In Situ X-Ray Analysis of Ice Crystal Fallout from Seeded Clouds

Abstract

A cloud chamber mounted on a commercial x-ray diffractometer has been used to obtain ice crystal shapes, numbers, and volume information from clouds seeded with AgI-NaI aerosols. Data obtainable from this system include automatic printout of integrated diffraction intensities, photocell traces for fog densities, four-point temperature readout for the chamber, and optional Formvar replication. Aerosol efficiency data for the AgI-NaI particles are comparable to those generally published with the exception of two anomalies: 1) a pronounced dip in the efficiency curve at ?12C interpreted to mark the transition from condensation-freezing to sublimation nucleation, and 2) a maximum in the curve at ??20C associated with a maximum in proportion of plates in the plate-column crystal mixture. At ?20C and an aerosol dilution density of 5 ? 106 cm?3 we observe about 5 ? 1015 active nuclei per gram of AgI. At dilution densities in the 108 cm?3 range the efficiencies are reduced an order of magnitude. X-ray and Formvar replica data indicate that plates are most common from ?17 to ?22C, with thick plates and short columns occurring above this temperature and long hollow columns and frozen water drops at progressively higher temperatures. We also observe the mean of the volume ratios of plates to columns for all runs to be 0.5, in fair agreement with other observations.