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"3.
years appears to ve the average time they spend before descending
to the ground, corresponding to an average annual rate of about
ten to twenty percent of the amount in the stratospnere at any
given time.
descend.
It is not clear as to just how they do finally
It seems possible that the general mixing of the
stratospheric dic with the tropospheric air, which occurs as
the tropopause shifts up and down with the season as well as what
is brought about by the jet streams, constitute the main
mechanisms.
The descent of the stratospheric fallout apparently
is never due to gravity but rathex to the buik mixing of
stratospheric air with tropospheric air which brings the radio-
active faliout particles down from the stratosphere into the
troposphere where the weather finally takes over.
This mecnani.m
makes the percentage fallout rate the same for all particles tco
small to fali of their own weight -- and the same as would be
expected for gases, providing some means of rapidly cemoving
the gases from the troposphere exists, so the reverse process of
troposphere to stratosphere transfer does not confuse the issue.
The world-wide fallout from the stratosphere descends very
slowly and one of the questions unanswered at this time is just
at what rate it does descend.
There have been various estimates
from 109% per year to 20% or even higher.
But everyone is agreed
that che stratosphere does hold its cadioactive fallout for a
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