-~ 2.
determining regions of greater or lesser concentration.
(26, 27)
Tropospheric fallout has contributed significantly to the radioactive debris now encountered over the northern hemisphere as a result
of weapons testing in the Pacific, in Nevada and by the USSR.
Stratospheric fallout has somewhat different characteristics and
distributions.
It consists of particles which rise into the atmosphere
but which do not fall out either as near-in or early fallout or as
tropospheric or latitudinal fallout during the first month or two following their formation but because of their small size are removed from
the upper atmosphere so slowly that their average period of suspension
is a matter of years.
Machta stated that it is not kmown whether theyare
earried down into the tropopause only by sir currents or are also carried
down by theirown weight. He conjectured, however, that the principal
factor in removal is downward atmospheric motions, though the particles
may in addition settle downwards at a rate of a mile or so a year.
Un-
fortunately, knowledge of atmospheric movement in the stratosphere is
still very primitive and it will take several more years of intense
effort, much of it associated with studies of nuclear weapons tests debris,
before one can mike more definitive statements on this point.
There are two major hypotheses as to the distribution pattern of
stratospheric fallout which are being considered at the present time.
One is the relatively simple one of relatively rapid horizontal mixing
with more or less uniform passage of the debris through the tropopause,
thence to be relatively uniformly distributed over the surface of the
earth as a result of tropospheric weather phenomena.
In this case,
there would be essentially no fallout where there is no rain at all and