-3Experience has shown that an atomic device ex- ploded on the surface distributes about 80 percent of its fission products on the ground within a few hundred miles of the burst point. A somewhat larger percentage takes. part in the close-in fallout from an underground burst, and a smaller percentage will be scavenged from a near-~ surface burst or tower shot. The tower shot is, in a sense, a special case of a surface burst, since the material of the tower it- self is mixed with the fission products in the fireball to a greater or lesser degree, depending on the yield. Experience with tower shots indicates that even in cases where the fireball does not touch the ground a few percent of the radioactive fission products come down as close-in fallout. The fraction which takes part in the close-in fallout from a surface burst over deep ocean water appears to be somewhere between 20 and 50 percent. This is less than the fraction of close-in fallout occurring from a corresponding surface burst over land, due to the evaporation of many of the drops before they reach the ground. Presumably this fraction is also affected by the prevailing humidity and temperature structure of the atmosphere through which the drops must fall. As the depth of the water is decreased, the point is reached where the fireball extends downward to the bottom and picks up bottom material. In such shallow water one would expect a higher percentage of close-in fallout than in deep water. Experience in the Pacific indicates that such is indeed the case, and that in fact there would be very little difference in the fallout between a large-yield device in very shallow water and a true surface shot.2/ The second type of fallout is the material which, though not coarse enough to fall of its own weight in the first few hours, is, nevertheless, left in the lower layer of the atmosphere, known as the troposphere, where ordinary weather phenomena occur. l/ "Close-In Fallout," W. W. Kellogg, R. R. Rapp, and S. M. Greenfield, P-822-AEC, March 12, 1956. (more) -/23