Preliminary Results on the Aggregation of Ice Crystals 
R. E. HauucGren anp C. L. Hosier 
Pennsylvania State University, University Park, Pa. * 
Abstract—A brief description of the experimental apparatus and procedure is given. 
Preliminary results of measurements of the collection efficiency of a 170-micron sphere 
as a function of temperature show a decrease of collection efficiency with tempera- 
ture from 0.17 at —4°C to 0.04 at —20°C. The results are discussed with reference to 
growth of snowflakes, charge separation in thunderstorms and the bright band. 
Introduction—The study of the mechanisms 
involved in the growth of cloud particles into 
precipitation particles has been one of the most 
important phases of cloud physics. The growth 
of water drops by condensation and coalescence 
has been treated extensively both theoretically 
and experimentally. Although all of the investi- 
gations are not In complete agreement and more 
work is needed for the smaller drops, one can 
make a reasonable estimate, from existing in- 
formation, of the time required for a small drop 
to grow to the size of observed precipitation 
particles. In addition, the growth of ice crystals 
by sublimation has received considerable atten- 
tion; however, the importance of the aggrega- 
tion or conglomeration of ice crystals has been 
grossly neglected. In this case, in addition to the 
aerodynamic considerations, it 1s necessary to 
establish the environmental conditions in which 
two ice crystals will actually stick together. 
The results of field experiments by Cunning- 
ham and Atlas [1953] indicated that in some 
storm systems the ice phase accounts for as much 
as two-thirds of the precipitation. It appears 
that a considerable amount was caused by the 
aggregation of ice crystals; however, it is quite 
difficult to separate the growth by sublimation 
and by aggregation in this particular experiment 
and to specify the conditions which are most 
conducive to aggregation. Several radar studies 
also indicate the importance of aggregation. In 
particular the attempts to justify the change in 
the echo intensity above the zero degree isotherm 
by sublimational growth alone seems to require 
unreasonable rates of growth of the ice crystals, 
*The research leading to this paper was sup- 
ported by the National Science Foundation grant 
G-3477. 
“Contribution No. 59-55, College of Mineral In- 
dustries. 
257 
and it appears that the aggregation of ice erys- 
tals must make an important contribution to the 
change in intensity of the echo. Very elaborate 
studies at MeGill University [Douglas and oth- 
ers, 1957] of snow generating cells using verti- 
cally pointed radar, which gives a height-time 
record of snow echoes from which the terminal 
fall velocity of the particles could be determined, 
have shown that aggregates of ice crystals exist 
at very low temperatures. They found aggregates 
at temperatures as low as —34°C. In the cases 
they studied they found no preferred tempera- 
ture at which snow cells form, but did find that 
the aggregation generally occurred within the 
first few thousand feet above a frontal surface. 
Preliminary laboratory experiments carried 
out by Hosler and others [1957] have shown that 
the aggregation of ice crystals may be important 
at temperatures lower than had been expected, 
and that the temperature and the vapor pres- 
sure are extremely important in determining 
whether or not a collision between two ice par- 
ticles will result in aggregation. They carefully 
manipulated two ice spheres together and meas- 
ured the force required to pull them apart after 
being in contact for one minute. The force re- 
quired to separate the spheres as a function of 
temperature and at ice saturation is reproduced 
in Figure 1. These measurements, although not 
realistically simulating two small ice crystals, in- 
dicate that aggregation should not be very im- 
portant at ice saturation at temperatures below 
—25°C. Their measurements at near water satu- 
ration gave only a slight decrease with tempera- 
ture in the force required to separate the spheres. 
With this in mind, we have developed an ap- 
paratus in which a small ice sphere could be 
fixed in a flow of ice-crystal-laden air, and the 
growth of the sphere observed at various tem- 
peratures and vapor pressures. 
