‘the IVY series and the collections of the UPSHOT-KNOTIOLE series ‘ls that the IVY samples were largely of material that stabilized in the stem of the cloud while many of the samples of the domestic test contained particles from the mushroom, therefore, the different distribution of activity and particles with height may be significant in the subsequent ground deposition. The complex of unknown variables appears to make further speculation unjustified at this time, The prodominsn® factors nay eventually be determined, but must “await a complete analysis of the data. il 6.2 INCREASED RADIOACTIVE DEPOSITION IN RAIN ‘Early in the fallout monitoring program of the Atomic Energy Commission it became evident that surface deposition was enhanced - by precipitation. This section summarizes some results of studies . of radioactivity deposited inrain. ' A study was made for the Buster-Jangle tests (2) to determine the increase of radioactivity collected on trays during periods of rain compared to collections made during no rain. The results indicated about a tenfold increase of activity in rain. No gummed paper comparisons were made. A similar comparison was made with ' the gummed paper samples collected at a group of stations in the United States east of the Mississippi River, during the Tumbler‘Snapper series (10). A 30-day period was chosen to include days when debris from one or more tests of the series was airborne over the eastern part of the United States. An increase of a factor of - . 8.3 was found when the average activity of collections in rain was compared with activity in no rain (trace of rain was excluded). similar study was made for the entire United States for the IVY A. tests using a 15-day period when MIKE debris was airborne over the United States (13 to 26 November 1952), and the average increase of activity in rain was by a factor of 3k. These results suggest that the debris from domestic tests may consist of large particles that are mre efficiently scavenged by rain, while only the smaller particles are airborne by the time the debris of the Pacific tests reached the United States - the smaller increase in rain reflecting the smaller collection efficiency for the small particles. If that were true, one would expect rain in the North Pacific area to bring about an order of - Magnitude increase of surface deposition. Unfortmately, this figure is not available because the averages are unduly biased by a.few very high. collections made within a few days Following the ; IVY tests.