-5- fraction of particulate matter which is of just the right size to descend in a matter of weeks. The division into the two types, the local and the stratospheric world-wide, is very sharp and marked, however, and to a very considerable approximation one can say that the megaton weapons yield the bulk of their fallout in these two categories. A weapon involving one megaton of fission would if fired in the air place most of its radioactivity in the stratosphere, and this in contrast to the tropospheric fallout appears to be widely distributed latitudinally and does not descend through the tropopause into the troposphere until months and years--an average time of about 10 years--have elapsed. The resultant . pattern of fallout appears to be essentially an approximately uniform world-wide distribution. The long time spent in the stratosphere is probably due to the absence of such scavenging agents as rain or snow, so the particles either must fall-out of their own weight or diffuse downward by slight eddy motion, either of these processes being of their very nature slow. After passing through the tropopause irto the troposphere they will be rained out rather quickly in a matter of days or weeks. Because of the long residence time in the air, this type of fallout is particularly harmless as a gamma ray hazard, since only the Cs137 is left. The amount of Cs137 is about the same in millicuries as the sr99, so one megaton of fission thus di gtrivutgd throughout the stratosphere would yield about 3 mc/nt of either Sr? or Csi3 Just as in the case of Sr99, the rate of deposition of about 10 percent of the reservoir per ygar POofmela Py a stratospheric fallout rate of Csl37 of 0.05 mc/mi*/yr in the beginning. This rate decreases to half, or 0.025 me/mi2/yr, at about 7 years as the stratospheric reservoir becomes depleted. After the Castle test series was completed there were about 24 megatons of fission erates in bbe stratosphere, corresponding to about 12 mc[mit195 Sr on the average and about the same amount of Csl37. The subsequent stratospheric world-wide fallout rate appears to have been a little over 1 mc/mi2/yr all over the world. In addition, of course, local fallout and (more) 126