The International classification for snow

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The International classification for snow

THE, INTERNATIONAL CLASSIFICATION FOR SNOW

(With Special Reference to Snow on the Ground)

Issued by

The Commission on Snow and Ice

of the

International Association of Hydrology

Published a s Technical Memorandum No. 31

by the

Associate Committee on Soil and Snow Mechanics,

National Research Council, Ottawa, Canada.

F O R E WORD

In r e c e n t y e a r s the need f o r a s t a n d a r d s y s t e m of c l a s s i - fying s o l i d p r e c i p i t a t i o n a n d f a l l e n snow h a s b e c o m e p r e s s i n g . Mechanized t r a n s p o r t o v e r v a s t s n o w - r i d d e n a r e a s , t h e u s e f u l n e s s of a c c u r a t e snow s u r v e y s i n the mountain a n d upland r e g i o n s of the w o r l d , and t h e i n t e n s i f i e d s c i e n t i f i c s t u d y of lying snow

-

a l l t h e s e f a c t o r s m a d e i t i m p e l l i n g t o d e v i s e s o m e r e l i a b l e a n d s i m p l e method of r e p o r t i n g a n d r e c o r d i n g snow.

T h e s e e d of the I n t e r n a t i o n a l Snow C l a s s i f i c a t l o n , which had long b e e n g e r m i n a t i n g , took f i r m r o o t d u r i n g t h e m e e t i n g s of the t h e n I n t e r n a t i o n a l C o m m i s s i o n on Snow a n d G l a c i e r s i n O s l o i n 1948. On t h a t o c c a s i o n t h r e e s e p a r a t e p a p e r s on snow d a t a w e r e r e a d and i t b e c a m e c l e a r t h a t a m a s t e r s y s t e m m u s t be d e v i s e d . A c c o r d i n g l y , a c o m m i t t e e w a s s e t up by t h e C o m m i s s i o n c o n s i s t i n g of D r . V. J. S c h a e f e r , M r . G. J. Klein, a n d D r . M. R . de Q u e r v a i n with i n s t r u c t i o n s t o produce a s y s t e m that would be g e n e r a l l y a n d i n t e r n a t i o n a l l y a c c e p t a b l e .

With i m m e n s e l a b o u r a n d application, a n d a f t e r c o n s u l t a - - -

t i o n s with c o r p o r a t e bodies and p r i v a t e individuals, a t e n t a t i v e s c h e m e was produced. T h i s w a s placed b e f o r e the C o m m i s s i o n a t B r u s s e l s i n 1951. A f t e r s o m e f i n a l a m e n d m e n t s , a u t h o r i t y w a s given for i t s publication. The C l a s s i f i c a t i o n i s now being c o n - s i d e r e d by the World M e t e o r o l o g i c a l O r g a n i s a t i o n through i t s T e c h n i c a l C o m m i s s i o n o n A e r o l o g y ( C . A e ) a n d i t i s hoped t h a t i t will r e c e i v e a l s o the a p p r o v a l of t h i s body. T h i s publication h a s been g r e a t l y f a c i l i t a t e d by the National R e s e a r c h Council of Canada through t h e good o f f i c e s of M r . R . F. L e g g e t . C h a i r m a n of i t s A s s o c i a t e C o m m i t t e e on Soil a n d Snow M e c h a n i c s .

On behalf of t h e C o m m i s s i o n on Snow a n d I c e I g r a t e f u l l y acknowledge the debt we a l l owe t o the a u t h o r s of t h i s w o r k .

November 13, 1952 London, England

G e r a l d Seligman, P r e s i d e n t ,

( i i )

INTRODUCTION

T h e c l a s s i f i c a t i o n p r e s e n t e d h e r e i n h a s been developed by the C o m m i t t e e on Snow C l a s s i f i c a t i o n i n t h e hope t h a t i t m a y b e c o m e t h e g e n e r a l l y a c c e p t e d i n t e r n a t i o n a l s y s t e m f o r c l a s s i f y i n g snow. T h e C o m m i t t e e , which w a s f o r m e d a t t h e O s l o C o n f e r e n c e of the I n t e r n a t i o n a l Union of G e o d e s y a n d G e o p h y s i c s , r e g a r d e d t h e q u e s t i o n of a n i n t e r n a t i o n a l n o m e n c l a t u r e f o r snow a s being o u t s i d e i t s p r o v i n c e . . F o r t h i s r e a s o n s y m b o l s w e r e u s e d t o d e s i g n a t e t h e v a r i o u s c l a s s e s a n d b a s i c f e a t u r e s of snow, m a k i n g t h e c l a s s i f i - c a t i o n independent of language a n d t h e r e f o r e m o r e convenient f o r i n t e r n a t i o n a l u s e . Although t h e s y m b o l s m a y a l s o be u s e d f o r t e l e t y p e m e s s a g e s , i t should be e m p h a s i z e d t h a t t h e l a t t e r u s e w a s not the p r i m a r y r e a s o n f o r a d o p t i n g t h e s y m b o l s . T h e c l a s s i f i c a t i o n i s s i m i l a r t o s e v e r a l m o r e o r l e s s p a r a l l e l s y s t e m s which have b e e n d e v e l o p e d i n d i f f e r e n t c o u n t r i e s a n d h a v e b e e n i n u s e d u r i n g t h e p a s t one o r two d e c a d e s . T h e b e s t f e a t u r e s of t h e s e s y s t e m s h a v e b e e n c o m b i n e d a n d m o d i f i e d t o f o r m a c o - h e r e n t a n d r e a s o n a b l y s i m p l e c l a s s i f i c a t i o n . Much thought a n d e f f o r t have b e e n put i n t o i t s g e n e r a l a r r a n g e m e n t a n d d e t a i l s , a n d m a n y g r o u p s a n d i n d i v i d u a l s d i r e c t l y c o n c e r n e d with snow r e s e a r c h h a v e b e e n c o n s u l t e d with a view t o m a k i n g t h e c l a s s i f i c a t i o n a s s u i t a b l e a n d g e n e r a l l y a c c e p t a b l e a s p o s s i b l e . If t h e c l a s s i f i c a t i o n i s a s w e l l r e c e i v e d a s i t w a s i n i t s tentative d r a f t f o r m , t h e r e i s l i t t l e doubt t h a t i t will be a d o p t e d by m a n y g r o u p s engaged i n t h e s t u d y of snow and i t s r e l a t e d p r o b l e m s . A n i m p o r t a n t f e a t u r e of t h e c l a s s i f i c a t i o n i s t h a t i t h a s b e e n s e t up a s t h e b a s i c f r a m e w o r k which m a y be expanded o r c o n t r a c t e d t o s u i t t h e n e e d s of a n y p a r t i c u l a r g r o u p r a n g i n g f r o m s c i e n t i s t s t o s k i e r s . I t h a s a l s o b e e n a r r a n g e d s o t h a t m a n y of t h e o b s e r v a t i o n s m a y be m a d e e i t h e r with t h e a i d of s i m p l e i n s t r u - m e n t s o r , a l t e r n a t i v e l y , by v i s u a l m e t h o d s . S i n c e the two m e t h o d s a r e b a s i c a l l y p a r a l l e l , m e a s u r e m e n t s a n d v i s u a l o b s e r v a t i o n s m a y be c o m b i n e d i n v a r i o u s w a y s t o o b t a i n t h e d e g r e e of p r e c i s i o n r e q u i r e d i n a n y p a r t i c u l a r c l a s s of w o r k . Section

