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V. ANALYSIS PIBAL POINTS Ie FEB. 1965 , \ ~ A- 1600' .- 1000' PREDICTED _ MEASURED ....-- Ale TRACKS Figure 43c.-Plume Dimensions and Pibal Soundings for 18 February 1965. it does not seem likely that the natural freezing nuclei are being scavenged, but are being used. It would be desirable to confirm this by electron microscope technique with replicas taken from the Park Range; facilities for doing this work are availa- ble at the National Center for Atmospheric Re- search, Boulder, Colo. Of the snowcrystal replicas taken during the past winter season, 33 were examined in detail. About 50 percent showed collected water drops and/or riming and 50 percent did not. Those showing water drops and/or riming were collected during the early winter (January and early February); those without water drops and/or riming were col- lected in late February and March. It appears that the presence or absence of water drops characterizes a storm. Figure 48 is an example taken from the 28 January 1965 storm. About 40 percent of the replicas contain simple crystals only; the remaining 60 percent consist of 35 aggregates. The presence of water drops or riming on aggregates exceeded that on simple crystals by a 2: 1 ratio, and the dendritic form-which is pre- dominant among all other forms-is most favored for the collection of water drops. If artificial nucleation is effective, the quantity of such drops should decrease when artificial nuclei are provided, The seasonal history of supercooled drop population will show whether the effectiveness of nucleation operations can be expected-on a physical basis-to vary with time, and whether- through neutron activation analyses of replicas-the variation is due to the abundance of natural nuclei or to carry-over effects of AgI nuclei. The measurement of Ag concentration in artifi- cial snow replicas3-snow formed by nucleating supercooled water drops in a cold box-show that 3 Schaefer, V. and Fuquay, ]., 1965: "The Detection of Silver Iodide in Snow by Neutron Activation Analysis", Journal of Recherches des Atmospheriques, Vol. II, April- June 1965, No.2, l!5mi R. - ------------ IOmi R. .~~ .;,," 'If !f.... .......'~..."A1 .. ""-, '?;J7'''-= . '- Y ."... .... - =."~:.~iI;"'.J'4$o...4t!i'~ . 'i. .,,4 ~. y~- OAK CREEK ( / ,/ ~ PREmCTEO _ MEASURED ------ TRUCK Figure 43d.-Plume Dimensions and Pibal Soundings for 22 February 1965.