radioactive particle, it is thus a function of the densities and diameters. The second method of rain scavenging which accounts for a smaller fraction of rain-out is the intermingling of an atomic cloud with a rain cloud. The rain cloud in this instance gathers up bomb debris ocr other contaminated particles into small droplets, which are then collected by large water drops and brought to earth as radioactive rain. This method of cleaning the air is most efficient in particle sizes of about 1 micron but is capable of scavenging any particles that are less than 2 microns in diameter. The mechanism of particle movement down from the stratosphere and into the troposphere is. not understood. In fact, it has not been definitely ascertained whether any of the small size stratospheric bomb debris has come to the ground. Such data, when available, would yield important information concerning the exchange of matter between the stratosphere and the troposphere. drawn: One tentative conclusion can be if particles are so light as to remain in the stratosphere for long periods of time, there is an increased chance of the harmful isotopes being dispersed; hence the reconcentration of these isotopes would be unlikely. It may also be concluded that particles small enough to remain aloft for sufficient time to become a possible longrange threat are too small to be scavenged efficiently by rain when they reach rain-bearing levels. During thunderstorm formation, the rain cloud is sometimes swept up to the tropopause where the cooled water particles form ice crystals about any small debris or dust that might be in the air. Because the lower atmosphere air is drawn up into the cloud the ice crystals descend and ascend alternately wtil they become hailstones, or are captured by raindrops, and fall to the ground during a thunderstorm or hail storm. Measurements of radioactivity in large hailstones from a hail storm at Washington, p.c.,2/ following Shot 10 of UPSHOT-KNOTHOLE List, R.J., The Transport of Atomic Debris from OperationUPSHOT- KNOTHOLE, U.S. Weather Bureau Report, NYO-4602, June 1954, SECRET Restricted Data. 23