of such large particles (70 to 150 microns), it is possible to prepare
a simple wind vector plot indicating where the fall-out will touch the
ground from a given point in the atomic clouwi.
These wind plots have
been used previously under various names and have been described in
great detail (3, 4, 5, 6, 7), therefore no attempt will be made to
describe the method in this report.
However, such vector wind plots
have been used extensively by this writer to obtain a lot of indirect
_dnformation,
There is some indication that soil particle size decreases
with altitude in the cloud.
It should be clearly understood that the
particle sizes indicated above refer to the median soil particle diam
eter, and that the soil particle size spectrum is wide,
The fall-out
at a given spot may have comefrom different levels of the cloud, thus
further increasing the spread of the size spectrum,
The density of
particles at NPG average around 2.5 gn/cm’, but certainly not all the
particles would have the same density oor are they all spherical in
shape and this also increases the particle size distribution.
Strangely
enough, during the domestic test operations it was observed that many
particles in the size range of only several microns in diameter fell cut
within a few hours after bomb burst.
According to Stokes’ Law (even
when corrected for the Cunningham slip factor and for the variatien of
air viscosity with temperature) it would take a 5 micron particle several
months to reach the ground from 40,000 “.
The explanation is toe be
found in the fact that a large: quantity of soil is sucked up into the
cloud and as this soil subsequently falls back to the ground, it entrains and traps a lot of air and a lot of small sized primary fission
leben te ntl
UNCLASSIFIED
13
et