the cloud before transport by winds)
and the diffusion rate off the cloud
as it was moved by the winds.*
The primary fallout plot technique initially used on CASTLE was a
joint analysis of the hodograph and the 72-hour airborne parti@le trajec-
tory forecast to define the fallout area for the first l2-hourfperiod and
assess the orientation and areal extent of the cloud after theffirst 12
hours.
tempted;
For BRAVO, the method of elliptical approximations was[first atits use was extremely limited, however, because of undertainties
in its application to high-yield events.
The BRAVO data were
Used to re-
fine this method for subsequent events.
The method of elliptical approximations drew ellipses over [the hodograph between wind levels.
The minor axes of the ellipses werq determined
by the amount of change in wind direction between wind layers.
The radia-
tion intensity associated with the ellipses was based on scali
and past
experience.
the Ne-
The method was based largely on empirical data fr
vada Proving Ground (NPG), and it had been used successfully td
predict
the fallout fields from low-yield PPG detonations.
As the tests began, it became apparent that a more accurat
method was
The la
e differ-
ences between NPG and PPG shot yields and cloud heights made t
accuracy
needed to clearly define the fallout from high yields.
of using elliptical approximations for CASTLE shots suspect.
ere was no
assurance that the high-yield fallout mechanisms were described
at all.
For example, the trapping characteristics of the atmospheric
ndary with
the stratosphere
(the tropopause) could only be conjectured, a
even if
* Because little was known about the fallout mechanism of high-pield events
and because of errors in other parameters of the fallout fore
additional safety factor was assumed necessary, even though t
the factor could not be precisely stated. For CASTLE, the es
the diffusion-caused widening of the fallout-contaminated are
with dis-
tance was increased by the addition of a 15° sector on each s de of the
calculated fallout area.
{Reference 16, p. 61).
In previous operations the factor u ed was 10°
lil