shed considerable light on those problems. Specifically, it seems that the analysis of dispersion of atomic debris could be used a) to check atmospheric motionby a verification of computed tra- jectories, b) derive information relative to the collection efficiency (scavenging) of natural precipitation and c) evaluate turbulent diffusion on both a small and large scale. Since the MIKE cloud penetrated so far into the stratosphere there seemed ~ phteatetnn Ante Cn pe tha AL Ornate exchange ra han to be hope ofFF ob wiring [ata the aiifusive of matter between the stratosphere and ‘troposphere. Anticipation of important meteorological gains from this data, unfortunately, has been largely unfulfilled, because of the appalling number of uncontrolled and unmeasurable variables. Cone ' Sider first the check on winds by studying the accuracy ofcomputed trajectories. There were some cases in which the debris appeared *. to move more rapidly than estimated by computations based on wind observations, both during Pacific and during continental tests. Aside from errors introduced by uncertainties in the wind field, inaccuracies will occur for two reasons. First, meteorologically computed trajectories estimate the mean motion of the cloud, - omitting any forward diffusion. Second,. and more important, the first material to arrive at a point is often the result of complicated, unpredictable three-dimensional motion. The computed trajectories, — in other words, do not always portray the most rapicly~-moving segment. of the cloud and the meteorolegist faced with the accomplished fact . of unexpectedly fast transport can seldom deduce the three-dimensional ‘path for there are often numerous equally-probable solutions. There have also been cases studied where the computed trajectories overestimated the actual speed of debris, but this appears to be explained by the development of an undetected disturbance which moved the debris in an unexpected direction. Im summary, then, there have been many cases of observed fallout which indicate that the computed trajectories were in error, but it has not yet been possible to improve the meteorological techniques through this experience. _ The Ivy data again demonstrated the fact that a greater quanti ty of debris is deposited on days of rain than on days with no rain. _The increase in rain in the United States during the IW tests, however, was not as great as was the increase noted during continental tests. It is believed that the difference results from the fact that only smaller and less readily scavenged particles remain airborne ' by the time the IVY clouds arrived in the United States. Since there is no data available to verify this hypothesis, however» at must be regarded as only speculation. ‘There has been no attempt to employ ‘the spread of debris as Ww the ~ 8 observed at the surface of the earth to estimate the coefficients of . eddy diffusion. The reasons are several. Most important is the fact - 3 '