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 several atolls, with Rongelap at 80 nmi (148 km) the closest. Storms are infrequent, although typhoons occur; nevertheless, both wind and sea are continuous erosional agents. Although possible at any time, most tropical storms occur from September to December. Cumulus clouds are abundant in the area.

The Bikini region incorporates three basic wind systems. Northeast trade winds extend from the surface to 25,000 to 30,000 feet (7.6 to 9.1 km), upper westerlies from the top of the trades to the base of the tropopause at 55,000 to 60,000 feet (16.8 to 18.3 km), and Krakatoa easterlies from the tropopause up into the stratosphere. These systems are all basically east-to-west or west-to-east air currents. Day-to-day changes reflect relatively small north-south components, which are markedly variable. Greatest variation occurs in the upper westerlies, particularly during late summer and fall.

Steady northeast trade winds in the lower levels cause water at the surface of the lagoon to flow from northeast to southwest, where it sinks to the bottom and returns along the lower levels of the lagoon, rises to the surface along the eastern arc of the reefs and islands, and is moved by winds to the southwest again. Lagoon waters moving in this closed loop also mix with those of the open ocean, resulting in a flushing action.

At Bikini, ocean water flows in over northern and eastern reefs and flows out of the western portion of Eneu Channel. Water exchanges over the western reefs with the tides, ocean water flowing in and mixing with the flood and lagoon water flowing out with the ebb. The net rate of flushing of Bikini waters is such that one-half of the lagoon waters is replaced by ocean water in 22 days and the original volume will account for only 10 percent of the lagoon volume after 2-1/2 months (Reference C.9.209, p. F-25).

During CROSSROADS, the Marshall Islands were under the jurisdiction of a U.S. military governor who reported to the Chief of Naval Operations and ultimately to the Secretary of the Navy. Since July 1947 these islands have been part of the Trust Territory of the Pacific Islands, a strategic area trusteeship of the United Nations, administered by the United States (Reference A.8, pp. 507-551).

In order to prepare Bikini Atoll for test operations, a considerable amount of work was required in the lagoon and on the principal islands. First, it was necessary to clear the lagoon of Japanese mines. On 10 March a survey unit arrived and began hydrographic and land surveys to augment the data recorded on the available Japanese charts. After the survey several coral heads were blasted out to permit safe navigation of large ships and to permit proper placement of ships in the proposed target arrays. Navigational and mooring buoys were laid in the lagoon and beacons placed on shore. On the islands, photographic towers (Figure 7), recording stations, recreational facilities (Figure 8), and landing facilities were constructed. This work was started on 20 March with the arrival of the 53rd Special Naval Construction Battalion, assisted by elements of the service groups and minesweeping units of the Pacific Fleet (Reference C.9.206, p. V-(B)-4).

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