[VoL. 91 of environ- ictivity in a ince, appar- d at Novya from these lantic Coast » the Missising air. The _ September 19641 YARTIN : RADIOECOLOGY AND STUDY OF ENVIRONMENTAL RADIATION 297 in the mid-latitudes of the northern hemisphere is higher in late winter and early spring than at any other time of year (Fig. 4). Several hypotheses have been offered to explain these latitudinal and seasonal variations in the amounts and rate of stratospheric fallout but several important issues have not yet been resolved (Arnold and Martell 1959, Libby 1956a, 1956b, 1957, 1958, 1959, Machta 1959, Machta and List 1956a, 1956b, 1958, Martell and Drevinsky 1960, and Stewart et al. 1957). Estimates of the residence time or effective half-life of radioactive debris in the stratosphere have ranged from ‘‘a matter of months’’ (Martell and Drevinsky 1960) to as much as 5 to 10 years (Libby 1956a). Apparently, the residence time and rate of stratospheric fallout are not uniform but tend to decrease with decreasing altitude and with increasing latitude of introduction. The rate of stratospheric fallout is highest for materials introduced in the lowest part of the stratosphere at high latitudes and lowest for materials introduced at higher altitudes near the equator. In both the troposphere and the stratosphere the transport of materials is toward the northeast at mid-latitudes and toward the southwest at higher latitudes. In mid-latitudes, the transfer of the stratospheric materials into the troposphere probably occurs in the vicinity of the subtropical jet stream. At high latitudes, this transfer occurs in high pressure (anti-cyclone) areas __— ¥ O Mar 1961 July 1961 rage monthly “it was 500 ning 1962). surface or 1e sufficient debris into balls which y a nuclear sphere. ratospheric deposition ude (Alex159, Health e of fallout where there is a pronounced subsidence of stratospheric air during late winter and early spring. In both hemispheres the rate at which stratospheric materials are removed from the troposphereis closely related to precipitation patterns. The inter-hemispheric exchange of atmospheric debris probably occurs in conjunction with the seasonal, latitudinal migration of global high and low pressure belts and their associated wind systems. The actual processes involved in these exchanges between the stratosphere and troposphere and between the northern and southern hemispheres are imperfectly understood. Studies now in progress may help to resolve a variety of questions concerning the mechanismsof stratospheric and tropospheric fallout, but the solution of these problems will probably be a greater boon to meteorology than to radioecology. The critical problems in radioecology are related to the measurable aspeets of fallout rate and the distribution of particular radionuclides. The radionuclides generally considered to be most important in the evaluation of the potential biclogical hazards of world-wide fallout are C-14, Sr-90 and Cs187. The C-14 produced by nuclear detonations and suspended in the atmosphere as C“O, mayeventually be distributed uniformly in the atmosphere of both hemispheres. Most of the Sr-90 and Cs-137 will probably be deposited in the northern hemisphere between 30° and 60° latitude. A better understanding of the mechanisms which determine the rate and geographical distribution of world-wide fallout would be of particular value