2. The stratospheric debris descends very slowly, unless, of course, it is so large as to fall in the first few hours. This paper is concerned only with the world-wide fallout, that is, the fallout which does not occur in the first few hours, and excludes the local fallout which constitutes the famous eliiptical pattern which is so hazardous becauseof its radiation intensity, but which in test operations is carefully restricted to test areas. It is worth mentioning in passing that the local fallout may be the principal hazard in the case of nuclear war. Most serious attention should be paid to it in civilian defense programs. The world-wide fallout from the stratosphere is literally world-wide in that the rate of descent of the tiny particles produced by the detonations is so small that something like ten years or somewhat less probably is the average time they spend before descending to the ground, corresponding to an average annual rate of about 10 per cent of the amountin the stratosphere at any given time. It is not clear as to just how they do finally descend. It seems prob- able that general mixing of the stratospheric air with the tropospheric air which occurs as the tropopause shifts with season and as is brought about by the jet streams constitutes the main mechanism, and that the descent of the stratospheric fallout is never mainly due to gravity; but rather the bulk mixing of stratospheric air with tropospheric air brings the radioactive fallout particles down from the stratosphere into the troposphere where tropospheric weatherfinally takes over. This mechanism makes the percentage fallout rate the samefor all particles too small to fall of their own weight, and the same as would be expected for gases providing some means of rapidly removing the gases from the troposphere exists, so the reverse process of troposphere to stratosphere transfer does not confuse the issue. 3. World-wide radioactive fallout in the troposphere is restricted to the general latitude of the detonations for the reason that the residence time in the troposphere is about thirty days.!?! The lifetime of fine particulates in the troposphere appears to be determined by the cleansing action of the water droplets in the clouds. For those particulates which are below one micron in diameter, Greenfield” calculates that the mean residence time of a one micron particle ina typical cloud of water droplets of 20 1 diameter may vary between 50 and 300 hr, but that a particle of 0.04 . diameter will last only 30 to 60 hr, and that a particle of 0.01 , diameter will last only 15 to 20 hr. The theory calculates the diffusion due to Brownian motion and says that it is just this motion induced bythe collisions with the air molecules which makes possible the contact between the fallout particles and the cloud drops. Since this theory is based on first principles with the single assumption that the fallout particle sticks to the water droplet on impact——an assumption so plausible as to be almost beyond doubt —it is no surprise to learn experimentally that the Greenfield theory appears to be correct. There is essentially no world-wide fallout in the absence of rainfall; i.e., in desert regions, except for a little that sticks to tree leaves, blades of grass, and general surfaces, by the same type of mechanism Greenfield describes in the case of clouds. Thus we seethat it is the moisture in the troposphere which assures the short lifetime of the world-wide fallout particles, and that when the stratospheric air which contains essentially no moisture and therefore has no cleansing mechanism descendsinto the troposphere, the tropospheric moisture proceeds to clean it up. On this model, we see that for submicron fallout particles, weather phenomena are controlling, and that the bombs which have insufficient energy to push their fireballs above the troposphere will have their world-wide fallout brought down in raindrops in a matter of about a month, in extreme contrast with the stratospheric material which apparently stays aloft for something like ten years on the average. The contrast between these two lifetimes means that the concentration of radioactive fallout in the stratospheric air in terms of equal densities of air is always much higher than in tropospheric air. This has been experimentally observed to be true.” In fact, the stratospheric content is about one hundred-fold higher than that of the troposphere corresponding to the much longer stratospheric residence time. Later in this paper new data on the fallout content of the stratosphere are given. It is inherent in the Greenfield mechanism that the total world-wide fallout will be propor- tional to rainfall if other factors are not allowed to vary. Thus we find that the Mediterranean basin’ affords a good example of the truth of this principle. Other regions are the Northeastern United States, the Southeastern United States, the Northwestern United States and the South- western United States.”* It is now well established that desert areas have verylittle fallout. 4. After falling to the ground in the form of rain or being picked up on the surface of the leaves of grass or trees by the same type of Brownian motion accretion mechanism causing 255