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