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18.
RADIATION STANDARDS, INCLUDING FALLOUT
The rising current through the equatorial tropopause is still present, but
the height to which the fountain rises is very much lower over the Equator.
Elsewhere the transport process is purely mixing, a mechanism which can
leave the highest concentration at the source position while diffusing it away
at the edges. There is still evidence from water vapor and ozone observations
that some kind of rising motion occurs near the equatorial tropopause. This
upward movement is now restricted to just 5,000 or 10,000 feet above the tropopause, the maximum extent allowed by the tungsten data.
An alternative solution was proposed by the HASP analysts and is illustrated in figure 14. In 1958 and 1959, the HASP sampling program had already detected the downward slope of the zone of maximum concentration of
the tungsten and the persistence of the maximum concentration at the latitude
of injection. This fitted their original view that virtually all of the movements of bomb debris could be described by a model in which only mixing
processes were present. The mixing is indicated by arrows in both directions
along surfaces which slope downward towardthe poles.
Not all of the details of the HASP model are shownin the figure. Thus, it
is argued that the horizontal mixing close to the Equator is slower than away
from the Equator. Hence, the U.S. injections at 11° N., at the Eniwetok Proving Grounds, have an easier time mixing into the Northern than into the Southern Hemispheres; and more fallout is observed in the Northern Hemisphere
from this source.
Second, the thermal stability just above the equatorial
tropopause will inhibit an exit into the equatorial troposphere. In fact, the
HASP analysts favor an exit through the breaks in the tropopause as the
main mode of entry into the troposphere. The maximum “storminess” occurs
in the winter and early spring period which accounts for the seasonal variation in fallout. The HASP model is appealing in its simplicity of having only
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one mode of transport mixing, and does explain the observed fallout features
without undue difficulties.
Before leaving the subject of movements of air in the stratosphere, one further figure, 15, may be of interest. This shows the stratospheric distribution
of rhodium 102, the tracer that was added te the very high atmosphere, over
300,000 feet according to reports. in August 1958 over Johnston Island. The
squares are the observation points and the solid-dashed lines are the tropo-
pause. It is apparent that the highest rhodium concentrations up to about
70,000 feet lie in the polar regions at the highest observable altitudes. Further
the concentration in the Southern Hemisphere is at least as high as in the
Northern Hemisphere despite Johnston Island being 17° in the Northern Hemisphere.
It is apparent that
the Johnston Island recket injection debris was, more
or less equally partitioned between hemispheres. Since there is a possibility
of large-scale, high-altitude radioactivity injections during a nuclear war by
antimissile missiles, it is of some consequence to know whether an injection
at say, 45° N. would behave like that of the Johnston Island tests. At present
we think it will
There is also a lack of agreement on an equally important subject, the
region of exit from the stratosphere.
It has been suggested that the trans-
fer of radioactivity and tracer material from the stratosphere into the troposphere can occur by
processes are:
at
least three mechanisms.
Three important exchange
1. Through the gap which is normally situated between the tropical and
polar tropopause.
2. By a continuous and gradual mixing across the tropopause.
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3. By the day-to-day or seasonal variability in tropopause height which
may leave behind pools of stratospheric air in the troposphere.
Which of these three mechanisms. or others, dominate the transfer of stratospheric radioactivity into the troposphere is still open to debate. Meteorolo-