Cloud Tracking Tracking the debris with aircraft was the primary method planned to verify radioactive cloud travel in the predicted direction. of survey were used to determine the cloud location: (1) of WB-29s scheduled specifically for tracking purposes, Three types "Wilson" flights (2) inflight data from all aircraft flying between Bikini and Enewetak for the first 24 hours after detonation, and (3) radio intercept of reports from and be- tween cloud-sampler aircraft. The major areas of concern were (in order of importance): 1. Downwind,* especially Enewetak and Ujelang 2. Upwind 3. Upwind of the native-populated atolls to the southeast 4. Air and surface transportation routes through Wake and the Marshall Islands. Figures 21 and 22 show the planned flight paths of the Wilson cloudtracking aircraft. Clouds more than 24 hours old were considered suffi- ciently dissipated to be harmless. The 24-hour time period translated into a distance of approximately 500 nmi (926 km). Cloud Sampling Cloud-sampling operations were undertaken to obtain scientifically valuable data for the evaluation of nuclear explosions. These data were collected as gaseous fractions and samples of particulate material from the nuclear clouds. Operation IVY was a significant milestone in the use of manned aircraft for cloud-sampling operations. LASL scientists placed a heavy support requirement on the Air Force to sample the cloud produced by the world's first thermonuclear device, IVY-MIKE, and this requirement was carried over into the CASTLE series. Planning between the LASL and the Air Force Special Weapons Center (AFSWC) was undertaken in 1951. Several different types of aircraft, in- Cluding the B-36, B-47, B-45, F-89, and F-84 models were considered for * Refers to the low-altitude trade winds, 132 rather than winds aloft.