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trajectory endpoint under these assumptions was found to be over eastern Kansas at 24 hours,
and over northeast North Carolina at 48 hours. Around each of these mean endpointsa circle
can be drawn inside of which half the actual endpoints in a large number of tests would be
expected to fall. The radii of the 24-hour and 48-hourcircles are approximately 800 and 1,300
miles respectively. Insofar as cloud location data from airplane tracking are available, they are
consistent with this meteorological prediction. From these results the probability of fallout from
Nevada tests appears to be substantially the same at all points in the eastern United States.
The debris in a single mushroom is progressively diluted, chiefly through the effects of wind
velocity shear with altitude. Statistical calculations have been performed on the basis of meteorological records to gain a quantitive understanding of this mode of dispersal as pertinent to
intervals of 12 and 24 hours after Nevada shots in the spring season. Among other more detailed
conclusions, the results show that after 12 hours the 200 mb (c.40,000 ft.) endpoint is found,
on the average, 120 miles east of the 300 mb endpoint; after 24 hours, 175 miles southeast. If
the debris cloud is initially a sphere tangent at top and bottom to the 200 mb and 300 mbplanes,
respectively, it will be smeared out by wind velocity shear into a 120-mile ribbon at 12 hours, and
a 175-mile ribbon at 24 hours. Additional dispersal of the debris is caused by gravity, diffusion,
and turbulence.
3. Effect of rain on fallout:
Calculations and experimental observations have been performed to determine the influence
of rain on fallout. Calculations, however, depend on so many parameters which are inaccurately known that they can probably be relied upon only for qualitative conclusions.
Some of the conclusions enumerated in a study by the RAND Corporation on the basis
of both theory and experiments, are these: (1) The most important mode of action of rain is
mechanical scavenging of debris by falling raindrops; in comparison, mechanical scavenging
by cloud droplets, rain induced down drafts, and electrostatic scavenging by raindrops or cloud
droplets are small effects. (2) Fraction of activity removed from air by rain falling through
it is dependent mainly on total rainfall, not intensity of rainfall. (3) To the extent that quantitative predictions are reliable, half the activity in air through which rain falls would be removed
by about 0.04 inches of rain. (4) Mushrooms from bursts of greater than 6 to 8 KT would be
altogether above rain bearing layer (maximum altitude about 20,000 feet) except in the case
of thunderstorms (sometimes extend to 50,000 feet).
Experimental comparisons between rainout and dry fallout may be rather inaccurate because
of differences in collector efficiency for the two conditions. In analyzing a set of the New York
laboratory’s gummed paper fallout collections, the activity of papers exposed during rainy days
was compared with those exposed on dry days. Papers exposed during rain were on the average
2.5 times more active than the dry collections. The efficiency of the gummed paperfor rainfall
collections has been estimated in simulated rainfall experiments to be 80%, 50%, and 20% for
particles of 2,1, and 0.4 micron diameter, respectively, but average efficiency for field conditions
is unknown. Since rain occurs during only a small fraction of any specified period at any one
place, it may be that cumulative dry fallout is greater than cumulative rainout.
No experimental observations on rainout close in to a shot are available since tests are
conducted only when local weatheris dry.
Rain has been analyzed for its Sr-90 content by both Libby ® and Kulp.* Libby’s series in-
cluded 20 samples from the Chicago area collected between November 1952 and October 1953, and
Kulp’s included 14 samples from N. Y. between January 11 and April 1, 1954. Average activity
except during test series was about 4 dpm Sr-90/gal, but rose both in June 1953 and April 1954 to
peaks of approximately 100 dpm Sr-90/gal.
Footnote references on pp. 42-43.
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