WORLDWIDE EFFECTS OF ATOMIC WEAPONS DISTRIBUTION OF RADIOACTIVE DEBRIS the north or south by the large-scale disturbances. The scale of motion of the order of magnitude of the cloud will tend to distort the cloud, stretching and twisting it into wisps and clumps. Scales of motion smaller than the cloud will stir it and expandit in an eddy diffusion process. ‘This is a very much simplified version of what happens, and it neglects the disparity between the horizontal and vertical motions. Horizontal motions in the atmosphere are several orders of magnitude greater than lower layers of the air. (A discussion of the settling of particles in a standard atmosphere, neglecting convection effects, is given in Appendix HI.) A cloud as large as the one described above can be acted on by the entire spectrum of atmospheric motions. In this event all the motions of 24 vertical mpttons, but the change in horizontal motion in the vertical, 1.e., the vertical shear, is several orders of magnitude greater than the change of horizontal motion in the horizontal. Thus, the portions of the cloud at various heights will be moved with different winds. If the cloud extends ee Sime we ae Om pe from 30,000 to 40,000 ft, the top of the cloud will, on the average, be about 300 km away from the bottom of the cloud after 24 hr. One additional type of atmospheric motion is convection. The most dramatic display of atmospheric convection is, of course, the thunderstorm, but many less spectacular forms of convection actto stir the entire troposphere rather thoroughly. If the cloud enters an area of moderate-to- violent convection, it can spread the cloud through the entire depth of the troposphere in a matter of a few hours. The extreme variability and lack of adequate observations on some scales of motion of the atmosphere makeit difficult to assign numbers to these atmospheric effects, but from the limited knowledge available an attempt will be made to give ordet-of-magnitude estimates. Consider the history of an atomic cloud in the middle latitudes in winter, with the cloud stabilizing between 30,000 and 40,000 ft and not encountering a con- vective area. The cloud will move eastward about 1000 mi in 24 hr and will be about 700 mi north or south of its original latitude. The top of the cloud will be about 300 mi away from the bottom. The cloud will have spread, at each altitude, to a radius of about 40 mi and may have moved vertically a few thousand feet. Thus at the end of 24 hr the cloud will be contained in a ribbon about 300 mi long, 4 mi wide, 15,000 ft thick, and About T200 Mi from its Oripinal wit wiht not be distributed in any regular way, but will consist of clumps and wisps of high concentration with areas of low concentration in between them. If convection affects the cloud, it will be mixed throughout the troposphere and will be extended even more in the horizontal by the shears in the - 25 the atmosphere, except the mean west-east flow, can be thought of as turbulent eddies. Defant' has computed an eddy diffusion coefficient for this motion, and by using his value of 10" cm?/sec, it was estimated that the spread of the cloud after 1 week would occupy an area of about $000 mi in diameter. The processes, other than fall, that can bring the debris to the ground are scavenging by rain and mass transportof air downward. Data gathered by the AEC New York Operations Office (NYOO) indicate that by far the most important of these is the rain scavenging. The data analyzed to date do not provide a means of measuring the scale of the mass transport. They do show, because small particles are brought down in dry air, that mass transport must, at times, be operative. During the time that the cloud is diffusing, it is subjected to the scavenging action of rain and to the removal of debris to the ground by mass transport and fall. For detonations resulting in a large percentage of activity on large particles, the maximum deposition will result from the physical fall of the particles close to the point of detonation. For detonation resulting in a majority of the activity on small particles, the fall will be less important, and the dispersive processes of the atmosphere will spread the debris over large areas. If the debrisis initially contained in the troposphere, in mid-latitudes, most of it will tend to remain in the tropo- sphere and becarried primarily in the westerly circulation of mid-latitudes. There are, however, several ways for the material to be carried into the southern hemisphere in the troposphere: one well-organized transport in the Indian Monsoon and many small-scale motions can carry the debris south at almost any Jongitude. Debris thrown into the stratosphere can easily be brought down either through breaks in the tropopause or through a process of eddy diffusion through the tropopause. In either event, however, such transport is ex- pected to be slower than transport in the troposphere.