Tare complex with electronic gages. There was a definite and consistent variation in
the data between the two lines.
It is recognized that a moisture-laden air will attenuate pressures in the blast wave,
simply because blast energy will be lost by an amount proportional to that which is necessary to evaporate the suspended water droplets or rain in the path of the shock. Studies
on the problem by the two projects concerned indicated that a moderate showercould
contribute sufficient water content to the air to account for the deviation in the pressure-
distance curves of the two blast lines (described in the Project 1.24 and 1.2b reports,
see Appendix).
2.3.5 Comparison with the 2W Theory. It was anticipated that sufficient data would
be obtained from Castle to allow a quantitative comparison to be made, for surface bursts,
with the ideal case. Theoretically, such a burst over a perfectly reflecting plane should
act like one of twice its yield in free air. Data from previous surface bursts, Jangle
Surface and Ivy Mike, did not entirely confirm this theory. The question was the value
of the reflection factor —of necessity between 1 and 2. From Castle data, it appeared
to be certainly less than 2— probably between 1.6 and 2.
The difficulty, and the reason a more-definite figure cannot be assigned, lies with
the determination of yield of the multi-stage devices; fireball and time-of-arrival meth-
ods used to estimate yield involve the 2W assumption.
A method independent of this as-
sumption is necessary. Unfortunately, only radiochemical analysis, which determines
only the fission yield of a device, satisfies this restriction.
Figure 2.4 shows a pressure-distance plot of all the surface overpressures scaied to
1 kt at standard sea-level conditions, along with similar data from Jangle Surface and
Ivy Mike, compared to the 1W and 2W free-air composite curves. All measured data
were scaled to 1 kt at sea-level conditions. The solid line represents a composite
pressure-distance curve for a 1-kt surface burst based primarily on Castle measurements. Yields used for data reduction were based on a radius-time history of the fireball (involving the 2W assumption)!. All arrival-time data are compared in Figure 2.5
on a similar basis.
There were no apparent effects due to refraction observed during Operation Castle.
In fact, Figure 2.4 indicates that overpressures at long ranges fall closer to the 2W freeair curve than do overpressures at closer ranges.
2.4
ABOVE-SURFACE MEASUREMENTS
The results of Ivy King confirmed the scaling laws for free-air pressures up to a
yield of 540 kt. Data obtained from the Mike event, however, were confined to the lowpressure region. There was reason to suspect that for high yields, an altitude correction
must be made for propagation vertically through a nonhomogeneous atmosphere. Castle,
then, presented an opportunity to document pressures in the air above megaton-yield
surface shots.
These phenomena include a definition or delineation of the shock from a
surface burst as it propagates through the iow levels of the atmosphere out to long ranges.
2.4.1 Pressures. The smoke-rocket.and direct-shock photography techniques were
used for pressure-distance determination in the air and along the surface. In general,
1On Redwing, considerable data was obtained from two land-surfaee bursts, one a kiloton
burst of medium yield determined by radiochemical analysis.
burst curve was drawn from the data—-it scaled about 1.6W.
A composite land-surface