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DISCUSSION 
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Discussion 
Dr. U. Nakaya—How do you measure these 
ice crystals? 
Dr. C. L. Hosler—We have a man who sits 
there eight hours a day and measures these erys- 
tals. We have made replicas in Formyar. We 
have spent a year and a half trymg to measure 
the actual number and size of the crystals in 
our cloud. It has been our most difficult problem 
to make sure we had a representative sample 
and to know what we had in the test section. 
We had inserted a collection device above and 
below the test section. We are finally fairly con- 
vinced we are actually measuring the population 
of test erystals going through our section. The 
process of collection of individual crystals is 
observed by means of a microscope attached to 
the test section. Their thicknesses and lengths 
are determined throughout the test. In addition, 
we have these samples collected by actually ro- 
tating out a section of the tunnel and by gravi- 
tational settling collecting them in a Formvar 
solution and making plastic replicas. 
Dr. W. Hitschfeld—I think these are very 
beautiful data, but I am still surprised to find 
that the collection efficiencies you measured are 
so low. 
Dr. Hosler—Do not put too much weight on 
these efficiencies because we computed them 
using an average cross-sectional area. In reality, 
they present weird-shaped aggregates with a lot 
of holes in them. Maybe one-half of this average 
area is actually occupied by ice and one-half is 
holes. 
Dr. Hitschfeld—So you could double these 
collection efficiencies ? 
Dr. Hosler—Yes, now we are trying to meas- 
ure the true area of these crystals, but we have 
not accomplished it yet. 
Dr. B. J. Mason—Those values on the right 
in Table 1 really do not represent ‘sticking’ alone, 
because they are really partly due to aggregation 
and partly due to sticking. The imterpretation 
of exactly what those figures mean will be diffi- 
cult because the hydrodynamics of the problem 
is changing all the time depending on the shape 
of the erystals one is using. But, nevertheless, 
what you are measuring is the overall thing 
which meteorologically is important. Of second- 
ary importance perhaps is whether you noticed 
any major difference, even in a qualitative way 
when you changed from needles, say, to plates. 
Dr. Hosler—One observes very interesting 
things. The bigger the crystals are, the more 
quickly this aggregate assumes a greater area. 
Some detailed observations appear to be quite 
important: We were afraid, for instance, when 
long extentions would build out, that they would 
break off and fall away. Instead, they build out 
to a certain length and then fold right back into 
the aggregate. Then it merely fills in and gives 
a graupel-type buildup. 
Dr. Helmut Weickmann—I have the impres- 
sion that your experiment is a good analogy to 
what really happens in nature during the ag- 
gregation in a convective cloud layer. Such a 
layer has a little convective activity with up and 
down draft velocities of one-half to one meter per 
second. This is enough to support some of the 
crystals. The ensuing small-scale turbulence cre- 
ates a much better chance to aggregate than if 
they just meet each other during a straight fall. 
If they go up and down, veering left and right, 
they meet each other much more frequently. 
Dr. Hosler—In addition to this, when falling 
through a cloud layer a water film may form on 
the crystals which greatly assists the sticking. 
