{
that for a particle of .04 micron diameter it wiii be between
39 to 60 hours, and that for a particle of .31 micron diameter
it wili te between 15 to 2) hours.
The theory caiculates the
diffusion due to the Brownian motion and says tnat it is just
fis motion induced by the collisions with the air molecules
which makes possible the contact between the fallout particles
and the cloud dcops.
Since this theory is based on first
principles with the single assumption that the fallout particle
sticks to the water droplet on impact -- an assumption sc
plausible as to be aimost beyond doubt -- it is no surprise to
learn experimentally that the Greenfield theory appears to be
correct,
There is essentially no world-wide fallout in the ebsence
of rainfall; i.e., in desert regions -- except fer a little
that sticks to tree leaves, blades of grass, and general surfaces,
by the same type of mechanism
of clouds.
Greenfield describes in the case
Thus we see that it i5 the moisture in the tropo-
sphere which assures the short Lifetime of the world-wide
fallout particies and, that when the stratospheric air which
contains essentially no moisture* and, therefore, has no cleansing
*Note:
The total water in the wtratosphere is about .01 gms / em
while that in the troposphere is atout 2 gms /em@, 2u0
times as much.
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