I,

o n s o l i d p r e c i p i t a t i o n , i s not i n t e n d e d t o r e p l a c e t h a t p a r t of t h e I n t e r n a t i o n a l M e t e o r o l o g i c a l Code which d e a l s with snow, h a i l , e t c . , a n d c a r e w a s t a k e n t o a v o i d a n y c o n f l i c t between t h e two s y s t e m s . Section I is b a s e d on t h e f o r m of t h e c r y s t a l o r p a r t i c l e and r e g a r d s f o r m a n d s i z e of a p a r t i c l e a s two s e p a r a t e f e a t u r e s .

( i i i ) S e c t i o n [ I , d e a l i n g w i t h d e p o s i t e d s n o w , i s I j a s e d o n t h e f u n d a m e n t a l f c a t u r c s w h i c l i d c t c l - ~ n i rlc t h e p h y s i c a l c h ; ~ r a c : t c . r i s t i c s of a m a s s o l s n o w a n d d i s t i n g u i s h o n < . typc: f r o m anothcr l-. S e c t i o n s 1 a n d I1 m a y b e r e g a r d e d a s t h e f u n d a r n e n t a I p a r t of t h e c l a s s i f i c a t i o n . S e c t i o n I11 d e a l s w i t h m e a s u r e m e n t s w h i c h a r e f r e q u e n t l y r e q u i r e d i n d e s c r i b i n g a s n o w c o v e r , w h i l e S e c t i o n 1V p r o v i d e s a m e a n s f o r d e s c r i b i n g c e r t a i n f e a t u r e s of t h e s n o w s u r f a c e w h i c h m a y b e s i g n i f i c a n t i n s o m e p r o b l e m s . S u b s e c t i o n 1 o f S e c t i o n I V , d e a l i n g w i t h s u r f a c e d e p o s i t s s u c h a s s u r f a c e h o a r , h a s b e e n i n c l u d e d b e c a u s e of i t s r e l a t i o n t o s n o w l y i n g o n t h e g r o u n d . T h e i c e d e p o s i t s w h i c h f o r m o n t h e w i n g s of a i r c r a f t a r e o f a s i m i l a r k i n d , a n d s p e c i a l i s t s i n t h i s f i e l d a r e u s i n g a m o r e d e t a i l e d c l a s s i f i c a t i o n t h a n t h e c l a s s i f i

-

c a t i o n f o r s u r f a c e d e p o s i t s p r e s e n t e d h e r e . I t m a y b e r e g a r d e d a s a n e x a m p l e of a n e x p a n s i o n o f a s e c t i o n of t h e p r e s e n t c l a s s i f i - c a t i o n . A n a t t e m p t h a s b e e n m a d e t o d e s c r i b e c l e a r l y e a c h c l a s s o r f e a t u r e o f s n o w i n t h e b o d y of t h e c l a s s i f i c a t i o n . T h e c l a s s i f i - c a t i o n h a s a l s o b e e n p r e s e n t e d i n a n a b s t r a c t form w h i c h i s s u i t a b l e f o r c o n v e n i e n t r e f e r e n c e i n t h e f i e l d . A n u m b e r of e x a m p l e s e x p l a i n i n g t h e u s e o f t h e s y m b o l s a r e g i v e n i n t h e t e x t , w h i l e t h e u s e of t h e g r a p h i c s y m b o l s i s i l l u s t r a t e d i n A p p e n d i x 11. T h o s e i n t e r e s t e d i n t h e E n g l i s h t e r m i n o l o g y c o m m o n l y u s e d i n d e s c r i b i n g s n o w s h o u l d r e f e r t o " S n o w S l r u c t u r e a n d S k i F i e l d s " b y G. S e l i g m a n , M a c M i l l a r l a n d C o . , L i m i t e d , L o n d o n , 1 9 3 6 . I f a n o m e n c l a t u r e f o r s n o w w e r e i n c l u d e d a s a n a p p e n d i x t o t h i s c l a s s i - f i c a t i o n , i t w o u l d c l o s e l y f o l l o w t h e n o m e n c l a t u r e g i v e n by S e l i g m a n .

SECTION I

SOLID PRECIPITATION

The t e r m "solid precipitation" r e f e r s to the v a r i o u s kinds of solid w a t e r p a r t i c l e s which develop in the a t m o s p h e r e and fall e a r t h w a r d s , for e x a m p l e , snow c r y s t a l s , i c e pellets. When a s h a r p distinction i s drawn between falling and deposited p a r t i c l e s , the t e r m applies to precipitation while i t r e m a i n s a i r b o r n e ; but in the classification p r e - sented h e r e , "solid precipitation" i s taken t o a l s o include f r e s h l y deposited p a r t i c l e s which have not undergone any perceptible t r a n s f o r m a t i o n subsequent to being deposited upon the e a r t h .

