(1) The characteristics of clouds from nuclear explosions which would afford an estimate of the sampling altitude required and the gamma radiation intensity within the cloud as function of time after burst. (2) A quantitative estimate of the radiation exposure required in the cloud for the aircraft to collect the amount of material desired, (3) A quantitative estimate of the additional radiation exposure to be received by the pilot on return to base, These factors were discussed and related to the planning of a sampling mission, 5 Characteristics of Clouds x From characteristics of bomb clouds, such as the height to which the primary material rose and the level and persistence of gamma radiation, personnel evaluated sampling capabilityin terms of the altitudecapability of the aircraft available, the degree of risk to overexposure to radiation and the flight time in the cloud required to collect the sample, The sampling altitudes required were specified by the altitude interval in ' which the primary cloud was stabilized and, in general, comprised altitudes within this interval for which dispersal of the cloud by diffusion and wind shear was low, Fortunately, under visual flight conditions samnling participants-easily recognized the primary bomb material by its reddish brown color in transmitted light arising largely from the presence of nitrogen dioxide, sometimes augmented by oxides of iron. In addition to these substances, the cloud was composed of condensed oxides of the fission fn CO 1 . AFWL/HO SWEH-2-003h a 201