On the Structure of Hail Stones

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Technical Translation (National Research Council of Canada), 1955

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On the Structure of Hail Stones

List, R.; de Quervain, M. R.

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The Division of Building Research, through its Snow and Ice Section. has a keen interest in the fundamental properties of ice and snow particles

involved in precipitation. The Division has there-

fore much interest in this paper on the structure of hail stones which has been kindly translated by

h l r . D, A. Sinclair, to whom the Division expresses its thanks.

The Division has a further interest in the

paper in that one of the authors (~r. Marcel de

~uervain) spent a year as a guest scientist with

the Division and prepared a report which will long

be the foundation upon which the snow and ice re- search programme of the Division will be based.

It is hoped that the paper will be of

interest to others in Canada who are concerned with the steadily mounting interest in this country in snow and ice research.

Ottawa,

July

1955.

R. F.. Legget,

Title:

NATIONAL RESEARCH COLJNCIL OF CANADA

Technical Translation TT-551

On the structure of hail stones.

( Z W Struktw von Hagelkbrnern. )

Authors : R. List and ill. de Quervain.

Reference: Z. angew. 1,dath. u. Phys.

4:

492-496,

1953.

Also as Wissenschaftliche Mitteilungen Nr.

3,

w s s i s c h e s Institut ftir Schnee- und

Lav~inenforschung, Weissf luh joch, Davos.

(scientific Report No.

3

of the Federal

Institute for Snow and Avalanche ~esearch).

Translator: D. A. Sinclair, Translations Section, N.R. C. Library.

Translated with permission.

During a thunderstorm in the area of Weissfluhjoch hail stones were collected for crystallographic investigation, Thin sections cut out of several specimens and inspected in polarized light reveal, in all cases but one,

a polycrystalline structure of the hail stones.

The normal pattern of the sections shows a nwnber of small crystals probably forming the

centre of growth and an adjacent or surrounding

ON THE STRUCTURE OF HAIL STONES

During a storm accompanied by hail (24th June 1953),

hail stones were collected on I"?eissfluhjoch and various types

of them were studied.

The fall of hail was characterized by the f o l l o v r ~ meteorological data:

Place: Weissfluhjoch (2670

m.

above sea level) near Davos.

General weather situation: unsettled humidity, cold air at high altitude, a generally storqy atmosphere, sub- sequent front storm. The hail belonged to the re- treating cold front with thunderstorms.

Temperature: falling, between the hours of' 1.20 p.m. and

ge30

p.m., from

6.

lo to 0.20ce

Clouds: cunmlo-nunibus with alto-stratus 8-10/10. Shortly

after the hail had fallen the station was fog-bound.

Wind: 3.30 p.mo

,

northwest at

4

m./sec., increasing until

5.20

p.m. up to 11 m. /set, and veering north; after a brief period of calm, squalls from the south with

velocities up to 22 rn./sec.

(5.23

p.m. to

5.35

p.m. ),

with fall of hail, then veering west to northwest

and decreasing in velocity to

5

m./sec.

(6.40

p.m. ).

Precipitation: 10.50 a.m, to 4.02 p.m.

-

rain 0.9 mm.

5.23

p.m. to

5.35

porno

-

hail without rain, then mixed with a small amount of rain

5.35

porn. to bolo a.m.

-

rain with snow.

The collected hail stones, some of which are shown

1. Spherical grains of transparent ice (~ig. l, stone 3) ;

2. Spherical to egg-shaped stones with an opaque nucleus

and a (thin) transparent shell (~ig. 1, stone

5)

;

3. Spherical forms merging into blunt cones (pig. 1,stone 1);

4.

Spherical, opaque stones with transparent cap (Fig. 2,

stones 7 and

9);

5.

Opaque, cloudy, l'reniformll shapes (Fig. 2, stones 6 and 8);

6 _{I r r e m a r , partiaily flat-surfaced transparent stones}

(Fig. 1, stone

4)

;

7.

Hemispheres (stones 10 and 12) and chipped forms (stones 11

and 13)9 transparent to partially opaque ( ~ f g , 3).

In order to obtain more information concerning the

inner structure of the collected hail stones, thin sections were manuf'actured of the main individual types.