When different c l a s s e s of solid precipitation o c c u r together, t h e r e l a t i v e number

of each type may be given a s the number of tenths, o r hundredths, e . g., 8FZD4

+

2F8D1.5

designates 80 p e r c e n t s t e l l a r c r y s t a l s of 4 m m . a v e r a g e d i a m e t e r mixed with 20 per c e n t graupel of 1 . 5 m m . a v e r a g e d i a m e t e r .

The International Meteorological Code m a y be used to indicate t h e intensity and duration of the precipitation a s well a s o t h e r weather data when t h i s information i s r e q u i r e d .

The method of classifying solid precipitation is outlined in subsections 1 and 2.

1. Type of P a r t i c l e

Graphic symbol f o r snow i n g e n e r a l , i . e . Fl t o F 7 TABLE I

G e n e r a l symbol F

CLASSES O F SOLID PRECIPITATION

Description Symbol Graphic

Symbol P l a t e

-

1

A plate i s a thin, plate -like snow c r y s t a l the f o r m of which m o r e o r l e s s r e s e m b l e s a hexagon o r , in r a r e c a s e s , a

triangle. Generally a l l edges o r a l t e r n a t i v e edges of the

0

plate a r e s i m i l a r i n pattern a n d length.

S t e l l a r C r y s t a l 2

A s t e l l a r c r y s t a l i s a thin, flat snow c r y s t a l i n the f o r m of a conventionalized s t a r . I t generally has s i x a r m s but

-)IC-

s t e l l a r c r y s t a l s with t h r e e o r twelve a r m s o c c u r o c c a - sionally. The a r m s may lie i n a single plane o r in closely spaced p a r a l l e l planes in which c a s e the a r m s a r e i n t e r -

connected by a v e r y s h o r t c'olumn.

Column 3

₀

A column i s a relatively s h o r t p r i s m a t i c c r y s t a l , e i t h e r solid o r hollow, with plane, pyramidal, truncated o r hollow ends. P y r a m i d s , which m a y be r e g a r d e d as a p a r t i c u l a r c a s e , and combinations of columns a r e included in this c l a s s .

( T a b l e 1, continued) D e s c r i p t i o n S y m b o l Symbol Needle 4 -C-C A needle i s a v e r y s l e n d e r n e e d l e - l i k e snow p a r t i c l e of a p p r o x i m a t e l y c y l i n d r i c a l f o r m . T h i s c l a s s i n c l u d e s hollow bundles of p a r a l l e l n e e d l e s , which a r e v e r y c o m m o n , a n d combinations of n e e d l e s a r r a n g e d i n a n y of a wide v a r i e t y of fashions. S p a t i a l D e n d r i t e 5 A s p a t i a l d e n d r i t e i s a c o m p l e x snow c r y s t a l with f e r n - l i k e a r m s which do not f i e i n a plane o r i n p a r a l l e l p l a n e s but extend i n m a n y d i r e c t i o n s f r o m a c e n t r a l n u c l e u s . I t s

69

g e n e r a l f o r m is roughly s p h e r i c a l .

Capped Column

6

A capped c o l u m n i s a c o l u m n with p l a t e s of hexagonal o r

t=l

s t e l l a r f o r m a t i t s e n d s an'd, i n m a n y c a s e s , with a d d i t i o n a l

p l a t e s a t i n t e r m e d i a t e p o s i t i o n s . T h e p l a t e s a r e a r r a n g e d n o r m a l to the p r i n c i p a l a x i s of the column. O c c a s i o n a l l y only one end of t h e c o l u m n is capped i n t h i s m a n n e r .

I r r e g u l a r C r y s t a l 7 An i r r e g u l a r c r y s t a l is a snow p a r t i c l e m a d e u p of a n u m -

n

b e r of s m a l l c r y s t a l s grown t o g e t h e r i n a r a n d o m f a s h i o n . G e n e r a l l y t h e component c r y s t a l s a r e s o s m a l l t h a t t h e c r y s t a l l i n e f o r m of t h e p a r t i c l e c a n o n l y be s e e n with t h e a i d of a magnifying g l a s s o r m i c r o s c o p e . G r a u p e l 8

G r a u p e l , which i n c l u d e s t h e s o f t h a i l , s m a l l hail, a n d snow p e l l e t s of the m e t e o r o l o g i s t , is a snow c r y s t a l o r p a r t i c l e coated with a heavy d e p o s i t of r i m e . I t m a y r e t a i n s o m e

2i

evidence of the outline of t h e o r i g i n a l c r y s t a l although t h e m o s t c o m m o n type h a s a f o r m which i s a p p r o x i m a t e l y s p h e r i c a l . I c e P e l l e t 9 I c e p e l l e t s a r e f r e q u e n t l y c a l l e d " s l e e t " i n N o r t h A m e r i c a . An i c e p e l l e t i s a t r a n s p a r e n t s p h e r o i d o f i c e and is u s u a l l y f a i r l y s m a l l . S o m e i c e p e l l e t s d o n o t h a v e a f r o z e n c e n t r e which i n d i c a t e s that, a t l e a s t i n s o m e c a s e s , f r e e z i n g t a k e s place f r o m the s u r f a c e i n w a r d s .

( T a b l e I , c o n t i n u e d ) D e s c r i p t i o n S y m b o l G r a p h i c S y m b o l H a i l

-

0 A h a i l s t o n e * i s a g r a i n of i c e , g e n e r a l l y h a v i n g a l a m i n a r s t r u c t u r e a n d c h a r a c t e r i z e d b y i t s s m o o t h g l a z e d s u r f a c e a n d i t s t r a n s l u c e n t o r m i l k y - w h i t e c e n t r e . H a i l i s u s u a l l y a s s o c i a t e d with t h o s e a t m o s p h e r i c c o n d i t i o n s w h i c h a c c o m p a n y t h u n d e r s t o r m s . H a i l s t o n e s a r e s o m e t i m e s q u i t e l a r g e .

*

_{I n E n g l i s h , h a i l , l i k e r a i n , r e f e r s t o a n u m b e r a t o n e} t i m e , w h i l e h a i l s t o n e , l i k e r a i n d r o p , r e f e r s t o a n i n d i v i d u a l .

