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Fig. 2—Fission-product radioactivity in the ground-level air at some
sites in the southern hemisphere.
The large spring peaks in the northern hemisphere following intensive periods of testing in the fall of 1958 and the fall of 1961 are
obvious. The rapid decrease in activity levels in the months following
the peak concentrations and the supposed cessation of stratospheric
subsidence indicates that the averageresidence time for tropospheric
debris in the northern hemisphere is something less than 30 days. The
smaller spring peaks during 1960 and 1961 during the moratorium on
nuclear testing observed by the United States, the Union of Soviet
Socialist Republics, and the United Kingdom are also apparent but
superimposed on them is activity from French tests held in the Sahara.