226 
a small drop of mercury with liquid air or dry ice and 
then passing it through saturated air at —15C, ice 
crystals are generated while the mercury is in the solid 
state. The instant it melts (melting point of Hg is 
—38.89C) ice crystals no longer are formed. Further 
poo ; ~ opuery 
Fic. 9.—Typical hexagonal plates formed by dry-ice seeding 
of a supercooled cloud. 
evidence that the critical transition temperature for the 
spontaneous nucleation of ice crystals is close to —38.9C 
may be demonstrated by using a sealed cold chamber 
with remote controls to add moisture to the air. As the 
air in the chamber is warmed and cooled between —37C 
and —41C, ice crystals suddenly appear at a tempera- 
ture of —38.9C + 0.1C. If the air remains colder than 
the critical temperature, snow forms and falls to the 
floor of the chamber continuously. This proves that 
foreign particles cannot be a factor in this process, since 
they would be quickly exhausted from the air by pre- 
cipitation. A further interesting fact is that when the 
temperature passes from —38C to —39C many of the 
supercooled waterdrops present become frozen and serve 
as condensation centers for ice-crystal formation. Asym- 
metric crystals grow on such particles in such a manner 
as to suggest that the cirrus-type crystals shown in 
Fig. 10 probably grow on cloud droplets which freeze 
when they supercool to —39C and are seeded by sev- 
eral very small ice crystals spontaneously generated in 
their vicinity. 
Another effect related to this critical transition tem- 
perature has been demonstrated by Vonnegut [38]. By 
suddenly expanding a cubic centimeter of air so that 
the rapid adiabatic expansion cools the air below —39C 
it is possible to form 10” ice crystals within a small 
fraction of a second. 
Since it is unusual for ordinary air to contain more 
than 1 X 106 cm foreign particles of all kinds it is 
obvious that spontaneous nucleation occurs under these 
conditions. 
4. Fragmentation Nuclei. A snow crystal formed by 
any of the preceding processes produces some interest- 
ing effects when in the presence of a supercooled cloud. 
CLOUD PHYSICS 
Although the mechanism is not yet clearly understood, 
the effects may be produced experimentally [34]. They 
involve the formation of tiny ice particles in the im- 
mediate vicinity of the larger crystal, each of which 
subsequently grows to form new snow particles. It is 
believed at present that fragile dendritic growths in 
(c) 
0.1 CM j 
Pe 
_ PREV RE Et SRE Naa ULRRES SCRE CORIO 
Fie. 10.—Compound columns from cirrus clouds probably 
growing on a frozen cloud droplet. 
the form of fine needles, and frostlike structures of the 
type shown in Fig. 11 are required for the development 
of fragmentation nuclei. 
Contact by liquid cloud droplets may produce ther- 
mal strains leading to the fracture of the delicate por- 
tions of such erystals. Collision with other particles 
could also lead to the formation of such fragments. In 
many snowstorms, especially those having convective 
activity, high winds, and supercooled clouds, broken 
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