Page:Operation Crossroads 1946.pdf/64

 WEATHER PREDICTION

Accurate weather predictions at least 24 hours in advance were needed to allow the task force to complete the complex final preparations for a detonation and to give reasonable assurance that radiological safety could be maintained. Cloud cover had to be at a minimum for the ABLE airdrop to allow the bombardier to see the target ship. Wind direction, not only near the surface but up to 60,000 feet (18.3 km), had to be such that it would not carry fallout over the task force. Moreover, wind direction had to be fairly steady so that fallout areas would be predictable. Tropical meteorology was not well developed at that time, and detailed data of past weather patterns at Bikini were lacking. The exacting forecasting requirements for CROSSROADS posed a major challenge.

The official forecast issued the day before a planned detonation and used as a major element in the decision to proceed included: the amount, in tenths of sky coverage, of low, middle, and high clouds; the altitude of the base and top of the low clouds and the altitude of other cloud layers; precipitation (if expected); the wind direction and velocity in 5,000-foot (1.5-km) increments from the surface to 60,000 feet (18.3 km); height of the tropopause; and visibility, temperature, and relative humidity (Reference C.9.207, p. VII-(O)-17).

Responsibility for furnishing weather forecasts or weather advice for task force operations was vested in the Staff Aerological Unit located on Mount McKinley. The unit was to prepare special forecasts for the Radiological Safety Unit to help anticipate movement of the radioactive cloud (Reference B.0.1, p. T-2). Because of lack of space on Mount McKinley, a significant portion of the personnel doing weather data analysis was stationed on Kwajalein at the Weather Central. To supply upper air and surface data, aerological units of from four to six personnel were stationed on USS Shangri-La (CV-38), USS Saidor (CVE-117), USS Fall River (CA-131), and USS Albemarle (AV-5). These personnel also provided weather briefings to task group commanders and aircrews. To gather surface data, one-man aerological units were stationed aboard USS Orca (AVP-49), Bowditch, USS Kenneth Whiting (AV-14), USS Blue Ridge (AGC-2), and USS Appalachian (AGC-1). Weather Central received reports daily or more often from weather stations on Wake, Enewetak, Tarawa, Majuro, Kwajalein, and Marcus islands and from two weather ships northeast and northwest of the Marshall Islands at 12°45'N, 180°0'W and 12°0'N, 153°40'E., respectively (Reference C.9.207, p. VII-(O)-22). Data from more distant U.S. and foreign weather stations funneled through Fleet Weather Central in Hawaii were also used.

At least one B-29 and one PB4Y-2 flew out of Kwajalein each day for weather reconnaissance, the B-29s usually toward the east and the PB4Y-2s toward the west. More flights were scheduled as necessary. On ABLE and BAKER days, three flights passed through the Bikini area.

The weather forecast for the following day was presented to CJTF 1 each day at 0830. From that he decided if the weather would allow the next day's planned operations. A second briefing for the commander was held daily at 2200. On the basis of this briefing, he decided whether to hold to the morning's decision or alter it. Continued weather input was provided the commander (Reference C.9.207, pp. VII-(O)-9, through VII-(O)-19).

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