In order to correctly assess the degree of danger posed to humanity by environmental contamination from radioactive products of nuclear explosions and wastes from the atomic industry, and to develop appropriate protective measures, it is necessary to study the laws governing the dispersion of radioactive aerosols and gases in the atmosphere and in other spheres of the Earth. For this purpose, the latest methods and achievements of meteorology, atmospheric physics and chemistry, and aerosol physics and chemistry have been widely applied. In such studies, observational data on the dispersion of naturally occurring “tracer substances” (tracers), such as carbon dioxide, ozone, water vapor, and natural radioactive isotopes, in the atmosphere and other geospheres are of great importance. At the same time, natural and artificial tracers, especially radioactive tracers, have become powerful new tools for the qualitative and quantitative study of the movement of media such as the atmosphere and hydrosphere, as well as exchanges between different reservoirs and geospheres, for example, air-mass exchange between the troposphere and stratosphere or carbon exchange between the atmosphere and the oceans. The study of various Earth-system processes using radioactive tracers has given rise to such disciplines as nuclear geophysics, nuclear geochemistry, and nuclear meteorology. In recent years, nuclear meteorology has developed rapidly both in the Soviet Union and throughout the world. The first monographs in this field have already been published. To facilitate the exchange of methods and research results, two conferences on nuclear meteorology were held: one in Vilnius in June 1960 and another in Obninsk in February 1964. This volume contains the papers presented at the second conference, which was convened by the State Committee for the Utilization of Atomic Energy of the USSR and the Main Administration of Hydrometeorology under the Council of Ministers of the USSR, and held at the Applied Geophysics Institute of the Hydrometeorological Administration. Specialists working on problems of nuclear meteorology and related disciplines participated in the meeting. Approximately sixty reports were presented, addressing the following principal topics: Natural radioactivity in the atmosphere and its application to the study of meteorological processes. The physicochemical properties of anthropogenic radioactive aerosols in the atmosphere, especially so-called “hot particles.” The global dispersion of radioactive products from nuclear explosions in the atmosphere and their deposition on the Earth's surface, including several reports devoted specifically to the dispersion of these products in the hydrosphere. Radioactive contamination of the lower atmosphere from local sources. Instruments and methods for measuring environmental radioactive contamination. Reports on natural radioactivity examined the distribution of measured concentrations of atmospheric radioactive emanations and their decay products, as well as the use of these data in studying meteorological processes at different spatial scales. Measurements of radon exhalation rates were also presented. Reports dealing with the physicochemical properties of radioactive aerosols discussed the particle-size composition of artificial radioactive aerosols in the atmosphere, the separation of fission fragments during nuclear explosions, and the processes by which clouds and precipitation remove radioactive aerosols from the atmosphere. Reports on global fallout presented systematic measurements of atmospheric radioactive contamination levels and deposition of nuclear-explosion products at various locations in the Soviet Union and other countries. These data were used to investigate general problems of atmospheric circulation, including exchanges between the Northern and Southern Hemispheres, exchanges between the stratosphere and troposphere, and zonal exchange processes in polar regions, as well as various meteorological problems. Reports on turbulent diffusion of contaminants in the lower troposphere made it possible to address practical problems connected with incorporating meteorological conditions into chimney design and determining permissible levels of radioactive emissions into the atmosphere. Reports devoted to the methodology of radioactive-contamination studies demonstrated that significant advances had recently been achieved in radiation measurement and radiochemistry. Among these developments were the application of gamma-ray spectrometry for determining the composition of radioactive mixtures, the measurement of ultra-low levels of radioactivity, and the use of flame photometry in the radiochemical analysis of samples, all of which have already found broad use in large-scale monitoring programs. The conference concluded that nuclear meteorology had now become a fully independent branch of atmospheric physics and chemistry. Nuclear meteorology investigates the production, distribution, and removal of radioactive isotopes from the atmosphere, the influence of meteorological factors on these processes, and the use of naturally occurring and anthropogenic radioactive isotopes in the atmosphere as tracers in meteorological research. The conference expressed satisfaction that the Treaty Banning Nuclear Weapon Tests in the Atmosphere, Outer Space, and Under Water, concluded at the initiative of the Soviet Government, had halted further environmental contamination by radioactive products of nuclear explosions. This created more favorable conditions for studying the global fallout of nuclear-explosion products, for using naturally occurring atmospheric radioactive isotopes for scientific purposes, and for investigating the dispersion of radioactive contaminants in the atmosphere and hydrosphere arising from the peaceful atomic industry. These issues form the basis of the various chapters in this volume. Each section begins with a review paper that summarizes the current state of the problem on the basis of Soviet and international scientific literature. The remaining reports contain experimental results obtained in recent years, theoretical syntheses, and recommended measurement methods suitable for large-scale observation programs. The principal comments raised during the discussions were incorporated into the text during editorial preparation, and therefore the discussions themselves are not published separately in the volume. The conference resolution emphasized the necessity of carefully coordinating research in nuclear meteorology and related fields in order to obtain more complete information on environmental radioactivity levels and to broaden the study of the qualitative, and especially quantitative, laws governing the dispersion of radioactive substances in the atmosphere and other geospheres.