reproduced for a detonation over continental land areas or built-up cities but in general the effects would be similar. DISTRIBUTION OF RADIOACTIVE PARTICLES . Tk For nominal bombs (in the range of 20 kiloton yield) the atomic cloud will not rise above the tropopause. (The tropopause marks the level below which is the turbulent air flow of the troposphere and above which is the relatively stable nonturbulent air of the stratosphere). The cloud from a high yield weapon will penetrate into the stratosphere as illustrated by the photograph on page ___—s off:_ the detonation during Operation Ivy in the fall of 1952. Two minutes after the explosion the cloud had risen to 40,000 feet and ten minutes later neared its maximum height of over 100,000 feet. The smaller particles carried into the stratosphere will settle only very slowly until they reach the troposphere where the turbulent air and rainfall will carry them much more rapidly to the earth's surface. The stratospheric storage is uniquely significant since the mixture of radioisotopes present there is enriched in strontium-90, the element of most concern for long-term hazards. This is because stron- tium-90 has a gaseous precursor krypton-90 with a half-life of 25 seconds. Thus, at the time when conditions are optimum in the fireball for the oxides and halides to become associated with molten inert particles, only a fraction of strontium-90 has formed and the gaseous krypton parent is largely carried into the stratosphere. This results