of injection of the debris.
Such conditions do not, of course, prevail in
the stratosphere.
Proposed models of stratospheric transport and removal have been devel-
oped by several investigations to explain the cbserved Sr-90 patterns at the
ground.
Three such models will be mentioned here.
The first, due to Dr. Libby, assumes that in the main, debris is uniformly
distributed throughout the stratosphere relatively quickly, that a fixed
fraction per year of the stratospheric debris enters the troposphere more or
less uniformly over the world, and that deposition is principally in precipitation (this last assumption is common to all models).
Irregularities in the
observed distribution such as the large Sr-90 deposition observed in the north
temperate latitudes and apparent changes in the rate of deposition at certain
stations are assumed to be the result of tropospheric fallout from kiloton-
yield tests in Nevada and the USSR.
.
Martell has modified this simple model to allow for a difference between
the behavior of debris from the large U. S. Pacific shots and the USSR
thermonuclear bursts which inject their debris into the polar or temperate
latitude stratosphere.
It is his contention that the U. S. Pacific bursts
contribute to a relatively uniform world-wide stratospheric fallout, while
the stratospheric debris from the USSR thermonuclear bursts has a shorter
residence time in the stratosphere, of the order of a few months.
He con-
cludes that the increased fallout in the north temperate latitudes is due
to stratospheric fallout from Russian debris which was injected into the
lowest layers of the stratosphere.
neo cy
Machta and List (22) in this country and Stewart and others in Ingland have
developed a model originally proposed by Brewer and Dobson based in part on
independent meteorological evidence from water vapor and ozone distribution,
which calls for a principal source of stratospheric air in the ascending
currents of the tropics, with a slow poleward drift, particularly in the
winter hemisphere, and a sink over the temperate and polar regions.
This
cireulation, together with the known precipitation patterns, is used to
explain the observed fallout of Sr~90 and the apparent increase in the rate
of fallout during the spring season.
Each of these models would predict a somewhat different distribution of
Sr-90 in the stratosphere.
Dr. Libby's would, of course, call for a uniform
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