METEOROLOGY—FALLOUT AND WEATHERING By Lesrer Macnra and Kenneru M. Nacuer US. Weather Bureau, Washington, D.C. INTRODUCTION locus of points at which particles from a given Meteorology plays two roles in the study of the biological effects of nuclear radiations on on the chart. Further, from the same informstion, it is also possible to derive the locus on man. First, winds and rain govern the trans- port of the fission products to man’s environment. Second, after settling on the ground, the fallout particles can have their effects modified by rain washing and wind erosion. It is the purpose of this paper to discuss both roles. Research in the Weather Bureau has been devoted largely to the first problem, namely predicting the fallout. Accordingly, in the absence offirst-hand research, the discussion of weath- ering will be more goneral. TRANSPORT There are two aspects of the problem of pre- dicting dosages of radioactivity on the ground. In the first: place, the initial distribution of radioactivity in the stabilized atomic cloud on various particle sizes, at different altitudes must be given. Then, with this distribution as the altitude will fall. These are the radial lines the ground of particles of the same size (or, really, fall rate), also shown on the figure and labelled according to their diameters in microns. The heavy line shows the path that the 100micron particle takes in falling from 40,000 feet to the ground. The heavy dashed lines are isolines of observed radiation intensities, in milliroentgens per hour 12 hours after the burst. Although the actual procedure is more complicated because of the finite lateral width of the cloud, the theory of producing a model of cloud radioactivity can be illustrated from this figure. The procedure is that of associating the amountof radioactivity at a given range of cloud altitude and particle size with the corresponding radiation intensity on the ground. For example, the particles in the shaded area (those between 87 and 100 microns in diameter which were initially between 30,000 and 35,000 In theory, it might be possible to deduce the distribution of radioactive particle sizes and feet) have caused an average dose rate of about 50 mr/hr. It is to be noted that this mapping procedure bypasses the determination of the number of radioactive particles and their specific activities, In fact, since the radiation js necessaryto rely on the findings from previous ground with conventional hand radiationmeasuring instruments (or, less frequently, by starting point, the particles are tracked downward according to their settling velocity and horizontally according to the winds. their specific activities in the atomic cloud from the explosion kinetics, thermodynamics, and available scavenging agents, but in practice, it nuclear explosions. Figure 1 shows, in principle, how this is accomplished. From considerations of the settling speed of the particles and the winds, it is a straightforward process to obtain the intensity lines used in this type of analysis are obtained for Nevada tests by monitoring the aircraft surveillance), the effectof shielding due to rough terrain is already included in any forecast derived from such information. This technique of preparing forecasts of radiation intensities from cloud models is now used by 3