Thomas N. White Loe -lamos Seientifie Laboratory July 195 1. The method of caloula the hasty cutgrowth of a more — loeal fallecut, as described here, is ex method that had been unexrectadly- successful in accounting for the Mu.fO fall-out pattern in the ilinzinse. nongelap-icngerik ares. «8 the tine of the last shot (on Eniwetok ..tol1) approached, the problem of foreeastine loeal fall-out became more acute. Sinso the method attempted to take account of the initial sise end share of the cloud, it seemed that it should be suitable for local forecasting. With the aid of Dr. Gaelen Felt, the method was simplified to the extent that am atoll pattern could be estimated within about an hour. The simplified method was tested egainst the Bikini satterns produsad by ROMEO, UNION and Y/NKER ani found satisfactory, and the moticd was used in forecasting for NECTAR. 2. The follewlng descripticn covers the simplified method only. The more complex method wrrante further study which will be reported olsc~ where. ° 3. aseumptions: (a) The initdal cloud (efter risc is. practically scamrlotad) is divided into horisontal slices, cach of 10,000 f% dopth, with cmtors at 10,000, 20,000, = - = = 70,000 f& altitude, with tho contral concontration (radlowactivity per unit volume) independant of altitude. (bd) In cach layer all of the astivity lies in a horiscntal Plane thru the canter. (0) In each layer, the concantration falls off latorally according to the law of normal distribution of errors ¢ (7) #Coe Tae & . wnere 6, is the initial contra] concentration, r is distanco from cuntar, and For fs the initial spread parametor (analogous to standard deviation). altitudes 10,000 thru 40,000 ft, a, 21.9 miles; 50,000 thru 70,000 ft, & 3 5.8 niles. REL De18