AGGREGATION OF ICE CRYSTALS 259 
walls and by impingement do not affect the ac- 
curacy of the sample. From samples taken only 
a few seconds apart, it appears that this method 
of sampling is reproducible and that the accu- 
racy is very good. Another attempt to sample in 
the smaller tube immediately above the observa- 
tional area has proved unsuccessful although we 
still have some hope of resolving the difficulties 
with this particular sampler. 
The method currently being developed for 
maintaining varying degrees of supersaturation 
with respect to ice is to mix air at a known flow 
rate, temperature, and dewpoint with the ice- 
saturated air. The rate of flow of air is measured 
by means of a sensitive flowmeter. This method 
appears favorable for varying the vapor pres- 
sure with reference to ice saturation and water 
saturation, but will be limited in the sense that 
we will not be able to determine absolutely our 
position above ice saturation due to deposition 
on the walls of the tube and the ice crystals. 
Experimental procedure—A cloud is formed 
within the inner chamber by a continuous source 
of vapor and then the cloud is slowly lifted 
through the larger tube. A period of ten sec- 
onds is required for the cloud to travel the 
length of the tube. During this time a small ice 
sphere is inserted into the observational area 
and the motors turned on so that the ice crystal 
cloud is carried past the ice sphere at a known 
velocity. The width and height of the aggregate 
is measured several times throughout the period 
of the run which in the cases reported here var- 
ied from three to six minutes. The cloud is sam- 
pled at the middle and end of the run. Immedi- 
ately prior to sampling, the cloud is stopped in 
order to prevent impingement of the cloud on 
the tube. A cover glass is then removed from the 
plastic solution, placed on a holder and inserted 
at the bottom of the volume to be sampled. The 
section is then shifted and the run continued. In 
the case of the final slide of the run, the tube is 
not shifted, but a known volume is closed and the 
cover glass inserted. Several minutes are allowed 
for the ice crystals to precipitate onto the cover 
glass. The cover glass and holder are then re- 
moved and left to dry. A complete analysis for 
concentration, size distribution, and type of ice 
crystal is made from the slide. From the con- 
centration, velocity in the tube, length of run, 
and the average area of the aggregate (actually 
the area of a circle of the diameter of the aggre- 
gate), we obtain the number of ice crystals for 
which collision was possible. At the end of the 
COMMERCIAL 
4 cu. ft. 
Cloud volume 
Mixing fon 
Fie. 3—Schematic of the apparatus 
run the aggregate of ice crystals is melted and 
the diameter of the drop is again measured. From 
the increase in size, the mass can be determined, 
and using the average volume of the crystals 
counted, we obtain the actual number of crystals 
that adhered to the sphere. The volume of the 
crystal is determined by measuring the diameter 
of the ice erystal and from average measure- 
ments of the thickness obtained from a side view 
of the erystal through the microscope. At pres- 
ent a technique is being developed so that the 
thickness can be determined by shadowing the 
replicas and measurements made from electron 
photomicrographs. With the two, we define the 
ratio of erystals collected to the total number of 
crystals in the path of the collector. 
Preliminary results—A summary of the aver- 
age measurements of some of the parameters 
necessary to compute the ratio is shown in Ta- 
ble 1, along with the ratio at several tempera- 
tures. All of the measurements reported here 
