STATUS OF GLOBAL RADIGACTIVE-FALLOUT PREDICTIONS LESTER MACHTA U, S. Weather Bureau, Department of Commerce, Washington, D. C. ABSTRACT The atmosphere transports, dilutes, and removes radioactive contaminants. Contaminants added almost anywhere in the earth’s atmo- sphere can be brought to man’s environment or food. The details of individual trajectories and precipitation-scavenging areas play prominent roles for tropospheric sources, and the statistical behavior of the Stratosphere controls the long-term transport and dilution of radioactive material at higher levels. INTRODUCTION The objective of this paper is to summarize certain aspects of predictions of man-made radioactive fallout. Included is adiscussion of the fate of radioactive gases and particulates injected into both the troposphere and the stratosphere. However, a number of important items are entirely or largely omitted; among these are local fallout and closein contamination, stratosphere—troposphere exchange, precipitationscavenging mechanisms, special removal mechanisms such as might exist over the oceans, redeposition, and others. TROPOSPHERIC FALLOUT Predictions of the fate of radioactivity introduced into the tropo- Sphere fall in two categories, The first is a forecast of the generalized fallout made well in advance of an event in order to plan for or to ap- 369