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much lonver time than the lower part of the atmosphere.
In fact,
the stratospheric material has a residence time of something
like several years and we shall estimate, in the course of the
discussion, that this figure is something like six years, whereas
the troposphere has a mean residence time 2f£ about one month
with the lower 10,000 feet of it being washed clean on the average
about every three days.
Between 10,090 feet and the tropopause,
which is at something like 40,000 to 50,000 feat, the residence
time is perhaps 45 days for a mean time for the troposphere
of about one month.
Thus, we see that radicactive faliout which
is injected into the troposphere is restricted to the general
latitude of the detonations for the reason that the residence
time is so short that it doesn't have time to mix eppreciably.
latitudinally.
The principal mechanism for removal from the troposphere to
the surface is rain.
The tiny fsilout particles kit cloud
droplets and stick to then.
Because the particles are so small
(perhaps a few hundred atomic diameters) they are subject to a
violent random jiggling motion due to collisions with air
molecuies.
droplets.
It is this motion which causes them to hit the cloud
This motion is cai.ed the Brownian motion.
In fact,
for a particle one micron in diameter, Greenfield _/ calculates
that the mean residence time in a typical cloud of water droplets of 20 microns diameter wili lie between 50 and 300 hours,
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