58
RADIATION STANDARDS, INCLUDING FALLOUT
for the lack of a seasonal variation in the South Hemisphere depends on a different behavior of the Southern Hemisphere stratosphere. There is evidence
gleaned from the International Geophysical Year weather data that differences
do exist.
For example, both hemispheres in winter have a quasi-circumpolar vortex
in the polar regions, close to, but not necessarily symmetrical about the pole,
with strong winds observed to extend upward from 70,000 feet. In the Northern
Hemisphere this vortex breaks down into waves and is accompanied by sudden
temperature increases in a few days which have been observed to occur sometime between late January and April, either before or after the vernal equinox:
(the first appearance of sunlight at the north pole). In the Southern Hemi, .
sphere there can either be a sudden or a gradual temperature change neither‘. .
of which has been observed to occur until after the vernal equinox.
To complicate matters this past winter and spring, the Northern Hemisphere
vortex did not undergo a sudden breakdown but was similar to the gradual
one found at times in the Southern Hemisphere. The importance of this
phenomenon may be greater than one realizes. Many meteorologists attribute
the sinking motions or mixing in the polar regions to the sudden breakdowns
of the cireumpolar vortex. This sinking or mixing is believed to be the method
by which radioactivity and other tracers are dumped into the lower atmosphere.
Time will tell how seriously this disparity of 1962 from previous years in the
Northern Hemisphere stratosphere will affect the pattern of the spring 1962,
fallout.
Before turning to the latest fallout data from Soviet 1961 tests, some comments on special injections marked with unique tracers may be of interest.
The seasonal variations of the tungsten, which it will be recalled was injected
in the lower equatorial stratosphere, indicated a peak in the Northern Hemi:
sphere spring but no apparent peak during the Southern Hemisphere spring,
It could be noted that the fallout of radiotungsten was considerably greater
in the Northern than in the Southern Hemisphere so that the eross-equatorial
tropospheric flow may possibly have accounted for the disruption of the Southern Hemisphere seasonal variation. Figure 9, however, shows data for an
isotope, rhodium 102, which contains a seasonal trend of the expected type in
both hemispheres. The rhodium was derived from the August 12, 1958, rocket
nuclear event at Johnston Island at a latitude of 17° N
Later we will note that the rhodium appeared in the stratosphere near the
poles of both hemispheres in about equal concentrations. This isotope, then.
is not higher in the Northern Hemisphere as were all of the other previously
viewed tracers. There is clearly a spring maximum and an autumn minimum
in the appropriate stations at Santiago, Chile, and at Argonne National Labora.
tory in Dlinois, both slightly delayed. This result strongly supports the first
explanation for the absence of a seasonal trend in the Southern Hemisphere—
the one which blames the lower atmospheric cross-equatorial mixing for confusing a regular seasonal trend from the stratosphere.
Figure 10 now brings up to date the recent fallout. The lag in analyzing
fallout has restricted the very recent results to one station, air concentratior
measurements of cesium 137 at Argonne National Laboratory from Soviet 1961
tests. Where data are available for earlier months at other locations there is
substantive agreement with the Argonne results. One way view the cesiwr
and strontium trends interchangeably since both have about the same half-lives
although the concentrations of cesium 137 are about twice those of strontium 90
The curve indicated as 1961 U.S.S.R. shows that there is an upward trenc
in the cesium 137 beginning in October 1961 and continuing through May 196:
as predicted from earlier findings.
For comparison, two other curves are given. The high-peaked curve shows
the history of cesium 137 fraction attributable to the Soviet October 1958 series
Tt is evident that the fallout in 1962 is about the same or perhaps slightly lowe1
than occurred after the Soviet October 1958 tests in the Chiacgo area. Ir
1960-61, the levels of cesium 137 had decreased very considerably as seen ir
the lowest curve. The new atmospheric tests in 1961 raised the levels by abou
a factor of 5 to 10. The full history of the spring 1962 must await more com
plete results. It should be noted that the fission yield of the 1958 Soviet serie:
was about half that of the preliminary estimate for the 1961 Soviet tests anc
simple extrapolation would suggest a doubling of fallout in 1962 over 1959
This now seems to be unlikely, probably due to the greater altitude of thi
nuclear clouds.
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