For surface detonations, the cloud is generally considered to have about 90% of the activity in the mushroom and 10% in the stem. How- ever, some U.S. investigators estimate a greater proportion of the activity in the stem for surface bursts. The British have estimated 70% in the mushroom and 30% in the stem for their tower shots. This question is presently unresolved; however, the REDWING test series in the spring of 1956 is designed to provide definitive answers. Particles less than 10 microns in diameter are not usually considered of importance in close-in fall-out patterns, since their slow rate of fall would not bring these particles down from the mushroom soon enough to contribute any appreciable activity in the immediate vicinity of the burst point. Instead, such particles are carried by the winds and deposited beyond the local contour system. Once a shape for a cloud model has been agreed upon, then it is reasonable to assume that there is an even distribution of activity, but not necessarily of fission product .~composition, throughout the cloud. The various horizontal increments of cloud layers or segments will each contain particles of all sizes, but the increments at lower altitudes will contain a higher proportion of heavy particles. This permits the assumption to be made that the “hot spot” ordinarily found close to ground zero results largely from fall-out from the stem or the bottom increment of the mushroom, where heavy particles tend to predominate. An atomic cloud with the same same general characteristics described in this section forms as a consequence of all ground surface detonations. A high air burst results in a similar mushroom, but the stem is minimal and the large dust cloud that follows the atomic cloud from ground surface bursts is likely to be absent, or if present, strug out as a long ribbon and much reduced in dust content. For an underwater burst in shallow water, a water colum is formed in a manner similar to the dust column formed by an underground burst. The height to which the water coliwm rises increases with the energy of the explosion, and decreases with increasing depth of deton20