THE FORMATION OF ICE CRYSTALS 213 
the plane parts observable in this crystal are the ex- 
tensions of the side planes of the columns, which form 
the skeleton of this crystal. 
10. Rimed erystal, &. Supercooled cloud droplets 
are sometimes frozen to a snow crystal, and thus a 
rimed crystal is obtained. When many droplets become 
attached to a plane crystal, it turns into a thick plate 
(R2), sometimes half a millimeter in thickness. The 
graupel bearing a trace of hexagonal symmetry (R4a) 
is composed of a spatial hexagonal crystal P5a with 
numerous droplets attached, the graupel-like snow of 
hexagonal type R3a being the intermediate stage. The 
lump graupel 4b is made in a similar manner from the 
spatial assemblage of radiating type P5b, through the 
by different groups. In 1949, this Committee on Snow 
Classification proposed a tentative snow classification 
covering solid precipitation and the deposited snow. 
This tentative classification for solid precipitation is 
considered to be more convenient and adequate for 
practical purposes than similar ones so far proposed. 
It is shown in Table II. 
In Table II, each class and basic feature of snow is 
designated by a code symbol with a view toward making 
the classification international (7.e., independent of lan- 
guage). The description is made very simple. For ex- 
ample, FirD1.5 or QO 1.5 means plate crystals with 
water droplets attached, the average size being 1.5 mm. 
F2fwDd or & d means a cluster of stellar crystals 
Tasie II. PrRacrican CLassIFICATION OF SOLID PRECIPITATION 
Code Graphic symbol Term Remarks 
1 O Plates Pia, P1b, Pic, P4. 
Pid, Pie, Pif, P1g, Pih, P11, P2a, P2b, P2c, P3a, P3b, 
2 sk Stellar crystals PL 
s 3 = Columns Cla, Clb, Cic, C2a, C2b. 
3 4 | Needles Nia, Nib, N2. 
= § & Spatial dendrites P&a, Pd5b. 
S. 6 — Capped columns CPia, CP1b, CPic, CP2a, CP2b. 
— 7 Va Irregular crystals CP3, S, L1, 12, 13: 
Q 8 @ Graupel (snow pellet) R4a, R4b, R4c. 
= 9 Seat G let) United States Weather Bureau definition; frozen 
= A Nay Soest raindrops, fairly small and transparent. 
0 A Hail Solid precipitation formed by the successive freezing 
of water layers. 
3 % m 2K Broken Broken crystals of type 1, 2, etc. 
2 38 r > Rimed R1, R2, R3. 
= 5 4 ij (Ck) Flake Clusters of crystals of type 1, 2, ete. 
ae w kK Wet Wet or partially melted crystals of type 1, 2, etc. 
a a 0 -0.49 mm Very small The size of particle means the greatest extension of a 
a9 b 0.5-0.99 Small particle (or average when many are considered) in 
oe c 1.0-1.99 Medium millimeters. For a cluster of crystals it refers to 
cS Be d 2.0-3.99 Large the average size of the crystals composing the 
R e 4.0 or larger Very large flake. 
stage of graupel-like snow &3b. The cone-like form 
R4c is considered to be due to the rotational motion 
around the vertical axis during its fall. 
11. Irregular snow particles, 7. Snow particles are 
sometimes observed which do not show any regular 
erystalline form. There are many varieties: one type 
looks like an assemblage of pieces of ice (1); another 
type has many water droplets attached (/2), etc. 
Practical Classification of Snow Crystals. The general 
classification described in the preceding paragraph is 
not adequate for practical purposes. At the Oslo meet- 
ing of the International Commission on Snow and Ice 
in 1948, a committee was appointed to prepare a practi- 
cal classification of snow, acceptable not only to scien- 
tists but also to others interested in snow. The aim was 
to promote uniformity in the method of describing 
snow and to simplify the correlation of data obtained 
partially melted, the average size of the crystals com- 
posing the flake being large, or between 2.0 and 3.9 mm. 
ARTIFICIAL PRODUCTION OF SNOW CRYSTALS 
Method for Making Snow Crystals in the Laboratory 
[11]. The artificial production of snow crystals means 
the production of frost crystals freely suspended in the 
air in a cold chamber laboratory. It takes at least from 
one-half to one hour for the complete development of a 
snow crystal. A thin filament was used to keep the 
crystal suspended in air for such a time. 
In order to obtain nearly steady convection of water 
vapor, the apparatus shown in Fig. 17 was designed. 
Two concentric glass cylinders are held vertically, so 
that warm water vapor is driven upward inside the 
inner tube, while the cooled air comes down through 
the space between the two cylinders. The water in the 
