316 
considerations are called for when extra humidity 
is injected. 
From these particulars it may be seen how the 
cold output which is too high during stable oper- 
ation makes possible large periodic temperature 
changes during unstable operation of the tun- 
nel.—To what extent, however, the variations 
brought about in our experimental atmosphere 
may suffice to simulate the growth of natural 
hailstones, is a question which can only be prop- 
erly answered by experiment and appropriate 
comparison. 
PROBLEMS OF MEASUREMENT 
That subcooled water clouds may be produced 
in the way we have shown, does not, of course, 
solve the problem of how the relevant conditions 
are to be measured. For measuring the tempera- 
ture alone a great variety of methods is therefore 
being tested and used according to need; and as 
the state of research shall dictate, new types of 
measurement can be tried out and new measuring 
apparatus built into the tunnel. 
PROSPECTS 
The performance of a plant has also to be 
judged from the point of view of how far it broad- 
ens our knowledge of known phenomena or helps 
us to discover new instances of regularity and 
law. In the latter connection the first experiments 
with particularly high water contents (up to 20 
g/m’) have indicated the following interesting 
effects: 
(1) It ean easily be shown that a close relation- 
ship must prevail between the conditions in which 
hailstones arise and their shape. But since differ- 
ent growth phases become overlaid the connec- 
tions are sometimes noticeably complicated. 
(2) A number of authors have assumed, when 
postulating a hail theory, that no more water can 
ROLAND LIST 
accumulate on a hailstone than can become frozen 
as a result of the warmth generated by freezing 
escaping into the surrounding air. The surplus 
water would then be carried away by the air cur- 
rent. These assumptions have proved to be quite 
arbitrary, since this surplus water can to a large 
extent be incorporated into the ice structure of a 
hailstone. This fact suggests a basic revision in 
the way the origin of large hailstones is explained. 
At the same time it also becomes obvious that 
nature produces hailstones of density greater than 
that of ice [List, 1959c]. 
The hail tunnel was planned and built under 
close cooperation with the firm Sulzer AG, Win- 
terthur, as part of the research program of the 
Swiss Commission for the Study of Hail Forma- 
tion and Prevention. I should like to express my 
particular thanks to my distinguished Principal, 
M. de Quervain, under whose helpful guidance 
I have been able to design, build and run the 
wind tunnel. The plant was paid for by the 
“Schweiz. Nationalfonds.” 
REFERENCES 
List, R., anp M. pre QureRvatn, Zur Struktur von 
Hagelkérnern, Zs. angew. Math. Phys., 4, 3-6, 
1953. 
List, R., anp M. pE Quervarn, Untersuchung 
liber die Wirksamkeit eines elektrostatischen 
Filters gegeniiber kleinsten Silberjodidteilchen, 
Helvetica Physica Acia, 29, 424-426, 1956. 
List, R., Kennzeichen atmosphiarischer Eisparti- 
keln, 1. Teil, Zs. angew. Math. Phys., 9a, 180- 
192, 1958a. 
List, R., Kennzeichen atmosphirischer Eisparti- 
keln, 2. Teil, Zs. angew. Math. Phys., 9a, 217- 
234, 1958b. 
List, R., Zur Aerodynamik von Hagelkérnern, Zs. 
angew. Math. Phys., 10, 143-159, 1959a. 
List, R., Der Hagelversuchskanal, Zs. angew. 
Math. Phys., 10, 381-415, 1959b. 
List, R., Wachstum von Eis-Wassergemischen im 
Hagelversuchskanal, Helvetica Physica Acta, 32, 
293-296, 1959¢. 
Discussion 
(Note: Discussion of this paper is combined with those of the preceding and following papers at 
the end of the following paper.) 
