- | OEER TReS TR — easece _—_—_— — 0018933 — - currents in the rising air, Some of the large particles will be heavy exough s0 thst they will have a net downward velocity even though the cloud 2s a whole iz moving upward, They will coztribute to the falicut in the immedi- — /During this time, volatile fissicn produc ate vicinity of ground zero (Reference 20),may be fractiorzted from lees volatile fission products by a kind of "fractional distillation" process within the hot cloud Once the upward motion kuz ceaged, the particles in the cloud will begin to settle cut at rates determined by their denaity, dimensions and shapes and by the viscosity and density of the 2ir (Reference Zl), The terminzl velocities for small spheres can be accurately calculated when the dependence of the drag coefficient cn Reyneid's nuinber is known, Ir- regular or angular particles will fell more slowly then spheres of the came weight, but their velocities camnct be estimzted as well due to uncertzinty in the shape factors (Reference 22), The particles which mzke up the lec] fallout fsllow traiccterzits to the surface governed by their fall rates aad by the mean wind vector between their points of origin in the cloud and the greund level. Locaticns can be specified by reference to 2 surface coordinate system mide up of height lines and size lines, The height lines are the losi ef the points of erri- val of all particles originating at given heights on the axis of the cloud, The size lines connect the arrivel points of particles of the same size from different 2ltitudes, Time and spice varizticn cf the winds will change the magnitude znd direction of the mein wind vector and verticel rncticns