640
FERBER
For air bursts it appears reasonably certain that less than 1% of
the total activity is present in the stem and that less than 0,1% stabilizes between the surface of the earth and one-half of the cloud-top
altitude. The fraction of activity per unit altitude increases with height
throughout the stem, and the region of maximum vertical gradient,
which might be termed “the radiological base of the cloud,” probably
occurs somewhat above the visual cloud base. The peak activity per
unit altitude is assumed to occur between 75 and 85% of the distance
from the surface to the cloud top, It is also assumed that for air bursts
the distribution of activity relative to the cloud-top height does not
vary with nuclear yield, burst height, or atmospheric conditions.
Actually, the interaction of these factors must exert some in-
fluence on the activity distribution. The estimated top and base heights
of the Dominic I clouds indicate that the ratio of base height to top
height has a tendency to decrease with increasing yield. However, it is
uncertain to what extent these indications are valid since the variation
among detonations of about the same yield is almost as great as that
for the range of yields from 40 kt to several megatons. The mean
ratio is 63%, with individual clouds varying from 53 to 73%. Some of
the variation may be attributed to errors in the estimates of the base
and top heights, but part of it is undoubtedly real. There is a similar
uncertainty concerning the height of the radiological base.
Partition of Activity Between Stratosphere and Troposphere
The height of the tropopause varies with latitude, season, and
daily atmospheric changes. The daily and seasonal variations are less
in tropical latitudes than elsewhere. The tropopause heights averaged
about 54,000 ft above sea level for the Dominic I tests, varying between
50,000 and 58,000 ft on individual shot days. This variation is representative of the tropical tropopause.
With the use of the activity distribution in Fig. 5, a mean tropo-
pause height of 54,000 ft, and the mean cloud-height curve in Fig. 1, a
typical curve of the percent of totaldebris in the troposphere as a func-
tion of yield has been calculated. The curve, shown in Fig. 6, is intended
to be valid at the time of cloud stabilization for air bursts in a tropical
atmosphere. Another curve has been drawnto indicate the likely maxi-
mum tropospheric fraction, based on a high tropopause (58,000 ft) and
low cloud heights (from the lower curve in Fig. 1). These extremes do
not represent absolute maximums since higher tropopauses and lower
clouds may occur occasionally. In addition, the uncertainties in the
activity—height curve (Fig. 5) make it impossible to define a meaningful and useful absolute maximum tropospheric fraction. No attempt
has been made to estimate the minimum tropospheric fraction, but, in
the megaton-yield range, it could be several orders of magnitude below
the typical fraction.