DISCUSSION 
measurements have established that not only are 
there growth phases which in places become over- 
laid, but that many times a water layer forms 
that is stabilized by a subsequent ice framework; 
it is this which, together with the action of 
capillary forces, is responsible for maintaining 
the fluid phase where it is [List, 1959]. This 
phase, which is apparently of such prime im- 
portance, means that even the usual and seem- 
ingly reasonable classification that rapid growth 
produces opaque ice layers must be given up. 
Very rapid particle increase may be connected 
with a two-phase ice-water addition, with the 
water involved only able to freeze at a later 
stage, when it forms a frequently clear zone in 
conjunction with the original ice framework. 
A technique for estabilshing such distinctions 
after the event has not yet been found, although 
the requisite comparative experiments are in 
progress. 
REFERENCES 
Dessens, H., La Gréle, Association d’études des 
moyens de lutle contre les fléaux atmosphériques, 
no. 7, pp. 3-17, 1959. 
pe Quervarn, M., Die Metamorphose des Schnee- 
kristalls, Verhandlungen der Schweiz. Naturfor- 
schenden Gesellschaft, pp. 114-122, 1950. 
List, R., Iennzeichen atmosphiirischer Hisparti- 
keln, pt. 1, Zs. f. Angew. Math. Phys., 9a, 180- 
192, 1958a. 
List, R., Kennzeichen atmosphirischer Hisparti- 
keln, pt. 2, Zs. Angew. Math. Phys., 9a, 217-234, 
1958b. 
List, R., Wachstum von Eis-Wassergemischen im 
Hagelversuchskanal, Helvetica Physica Acta, 32, 
293-296, 1959. 
Mason, B. J., The Physics of Clouds, Clarendon 
Press, Oxford, 481 pp., 1958. 
Discussion 
(This discussion relates to the three immediately 
preceding papers.) 
Dr. C. L. Hosler—Do the thin sections elimi- 
nate the possibility that a large portion of the 
growth is due to the collection of the ice crystals 
rather than entirely to supercooled water? 
Mr. R. List—We have not yet made experi- 
ments which could show very clearly that that 
would not be the case. But our experiments 
show that it is not necessary to have snow crys- 
tals which can aggregate. 
Dr. Hosler—My picture would be one of ag- 
gregation of crystals; then filling in of the spaces 
by water of the supercooled droplets. 
Mr. List—Only when the surface is wet can 
ice particles aggregate. When the particles, big 
or small, are dry they bounce off. 
Dr. Choji Magono—The pictures shown by 
you are actual hail stones? 
Mr. R. List—Yes, and the center of natural 
hailstones can be recognized in 80% of the cases 
as graupel. 
Mr. D. Blanchard—I would like to take ex- 
ception to a statement Dr. List made about liquid 
water being shed from the hail pellet itself. You 
said that the liquid water is itself absorbed into 
the hail stones or is shed. At the first meeting 
here at Woods Hole, I presented a paper (The 
Supercooling, Freezing, and Melting of Giant 
Waterdrops at Terminal Velocity in Air, Arti- 
ficial Stimulation of Rain, pp. 233-249) report- 
ing on some experiments I did. One day, I tried 
the reverse process, taking an ice sphere, sus- 
pending it in the wind tunnel, to see what hap- 
pens when the ice melts. When the melting oc- 
curred, a water ring would form around the 
center of the sphere. Of course, eventually water 
will be shed, but first there will be a horizontal 
ring around the center. 
Mr, List—Horizontal rings can be observed 
when the rotating axis of the growing stone is 
stable. That means the form of hail stones de- 
pends on the icing conditions. When the water 
freezes slowly, it has time to soak into the in- 
terior and finally to form corresponding to the 
aerodynamic pressure distribution a ring around 
the rim of the stone. Then one finally arrives at 
plates which can rotate about the vertical axis. 
One can see this rotation also in the frozen 
water droplets. They are arranged around the 
vertical axis. 
From the floor—Do we know enough about 
what a frozen water drop looks like when it is 
freezing? I wonder whether your analysis would 
indicate whether the center of the stone was 
formed by the particle which Ludlam talks 
about, that is, by freezing of a water drop 
having a radius something of the order of 30 to 
50 microns. If this froze because of a violent 
collision of two such drop sizes, or because of the 
crystallization of a single water drop or if the 
center was formed from the conglomeration of a 
number of frozen particles which resulted from 
