radioactive particle, it is thus a function of the densities and diameters.
The second method of rain scavenging which accounts for a smaller
fraction of rain-out is the intermingling of an atomic cloud with a rain
cloud.
The rain cloud in this instance gathers up bomb debris ocr other
contaminated particles into small droplets, which are then collected by
large water drops and brought to earth as radioactive rain.
This method
of cleaning the air is most efficient in particle sizes of about 1
micron but is capable of scavenging any particles that are less than 2
microns in diameter.
The mechanism of particle movement down from the stratosphere and
into the troposphere is. not understood.
In fact, it has not been
definitely ascertained whether any of the small size stratospheric
bomb debris has come to the ground.
Such data, when available, would
yield important information concerning the exchange of matter between
the stratosphere and the troposphere.
drawn:
One tentative conclusion can be
if particles are so light as to remain in the stratosphere for
long periods of time, there is an increased chance of the harmful isotopes being dispersed; hence the reconcentration of these isotopes
would be unlikely.
It may also be concluded that particles small
enough to remain aloft for sufficient time to become a possible longrange threat are too small to be scavenged efficiently by rain when
they reach rain-bearing levels.
During thunderstorm formation, the rain cloud is sometimes swept
up to the tropopause where the cooled water particles form ice crystals
about any small debris or dust that might be in the air.
Because the
lower atmosphere air is drawn up into the cloud the ice crystals descend and ascend alternately wtil they become hailstones, or are
captured by raindrops, and fall to the ground during a thunderstorm or
hail storm.
Measurements of radioactivity in large hailstones from a
hail storm at Washington, p.c.,2/ following Shot 10 of UPSHOT-KNOTHOLE
List, R.J., The Transport of Atomic Debris from OperationUPSHOT-
KNOTHOLE, U.S. Weather Bureau Report, NYO-4602, June 1954, SECRET
Restricted Data.
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