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I I I I I I I I I I I I I I I I I I I a bit more northerly than on 7 December and the Emerald wind is tbe same, During this period, a tracer experi:ment was performed in con- junction with the regular seeding plan. Lithium stearate powde l' (inert as both a condensation nucleus and as an ice nucleus) was ernitted alongside the silver iodide generator and was detected at Rabbit Ea.rs by an aerosol flame photometer in concentrations two order of magnitude above background. The significance of these data is uncertain as the tracer was emitted only once under the conditions of this case - the only time the flame photometer was available. Two ideas might help to explain the presence of the tracer and absence of ice nuclei. One is that nearly all of the silver iodide parti- cles are effective as ice nuclei and result in snow crystals whichs with any appreciable fall velocity, do not enter the counter. The lithium tracer particles are not effective as ice nuclei and would not be so affect- ed. The second idea is based on preferential scavenging of the particles by the snow which was falling during the te st. The mean diameters of the silver iodide and lithium stearate particles are about 0.05-0, IF and 2~5/~t respectively. Because of these different sizes, one might expect diffe rent scavenging efficiencie s both from a mechanical and diffusive viewpoint '. But at this time: it appears that the most likely explanation for tbe favorable wind/low nuclei count case is that the nuclei are being swept upward by local 'Ie rtical motions, This case may be the only one in which artificial nuclei are being carried to significant altitudes above the target. Although the counter at Buffalo Pass was operated much less often than the one at Rabbit Ears, the data show basically the same results" Most often.. the Emerald wind appeared to be the factor controlling the - 37 -