0018933 currents in the rising air. Someof the large particles will be heavy enough so that they will have a net downward velocity even though the cloud as a whole is moving upward, They will contribute to the fallout in the immedi- ate vicinity of ground zero (Reference 20), Once the upward motion has ceased, the particles in the cloud will begin to settle out at rates determined by their density, dimensions and shapes and by the viscosity and density of the air (Reference 21'}, The terminal velocities for small spheres can be accurately calculated when the dependence of the drag coefficient om Reynold's number is known, Ir- regular or angular particles will fall more slowly than spheres of the same weight, but their velocities cannot be estimated as well due to uncertainty in the shape factors (Reference 22), The particles which make up the local fallout follow trajectories to the surface governed by their fail rates and by the mean wind vector between their points of origin in the cloud and the ground level. Locations can be specified by reference to a surface coordinate system made up of height lines and size lines, The height lines are the loci of the points of arri- val of all particles originating at givem heights on the axis of the cloud, The sizelines connect the arrival points of particles of the same size from different altitudes, Time and space variation of the winds will change n wind vector, and vertical motions the magnitude and direction of the mea SAN BRONO ERE 2.\