particle. These studies show that regardless of the method of sampling or the manner of handling the data, approximately 90% of the radioactivity incorporated in particles is carried by particles larger than 20 microns. A study conducted at UPSHOT-KNOTHOLEY indicated that the activity per active particle was approximately a function of particle volume for particles less than 150 microns in diameter and of surface area for particles lerger than 150 microns. C. The Radioactive Cloud. The release of large amounts of thermal energy that follows the detonation of an atomic device carries the fission product radio- activity by convection high into the atmosphere. The yield of the bomb, the behavior of the fireball in the first few seconds, and the stability of the atmosphere all influence the rise of the cloud and, conversely, the return of the radioactivity to earth. When an atomic explosion takes place over land, the energy yield is the most important single factor in determining the ultimate cloud height, while the amount of particulate matter from the ground which is contaminated by the burst and carried aloft is determined by the area of the ground contacted by the fireball. For a contact surface burst, the fireball is roughly hemispherical in shape, and grows to its maximum diameter during the “hover time”. The hover time is that interval during which the buoyant forces resulting from the heat of the fireball act to accelerate the fireball and cloud upward. These buoyant forces are essentially independent of the radial forces. After the brief hover period, the fireball leaves the ground and starts to ascend at about five times the acceleration of gravity. After several seconds during which the fireball is rising, the typical mushroom cap and stem are formed. There is a great deal of toroidal motion both in the cap and the stem acting independent of the upward motion, but concurrent with it as shown in Figure 2. At this 77 Rainey, C.T., Neel, J.W., Mork,H.N., Larson, K.H., Distribution and Characteristics of Fall-out at Distances Greater than Ten Miles from Ground Zero, March and April 1953, WI-811, SECRET Restricted Data. 16