RADIOACTIVITY VS. HEIGHT IN NUCLEAR CLOUDS
643
through August 1962. The difference between the expected and the measured amounts may be due to the uncertainties in the estimates of the
tropospheric fraction and the deposition. However, there are several
reasons for believing that the amount deposited in the latitude band was
actually less than that estimated from the tropospheric fraction at time
of cloud stabilization.
First, some of the debris whichinitially stabilized below the tropo-
pause may have ascendedinto the stratosphere in convective cells or as
a result of thermally induced direct circulation. Second, some debris
was transported to mid-latitudes at altitudes below the tropical tropopause. Since there is a polar tropopause in mid-latitudes, generally
between 30,000 and 40,000 ft, the debris which was transported away
from the equatorial region at altitudes from about 40,000 to 55,000 ft
would have become incorporated into the mid-latitude stratosphere. An
interesting example of such transport was provided by the interception
of one of the Dominic I clouds by sampling aircraft over the western
United States.’? In addition, sampling of the lower stratosphere over
the United States indicates that it contained fresh debris from the
Dominic I tests during most of the month of May 1962. Finally, the evi-
dence for a half-residence time of one month for tropospheric debris
may actually apply only to debris below the polar tropopause. The
residence time for debris inthe troposphere, above 40,000 ft, in tropical
latitudes has not been established. Only a very small fraction of the
debris from the Dominic I tests stabilized below 40,000 ft. The fraction
was much smaller than that for previous Pacific test series, which
consisted primarily of surface bursts.
In any case, it has become increasingly evident that the potential
hazard due to short-lived fission products is not attributable solely to
the portion initially injected in the troposphere since there is an exchange of air between the stratosphere and troposphere. Therefore the
three-dimensional trajectory of the debris-laden air would have to be
considered in determining the fate of a particular debris cloud.
It has also been shown’ that severe thunderstormsthat penetrate
the lower stratosphere provide an effective mechanism for bringing
stratospheric debris directly to the ground, It appears that thunder-
storm scavenging of stratospheric debris from the Dominic I tests
accounted for most of the ‘*'I found in milk in the midwestern United
States in May 1962,
RECOMMENDATIONS FOR FUTURE WORK
Project Stemwinder has shown in-cloud dose-rate monitoring by
aircraft to be a relatively simple and economical wayto obtain infor-
mation on the distribution of radioactive debris in nuclear clouds.