METEOROLOGICAL INFLUENCES ON sr CONCENTRATIONS
591
low. This observed reversal may be considered one predominantly
distinguishing feature between convective activity and large-scale uplift.
A major feature has been the peak-concentration dependence upon
the location of the cloud system with respect to the jet stream and the
tropopause; this dependence uponthe location of the cloud system with
respect to the tropopause was especially noticeable during the 1961
period, During the three-year period of relatively minor new input of
%Sr into the stratosphere by atmospheric weapons testing following the
1958 testing moratorium, the air concentration of nuclear debris may
have been at a maximum uniform value over large air masses.
Figure 4 shows the peak concentrations in the seven 1961 showers
related to the top of the cumulus cloud with respect to the tropopause
height and the horizontal distance to the jet stream.*’ This dependence
is consistent with the supposition that the nuclear-debris concentration
in air at the tropopause and near the jet stream is greater at any given
time than at lower levels in the troposphere. The observations, however, may also be consistent with the supposition that layers of dry air
of recent stratospheric origin may be present isthe mivure troposphere
at times, resulting in layers of large nuclear-debris concentration
between large volumes of air at low nuclear-debris concentration. The
growth of a cumulus cloud through such layers and the extent of entrainment of the debris as new nucleation particles would, of course, also
be dependent upon the maximum growth of the cumulus clouds. The
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DISTANCE FROM JET AXIS. MILES
Fig. 4—Peak 9S + concentration in disintegrations per minute per liter
in convective-showeyr precipitation plotted at relative positions of maxtmum cloud tops and distances from jet core and superimposed on atr-
concentration plot in disintegrations per minute per 1000 scf from
Giles.?
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