About 2.85 seconds after the first visible effects on the surface, a condensation clouz appeared directly above the burst as a result of the passage of the air shock wave. A second, less clearly defined condensation cloud appeared about 8.5 seconds after SZT. The spray domes were broken up by large irregular plumes at approximately 10 seconds after SZT. The first plumes were predominantly vertical and denser than the spray in the domes. These plumes reached a maximum height of 1,450 feet and an aggregate diameter of 2,000 feet at 19 seconds. A second group of plumes, appearing at lower levels, developed at approximately 16 seconds. The entire plume formation fell back to the surface covering an area 3,100 feet in diameter at 25 seconds. A third plume formation became visible at 35 seconds, reaching a height of about 770 feet. The plumes broke up into a fine spray, spreading out radially to form a base surge that was first evident at 13 seconds. The development of the base surge from both the primary and secondary plumes was continuous. The base surge was clearly identifiable at approximately 26 seconds, attaining a maximum height of 1,900 feet at 4 minutes. The base surge moved downwind as a low-lying cloud and remained visible for about 23 minutes.” A white circular patch of foam was observed about surface zero after the base surge had thinned sufficiently to make it visible. At 1.5 minutes, the patch’s diameter was 6,300 feet: it continued to expand to a diameter of 10,400 feet at 13 minutes, at which time it became dif- ficult to distinguish. The YAG-39, which was steaming at approximately 10 knots, encountered a radioactive cloud about 13 minutes after zero time at approximately 28,000 feet downwind (bearing 200° T from surface zero), Gamma intensity measurements of greater than 400 r/hr were recorded during the period from 16 to 19 minutes. By 22 minutes, the radiaticn field had dropped off to a level of 1 r/hr. Assuming that the observed decay exponent (—1.5) is valid for early times, the radiation intensity was estimated to be about 10,000 r/hr at 2 minutes after SZT. The overall dose accumulated topside on the YAG-39 with the washdown system in operation was about 30 r. Phosphate glass dosimeters placed tn the vicinity of YC-473 (shot barge) recorded total gamma doses of approximately 3,600 r. 415