dailies iat pore _ approximation, a theoretical relationship between the wind field and the pressure field which is extremely useful at higher latitudes, so that reliance on wind observations alone is necessary . and little further assistance is derived from observations of ies dhadh suit al wht. Wtsckentoeni - the pressure field. — .The wind structure over the Marshall Islands area during the test period can be considered in three broad lavers: the trade wind layer of steady eastnortheast winds extending from the surface to about 25,000 feet; the upper troposphere, from 25,000 to about 55,000 feet, which consists of a series of moving large- scale vortices; and the lower stratosphere, from 55,000 to 130,000 feet, containing variable east or west winds with speeds decreasing. from values as high as SO knots near the base of the . _ layer to light (and uncertain) values above. - Unfortunately, wind data over the Pacific is almost com- pletely lacking at heights above 100,000 feet and only sporadic _ soundings are availeble in the remainder of the lower stratosphere. ~ Even climatological mean winds are not available at these great heights. The paucity of data is critical in view of the great height reached by the MIKE cloud and the fact that the bulk of ‘the airborne debris is probably contained in the mushroom. The relatively small mmber of observations make it necessary to place special emphasis on the analysis of flow patterns that can only be achieved under research conditions, paying particular attention to time continuity. Figure 1.1 illustrates the 30,000-foot flow patterns for 0300 GCT and 1500 GCT, 1 November 1953, ani shows the number of observations typically available at this level. Meteoro-— logical data for the Marshall Islands area was obtained from a publication by the Joint Task Force (6), the remaining data from conventional. weather teletypewriter sources. - Mapa of the airflow patterns at several elevations ares-used to prepare trajectories of the various portions of the atomic cloud. In general, the average error of the trajectories over. the United States has been found to beabout 20% of the path length ‘at 30,000 feet. The lesser amounts of data will undoubtedly | ‘increase this error over the Pacific, except in trade wind layer, where the regularity of the winds increases the accuracy. - Another meteorological feature of most of the tropical Pacific region, which differs from conditions in the United States, is the precipitation regime. Rainfall in tropical areas occurs in frequent showers (rather than large areas of steady rain) derived, in general, from clouds embedded in the trade wind layers, although thunderstorms in disturbed situations do extend to 55,000 or 60,000 - feet. — _