network of miniature ice bridges; but if considerable melting
occurs, an icy cru st or ice ;Nil1 be [nrmad.
113. The r-epe ated thawing and r-e.rr-ee z tng which usually
occurs in late spr Lng , wnen the t.emp er-a tur-e is above freezing
during the d ay and b eLcw freez1.. ng at nigh t., produces a very
coarse type of snow. During the thawing period, the smaller
grains malt more readJly than the larger grains. If a
mede-r a te amount of IDtJ.l:Glrlg occure, cher-e will be no appreciable
run-o.ff an d ne arLy all the thew water' will be d.1.str1buted in
a fairly even coating on tee remaiL:I.:ngg.cains.so "Ghat> when
freezing takes place, the size of the larger grains will be
greater' than at the ｢･ｧｩＮｮｮＺＱｲｾｧ of' the cycle. Thus the larger
grains grow at the exp en se of the smaller grains. In its
thawed condition this coarse type of snow Ｑｾ ca11.ed "spring
snow"; when it is fl'ozen it l!3 called Iispring crustH 0
Wet Snow
1140 Th.6 molecular at.t.r-act.Lcne whioh occur at the sur-»
face of contaot between two liquids, or between a liquid and a. e/"s.pT.Jduce effects which appear to be due to a tension in
ｴＮＬｨＨ［Ｌ［ＺＯＨＧｌｾ｣･ of cont.a ct , This phenomenon is known as surface
tension. It acts at the alr-to-water surfaces of' water drops
and ｣｡ｵｳｾｳ them to cling to solid objects wetted by the drops.
When a water drop wets sever-aj small grains, such as gr-af.ns
of 'sand, the forces due to surface tension draw the grains
to-gether. In wet snow, there are a large number of alr-to-water
surfaces, with the result tllat surface tension has considerable effect on the proper-ties of the snow and is largely responsible for the cohesion of wet snow.
115, The ｭｯｵｬ､ｩｮｾ properties of ｳｦｵｾ､ afford a good ･ｸｾ
ample of sUr'fac9 tenslon effectso Dry, very fine sand cannot
be moulded, but when it is dampened and pressed ｴｯｧ･ｴｨ･ｲｾ it
has considerable cohesion whteh gives it exceLLen t moulding
properties. However, Lf all the air r.n the sand is expelled
by flooding with water, there will be no ｡ｩｲＭｴｯｾｷ｡ｴ･ｲ surfaces
at which surface tension can act, Sand flooded with water has
no cohe e i.on but tends to flow like a l:tquid. 'lnus there is a
certain proportion of sand and water which gives maximum
cohe-i.on and optimum mouLdLrrg properties.
116. Grain size also affects the cohesion. If we
con-sider two equal volumes of sand, each wetted to give maximum
cohesion but each of different grain sLze , the s arid of finer
grains will eontain a greater number of surface "tension bonds
and will therefore have greater cohesion. For instance, there
is practically no cohesion in a pile of small pebbles. no matter what amount of water is used to wet the pebbles.
117. 'TIle sur-f'ac e ter.sior; c,f'fi-' r;;;; in Sj:.lOW are VtJry sImi
Laz-to those in sand. t.cose dJ. ｾＺｲＮｖｊｩｉＧ［ J:1& very 11t t Le cohesionｾ =
in other words it is ､ｩｦｦＱｾｵｬｴ ｾｯ make a snow ball of dry snow.
On the other hand, slightly ｷｾｴ Zi.8W snow compo sed of ｣ｲｹｾｴ｡ｬｳＬ
Which have a large number of slender br anche a , has high cohesion,
but if its feathery s t.ruc tur-e ＺＱｾｾ de s t.r-oyed by further melting
it will have ccnsfd6rably Le53 '':OhEL:d..on. Coarse spring snow
has rela. tively low cohe si on , an d slush has ｰｲｾｷｴｩＡＢＬ｡ｬｬｹ none.
1180 When s l : 1 y wet to moder at e'i y wet new snow 1s
pressed togethery the ｦｾ rys'tals become broken and 1t e much closer
together. This greatly ｴｮＨｾＺｲﾷ･ｄＮｂ･ s the number- of'9u!face tellai on
bonds wit;;h the r-esu'l.t that t.he cohe ston becomes \i'ery much. greater.
