0018933 of cold air and loss of energy in overcoming atmospheric drag (References 14, 17, 18). The diameter of the ring increases rapidly during the ascent and the cloud spreads out laterally to a large area as its upward velocity decreases. For smaller yields the cloud stops at the tropopause or be- low, but for megaton-range weapons the top may penetrate several thousand feet into the stratosphere. The time to maximum altitude is some- what less than ten minutes, A knowledge of the distribution of activity and particles within the stabilized cloud is needed for the establishment of a rational fallout model; however, the collection of a suitable set of samples which could ” be used to determine these quantities experimentally presents a formidable operational problem which has not yet been solved, Several distri- butions have been assumed in an effort to match the fallout patterns on the ground, but it is not known how closely these models correspond to the actual structure of the cloud, Considering the method of formation, it might perhaps be anticipated that the activity would be greatest in am anchor ring centered on the axis of the cloud. Some evidence for this structure was obtained at Operation REDWING with rockets with telemetering ionization chambers (Reference 19). 1.2.3 Transport and Distribution, During the ascent of the nuclear cloud the particles present are acted on by body forces and by the vertical SAN BRUNO FRC 20