be exceeded in other countries unless particular factors of environment intervene since the U. S. probably has the highest total fallout in the world. It seems very unlikely, however, that environmental factors could increase the level cver the WU. S. by more than a factor of 2 or 3. B. Effect of Rainfsil. As mentioned earlier, there is excellent reason te suppose that the deposition from the troposphere on the earth’s surface is best accomplished by rain. By rain is meant not heavy rain but anything which in- volves the settling of weter droplets. This might in-~ clude fog or mist. The suggestion has been made3/ that the small size of the stratospheric fallout particles gives them a very high mobility due to molecular motion since, in fact, they probably are almost molecular in dimensions. This high mobility of the particles makes it probable that direct coxtact of the fallout particles with water droplets will occur. One imagines on this theory that the tiny particles pass through the tropopause from the stratosphere, and then meet water droplets ina cloud or mist or rain in the course of their rapid random motion due to collisions with the air molecules. So, rather than the classicsl langmuir mechanism of the rain sweeping out the air through whicu it falls by colliding with the particles themselves, the particles probably collide with the water dreplets either before or during the rainstorm, prcbarly most importantly before. It is clear from this mechanism that fog and mist may very well be very effective and that a cloud probvably gathers a considerable fraction of the fallout from the air in its bulk. In eny tase, some experimental evidence has been found for the efiect of raiafall on fallout by studying three particularly arid regions--the Imperial Valley in Califcrnia at the town of Brawley, and the 3/ Stanley Greenfield, Rard Corporation. (more)