1190 When moderate1y wet snow 1.8. subjected to high ｰｲＨｾｳＭ
sure, mo st of the air arid S011H'" of' the wate!' is forred outｾ and
the remaining water become s a thin fjlIn between the snow gr-at.n e ,
On release of the pressure, the fllm of water f'r-eezee and the
snow becomes very hard and icy. Lce may form in th.:i.s way
be-tween the wheels and tracks of track type vehicles and 6f'ten
precludes ｴｨ･ｨｾ use on fairly wet snow" In some cases, nhe ice
cauaesexcesslve stretching or breakage of the trackso This
diffi.culty may be ellm111ated by the use of a flexible tr-ack =
such ;:....J it rubber track = which frees 1t self of the ice by
flexlngo
120. It should be noted that the rate of seepage of thaw
water through snow 1s very lowo In most cases the rate 1s be=
tween 0.3 and 2 inches per m1nuteo Hoar
121. Hoar crystals are similar to snow ｣ｲｹｳｴｾＱｾＵ the
main difference b ei.ng that hoar- grows on fixed objects while
snow grows on dUBt p ar-t I c Les floating in the atmosphere0 S1.nce
hoar grows on objects that are usually fairly 1a1'g89 the ｰｲ･ｾ
sence of the object and of adjacent hoar crystals has an ｩｮｦｬｵｾ
enc e on the manner in which tile ci-yst a'l a grow and gives them
their char-acter-i at.Lc r crm ,
1220 Hoar which. grows on obje ct s above the snow surface,
such as trees, is called Hair hoar!!Q Its ｣ｲｹｾｴ｡ＱＸ are usually
no llow hexagonal pri sms when i t grows slowly9 and 8]
needles arranged in a haphazard manner when it grr)1NS x'apicrty,
1230 A common form of hoar, "eur-t'ac e ho s.r ", grows on
the snow surface0 The cond; tions that promo t.e 1.ts growth occur
on clear nights wlth little or no wind v.nen the humidity of' the
air is high and the air temperature is higher than that of the
snow0 It often grows ｮ･｡ｾ open water. Its crystals are flat
be mistaken for freshly fallen ;5!")()W" SU1'face hoar crystals ar-e
often as Lar-ge a s 1/211 Hl1Q in a
f"£HV ca s e a crystals several
inches in eliameter- have been obSE11'Ved, If it is formed during
a ligh.t wind, the two si.de s mo st ne arLy p ar-a l.r e I to the wind
direction will be slightly Lorig ez- than the other s ide s ,
1240 Hoar a Lao gX'OW8 in eavities wi thin the
snow-cover-and is then called I!dEYOth he tI It, 1s ｾｦＶｮｂｲ｡ＮｬＱｶ found in air
ｾ ' J ｾ
spaces ､ＺＱｲ･｣ｴＮｬｾｲ b e Low B hard old cz-ust ｡ｦｾＬ･ｉＢ a. lc,.'l.t, mode r-steLy
cold period wit..'10Ut thaws" Sini:.e 1t s ;:;;r'y s:::a18 grow very slow} y
they a'l.way s have solid 3ti!"faces. !U1d are e tthe r p La s or hollow
hexa.gona.l cup s •
125. 'I'here are some crystals of' old snow wb.lch are
analo-gous to hcar , althougp they BIte not: hollow like most depth hoar
crystals. They occur in the d e eper- p ar-u e oT the snow-cover- and
reach full development only after a fairly long period in whleh
the Lower- layers have not been subjected to meLtrng , These
cry-sta.ls are fairly c oar se and r-atrwr wealdy bonded together. 'l"hey
mayor may not have crysta,lline facets; in th e Bar-r-en s where
temperatures are quite low and calm c or.di td on s ar e rare, crystals with facets a.re the most commonG
R.ime
1260 Rime deposits are found not only on snowflakes but
also on vertica.l objects above ｴｨｾ snow surface and occasionally
on the snow surface0
1270 Rime is an accumulation of frozen water droplets
which generally requires a moderate to strong wind to promote
its r or-matLcn , For this r-ea son , rime deposits are umally h e av I»
est r.ear the top of vertical objects whereas hoar deposits,which are more fragile and grow in l1.ttle wind, are heaviest near the
snow sur-rac e , When rime is deposited dur i.ng a snow s t.orm, some
of the snowflakes become en t.rapp e d in the deposlt and give i t
the appearance of hoaru
NOMENCLATURE FOR SNOW
1280 ｾｨ･ snow nomenclatures which ha.ve been used at
various t i.me s by meteor-o l.ogd at s , glaciologists. skt.er-s and others
ha.ve not been ｩｾ complete ha.rmony, Occasionally the same ter:n
has been used by different groups, or in different localities, to describe entirely different forms of snow and this has led to a certain amount of confusiono
