THE MECHANISM OF HAIL FORMATION 307 
obvious reasons, be the same) may be justified 
on the basis of Prandtl’s considerations aimed 
at a better quantitative description of the ex- 
change of ponderable matter brought about by 
turbulence. It may be remarked in passing that 
Sutton [1947] has made successful use of this 
concept in deriving his formulas for the spread 
of atmospheric dust. By adopting such a pro- 
cedure, a density would be obtained for ice- 
forming nuclei, which would turn out to be mark- 
edly smaller in order of magnitude than that 
given by Ludlam. This density could clearly of- 
ten be realized in nature at temperatures less 
than —10°C, and there would also be no great 
difficulty in producing it artificially. 
These suggestions make it evidently necessary 
to say, that the possibilities of precipitation be- 
ing produced according to the Bergeron-Findei- 
sen process should not be rated so low as has 
sometimes been done more recently. This view 
is supported ultimately also by the Final Re- 
port of the U.S. Advisory Committee on 
Weather Control which states that it has been 
proved statistically with a satisfactory degree 
of certainty, that in orographic conditions an 
increase in rainfall amounting to between 10 and 
20% may be expected from artificial seeding with 
silver iodide by means of ground generators. 
Also the results of the first two experimental 
years of the Swiss Hail Suppression Project, in 
which a randomized ordinance has been con- 
sistently adhered to, likewise give indications for 
believing that a surprisingly large increase in 
precipitation can be effected by seeding; a re- 
port on this is, however, being presented in a 
separate paper. Dessens [1958] points out in a 
short article, in which he takes up the ideas 
propounded by Ludlam, that the effects known 
to be produced in clouds by seeding with ice- 
forming nuclei could never be observed in clouds 
seeded with condensation nuclei. And in a work 
published recently Schaefer and Dietrich [1959] 
have also confirmed in an impressive manner 
the influence on supercooled clouds by seeding 
with silver iodide particles. Finally, the excellent 
experiments designed by Schaefer must be re- 
ealled, in which, within a few seconds after 
dropping dry-ice particles through supercooled 
clouds, centimeter-broad dark bands appear 
along the sides of the threads of small ice crys- 
tals; after approximately another 30 sec the 
whole picture becomes blurred as turbulence sets 
in. 
These comments on the significance of the 
Bergeron-Findeisen mechanism have been put 
forward less to provide a more solid foundation 
for hail-prevention experiments by silver iodide 
seeding than to indicate from the point of view 
also of these experiments what kind of problems 
are involved in constructing any theoretical 
model to represent some part only of a weather 
process. It is important to stress here with par- 
ticular clarity that only experiment can provide 
the ultimate answer. This is no less true when 
it is a question of clarifying the processes which 
lead to the formation of hail, than im the practi- 
cal field of hail prevention. 
Recently List [1958] has published two arti- 
cles on investigations into the structure of differ- 
ent types of graupels and of large hailstones col- 
lected at different places in Switzerland in the 
years 1953 to 1957. The structural analyses have 
been carried out by means of techniques for 
producing very fine cross sections, which have 
been developed to a degree of great proficiency 
at the Swiss Federal Institute for Snow and Ava- 
lanche Research, Weissfluhjoch-Davos. This In- 
stitute houses also one of the research centers 
of the Federal Commission for the Study of Hail 
Formation and Prevention and it is here that 
for the last few months the Swiss Research Hail 
Tunnel has been operating, about which a sepa- 
rate report is presented as the next paper in this 
volume by the supervisor of the tunnel experi- 
ments. 
On the basis of pictures which have been made 
of the structure of actual hailstones, List ar- 
rives at the view that the hailstones which were 
examined and had dimensions of several centi- 
meters, all possessed in their original form as 
growth centers (to use the author’s own term) 
some type of rime or frost graupel, which may 
well in certain eases have come about through 
snow crystals entering a graupel phase. A par- 
ticularly pertinent example is offered by the 
hailstone 57.7 (see Fig. 1 and 2), of which struc- 
tural photographs are reproduced. There can be 
no doubt that this argument is entirely correct in 
this case, and above all his claim that the initial 
graupel cannot have developed from a drop of 
water which subsequently froze. Thus we are 
able to make a statement, which is certainly 
confirmed by experiment, but contradicts Lud- 
lam’s suppositions regarding the growth of hail- 
stones, as we described them at the beginning 
of this paper. Figure 3 is a picture of a fine 
cross section of a graupel which has reached such 
