18?) AND *H IN NATURAL WATERS
683
composition of the atmospheric moisture is to be expected. Very few
data on the isotopic composition of atmospheric moisture are avail-
able, but they are clearly of interest in view of the deductions above.
In the present context, we may consider that sea water contains no
tritium. On the other hand, a considerable amount of tritium has been
introduced into the stratosphere by the explosion of thermonuclear
devices, and nuclear industry produces an increasing amount of tritium
that is released into the lower troposphere. Recent measurements
(March 1964) in the atmospheric moisture in tropical latitudes (over
the Congo, see Fig. 4) show a marked increase of tritium with eleva-
tion, indicating a downward transfer of tritium due to vertical mixing.
The mere fact that tritium is found at upper levels in the tropics supports the idea that eddy motion is of greater importance for the vertical transfer than the meridional circulation cell. Considering now the
Simple example evaluated above, we see that with a source aloft the
isotope increase with elevation is twice as rapid as the decrease of the
moisture content, i.e., with A= (4km)', we obtain an increase by a
factor of 2 over a vertical interval of about 1.4 km. Of course, this
treatment is not directly applicable to the case of tritium since horizontal transfer processes, also of importance for tritium distribution,
become of importance in the upper troposphere and lower stratosphere.
The simple example indicates, however, the possible role of turbulence
for the vertical transfer of tritium in the lowest part of the troposphere
also. Qualitatively, we note that the isotopic exchange between falling
droplets and environment decreases the amount of tritium brought to
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