Local Fallout
Iocal fallout is characterized by the fact that the deposition
is largely by the settling of the particles dus to their weight.
Given the size, shape, and specific gravity of the particles, it
is possible to predict their fall velocity.
'
However, even in the absence of such exact inforration, it is
possible to produce useful results as to where and when particles
will reach the ground.
This is Because the relative tine of fall
as a finction of ThefeedAgia
tcentltive to the particle's physical
properties, and pecause-tt-4e-the -reletive—time-off92)-sithheight
whichtigns—out-to-beimportants.
There are at least two other processes which produce? a downward
flux of particles in the atmosphere.
First, vertic31 mixing acting
upon & clowl of particles at upper altitudes will transfer some of
thess to lower altitudes,
Although we cannot be sure, the magnitude
of usual tropospheric vertical eddy speeds is of the order of a few
tenths to a few miles per hour.
It is likely, therefore, that
particles which have settling speeds of a tenth to a hundreth of
the eddy velocity will hsve their vertical component of moti
controlled mainly by atmospheric turbulence while those with fall.
velocities equal to or greater than a few miles per hour will descend
mainly because of gravitational settling. The point of division for
a spherical particle of specific gravity of 2.5 is betweeh 50 and 80
riers in dianeter. Host of the sizes of particles fn local fallout
exceed this dianster.
A second msans of domnward transfer of the particles occurs by
precipitation.
Insofar as possible, nuclear tests by the United
States are conducted in the absence of naturally-occurring precipitation.
However, in the Pacific Proving Grounds, it is likely that the
huge amount of moisture entrained by the rising firsba3ll condensas
upon cooling and that sone dowmsard transfer of tha particles
results from scavenging of falling precipitation.
Department of Energy
Historian’s Office .
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