STRATOSPHERIC DISTRIBUTION OF NUCLEAR DEBRIS
305
high altitudes in early 1963 indicate that the main souce of debris was
the 1962 test series. This observation is one of the several lines of
evidence that has led us to the conclusion that almost all the debris
from U.S.S.R. weapons tests, especially that from tests performed before 1962, was injected into the lower polar stratosphere and that
virtually none stabilized at high altitudes.®
By early 1963 high concentrations of °*Srwerefound at all altitudes
sampled at San Angelo, from 65,000 to 105,000 ft. At 105,000 ft the
concentrations appeared to decrease sharply after April but then to rise
again gradually during the second and third quarters of the year to
about the same concentration as was found in April. Between September
1963 and February 1964, the concentrations at 105,000 ft decreased
significantly, but during early 1964 they again slowly began to increase.
At 65,000 to 70,000 ft, the "Sr concentrations remained fairly steady
during the first three quarters of 1963 but then decreased during the
last quarter of the year. During early 1964 the concentrations at 65,000
to 70,000 ft, like those at 105,000 ft, slowly began to increase again.
The changes in "Sr concentration observed at 65,000 to 70,000 ft
at San Angelo during late 1963 and early 1964 might be attributed to a
large-scale exchange of air between the northern polar and the tropical
Stratosphere during the autumn and winter seasons of the northern
hemisphere. The result was a decrease in *°Sr concentrations in the
vicinity of 31°N as polar air, containing high concentrations, was diluted
by tropical air, containing lower concentrations. With the breakdown
of the winter night circulation in early 1964, the exchange of air between polar and tropical regions probably slowed drastically. As a re-
sult of turbulent exchange within the polar stratosphere, the southward
migration of debris from the polar source region continued to bring
additional *“Sr to the vicinity of 31°N. However, the drastically re-~
duced exchange rates between the polar andtropical regions which were
in effect after the breakdown of the winter circulation resulted in an
inhibition of the movementof this *°Sr toward still lower latitudes, and
concentration began to build up at 31°N.
The changes in "Sr concentration observed at about 105,000 ft at
San Angelo during 1963 and early 1964 were more abrupt than those at
65,000 to 70,000 ft. These abrupt changes are attributable to the pres-
ence of large parcels of air containing low concentrations of nuclear
debris at the higher altitudes which had not yet mixed with the parcels
of air containing high concentrations. As a result it may have been
mid-1963
before concentrations at 105,000 ft were fairly uniform
throughout the northern polar stratosphere. When mixing between the
polar air and the tropical air of very low concentration began in late
1963, the concentrations at 31°N dropped rapidly.
Before accepting the hypotheses given in the preceding paragraphs
to explain the observations at San Angelo, it would be well to consider