CHAPTER6 METEOROLOGICAL CONSIDERATIONS AND FORECASTS OF FALL-OUT Knowledge of the mechanism of the fall-out phenomenonis necessary as a first step in the development of forecasting techniques that will satisfactorily define the gamma field created by the residual radioactive debris from a nuclear detonation, Fall-out gamma fields of military significance are known to develop with surface and underground or underwater nuclear explosions, and the problem of fixing both the location and extent of the resultant radiation field is paramount for either offensive or defensive operations. Solution of this problem requires knowledge of the shot location, an estimate of the resulting cloud height, and the wind speed and direction to an elevation equal to the height of the explosion cloud. 6.1 THEORIES OF FALL-OUT MECHANISM J. O. Hirschfelder’s analysis’ satisfactorily explains the mechanism of fall-out, except for the area immediately surrounding ground zero at Operation Ivy. The theory developed by Charles E. Adams’ accounts for the phenomenology of the fall-out in the area in the immediate vicinity of ground zero. It is believed that these theories in their respective areas accounted for the fall-out phenomena accurately at Operation Ivy. 6.2 PRIMARY FALL-OUT No data were collected downwind from ground zero, fall-out area as defined by the Hirschfelder analysis. Figure 6.1 represents the downwind The cross-wind data showing the arrival time to be independent of distance can be satisfactorily explained by the vertical-circulation theory as explained by Adamsin the Greenhouse fall-out studies. If a cloud chimney 5 miles in diameter is assumed to contain rising air currents, there is reason to believe descending currents exist around this upward con- vection column out to a distance equal to several column diameters, This vertical circulation is analogous to the circulation around a thunderstorm. A subsidence of this type would deposit particulate of heterogeneous mixture out to approximately 15 miles, and the time of deposition would be independentof distance. Therefore the primaryfall-out pattern is believed to have developed by two separate and distinct mechanisms: first, a subsidence extending out to several cloud diameters and, second, a downwind pattern determined by particle settling rates and the wind profile. This downwind pattern is based on the assumption that the particulate source is the cloud chimney from the 51