214 
reservoir & is warmed electrically. The cover C is a 
metal sheet and D is a plate of wood. After examining 
five kinds of filaments, a thin rabbit hair was chosen 
because it was the most suitable filament to which a few 
Fie. 17.—Apparatus for making artificial snow. 
isolated germs of snow crystals could be attached. The 
structure of rabbit hair was examined under a micro- 
scope of high magnification, and it was found that a 
few knobs occur at suitable intervals. These knobs 
seem to serve as the nuclei for snow formation. The 
whole apparatus is set in a thermostat placed in the 
cold chamber which is kept below —80C by a refrigerat- 
ing machine. 
The temperature of the water in the reservoir (T,,) is 
measured. This is a measure of the degree of super- 
saturation and can be controlled. The temperature of 
the air (7’,) where the crystal is formed is a function of 
T. and the temperature of the thermostat 7. For a 
given 7’, the air temperature 7’, is regulated by chang- 
ing T,. Snow crystals have been produced for various 
combinations of 7, and T.,, and the relationship be- 
CLOUD PHYSICS 
tween the crystal form and the external conditions has 
been studied. It was found that T, and 7’, are the two 
elements controlling the form of the erystals. 
The mode of air convection in the apparatus was 
studied by measuring the temperature distribution un- 
der operating conditions; one example is shown in 
Fig. 18. In this figure it is evident that the warm vapor 
! 
1 
\ 
1 
' 
i] 
1 
I 
1 
1 
1 
1 
1 
! 
U 
1 
U 
Fic. 18.—Convection of air in the apparatus (temperatures in 
degrees centigrade) ;7. = —23C, T, = +15C. 
rises through the inner cylinder and is cooled gradually 
as it ascends from the outlet. The accumulation of 
warm air in the upper part of the apparatus is shown by 
the existence of a high temperature region H. 
Three values of T., T,,, and T, were recorded during 
the course of formation of a snow crystal. They were 
plotted as a function of time and the resulting graph 
was taken to represent the history of the conditions 
leading to the production of the erystal in its final form. 
The Relation between the Crystal Form and the 
External Conditions. Inasmuch as 7, and 7, are the 
factors controlling the crystal form, a given type must 
be indicated by a point on the (7, T.,)-diagram. In 
fact, it was proved experimentally that a certain type 
of crystal occupies a certain region in the (7a, 7')-dia- 
gram. A crystal of complicated form which has been 
developed through successive stages of various condi- 
tions is represented by a succession of arrows in the 
(T., T»)-diagram. 
The relation between the crystal form and 7, and T,, 
has been examined for 700 crystals by Hanajima [7]. 
The results are plotted in the (Ta, T,,)-diagram shown in 
Fig. 19. Crystals are classified into eight types: den- 
