is reduced) the soil in the stem becomes tore active, thus producing heavy contamination immediately downwind. The total percentage fall- out increases with yield (when height is constant), but the percentage fall-out from the mushroom decreases with increasing yield. To a per=-; son who has not analyzed the total fall-out picture and who only chooses to utilize ground readings, the fall-out problem must appear even more complex than it really is. As a matter of fact, recently a set of empirical relations has been develeped on fall-out from tower shots utilizing only the ground readings from J/'K Test Operation. The air readings were not utilized out of impatience or lack of knowledge on how to use them. The T/S Test Cperation data were not used because they were more difficult to reduce, since most of the fall-out during T/S Test Cperations fortunately occurred North and Northeast of the Test Site where there are very few good roads and very little popula- tion. Sure enough a set of relations were developed which indicated intensity of fall-out to be independent of yield. Here is a good example of the need to evaluate all of the data before empirical relations are developed. B. Construction of the Forecast Fal.-out l. Particle Size Assume that the particle size distribution within a nominal bomb expucded at 300 ft is 100 microns if the maximum cloud height does not reach beyond 35,CCO ft msl. The maximum cloud height is a function of the yield, the height of the tropopause, the lapse rate of the atmosphere and the speed of the horizontal winds. Seite tee Lee 5001299 C3-3641], wee UNCLASSIFIED 39 _ A nominal bomb cloud will =sabggeiip .