GROWTH AND STRUCTURE OF GRAUPEL AND HAILSTONES 321 
Fie. 5—Thin section of a small hail particle under polarized light; diameter, 4.5 mm 
cretion of water drops which then freeze par- 
tially or entirely. 
(Small hail particles are, in fact, essentially 
graupel which have already reached a higher 
stage of density. To call them small hail makes 
sense only according to this manner of charac- 
terization, for there is otherwise no_ possible 
means of distinguishing small hail from hail- 
stones.) 
Figures 4-7 show thin sections of a graupel, a 
small hail particle (these are wet as a rule and 
freeze together before examination, with the re- 
sult shown in Fig. 5), and of a hailstone. 
With regard to Figure 3 note that in all cases 
observed there was no indication that graupel 
arise from frozen water drops. This possibility 
may, however, exist in nature; this would neces- 
sitate a corresponding modification of the model. 
The color pictures, Figures 8-11, should give 
an impression of the real appearance of a thin 
section under polarized hght. The range of 
variation in color is given by the thickness of 
the ice, where the brown tone sections corre- 
spond to 0.3 mm; the other ones are thicker. 
As the limits of the single crystals are much 
sharper the thinner the section, it is better to cut 
the ice slices as thin as possible, perhaps con- 
trary to the aesthetic point of view. 
The comparison with black and white repro- 
ductions of thin sections under polarized light 
(see Fig. 7 and 11 which were taken of the same 
thin section) show that the impression of the ar- 
rangement of the single ice crystals and the 
symmetries is more evident in the color pictures. 
CoNcLUDING COMMENTS 
The information which can be inferred from 
analysis of bubble arrangements and crystalline 
structure is of crucial significance but only 
qualitative. Our improved knowledge should, on 
the one hand, prevent us from carrying out 
calculations concerning layer formations on the 
basis of entirely untenable assumptions [J/ason, 
1958], and on the other hand encourage research 
which will yield quantitative results. It is to this 
end that investigations are to be carried out with 
the Swiss Hail Tunnel, which aims at explaining 
natural ice structure by creating similar struc- 
tures by artificial means. 
Whereas a year ago it seemed possible to 
understand growth behavior perfectly with the 
help of structural investigations, laboratory 
