DISCUSSION OF METEROLOGICAL FACTORS AND FALLOUT DISTRIBUTION “* Lester Machta Weather Bureau, U. S. Department of Commerce, Washington, D.C. 1 INTRODUCTION It is typical of nuclear tests that the radioactivity of the fission products has been released to the atmosphere. The deposition of these fission products on the earth’s surface is loosely termed “fallout.” The total quantity of fallout depends primarily on the total fission yield of the nuclear device, but the area in which the deposition occurs depends on a number of fea- tures, such as the atmospheric winds, the yield of the bomb, the terrain, and aititude of the explosion. It is the purpose of this discussion to review the atmospheric processes that transport the radioactive debris back to the ground. Fallout is assigned to three classes:! first, local or close-in, which is deposited within the first 24 hr after the detonation; second, intermediate or tropospheric, which is deposited largely within the first 30 to 60 days; and finally, delayed or stratospheric, which can take many years to be deposited. 2 LOCAL FALLOUT The main feature that distinguishes local fallout from other categories is its appreciable settling speed. The particles are large and heavy enoughto fall through the air. As the particles settle, they are transported by the winds. Particles originating at different altitudes are acted upon by differing winds, causing fallout in different areas. If the winds blow in approximately the same direction at all altitudes, as frequently occurs, the pattern is long and narrow. This gives rise to the familiar cigar-shaped pattern, with the larger particles, or those originating at lower levels, falling closer to the burst point. If, on the other hand, there is appreciable change of the wind direction with altitude, then the patterns may be very broad and may show no Similarity to a cigar.” If the winds are extremely light, the particles will settle back to earth close to ground zero and will make for very intense nearby radioactive areas. If the speeds are comparatively strong, the same particles will be carried to greater distances and will become diluted by being spread over larger areas with lower radiation intensities. Further, from day to day one finds that the wind direction changes, varying the general direction of the fallout area. The meteorological principles governing the prediction of local fallout are well known. 2 Although there is considerable uncertainty in the prediction of the winds, this is not the only uncertainty in predicting dosages on the ground. One must also associate a known amount of radioactivity with each particle size at every altitude in the nuclear cloud. It is impossible to obtain this radiological data from first principles based on the thermodynamics of the fireball and the chemical and physical properties of the entrained debris. Instead, one uses observed *Paper presented at Symposium on Low-Level Irradiation, American Association for the Advance- ment of Science, Indianapolis, Indiana, Dec. 30, 1957. 316 ”