Growth and Structure of Graupel and Hailstones 
Rowuanp List 
Swiss Federal Snow and Avalanche Research Institute, 
Weissfluhjoch-Davos, Switzerland 
Abstract—Interpretation of the structures of air bubbles and single ice crystals makes 
it possible to show that the growth of hailstones on graupel and small hail particles fol- 
lows certain laws of symmetry. For the present a number of general qualitative state- 
ments concerning corresponding glaciation conditions can be made, which explain these 
facts. 
INTRODUCTION 
The elucidation of the structure of natural ice 
particles is fundamental for an understanding of 
their growth. This principle takes precedence 
over any theoretical consideration, since in the 
case of atmospheric freezing processes, only ob- 
servation of the natural ice particles covers the 
full range of variation in shape, structure, and 
growth potential. 
An attempt is here made to give some coherent 
order to all the facts which have thus far been 
established from graupel and hailstones at 
Weissfluhjoch. Of course, so far it has not been 
possible that every type occurring in nature has 
been taken into account. Further observations 
or equally intensive parallel studies in other 
places will perhaps give more complete results. 
In particular what will be shown is the way 
in which the growth of graupel or hailstones is 
to be characterized and by what structural fac- 
tors this suggests. For specialized information 
reference may be made to the more detailed 
publications [List, 1958ab]. 
Tue THIN-Section TECHNIQUE 
The structural analysis of graupel and hail- 
stones is based primarily on the thin-section 
technique as it was developed by de Quervain 
[1950] for snow. His method enables, in particu- 
lar, layers 0.3-0.4 mm thick to be sawed out of 
ice particles of low density (0.1-0.7 g/m*), with- 
out the ice structure being disturbed. The layer 
obtained in this way provides us with knowledge 
of the amount of air contained in the ice and 
of the arrangement of the individual air bubbles. 
The use of polarized light shows up the crystal- 
line structure of the ice and indicates the ar- 
rangement and size of the individual single erys- 
tals. 
317 
The structural picture is strongly influenced, 
of course, by the zone and direction in which the 
section is taken from the graupel or hailstone. 
Normally the plane investigated should run 
through the growth center, the original nucleus 
of the particle, the growth directions of the in- 
dividual crystals bemg contained in this plane. 
This is called a main section; other sectional 
planes may be taken for special examination. 
Tur FUNDAMENTALS OF GROWTH IN 
ATMOSPHERIC Ick PARTICLES 
Crystallographic considerations—The crystal- 
lographic main axis, the c-axis of the individual 
ice crystallites, is as a rule approximately verti- 
cal to their direction of growth. This results 
from the speed of growth bemg dependent on 
direction; this determines the selection of start- 
ing points for new unit crystals. A further conse- 
quence of this characteristic is that the unit 
crystals, growing out symmetrically from the 
center like rays, exhibit a pyramidal or trun- 
cated pyramid form. Lengthwise sections are 
therefore generally triangular or trapezoidal, 
while sections taken at right angles to the di- 
rection of growth are rather polygonal. With 
practice all intermediate cuts can also be recog- 
nized distinctly. The arrangement of the crys- 
tallites yields a certain symmetry for which 
conclusions can be drawn as to the manner of 
growth. It is, for instance, possible in every 
hailstone to determine the center of growth, the 
oldest part of the whole particle, on the basis 
of the form and arrangement of the single crys- 
tals. So long as this center comprises the growth 
directions of the first generation of single crys- 
tals, it is regarded as symmetry-center I. It con- 
tinues to occupy this role so long as the condi- 
tions of growth, the type of glaciation, the shape 
of the particle, and its aerodynamics all remain 