The stone to be investigated is placed in a liquid which is indifferent to H20 (phthalic acid diethyl ester) and has a freezing temperatwe a few degrees below the freezing

point of water, At a temperatwe of -20°C. a disc 0.2 to

0.4

m.

thick is then cut from the stone by means of a circular saw.

By liquefying and dissolving away the embedded material, a

transparent specimen is then obtained for micrascopic analysis.

TPle thin sections were taken as nearly as possible through the geometric centre of the hail stone, and also through the axis of symmetry, where one existed, They show no exceptional structural features in transmitted rrrhite light,

apart from the arrangemnt of the air bdbles. In polarized

light, however, various structural types become visible,

corresponding the variety of external shapes. The stones

With respect to crystallite size distribution the following classifications be made (see Figs.

5, 7,

9, 11,

13

and

15

) :

1. Thin section, containing a single large stone, to which two smaller ones are joined or in which they are embedded (Fig. 9); 2. Thin sections with tivo grov.ps of structural components of

clearly different grain size:

(a) with fan-shaped structure, the small structural components clustered together to form the origin of the fan and the larger ones being attached to these (Bigs.

5

and 15);

(b ) with radially symmetrical arrangement of the two groups, small structural components at the centre

(0.1 to

1.5

rmn. diam), comparatively large stones adjacent to it

(1.5

to

4.0

rnm. diam), e.g. stone 8;

(c) transitional forms between(a) and@,

(d) thin sections in which there is no clear centre of small structural components in the above sense

(Figs.

7

and

13).

Classification 2(d) m y result from the fact that the cut just passes over the comparatively small structural nucleus. In hail stones which do not show outward symmetry (spherical form or distinct axes), the cluster of small crystals can be exposed by the cutting, only accidentally.

Observation of the bubble patterns leads to the same conclusion, especially if we base the observation on the thin

sections of hail stones 1 and

14

(Figs. 4 and 12). From these it may be supposed that the bubbles gather into the shape of a cone, the apex of which lies in the region of the s m l l

structural components. Fig.

4

would then be a section parallel to the cone axis (branched hyperbola) and Fig. 12 nould be one

perpendicular to the axis (circle). This assumption is backed up by other sections studied which show elliptical bubble

patterns. In the case of stones which have a radially sDm.etri-

oal arrange13ent of the structural component groups (types 2 ( b ) )

we do not expect to find such bubble patterns, because there are no preferred kinds of clustering for them

On the basis of kno1:rn ice grori7th phenomena it may be assumed that the rows of bubbles which show no visible rela- tionship to the individual crystals indicate growth directions within the hail stone. Thus the two groups of structural com- ponents can be regarded as two generations. Bubbles arranged in a distinct shell pattern have another meaning, as the

following experiment suggests, Surface freezing was carried out artificially on a n m b e r of different natural hail stones

suspended from a thread by repeatedly dipping them in water.

Owing to the low temperature of the stones and of the ambient air ( - 3 5 ' ~ ~ ) the freezing process proceeded simultaneously from the surface of the stone and the water surface, so that the bubbles were shifted to the place where the two layers

of ice growing tov~ards one another met. This is s h o w in

Figs, 16 and

17.

It would be difficult to come to any conclusions about the formation of the hail stones under investigation,

The origin of the 'tmonocrystal" (stone

4)

is particularly

obscure,

In

the case of stones

6

and 8, which, superficially,

appear to have grown together from smaller stones (Fig. 2),

the crystalline structure (e.g, Fig. 11) points rather to a

uniform growth. (In Fig. 10 the line running upwards to the

right from the indentation is due to a fracture which occurred

d u r i ~ preparation). Whether stones 10 to

13

represent

determined from the thin sections. The stones with conical bubble patterns may possibly have grown from conical sleet

(1)

particles, conf'imning the view of H. Israel

.

In any case these few remarks shorn that more thorough and far-reaching investigations are needed in order to obtain more accurate information about the origin and growth of hail

stones.

Reference

Israel, H. Neuere Anschauungen und Versuche zur ErklHrung

der Hagelbildung; (~ecent vievis and experiments

for clarification of hail formation). Die

Hail Stones from the Stomn of June

24,

1953 in the Area of Weissfluhjoch

Scale 1,3:1

Fig, 1, Stones

1-5

Fig, 2 , Stones

6-9

~ i g .

3,

Stones 10-13

Fig, 4, Stone 1 Fig, 5, Stone 1

Thin sections of hail stones under polarized light