The environment of the earth is a product of “weather”—of the transport
of moisture, of the actions between winds and oceans, of the cycling of
energy through biotic systems. Understanding of biological potentials of
atmospheric factors involves understanding of atmospheric motions
affecting transport and mixing of contaminants and the processes of
deposition of radionuclides from atmosphere to earth.
[Illustration: _Network of towers on the Atomic Energy Commission
reservation near Richland, Washington, used by atmospheric physicists in
measuring quantity, concentration, and dilution of radioactive materials
in the atmosphere._]
At some thousands of feet above the earth’s surface—at 30,000 to 40,000
feet in the middle and polar latitudes and at 50,000 to 60,000 feet in
the tropics—there is a level, the tropopause, at which air temperature,
rather than decreasing, becomes constant or increases with height. Below
this level is the troposphere, the turbulent zone of clouds, rain, and
fog. Above it is the stratosphere, where there is no turbulence and only
a slow mixing of dry and cloudless air. The stratosphere continues to a
height of about 100,000 feet. Investigators have noted the importance of
rain or snow in washing fallout particles from the air in the
troposphere. There is disagreement on the precise modes of distribution
of radioactive materials projected into the stratosphere.
In the detonation of low-yield nuclear devices, fission products are not
projected beyond the troposphere, and fallout is washed down in periods
of days or weeks. Because winds move principally in east-west
directions, tropospheric fallout appears on the earth in bands centered
approximately at the latitude of detonation. But when high-yield
explosions propel contaminants into the stratosphere, the pattern of
subsequent developments is less clear. It once was believed that fallout
from the stratosphere was distributed more or less evenly—though over
long periods of time—over the surface of the earth. The present view is
that fallout debris placed in the stratosphere remains in that
hemisphere in which the explosion occurs. This concept is based on an
atmospheric circulation theory that air enters the stratosphere at the
equator and descends again in temperate and polar latitudes each spring.
The theory presumes a much shorter “residence time” of stratospheric air
and thus a quicker return of fallout particles to the turbulent
troposphere.[16]
The presence of radionuclides in the atmosphere has provided clues to
cyclical movements of biological importance. During the period of
nuclear tests in the Pacific, observers noted spring “pulses”, or
increases, of strontium-90 deposition in the northern hemisphere, a
phenomenon difficult to verify or explain satisfactorily while testing
was proceeding. Later, when testing had been suspended, the spring peaks
reappeared. The observation seemed to support the theory that nuclear
debris injected into the stratosphere was descending years later through
a gap in the tropopause.
Samplings of nuclear debris by balloon have been under way for several
years at altitudes of 100,000 to 150,000 feet, and rocket-borne air
samplers and other systems have been developed for taking atmospheric
samples up to 200,000 feet.
Programs for studying airborne contamination from industrial
activities—operated at the more accessible but equally difficult levels
of the atmosphere—have been sponsored by the Atomic Energy Commission
near the Hanford Plant, Washington, and at the Oak Ridge, Argonne, and
Brookhaven National Laboratories in Tennessee, Illinois, and New York.
The Hanford studies were started before plutonium production was begun
in 1943, and findings on industrial stack-discharge rates established
patterns for meteorological programs at other sites.[17]