-

Modifying f e a t u r e s of c l a s s e s F 1 t o F O m a y b e i n c l u d e d b y a d d i n g o n e or m o r e of t h e f o l l o w i n g s u b s c r i p t s . T A B L E I1 MODIFYING F E A T U R E S 2. S i z e of P a r t i c l e F e a t u r e S y m b o l S u b s c r i p t

...

B r o k e n c r y s t a l s P R i m e - c o a t e d p a r t i c l e s n o t s u f f i c i e n t l y c o a t e d t o b e r c l a s s e d as g r a u p e l C l u s t e r s , s u c h a s c o m p o u n d s n o w f l a k e s , c o m p o s e d f of s e v e r a l i n d i v i d u a l s n o w ' c r y s t a l s W e t o r p a r t l y m e l t e d p a r t i c l e s w G e n e r a l s y m b o l D ? T h e s i z e of a c r y s t a l o r p a r t i c l e i s i t s g r e a t e s t e x t e n s i o n m e a s u r e d i n m i l l i - m e t e r s . When m a n y p a r t i c l e s a r e i n v o l v e d , e . g . , a c o m p o u n d s n o w f l a k e , i t r e f e r s t o t h e a v e r a g e s i z e of t h e i n d i v i d u a l p a r t i c l e s . E x a m p l e : F 4 f D 2 . 5 o r

-

f D 2 . 5 d e s i g n a t e s a c l u s t e r o r c l u s t e r s c o m p o s e d o f n e e d l e s , t h e a v e r a g e s i z e of t h e n e e d l e s b e i n g 2. 5 mm.

SECTION 11 DEPOSITED SNOW

A snow c o v e r i s g e n e r a l l y c o m p o s e d of l a y e r s of d i f f e r e n t t y p e s of snow e a c h of which i s m o r e o r l e s s homogeneous within i t s own b o u n d a r i e s . S e c t i o n I1 d e a l s with the c l a s s i f i c a t i o n of the type of snow i n a n y one l a y e r .

A m a s s of snow i s v e r y p o r o u s a n d i t m a y o r m a y n o t contain s o m e w a t e r i n t h e liquid s t a t e which i s u s u a l l y r e f e r r e d to as " f r e e w a t e r " . I n the g e n e r a l c a s e , t h e r e f o r e , snow m a y be r e g a r d e d as a m i x t u r e of i c e , a i r , a n d w a t e r , the i c e being i n t h e f o r m o f c r y s t a l s o r g r a i n s which a r e u s u a l l y i n t e r k n i t o r welded t o g e t h e r t o f o r m a s t r u c t u r e which p o s s e s s e s s o m e d e g r e e of s t r e n g t h . The p h y s i c a l c h a r a c t e r i s t i c s of a m a s s of snow, like t h o s e of m a n y o t h e r m a t e r i a l s , depend upon t h e r e l a t i v e p r o p o r t i o n s o f i t s c o n s t i t u e n t s , its s t r u c t u r e , a n d i t s t e m p e r a t u r e . T a k i n g p h y s i c a l c h a r a c t e r i s t i c s a s t h e c r i t e r i o n , the p r i m a r y f e a t u r e s which c l a s s i f y a type of d e p o s i t e d s n o w ' a r e t h o s e given i n T a b l e 111.

T A B L E 111

PRIMARY F E A T U R E S O F DEPOSITED SNOW

T h e a b o v e f e a t u r e s a r e d i s c u s s e d i n s o m e d e t a i l i n t h e following s u b s e c t i o n s .

-

u h +I

'

U

2

fn F e a t u r e Units Symbol Specific g r a v i t y , o r n o n - d i m e n s i o n a l 3 3

G

Density g / c m

,

o r k g / m

.

F r e e w a t e r content by weight, o r W s e e T a b l e I V I m p u r i t i e s $I by weight J G r a i n s h a p e s e e T a b l e V F G r a i n s i z e m i l l i m e t e r s D S t r e n g t h r e p r e s e n t e d by: C o m p r e s s i v e yield s t r e n g t h , g/crn2. K P T e n s i l e s t r e n g t h g / c ~ n 2

.

K z S h e a r s t r e n g t h a t z e r o g / c m 2 . K s n o r m a l s t r e s s , o r H a r d n e s s a c c o r d ~ n g t o i n s t r u m e n t R Snow t e m p e r a t u r e d e g r e e s C e n t i g r a d e T

1. Specific G r a v i t y G e n e r a l s y m b o l G S p e c i f i c g r a v i t y i s the r a t i o of the weight of a n y volume of a s u b s t a n c e t o the weight o f a n e q u a l volume of w a t e r a n d i s t h e r e f o r e n o n - d i m e n s i o n a l . D e n s i t y m a y be u s e d as a n a l t e r n a t i v e . I t should, h o w e v e r , be e x p r e s s e d e i t h e r in g r a m s p e r c u b i c c e n t i m e t e r o r i n k i l o g r a m s p e r cubic m e t e r ; the f o r m e r having the s a m e n u m e r i c a l value a s s p e c i f i c g r a v i t y while the l a t t e r a v o i d s the inconvenience of the d e c i m a l point. F o r e x a m p l e : a s p e c i f i c g r a v i t y of 0 . 2 3 5 is e q u i v a l e n t t o a d e n s i t y of 0. 235 g r a m s p e r cubic c e n t i m e t e r o r 235 k i l o g r a m s p e r cubic m e t e r . Using s y m b o l s , t h i s m a y be given a s G 0. 235 o r G 235 provided the d i m e n s i o n s u s e d a r e c l e a r l y indicated o r u n d e r s t o o d .

2. F r e e W a t e r Content G e n e r a l s y m b o l W

T h e r e a r e s e v e r a l m e t h o d s of m e a s u r i n g the free w a t e r c o n t e n t of snow b u t , s i n c e f a i r l y e l a b o r a t e a p p a r a t u s i s u s u a l l y r e q u i r e d in o r d e r t o o b t a i n r e a s o n a b l e a c c u r a c y , t h e s e m e t h o d s a r e g e n e r a l l y u s e d only in a l a b o r a t o r y . M e a s u r e m e n t s of f r e e w a t e r c o n t e n t a r e e x p r e s s e d a s a p e r c e n t a g e by weight.

