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A Defense Nuclear Agency Report on the Bravo Test Describing Wind Change

April 1, 1982
In his statement to the press following the detonation of the "Bravo" hydrogen bomb test, AEC Chairman Lewis Strauss claimed that the wind shifted unexpectedly just before the blast, which resulted in the contamination of inhabited islands to the east. But a report produced by the Defense Nuclear Agency in 1982, reveals that six hours before the dawn blast the task force knew that the winds had shifted and were blowing towards Rongerik.

[Extract from the report pages 201 & 202]

Source: Castle Series, 1984, The Defense Nuclear Agency, April 1, 1982

The first event of the CASTLE series, BRAVO, was scheduled for detonation on a small artificial island connected to Nam by a causeway. The island was built over the reef some 3,000 feet (914 meters) southwest of Nam. The device, provided by Los Alamos Scientific Laboratory (LASL), was to be detonated on 1 March 1954 if meteorological conditions were favorable.

Decision to Shoot
The preshot 5-day advisory message to the Atomic Energy Commission (AEC), the Joint Chiefs of Staff (JCS), and the Commander-in-Chief of the Pacific (CINCPAC) indicated that the BRAVO fallout would form a fan-shaped pattern from the northwest to northeast if the predicted winds held. Based on this favorable wind pattern, a search for transient shipping was conducted northwest of ground zero on BRAVO D-2. The flight pattern was centered on a bearing of 300deg. from ground zero and consisted of radar coverage by the patrol aircraft of a rectangular corridor 200 nmi (370 km) wide by 800 nmi (1,480 km) long. On BRAVO D-1 a search sector was set up on a heading of 330deg.. With radar coverage, this search encompassed a trapezoid 600 nmi (1,110 km) long with end lengths of 100 nmi (185 km) at ground zero and 200 nmi (370 km) at the outer end. Results of these searches were negative.

On BRAVO D-1 at 1100 hours, the task force predicted (in the H-18 advisory to the CINCPAC) "no significant fallout...for the populated Marshalls." Moreover, the task force predicted that no safety problems would exist except on air or surface routes in the sector 275deg. to 80deg. clockwise to a range of 450 nmi (833 km).

At the 1800 weather briefing, the predicted winds were less favorable; nevertheless, the decision to shoot was reaffirmed, but with another review of the winds scheduled for 2400. The USS Renshaw, acting as the air controlled between Enewetak and Bikini, was ordered repositioned from its planned 270deg. bearing, 90 nmi (167 km) range from Bikini to a more southerly 230deg. bearing 90 nmi (167 km) range from Bikini. The cloud track flight, designated "Wilson 2," was set up at 2200. It would hold west of Bikini at H+2 at 10,000 feet (3.05 km) in a position to intercept and warn of debris coming west toward Enewetak and Ujelang and then fly east after 2 hours, searching a sector between 55deg. and 85deg. through ground zero to find the cloud if it had not already appeared to the west.

The midnight briefing indicated less favorable winds at 10,000 to 25,000 foot (3.05 to 7.52 km) levels. Winds at 20,000 feet (6.10 km) were headed for Rongelap to the east. The predicted speed of these winds was low enough to be of no concern, although it was recognized that both Bikini and Eneman islands would probably be contaminated. The decision to shoot was reaffirmed, at least until the 0430 briefing. A burst-day flight to search for transient shipping was added at this time, to be centered on a 65deg. vector to a distance of 600 nmi (1,110 km).

At 0430 "no significant changes" in the winds had occurred, except at Bikini where the lower level winds were showing more "northerly and westerly components." The JTF Radsafe Office recommended that the fleet sortie area southeast of Bikini be moved outward from 30 to 50 nmi (56 to 93 km). This was ordered for smaller and slower units, but the larger vessels remained in their original areas to maintain good UHF communications with the firing bunker on Eneu and to be in good position for prompt reentry. The USS Estes also had to be within range for its role as the master Raydist navigation system station in controlling experimental aircraft flights near the burst point.

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