diameter and that from 50% to 90% of the fission product activity is carried by particles of this size range. The influence of atmospheric density can also be noted in the slopes of the curves of Figure 5 in that there is a slower rate of fall as the particle approaches the surface. Spherical particles under 100 microns in diameter thus fall in a manner described by Stokes' Law" with the terminal velocity of Hae Navin those particles in the upper cloud being twice the terminal velocity of free fall in the lower cloud from yields greater than 1 MI. For spheri- cal particles larger than 100 microns, as well as for irregulerlyshaped particles of all sizes, free fall is better fitted to a fall described aerodynamically rather than by Stokes' Law. During fall . through any one segment or layer of a cloud, the time of fall will be Peery the sum of the time for the loss in height, "gravity", and change in horizontal position, "vertical wind effect". Assuming wiiform distri- bution of the particle size spectrum and of chemical composition throughout the cloud, a projected fall-out field can be made by plotting the time required to travel from the initial placement of certain categories of particles in each cloud segment, to the growd. The activity deposited will vary, owing to the decay of activity in particles composed of very short half-lived elements, individual particle response to wind shear and eddy currents, and redistribution after deposition because of wind and water erosion. After stabilization, which occurs about 5 to 7 minutes after deton- ation, the atomic cloud is moved downwind from ground zero. The mush- room having cooled to a temperature about equal to that of the surround- ing atmosphere no longer rises, but does expand to a greater diameter. This lateral expansion results from the expenditure of the kinetic energy acquired during the turbulent rise of the fireball, and from diffusion effects. As this lateral growth proceeds, the cloud becomes jess sharply defined, and within a matter of hours, except under unusual atmospheric conditions, may no longer be identified visually. The * Stokee'Law: A mathematical treatment of small spherical particles falling through air in which the viscosity of the air opposes the pull of gravity. \ 2