In field t e s t s , r e l i a n c e u s u a l l y h a s t o be p l a c e d u p o n s i m p l e o b s e r v a t i o n s . T a b l e I V i s given a s the b a s i s of o b s e r v a t i o n s of t h i s kind a n d , a l t h o u g h t h i s m e t h o d w a s p r i m a r i l y intended f o r u s e i n the f i e l d , i t i s a l s o of c o n s i d e r a b l e value i n t h e l a b o r a t o r y .

TABLE I V

F R E E WATER CONTENT

G r a p h i c

T e r m R e m a r k s S y m b o l

Symbol

D r y U s u a l l y T i s below O°C, but d r y snow c a n o c c u r a t a n y a

t e m p e r a t u r e u p t o a n d including 0" C. When i t s s t r u c

-

t u r e i s b r o k e n down by c r u s h i n g a n d t h e l o o s e g r a i n s a r e lightly p r e s s e d t o g e t h e r a s i n m a k i n g a snow ball,

I

the g r a i n s have l i t t l e tendency to c l i n g t o e a c h o t h e r . M o i s t T = O°C. T h e w a t e r i s not visible e v e n with the a i d of

a magnifying g l a s s . When lightly c r u s h e d , t h e snow

h a s a d i s t i n c t tendency t o s t i c k t o g e t h e r .

m

Wet T = 0 ° C . T h e w a t e r c a n be r e c o g n i z e d by i t s m e n i s c u s c between a d j a c e n t snow g r a i n s , but w a t e r cannot b e

p r e s s e d o u t by m o d e r a t e l y s q u e e z i n g the snow i n t h e

m

h a n d s .

V e r y T = 0 ° C . T h e w a t e r c a n be p r e s s e d out by m o d e r a t e l y d Wet s q u e e z i n g the snow i n t h e h a n d s but t h e r e s t i l l i s a n

a p p r e c i a b l e a m o u n t of a i r confined within the snow

m

s t r u c t u r e .

S l u s h T = 0°C. Snow flooded with w a t e r a n d containing a e

3. I m p u r i t i e s G e n e r a l s y m b o l J

T h i s s u b s e c t i o n h a s b e e n i n c l u d e d in the c l a s s i f i c a t i o n i n o r d e r t o c o v e r t h o s e c a s e s in which the kind a n d a m o u n t of a n i m p u r i t y have a n i n f l u e n c e upon t h e p h y s i c a l c h a r a c t e r i s t i c s of t h e snow. I n t h e s e c a s e s t h e kind of i m p u r i t y s h o u l d b e fully d e s c r i b e d a n d i t s a m o u n t g i v e n a s a p e r c e n t a g e by weight. C o m m o n i m p u r i t i e s a r e : d u s t , s a n d , o r g a n i c m a t e r i a l , s a l t , e t c . G r a p h i c s y m b o l 4. G r a i n Shape G e n e r a l s y m b o l F I n the c l a s s i f i c a t i o n , n u m e r i c a l s y m b o l s ( F l , F 2 , e t c . ) h a v e b e e n u s e d f o r s o l i d p r e c i p i t a t i o n while a l p h a b e t i c a l s y m b o l s ( F a , F b , e t c . ) h a v e b e e n u s e d f o r d e p o s i t e d snow. H o w e v e r , w h e n e v e r a d i s t i n c t i o n b e t w e e n t h e v a r i o u s t y p e s of f r e s h l y d e p o s i t e d s n o w i s r e q u i r e d , t h e c l a s s i f i c a t i o n g i v e n i n T a b l e I m a y b e u s e d a n d , when n e c e s s a r y , t h e r e l a t i v e p r o p o r t i o n s of t h e v a r i o u s t y p e s m a y b e e x p r e s s e d a s t h e n u m b e r of t e n t h s a s e x p l a i n e d i n S e c t i o n I. T A B L E V GRAIN S H A P E D e s c r i p t i o n S y m b o l G r a p h i c S y m b o l C l a s s "a" a

C l a s s "a" r e f e r s t o f r e s h l y d e p o s i t e d snow c o m p o s e d of

I++]

c r y s t a l s , o r p a r t s of b r o k e n c r y s t a l s , of t y p e s F 1 t o F 7

( T a b l e I ) . Snow which h a s l o s t i t s c r y s t a l l i n e c h a r a c t e r while f a l l i n g t o e a r t h , a n d g r a u p e 1 , i c e p e l l e t s , a n d h a i l d o not belong t o t h i s c l a s s . C l a s s "a" s n o w i s g e n e r a l l y v e r y s o f t .

C l a s s "b" b

T h i s c l a s s r e f e r s t o snow d u r i n g i t s i n i t i a l s t a g e of settling. I t h a s not r e a c h e d t h e v e r y fine g r a i n - s i z e condition w h i c h is g e n e r a l l y r e g a r d e d as t h e c o n c l u s i o n of the i n i t i a l s t a g e of t r a n s f o r m a t i o n . Although i t h a s l o s t a g r e a t d e a l of its c r y s t a l l i n e c h a r a c t e r , s o m e c r y s t a l l i n e f e a t u r e s c a n b e o b s e r v e d . C l a s s "b" s n o w is u s u a l l y f a i r l y s o f t . C l a s s "c" c When s n o w is t r a n s f o r m e d by m e l t i n g , o r m e l t i n g followed by f r e e z i n g , i t c o m p l e t e l y l o s e s all c r y s t a l l i n e f e a t u r e s a n d i t s g r a i n s b e c o m e i r r e g u l a r a n d m o r e o r l e s s r o u n d e d i n f o r m . T h i s is C l a s s "c" s n o w . I t h a s no s p a r k l e e f f e c t e v e n i n b r i g h t s u n l i g h t a n d c a n b e r e a d i l y r e c o g n i z e d by i t s d u l l a p p e a r a n c e . I t i s u s u a l l y f a i r l y s o f t when w e t , but c a n be v e r y h a r d when f r o z e n . C l a s s "c" s n o w m a y h a v e a n y s i z e of g r a i n s f r o m v e r y f i n e t o v e r y c o a r s e .

.

