in the nearby fallout (within several hundred miles downwind) being partially depleted in strontium-90 while that at more distant areas will be enriched. | . The activity placed in the stratosphere circles and recircles of: — uf the earth, first at the same general latitude as the burst and then slowly spreading laterally. At the same time there will be a slow diffusion into the tropopause. Initially, there will be more deposition in the same hemisphere (northern or southern) in which the burst occurred but after many months the rate of deposition may become more generally uniform over the entire earth's surface. In terms of strontium-90 about 10 to 20 percent of the activity remaining in the stratosphere may descend each year. The distribution of the nearby fallout (up to several hundred miles downwind) from high yield weapons detonated near the earth's surface will be determined principally by particle size, initial position in the steam and cloud, and by the wind structure at various altitudes. The particle sizes and the distribution of these particles within the stem and cloud are principally functions of the yield of the bomb, the nature of the surface over which the burst occurs and the quantity of material vaporized. There are uncertainties in our knowledge but Figure 1 presents one generalized concept of such an initial distribution. Although the cloud may be 100 miles in diameter, the activity probably is not uniformly distributed, but rather is more concentrated near the central and lower portions of the cloud: