characterized by a temperature inversion. Particulate matter on reach- ing this layer requires more energy to rise further than has been necessary in its previous rise, and can no longer rise on the basis of the negative temperature gradient prevailing up to that layer. If it is assumed that the cloud mushroom contains in excess of 90% of the par- ticles and 90% of fission activity uniformly distributed, then from the available data on world-wide detonstions, 36,000 KT has been deposited above the tropopause, while approximately 4,500 KT was deposited below it as the contribution of the 10% of the radioactivity remaining in the stem, and the total activity of stem and mushroom. This takes into consideration the place and date of the known detonations, as well as their fission yields. Since these values are for the clouds at their time of stabilization, they include all the particles that would be found in local fall-out, 1.e., those with sufficient weight to be returned to earth within a 50 r infinity dose contour line, plus those which are small enough to be carried outside this contour. With the stabilized cloud in the air, only two sets of forces can act upon it: (1) atmospheric forces and (2) gravitational forces. The influence of these can be shown by the vector arrows in Figure 4. Tropopause Atmospheric Forces Wind Wind shear Eddy Moisture Gravitational Forces Particle size Particle shape Mush-j Free Fall 90% room Impaction Rain T T bo | Stem L 10% — Dose Rate ‘ j Surface of "Earth Figure 4. The Atomic Cloud Model and the Forces Exerted Upon It. 19