particles at this stage that have not already fallen out are so small as to respond less to gravity than to vertical wind components. Air- eraft monitoring has shown the activity at this time to be scattered into wispy patches. The physical process of diffusion involved in the movement and growth of a cloud results to a large extent from atmospheric turbulence. While little is actually kmown about diffusion, it results in the spread of clouds of finely divided particulate matter in all directions. process is not isotropic. The Diffusion is thus probably a part of the process by which isolated activity comes to the ground in areas remote from the primary cloud track. After two or three times around the earth, the cloud is diffused to the extent that activity may be collected at many different altitudes. However, airplanes equipped with sampling devices have proved non-uniform distribution of radioactivity by being unable to collect fission product activity on every flight. As the radioactive cloud passes over the earth's surface in this diffused form, portions of its activity continue to be deposited on the ground over periods of weeks and months and perhaps over years. This accounts for the world-wide distribution shown by collection of minute amounts of radioactivity at very remote sampling stations around the globe. Cloud trajectories have been plotted showing cloud paths from various nuclear detonations. These have been checked for travel more than half way around the wor1a.&/ The results of aircraft monitoring flights vectored to intersect the cloud met with sufficient success to prove that a nuclear detonation cloud has finite boundaries, radiological if not visual, as mich as thousands of miles from the point of origin. World-wide distribution of bomb contamination has been proven. The key to this distribution lies in the action of world meteorological conditions on diffused and probably dispersed bomb clouds which may B/ USAF Report of Operation Fitzwilliam (Secret), Nuclear Detonation by Airborne Filters, Vol II, SECRET, Restricted Data. 25 —-