( T a b l e V, continued) 5. G r a i n S i z e G e n e r a l s y m b o l D D e s c r i p t i o n Symbol G r a p h i c Symbol C l a s s "d" d At t e m p e r a t u r e s w e l l below f r e e z i n g a n d without a n y

[q

a p p a r e n t m e l t i n g , snow i s t r a n s f o r m e d i n t o C l a s s "d" by the p r o c e s s of s u b l i m a t i o n w h i c h p r o d u c e s i r r e g u l a r g r a i n s with f l a t f a c e t s . T h e s e f a c e t s give t h e snow a d i s t i n c t s p a r k l e e f f e c t in b r i g h t sunlight. I n t h e A r c t i c , w h e r e t e m p e r a t u r e s a r e low a n d p e r s i s t e n t winds a c c e l e r a t e t h e s u b l i m a t i o n , p r a c t i c a l l y a l l of t h e s e t t l e d snow i s C l a s s "dl1 a n d h a s a l m o s t a s m u c h s p a r k l e a s a d e p o s i t of F 1 c r y s t a l s . C l a s s "d" snow i s u s u a l l y f a i r l y h a r d . Depth H o a r e Depth h o a r i s c h a r a c t e r i z e d by i t s hollow c u p - s h a p e d c r y s t a l s . T h e s e c r y s t a l s a r e p r o d u c e d by a v e r y low r a t e of s u b l i m a t i o n d u r i n g a long u n i n t e r r u p t e d cold p e r i o d and a r e m o s t f r e q u e n t l y found d i r e c t l y below a m o r e o r l e s s i m p e r m e a b l e c r u s t in t h e l o w e r p a r t of the snow c o v e r . T h e s t r e n g t h of a l a y e r of depth h o a r i s v e r y low.

T h e g r a i n s i z e of a m o r e o r l e s s homogeneous m a s s of s n o w i s t h e a v e r a g e s i z e of i t s g r a l n s , taking t h e s i z e of a n individual g r a i n a s i t s g r e a t e s t e x t e n s i o n . A s i m p l e method siritable f o r f i e l d m e a s u r e m e n t s is t o p l a c e a f a i r s a m p l e of the g r a i n s on a p l a t e which h a s b e e n r u l e d in m i l l i m e t e r s . T h e a v e r a g e o r typical s i z e i s then e s t i m a t e d by c o m p a r i n g the s i z e of the g r a i n s with t h e s p a c i n g of the l i n e s on t h e p l a t e .

i

T h e g r a i n s i z e of d e p o s i t e d snow i s e x p r e s s e d i n m i l l i m e t e r s o r , a l t e r n a t i v e l y , by t h e u s e of the t e r m s o r s y m b o l s p r e s e n t e d in T a b l e VI.

TABLE VI

GRAIN SIZE O F DEPOSITED SNOW

L T e r m S y m b o l G r a i n S i z e Range V e r y fine a l e s s t h a n 0 . 5 m m . F i n e b 0 . 5 t o 1 . 0 m m . Medium c 1 . 0 t o 2 . 0 m m . C o a r s e d 2. 3 to 4 . 0 m m . V e r y c o a r s e e g r e a t e r than 4 . 0 m m .

6.

Strength of Structure General symbol K The yield strengths in compression, tension, and s h e a r of many m a t e r i a l s a r e

generally inter-related by simple r a t i o s , such a s 2: 2 : 1 which applies to ductile

m a t e r i a l s . In this c a s e , the three yield strengths a r e known when any one of them i s given. Investigations, s t i l l in p r o g r e s s , indicate that a relation of this kind e x i s t s

between the yield strengths of snow. It i s therefore proposed that the strength of

deposited snow be described by any one of the following:

(a) Compressive yield strength, i.e., the s t r e s s under a compressive load a t which initial collapse of the snow

2 s t r u c t u r e o c c u r s , g / c m

.

(b) Tensile strength, g/cm2.

2

(c) Shear strength a t z e r o normal s t r e s s , g/cm

.

(d) Hardness, in which case the correlation between the readings of the particular h a r d n e s s instrument and

(a), (b), o r ( c ) above should be given.

Since the technique and instruments required f o r measuring (a) and (d) a r e con- siderably l e s s complex than those required f o r measuring (b) and (c), compressive yield strength and hardness m e a s u r e m e n t s uan be expected to be used m o r e frequently than m e a s u r e m e n t s of tensile and s h e a r strength.

TABLE VII

STRENGTH O F DEPOSITED SNOW

T e r m Range of Kp, g / c m 2 Symbol Graphic Symbol

Very low 0

-

10 _a

₀

Low 10

-

100 _b

rzzzzl

Medium 100

-

1000 c

m333

High 1000

-

10,000 d

I

Very high G r e a t e r than 10,000 e

txzzl

Ice i

7. Snow T e m p e r a t u r e General symbol T

The t e m p e r a t u r e of snow should be given in d e g r e e s Centigrade. Sometimes i t

is desirable to r e c o r d other related t e m p e r a t u r e s ; the suggested symbols for the m o r e

.common ones a r e included below:

Snow t e m p e r a t u r e T d e g r e e s Centigrade

A i r t e m p e r a t u r e T a II I I

T e m p e r a t u r e of snow s u r f a c e TS I I I I

SECTION I11

SNOW COVER MEASUREMENTS

A c r o s s - s e c t i o n of a snow c o v e r m a y be d e s c r i b e d by c l a s s i f y i n g the snow i n e a c h l a y e r , a s outlined i n S e c t i o n 11, a n d b y giving t h e l o c a t i o n of t h e b o u n d a r i e s of the l a y e r s by m e a n s of m e a s u r e m e q t s . T h e location of a l a y e r b o u n d a r y i s g e n e r a l l y e s t a b l i s h e d by i t s v e r t i c a l c o - o r d i n a t e m e a s u r e d f r o m t h e s u r f a c e of the ground but, i n c e r t a i n c a s e s , w h e r e only the u p p e r p a r t of the snow c o v e r i s of i n t e r e s t o r w h e r e i t i s difficult t o u s e the ground a s the r e f e r e n c e , the snow s u r f a c e m a y be t a k e n as the r e f e r e n c e .

