THE FORMATION OF ICE CRYSTALS 
dritic, sector, plate, spatial assemblage of plates, cup or 
seroll, needle, irregular needle, and column. Region I 
in Fig. 19 is the condition for dendritic growth. It 
means that the necessary condition for dendritic de- 
| NEEDLE 
© SCROLL OR GUP 
© SECTOR AND PLATE 
@ THICK PLATE 
F * DENDRITIC 
! ® SPATIAL PLATES 
1 m™ COLUMN 
x IRREGULAR NEEDLE 
°C 
30 
25 
20 
oO =6) =||0) =(5 
Ta 
Fig. 19.—Relation between the crystal form and 7, and T'y. 
- 20 °C 
velopment is that 7, is between —14C and —17C and 
the supersaturation is above a certain lower limit. 
Contrary to what has hitherto been believed, the air 
temperature is an important factor controlling the crys- 
tal form. 
The degree of supersaturation s is taken as the ratio 
of the amount of vapor and droplets per unit volume 
(measured) and the amount of saturated vapor at 7. 
per unit volume (calculated). In other words s is a 
relative humidity in the range of supersaturation, where 
humidity is taken to mean the total water content in 
the atmosphere. The ratio s was measured by a gravi- 
metric method using P,O;, and Fig. 19 was transformed 
into Fig. 20, which represents the crystal form as a 
function of T, and s [8]. 
Figure 20 tells us more clearly that the chief factor 
controlling the form of the crystal is the air temperature, 
and that a given type of crystal can be obtained for a 
wide range of supersaturation. It has generally been 
believed that the degree of supersaturation determines 
the form of the crystal, but Fig. 20 shows that this is 
not the case, except within a certain range of tempera- 
ture. The transition of the plate form into the dendritic 
form occurs when the supersaturation exceeds about 
110 per cent, provided the temperature lies between 
215 
—14C and —17C. Above a supersaturation of 140 
per cent the crystal is attached with numerous water 
droplets and becomes a rimed crystal. 
%* DENDRITIC | NEEDLE 
© SECTOR AND PLATE * IRREGULAR NEEDLE 
@ THICK PLATE © COLUMN 
® SPATIAL PLATES © SCROLL OR CUP 
130 
(PER CENT) 
x 
120 
SUPERSATURATION 
0 
' 
1 
! 
! 
' 
! 
! 
1 
' 
' 
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' 
1 
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1 
1 
1 
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100 
=EloP(G} 
Ta 
Fic. 20.—Relation between the crystal form and 7, and s. 
The Growth of Snow Crystals. The acquisition of a 
detailed technique made it possible to produce all 
types of snow crystals almost at will. In the case of 
natural snow, the germ is first formed in the upper 
atmosphere as a result of the sublimation of water 
vapor on a nucleus or by the spontaneous transforma- 
tion of a supercooled droplet. While falling through the 
atmospheric layers of various temperatures and degrees 
of supersaturation, this germ gradually grows into a 
snow crystal of complicated structure and eventually 
reaches the ground. In the case of artificial snow, this 
development can be observed in the course of time 
since the primitive crystal is recognized at a spot on the 
rabbit hair. 
The course of formation is studied by taking photo- 
micrographs of the growing crystal from outside the 
apparatus at regular time intervals while keeping the 
crystal in the apparatus. In order to maintain the 
crystal at rest against the convection current, the 
rabbit hair is stretched on a frame of thin glass rods. 
