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] PROBLEMS AND PROJECTS