SESSION IIIB DISCUSSION
699
DANIELSEN: I agree with what was said.
DINGLE: I have the impression the process in connection with this
extrusion is very important in the generation of some of the radiation
“hot spots” that we See in the central part of the country.
REITER: I would tend to agree with you on that point, but I think
that,if a very crude division into dry fallout and rainout cases is made,
I would classify the generation of the hot spots as dry fallout cases
simply because the air moved in in a matter of two days, as a matter
of fact, from the tropopause to the surface, and then got caught ina
disturbance,
creating some wet “hot spots.” Some of these cases
probably could be explained that way, but I would, as far as the debris
transport then goes, rather classify them as dry fallout cases that got
sprinkled down through a series of coincidences,
DANIELSEN: I would like to add a comment. I have studied the
trajectories associated with a cyclonic development over the central
United States in the spring of 1956. As the vortex moved northeastward over the Great Lakes, dry air entered the northeast section of
the vortex, Trajectory analyses indicated that air moving rapidly
eastward across central Canada in the upper troposphere subsided
rapidly, turning anticyclonically until at low elevations its momentum
was reversed, Although the dry air ascended again after entering the
vortex, its relative humidity remained low and precipitation was cut
off. Therefore in the northeast quadrant of the low, where maximum
precipitation might be expected, only very light rain or drizzle was
reported. This rain may have been loaded with radioactivity, however,
caused by evaporation during its fall through the dry air. Definitely,
the anticyclonic branch of the descending flow can link into another
cyclonic flow, and, in general, both must be considered.
REITER: There is still one rather interesting point which Mr.
List made to me which I hope he will repeat.
LIST: It concerns rainfall scavenging. Since it usually occurs in
the very lowest levels of the storm, why do we propose that towering
cumulus clouds will bring debris down from the stratosphere? I believe
the reason is that, although the efficiency of the entrainment mecha-
nism may be very much less in the stratosphere, the concentrations
of radioactivity are several orders of magnitude higher; therefore,
even if the entrainment mechanism is an order of magnitude less
efficient, considerable amounts of radioactivity from the large res-
ervOir in the stratosphere would be brought down.
While I have the floor, I want to make some philosophical comments, i.e., that in previous years at meetings like this we were
concerned with gross features of fallout and considered ourselves
successful if we could tell what the fallout was going to be, given the