although carried aloft, is not heated by the fireball to the temperature necessary for radioactive particle formation, nor is it drawn aloft in time to mix with the fission products to form true radioactive particles. The "dry" scavenging action of these dust particles is known to be very inefficient at low atmospheric levels, and would be even more so at higher altitudes. Thug there is no known scavenging mechanism, ex- cept diffusion to the rain-bearing levels, for the very finely divided fission products originally deposited in the stratosphere by large yield weapon detonations. Rain as a means of bringing radioactive particles to the ground is at least several times more efficient than is dry scavenging. In order to utilize this method, however, the particles must be deposited in the rain-bearing levels -- i.e., below the -15° C isotherm -- or transported to this region by gravitational forces or atmospheric effects. All sur- face or air bursts with yields in excess of 8 KT result in clouds with the mushroom stabilized above the -15°C isotherm. ‘Thus, rain to be an effective scavenger depends upon efficient deposition of the radioactive particles in rain-bearing levels. An exception to this occurs in the formation of thunderstorms where localized moisture-bearing clouds are sometimes swept to great heights for short periods of time. ‘Clouds from small weapons (less than 5 KT) generally stabilize within the scavenging ability of the rain-out region under the -15% isothern. The “15°C isotherm for the North Temperate Zone lies between 15,000 and 20,000 feet, although thunderstorms can rise as high as 55,000 feet and last as mich as an how’. Frequency and amount of rain per month or per season are known within limits of useful accuracy for many areas of the world. The primary mechanism for rain scavenging appears to be fall of raindrops through a volume containing radioactive particles, with entrapment of particles in the falling raindrops. vary considerably in size. Raindrops are kmown to Since the “collection efficiency" of rain is a function of the rate of fall of the raindrop and the fall of the 27