T h e s y m b o l s H , H S a n d HN should b e u s e d f o r a l l v e r t i c a l m e a s u r e m e n t s r e g a r d - l e s s of w h e t h e r t h e y a r e t a k e n a t a place w h e r e t h e snow s u r f a c e i s h o r i z o n t a l o r i n c l i n e d . V e r t i c a l m e a s u r e m e n t s a r e p r e f e r r e d e v e n when the snow l i e s o n a s l o p e . If, h o w e v e r . the m e a s u r e m e n t s a r e t a k e n along a l i n e p e r p e n d i c u l a r t o a n i n c l i n e d snow s u r f a c e , t h i s f a c t should be i n d i c a t e d b y u s i n g t h e c o r r e s p o n d i n g s y m b o l s M , MS a n d MN.

T A B L E VIII

SNOW COVER MEASUREMENTS

T e r m D i m e n s i o n Symbol

Ver;tical c o - o r d i n a t e ( m e a s u r e d f r o m t h e g r o u n d ) c m H

T o t a i depth of snow c o v e r c m HS

Depth of daily new snowfall c m HN

M e a s u r e m e n t s c o r r e s p o n d i n g to t h o s e a b o v e but c m M , MS, t a k e n p e r p e n d i c u l a r l y t o a n i n c l i n e d snow a n d MN s u r f a c e Inclination of snow s u r f a c e a n g l e i n N d e g r e e s W a t e r e q u i v a l e n t of t h e snow c o v e r m m H W R a t i o of snow c o v e r e d a r e a t o t o t a l a r e a

-

t e n t h s Q Age of snow d e p o s i t i n d i c a t e A w h e t h e r h o u r s , d a y s . o r y e a r s

SECTION IV

SNOW SURFACE CONDITIONS

Surface d e p o s i t s , such a s s u r f a c e h o a r , and o t h e r f e a t u r e s of the snow s u r f a c e may be d e s c r i b e d a s indicated in the following s u b s e c t i o n s .

1. Surface Deposits G e n e r a l symbol V

T h e s e deposits a r e r e p r e s e n t e d by the s y m b o l s V1, V2, e t c . , to avoid confusion with the s y m b o l s F 1 ,

FZ,

e t c . , f o r solid precipitation.

TABLE I X SURFACE DEPOSITS Symbol Graphic T e r m and Description Symbol Surface Hoar 1

u

Surface h o a r i s a deposit of plane, p r i s m a t i c , o r d e n d r i t i c

c r y s t a l s formed by sublimation of w a t e r . vapour onto a n y

fixed object the t e m p e r a t u r e ofwhich i s below 0°C. _{P l a n e}

c r y s t a l s of s u r f a c e h o a r c a n be distinguished f r o m plates ( F l ) by t h e i r lack of s y m m e t r y .

Soft Rime 2

Soft r i m e i s a light, b r i t t l e f e a t h e r y deposit i n t e r m e d i a t e

between s u r f a c e hoar and h a r d r i m e ( s e e definition below)

v

and a p p e a r s to be a combination of the e l e m e n t s of both

hoar and r i m e .

H a r d Rime 3

H a r d r i m e i s a f r o z e n deposit of s m a l l , supercooled fog d r o p l e t s on any solid object. T h e d r o p l e t s f r e e z e i m m e

-

diately upon contact'with the object which gives r i m e i t s

V

v e r y fine pebbly t e x t u r e .

Glazed F r o s t o r Glaze 4

Glazed f r o s t i s a smooth, thin, i c e coating f o r m e d on any

C\3

solid object by r a i n d r o p s which have r u n together before f r e e z i n g , o r by thawing followed by f r e e z i n g .

2. Surface Roughness G e n e r a l symbol S

T h i s subsection does not r e f e r to r o u g h n e s s due to the g r a n u l a r n a t u r e of snow,but

to the roughness of a snow s u r f a c e caused by the e f f e c t s of wind,rain,unequal evaporation

o r unequal melting. The a v e r a g e depth of the i r r e g u l a r i t i e s , m e a s u r e d i n c m . , m a y be

-

11

-

TABLE X SURFACE ROUGHNESS T e r m Symbol G r a p h i c Symbol Smooth a Wavy b

w

Concave f u r r o w s c

-

Convex f u r r o w s d -, Random f u r r o w s e

-

3. P e n e t r a b i l i t y of S u r f a c e L a y e r s G e n e r a l s y m b o l P

Occasionally, a n a p p r o x i m a t e indication is r e q u i r e d o f the a b i l i t y of a snow c o v e r to s a t i s f a c t o r i l y s u p p o r t a c e r t a i n load. The depth of penetration of s o m e s u i t a b l e object, s u c h a s a s k i o r a m a n ' s foot, m a y be employed f o r t h i s p u r p o s e . The following s y m b o l s a r e suggested:

Depth of s k i t r a c k ( s k i e r s u p p o r t e d on one s k i ) P S

Depth of footprint (man standing on one foot) PP

T h e depth of penetration should be m e a s u r e d i n c e n t i m e t e r s o r m a y b e e x p r e s s e d by s y m b o l s

.

T A B L E XI

DEPTH O F PENETRATION

T e r m Depth Range, c m . Symbol

V e r y s m a l l l e s s than 0. 5 a

S m a l i 0 . 5 t o 2 b

Medium 2 t o 10 c

Deep 10 t o 30 d

ABSTRACT OF

THE C L A S S I F I C A T I O N F O R SNOW

1

SOLID P R E C I P I T A T I O N

L GRAPHIC T YPE OF PARTICLG S Y Y s O L P L A T E F

t

0

STELLAR CRYSTAL

F

2

*

COLUMN F 3

n

NEEDLE F 4

-

SPATIAL DENDRITE F 5

@

CAPPED COLUMN F 6

w

IRREGULAR CRYSTAL F 7

_>C\

GRAUPEL F 8

&

ICE PELLET _{F 9}

A

HAIL _{F O}

A

4 S I Z E OF P A R T I C L L

D

MEASURED IN MILLIMETERS. MODIFYING FEATURE SYMBOL SUBSCRIPT # CLUSTERS

f

BROKEN CRYSTALS

P

WET W RIME COATED CRYSTALS

r

I I D E P O S I T E D S N O W I l l SNOW COVER M E A S U R E M E N T S

.

