The time which the radioactive debris spends in the cloud depends most critically on one factor: the proximity of the explosion to the ground surface. The nature of the surface, whether it is soil or water, also plays a role. If the explosion has taken place right on the ground, on a soil surface, a lot of big, heavy dirt particles become incorporated into the fireball and begin to fall under the action of gravity even before the cloud stops rising. This fallout continues for a period of several hours to perhaps a half day. At the same time some of the radioactive fission products which have adhered to these dirt particles also fall out. This is the origin of the so-called close-in or local fallout, which extends for a distance downwind of the explosion of a few miles to a few hundred miles, according to the energy of the bomb and the strength of the winds. Approximately eighty per cent or so of all the fission products are accounted for by this close-in fallout in the case of a surface explosion. The shot on March 1, 1954 was of this variety. There are several possibilities for influencing the amount of close-in fallout. One is to explode the bomb over deep water. In this case the close-in fallout amounts to between thirty and fifty per cent. This is because many of the water drops to which radioactive particles have adhered evaporate before they hit the ground. Over shallow water, however, if the fireball actually touches the bottom, the close-in fallout resembles the case of a land explosion and is again about eighty per cent or so. The close-in fallout for underground or underwater explosions will be even higher than for the surface explosions. In fact a really deep underground or underwater explosion would be completely contained and no activity would be spread around. Another possibility for reducing the close-in fallout is to detonate the bomb on a tower so tall that the fireball cannot touch the surface. In this case the amount of close-in fallout is reduced from eighty per cent to approximately five per cent. Of course, it is not feasible to build towers for really big bombs whose fireballs may be a mile or so in diameter. In this case the bomb might be dropped from an airplane to produce the same effect. The Hiroshima explosion was an example of an air burst of a small bomb. The close-in fallout in that case was very small. Such radiation sickness as occurred there was due to the direct gamma rays and neutrons released in the explosion itself. In the case of a near-surface explosion, where the fireball almost touches the ground, the close-in fallout is also only about five per cent. This is a somewhat surprising fact since in this case photographs show large quantities of surface material being sucked up into the cloud, just as they are in a true surface explosion.