AGGREGATION OF ICE CRYSTALS 
of ice crystals surrounding the core of the cloud, 
might be explained in the following manner: 
Drops grow in the core of the Cumulus to a di- 
ameter that cannot reflect a radar signal, say 
40 microns, and upon arriving in the layer of ice 
erystals surrounding the Cumulus grow quite 
rapidly by means of the extensions to a size that 
can be detected by the radar. Unfortunately, 
present airborne sampling techniques do not en- 
able us to detect the presence of such small ag- 
gregates. 
These extensions would only make an impor- 
tant contribution to the area swept out by the 
aggregate when the collector is rather small. It 
seems reasonable that there is some critical di- 
mension after which the ‘fillmg in’ would pro- 
ceed at the same rate the extensions are grow- 
ing. 
For some time there has been a good deal of 
speculation as to how much of the increase in 
the radar signal above the bright band can be 
attributed to aggregation and how much to sub- 
limational growth of the ice crystal. With the 
above results, one can see that even at ice satu- 
ration considerable growth could be achieved 
through aggregation down to temperatures lower 
than had been thought. 
The above results also have some important 
implications with reference to charge separation 
in clouds. The graupel process, as it has been 
developed by Reynolds and others [1957], re- 
quires the separation of the charge by means of 
frictional contact between the larger ice parti- 
cle and the ice crystals in the cloud. In the ex- 
periments of Reynolds, which were carried out 
in an ambient temperature of between —12 and 
—17°C, with the growing graupel particle being 
three or more degrees warmer, one would expect 
a large number of the ice crystals actually to ad- 
here to the graupel particle. If one assumes that 
the 0.17 measured at —4°C represents 100% col- 
lection after collision, then even at temperatures 
down to about —15°C approximately one-half 
of the crystals adhere after collision even at ice 
saturation. In addition Reynolds and others 
[1957] observed the maximum charge generation 
in a cloud near water saturation which is where 
Hosler and others [1957] report only a slight 
change with temperature in the ability for ice 
crystals to adhere, and therefore, places the as- 
sumption (made by Reynolds) of the amount of 
charge separated per collision by assuming that 
none of the crystals adhered, in error by a con- 
261 
siderable amount. Our contemplated investiga- 
tions should elarify this particular point. 
It is interesting to speculate upon the role 
collection efficiencies of ice pellets in an ice or 
mixed cloud may play in charge separation m 
thunderstorms. Should the collision between a 
graupel or hail pellet and an ice crystal, without 
collection, be a necessary prerequisite to the 
rapid separation of charge, then a pellet falling 
through an all ice cloud at the lowest possible 
temperature would yield the largest charge sepa- 
ration. This is due to the observation that fewer 
crystals are collected at ice saturation than at 
higher vapor pressure, and that the lower the 
temperature, the fewer ice crystals that are col- 
lected. This would not agree with Reynolds and 
others [1957] observations, but it might well ex- 
plain the increased lightning in thunderstorms 
where cloud seeding has been employed to trans- 
form a greater proportion of the clouds to ice 
[Battan and Kassander, 1960]. If Reynolds’ ob- 
servations are correct, then in light of our ex- 
periment, another charge separating mechanism 
must be operative. However, it may be that the 
charges generated in Reynolds’ mixed clouds 
were due to the small amount of bounce-off that 
occurred and had he performed the experiment 
at high erystal concentrations in an all-ice ecrys- 
tal cloud he would have measured even more 
charge generation. 
With our projected measurements at ice super- 
saturations, we also hope to explain the result 
of some radar studies of snow generation cells 
carried out at MeGill [Douglas and others, 1956]. 
These observations indicated that the majority 
of the cells occurred just above a frontal sur- 
face where the uplift of the front would be most 
intense. Combined with the general uplift that 
was occurring under the situations studied, one 
would expect these stronger regions of uplift to 
be supersaturated with respect to ice, thereby 
making aggregation possible at all temperatures. 
In these studies the conditions were always such 
that the atmosphere was supersaturated with re- 
spect to ice, and for the several observations that 
indicated generating levels at very low tempera- 
tures, it appears the area was also somewhat 
unstable at the generating level, which would 
create convection and higher supersaturations 
with respect to ice. 
REFERENCES 
Bartan, L. J., anp A. R. Kassanper, Jr., Artificial 
nucleation of orographic Cumuli, this publica- 
tion, pp. 409-411, 1960. 
