surface to the cloud’s maximum elevation, with a heterogeneous particle-size mixture existing throughout. The points of arrival of particle sizes from all elevations define the downwind pattern with respect to both area and time of arrival. 6.3 SECONDARY FALL-OUT The winds in the Marshall Islands area above 90,000 ft are predominantly from the west at the time of the year of Operation Ivy.? The cloud from Mike shot rose to a height greater than 100,000 ft and was observed to move to the east. The few winds above 90,000 ft observed during the operation by the task force Weather Central were from the west. The arrival time of the secondary fall-out can be satisfactorily explained by assuming that the particulate originated in the uppermost portion of the cloud, carried eastward by the stratospheric winds. Since the particulate settled into the troposphere somewhere east of the Marshall Islands area, an examination of the troposphere wind pattern during the days follow- ing the detonation showed that the particulate would be carried back westward and deposited as secondaryfall-out in the area investigated. 6.4 THE EFFECT OF VERTICAL MIXING The arrival time of small particulate at distances beyond the area of subsidence has defied explanation by particle settling rates. This failure is especially evident when considering arrival times of secondary fall-out, It is suggested that for particles whose diameter and density establish slow settling rates the effect of vertical mixing in the atmosphere becomes the primary mechanism determining their deposition. REFERENCES 1. Los Alamos Scientific Laboratory, “The Effects of Atomic Weapons,” Appendix F, U. 3, Government Printing Office, Washington, 1950. 2. Charles E, Adams, Fail-out Phenomenology, Greenhouse Report, Annex 6.4, WT-4, August 1951, 3, C. E. Palmer, The Central Pacific Project, First Report, Institute of Geophysics, October 1951. 53