I N C L I N A T I O N OF SURFACE, N DEGREES WATER E Q U I V A L E N T O F COVER, H W mm.OF W A T E R L S N O W COVEREQ A R E A , T O T A L AREA Q T E N T H S AGE OF DEPOSIT, A HR., DAYS, E T C .

F E A T U R E S P E C I F I C G R A V I T Y F R E E W A T E R ,

%

G R A I N S H A P E

-

G R A I N S I Z E , mm. COMP. YLD. S T G T H . ~ / C ~ ~ C O - O R D I N A T E , cm. T O T A L D E P T H , cm. D A I L Y N E W _cm. SNOWFALL

I V SNOW SURFACE CONDITIONS

SYMBOL. G F D K V E R T ' C A L H H S H N S N O W T E M P E R A T U R E , T ' C I t o inclined s u r f a c e r M M S M N SURFACE b E P O S l T S Y M B O L G R A P H I C S Y M B O L S U B C L A S S I F I C A T I O N S U R F A C E R O U G H N E S S S Y M B O L G R A P H I C S Y M B O L

SKI T R A C K DEPTH (SKIER SUPPORTED ON ONE SKI) cm. P S FOOT PRINT DEPTH ( M A N STANDING ON ONE FOOT) cm. P P

o d r y

J+fl

F I - F7 c r y s t a l s

<

0 . 5 0-10 I C E L A Y E R

,

i

-

SURFACE HOAR V I U SURFACE PENETRABILITY R A N G E , cm. I M P U R I T I E S

J %

]

[-S M O O T H S a b m o l s t p a r t l y settled 0 . 5 - 1 10

-

lo2 S O F T R I M E V 2

v

a < 0 . 5 W A V Y S b

-

c W ~ m ~ ~ ~ w e t

1-

rounded grolns 1-2

O

I

I

X

X

1

~

~

to2- lo3 HARD RIME V 3

v

b 0 . 5

-

2 G L A Z E D FROST V 4

03

--

CONCAVE FURROWS S c d v e r y w e t

1 7 1

9'0;lte;ith 2- 4 lo3- lo4 c 2 - 10 e s l u s h

1

7

1

d e p t h hoor

>

4 ) lo4 C O N V E X FURROWS S d

-

FURROWS R A N D O M S e

-

d 10 -30 e

>

So

A P P E N D I X 1 L I S T O F

SYMBOLS

D e r i v e d f r o m : A l t e r ,

-

A g e

-

A g e of a s n o w d e p o s i t v e r y low, e t c . low, e t c . m e d i u m high, e t c . v e r y high, e t c . A l w a y s u s e d a f t e r t h e general s y m b o l of a p a r t i c u l a r q u a l i t y , s u c h as s p e c i f i c g r a v i t y t o i n d i c a t e t h e degree of t h a t q u a l i t y . D i a m e t e r

-

F o r m

-

S i z e of a p a r t i c l e S h a p e o r f o r m of a p a r t i c l e with f i g u r e : s o l i d p r e c i p i t a t i o n with l e t t e r : d e p o s i t e d s n o w F l a k e

-

G r a v i t y ,

-

G e w i c h t H e i g h t ,

-

H a u t e u r ,

-

H o h e

-

C l u s t e r of p a r t i c l e s ; s e e ' T a b l e I1 S p e c i f i c g r a v i t y , d e n s i t y V e r t i c a l c o - o r d i n a t e Depth of d a i l y new s n o w f a l l T o t a l d e p t h of s n o w c o v e r W a t e r e q u i v a l e n t of s n o w c o v e r s e e T a b l e VIII A n i c e l a y e r

-

I c e I m p u r i t i e s , I-tnpuret&s

-

K o h a s i o n

-

I m p u r i t i e s G e n e r a l s y m b o l f o r s t r e n g t h of d e p o s i t e d s n o w C o m p r e s s i v e y i e l d s t r e n g t h

1

S h e a r s t r e n g t h a t z e r o n o r m a l s t r e s s

I

see T a b l e VII T e n s i l e s t r e n g t h

J

M a c h t i g k e i t

-

M e a s u r e m e n t s t a k e n p e r p e n d i c u l a r l y t o a n i n c l i n e d s n o w s u r f a c e ; s e e T a b l e VIII I n c l i n a t i o n of a s n o w s u r f a c e

-

Nei gung P e n e t r a t e , ~ e ? n ; t r e r

-

-

G e n e r a l s y m b o l f o r p e n e t r a b i l i t y of s u r f a c e

LIST O F SYMBOLS, continued Derived f r o m : PP Depth of footprint

I

s e e Section IV

-

3 PS Depth of ski t r a c k P F r a g m e n t of a snow c r y s t a l ; s e e Table I1

-

P a r t a g e r ,

-

P a r t

Q Ratio of snow-covered a r e a to t o t a l a r e a

-

Quotient

R Snow h a r d n e s s , r e l a t e d to a p a r t i c u l a r i n s t r u m e n t

-

Resistance,

-

~ i ~ i d i t h

r Rime -coated particle; s e e Table I1

-

Rime

S G e n e r a l symbol for snow s u r f a c e

Sa to Se Snow s u r f a c e roughness; s e e Table X

Surface

-

r . -Snow t e m p e r a t u r e

-

T e m p e r a t u r e T a A i r t e m p e r a t u r e Tg Ground t e m p e r a t u r e T s T e m p e r a t u r e of the snow s u r f a c e

V Surface deposits, such as s u r f a c e hoar; see Section IV

-

1

W F r e e water content of snow

-

Water, W a s s e r

-

A v e r t i c a l s e c t i o n of a s n o w c o v e r m a y b e r e p r e s e n t e d g r a p h i c a l l y a s i l l u s t r a t e d b e l o w . 'The a p p r o p r i a t e s y m t m l s f o r a p a r t i c u l a r l a y e r a r e c o m b i n e d b y s u p e r p o s i t i o n , w h i l e s n o w t e m p e r a t u r e is plotted a s a c u r v e . Grairl s i z e and s p e c i f i c g r a v i t y m a y b e tabulated a s s h o w n i n the e x a m p l e o r r e p r e s e n t e d b y g r a p h s .