the upwind direction but were then transported downwind after their initial energy had been dissipated against existing surface winds. Alternatively, originally coherent masses of radioactive aerosol could nave been broken up by turbulence and small variations in wind structure. The simpler Umbrella dose rate records show no evidence of an intersurge decrement but indicate a comparatively large central decrement, a structure again corroborated by aerial photography. The fact that this central decrement is recorded at coracles experiencing only an inner edge transit (U 1.8, CL 3.1, DLL 6.6, DRR 3.9; Figures 3.80, 3.83, 3.85, and 3.94 respectively} indicates that the central decrement is at least as large as 3,000 feet in radius. Thus, both the photographic evidence and the simpler dose rate records suggest that a relatively large nonradioactive center was followed by the rapid passage of a compact, highly radioactive aerosol over the stations and then by the longer transit of a more diffuse cloud, which again probably represents base surge originally moving in an upwind direction. Similar conclusions on the general structure of both baSe surges are arrived at using the hypothetical surge center H (Section 3.3.4; Figures 3.120 through 3.127). Further analysis of the Umbrella central decrement is complicated by the presence of white water, by expansion of the base surge torus, and by the fact that no two stations record exactly the same transit. After radial expansion ceases, inward diffusion of the surge boundary might be expected to eradicate any central decrement; however, there is only indirect evidence for any such process. Of the four coracles providing central transit records, two overturned during transit; therefore, any comparison must be made between coracle and shipboard records. Because of the possible persistence of radioactive aerosols in the neighborhood of obstructions causing turbulence (Section 3.4.3), this particular comparison is not desirable. The minimum normalized dose rates during central transit for Coracles DL 6.2 and DL 16.0 are 29 r/hr at 3.70 minutes and 37 r/hr at 7.80 minutes, respectively, whereas those for the three destroyers in order of increasing distance from surface zero are 400 r/hr at 2.19 minutes, 160 r/hr at 2.30 minutes, and 55 r/hr at 3.91 minutes, respectively. All usable records suggest that ex- pansion of the central decrement ceased after about 3 minutes. Since the outer base surge boundary is photographically observed to continue radial expansion until at least 6 minutes, this earlier stabilization of the inner boundary may be the only evidence for inward diffusion. ‘ Coracle D 4.8, although overturned, was not quite in white water at the time of central transit; thus, its minimum normalized dose rate corrected for attenuation (110 r/hr) may be tentatively included in the above comparison. The continued persistence of gammaactivity after final transit of the surge photo-boundary is a phenomenon frequently observed for both shots. Generally, gamma records showing the longest persistence are those from coracles that experience central transits or are located where turbulence from target ships upwind is possible. A number of explanations for the observed persistence are possible; the simplest, however, is that turbulence resulting from passage of the base surge over the ocean surface and around large obstacles separates diffuse radioactive remnants, which stream out behind the surge. Indirect evidence of surface drag forces necessary to the formation of such remnants is implicit in the photographic observation that, in later time, the base surge torus tends toward an ellipse with its major axis in the direction of the surface wind (Reference 91). For brevity, these postulated remnants are re- ferred to as “tails.” Simple hydrodynamic considerations indicate that the length of such tails should increase with increasing distance downwind of surface zero and should decrease as the transit path approaches the crosswind edge of the base surge torus. Although interference by target ships must be considered in nearly all cases, the persistence in dose rate expected of the suggested tails roughly fits such predictions. In cases where target ships are involved (Table 3.11), prolonged gamma dose rates may be caused by both ship retardation and by streaming of surge remnants detained ir turbulent eddies generated by the superstructure (Section 3.4.2). Surge retardation by the target ships, apparent in base surge photography (Reference 91), is not detectable in surge arrival times derived from The prolonged dose rate records after surge transit.may, how- ever, be radiological evidence for such retardation. If the postulated tails following the base ic XQ 132 Xo the gamma dose rate records.