89, 2H, AND °H IN NATURAL WATERS
685
previously when the vertical transfer was considered, the turbulent
exchange processes tend to reduce the differences in 489 content from
one place to another, and the same presumably holds for horizontal
turbulent exchange. Dansgaard’s measurements also show that the
mean '89 concentrations vary only about 10 per mille within the latitude
belt 55°N to 55°S (excluding only some continental stations for which
other processes might be of importance) in spite of the fact that the
total water content of an air column may vary from an average of
about 6.5 g/cm? in the tropics to about 1 g/com? close to the polar re-
gions. A much more careful study of the variations of the isotopic
composition of rainwater must be conducted in conjunction with measurements of the isotopic composition of atmospheric moisture to
arrive at more conclusive results about the flux of water vapor through
the atmosphere.
The preceding discussion is, of course, very tentative in many
respects. It shows, however, that we may expect considerable differences in the distribution of various isotopes as a result of the relative
importance of different physical processes contributing to the transfer
processes. Data are still inadequate for a more quantitative evaluation
of these differences. Obviously systematic measurements of the isotopic composition of atmospheric water vapor as a function of elevation
and latitude seem very desirable, and simultaneous measurements of
particles
and
artificial radioactivity would provide additional im-
portant information. Studies of the isotopic composition of precipitation
as conducted at present are of course also of value, although they are
insufficient since such measurements represent an integration over the
condensation and evaporation processes, the result of which may be
difficult to interpret by itself.
It should be added that some simple studies of the tritium budget
of the atmosphere as compared with that of *Sr give some further
support to the general picture of the verticaltransfer affecting tritiated
water vapor and particular matter differently as previously indicated.
Machta? has shownthat the sum of the total deposition of Sr over land
and by rain over the oceans added to the amount remaining in the atmo-
sphere is about 80% of the total injected (summer 1963). A corresponding inventory of tritium? shows quite a different result. In this
inventory it is estimated that the direct air—sea transfer of tritium
must be about twice that due to precipitation to obtain an approximate
balance. This probably is a lower limit since only bomb-produced
tritium was considered in the computation.
REFERENCES
1. V. Starr and J. P. Peixoto, The Hemispheric Eddy Flux of Water Vapor and
Its Implications for the Mechanics of the General Circulation, Arch. Meteorol. Geophys. Bioklimatol., Ser. A, 14: 111-130 (1964).