1489, 2H, AND °H IN NATURAL WATERS
679
200
400
fam]
= 400
800
1000
80°N
7OPN
60°N
5O°N
40°N
30PN
Z20°N
10°N
oP
LATITUDE
Fig. 3—Mean meridional circulation during the winter. The tubes be-
tween the streamlines transport 25 x 10 tons/sec. [From E. Palmen
and L, Vuorela, Quart. J. Roy, Meteorol. Soc., 89: 134 (1963)./
vapor is also constantly transferred upwards owing to turbulent motions of the atmosphere. This latter transfer is likely considerably
more intense than that due to the meridional circulation cells. The
transfer upwards of water vapor due to a meridional circulation cell
equals the net horizontal convergence and amounts to a few hundred
millimeters of precipitation per year. According to Jacobs, ° the annual
precipitation in tropical latitudes in the neighborhood of the tropical
convergence zone is between 1000 and 2000 mm. Thus mostof the
water that falls as precipitation must have evaporated in the same
' latitudes. The lifetimes of water in the atmosphere is rather short.
Considering removal of water vapor from the atmosphere by precipitation only, neglecting the direct molecular exchange at the sea surface, and adopting 5 g/cm’ as a characteristic value for the total water
content of an air columnin tropical latitudes, we find that 1500 mm of
precipitation implies a mean residence time for water in the atmo-
sphere of about 12 days. This is obviously an upper limit since direct
molecular exchange may be appreciable.
With this general picture of the water transfer in the atmosphere
in mind, we may ask ourselves what variations of the isotopic compo-
sition of water results from the series of physical processes involved.
We shall next give a few simple deductions, on the basis of which we
shall be able to interpret some of the general features of the distribution of various nuclides in precipitation and atmospheric moisture as
depending upon evaporation, condensation, and turbulent transfer. We
shall also be able to indicate some measurements of special interest
for a better understanding of the transfer mechanisms in the atmosphere.