METEOROLOGICAL INTERPRETATION OF Sr? FALLOUT’ L. Machta and R. J. List Weather Bureau, U. S. Department of Commerce, Washington, D.C. In Lausanne, Switzerland, on Mar. 27, 1958, Dr. W. F. Libby of the USAEC presented some new information on physical aspects of the world-wide fallout problem and provided an interpretation of these data. It is the purpose of this paper to use this same information and provide a meteorological interpretation of the findings. It must be admitted, as Dr. Libby pointed out in his talk, that there is still room for differences and personal interpretation. 1 BACKGROUND It is a matter of observation, as will be shown later, that Sr®® fallout is nonuniform over the globe. It apparently possesses two types of variations: the first is due to rainfall differences and proximity to proving grounds and the second is a large scale nonuniformity. This latter is characterized by much more fallout in the temperature latitudes of the Northern HemiSphere than elsewhere. The question at issue is the cause of the higher fallout in temperature latitude observations. Dr. Libby’s argument is that the bump in the fallout profile in our latitudes is almost entirely due to tropospheric fallout—fallout which occurs within a month or two from tests conducted within the latitude band of the bump. Steward et al., Machta, and others have argued that the contribution of tropospheric fallout from tests in Nevada and in the U.S.S.R. falls far short of accounting for the temperate latitude bump. They also have claimed that radiochemical analysis suggests that the “age” of the fallout is too great to make most of the temperate latitude fallout tropospheric. However, it has been pointed out that fractionation of the fission products and difficulties in the radiochemistry of the radioisotopes could conceivably make these conclusions misleading. The AEC is currently obtaining “age” determinations from Ba!“ which, it is hoped, will permit a less ambiguous evaluation of the source of fallout in the temperate latitudes. This paper will continue the argument that the stratospheric component of fallout is markedly nonuniform, 2 OBSERVED FALLOUT The observed fallout patterns to be shown refer exclusively to Sr*® results unless other- wise noted. This fission product, as has been repeatedly indicated, represents the radionuclide with the greatest potential world-wide hazard from nuclear testing. It is produced in large quantities having about a 5 per cent fission yield, and a 28-year half life. Physically, it is in a particulate form, but the size of particles that have had an appreciable stratospheric residence time have never been measured. Rather, it is surmised that they must be small enough to remain airborne for years and must therefore be about 0.1 uw or smaller in diameter. Three types of routine and extensive sampling are in progress at ground levels. These are (1) soil analysis, (2) rainwater analysis, and (3) air concentration measurements. Soil * Presented at a Public Meeting sponsored by the Washington Chapter, Federation of American Scientists, May 1, 1958, on Radiation and its Effects, Washington, D. C. 327