260 HALLGREN AND HOSLER 
TaBLe 1—Average values of measurements 
| | Ratio: 
Increase , Final Size of crystal crystals 
Temperature | Hangth | im mass [Density of diameter Crpetal co" ype of crystal collested 
sphere | gate | crystals 
Volume Radius in path 
XO} sec 108 g | gm/cm* micron cm-3 | micron’ micron 
—4 197 3.2 0.05 450 | 3050 Needle | 1170 6.3 X 39 0.17 
—8 147 2.0 0.04 435 8100 Plates | 320 8.7 
| Columns |" Gr3) ie 0.12 
—12 175s || Oral 0.038 740 14700 Plates 350 | 9.0 
Columns 0.07 
— 20 297 2.07 0.02 570 7300 Plates 425 10.5 0.04 
were taken with a sphere with an initial diameter 
of 170 microns. The increase in crystal concen- 
tration with decreasing temperature accounts for 
the manner in which the amount of mass col- 
lected on the ice sphere and the final diameter of 
the aggregate vary. If reduced to a common 
crystal concentration and length of run, the mass 
collected and the final diameter is actually much 
less at the lower temperatures. The ratio of erys- 
tals collected to erystals in the path of the collec- 
tor decreases with decreasing temperature. The 
decrease with temperature is quite uniform, be- 
ing 0.17 at —4°C and 0.04 at —20°C. Whether 
or not the factor of four between the collection 
efficiency at —4°C and —20°C represents en- 
tirely a change in the number of ice crystals that 
stick after a collision is open to some question 
because of the method utilized in estimating the 
area swept by the aggregate. Actually, since the 
density of the aggregate is less at lower tempera- 
tures than at higher temperatures, one would 
expect that an ice crystal would have a better 
chance of actually being within the area and 
still not having a collision with another crystal 
at the lower temperatures. We are currently at- 
tempting to obtain a more realistic estimate of 
the actual area. However, the type of growth we 
observe probably is similar to that occurring in 
clouds since our densities are approximately the 
same as the density of snowflakes. Therefore, 
the results are satisfactory for computations of 
the growth of aggregates within clouds. Although 
we did not measure the charge accumulated by 
the aggregate, the studies of Reynolds and others 
[1957] indicate that it should be negligible since 
the cloud used in these particular runs contained 
only ice erystals and no supercooled droplets. 
It is interesting to note that the area pre- 
sented to the flow by the collector increases quite 
rapidly because the initial growth is quite open 
and results in a rapid decrease of the density of 
the aggregate. This could very well lead to a 
particle of sufficient size to reflect a radar signal 
in a rather:short period of time. Observations of 
an aggregate growing in our chamber show ex- 
tensions growing out from the sphere a consid- 
erable distance before any appreciable ‘filling in’ 
occurs. For example, in Table 1 we see that 
while the increase in mass was only sufficient to 
increase the diameter of the 170 micron sphere 
shghtly at —4°C, the final diameter of the ag- 
gregate was on the average 450 microns. Simi- 
larly, at —8°C the average final diameter of the 
aggregate was 435 microns, at —12°C the final 
diameter was 740 microns and at —20°C the 
final diameter was 570 microns. Actually, the in- 
crease would be more pronounced if the con- 
centration of ice particles had not increased with 
decreasing temperature. In addition, the average 
length of the run increased with decreasing tem- 
perature. It seems to be true when observing 
growth of an aggregate that the final size of the 
aggregate is considerably larger with larger ice 
crystals. In other words, the extension grows 
much more rapidly with the larger ice crystals 
(our crystals are largest at —4°C) and results 
in a much lower density growth initially than 
with smaller crystals. However, it is, at least 
with the present information, extremely difficult 
to make any estimate of the time required for 
the particle to grow to a particular size because 
the relation between the length of the extension 
and the size of the crystals has not yet been 
established. 
The rapid growth of the extensions could ex- 
plain the recent observations of Vonnegut and 
Moore [1958] which indicate that the initial echo 
on the radar scope is in the form of an inverted 
cup and that the echo develops very rapidly. 
This, combined with their observation of a veil 
