Field Observations of Ice Crystal Formation in Clouds at Warm Temperatures

Richard L Pitter, William G Finnegan

Abstract


A field study of the effect of treating a shear line convective cloud with a 20% aqueous solution of ammonium carbonate is described. The results indicate that the presence of certain soluble salts in growing ice crystals may be sufficient to initiate ice crystal multiplication in clouds at temperatures of -4 to -5 C, as measured by a Rosemount instrument, or -3 C, as measured by a reverse flow thermometer. The field results are consistent with laboratory experiments conducted in the Desert Research Institute’s cloud chamber, where ice multiplication was observed at temperatures as warm as -4 C. In the cloud chamber experiments, ice multiplication only occurred when crystals were growing rapidly and aggregating. Aggregation was more readily observed when certain soluble salts were added to the water which formed the supercooled cloud. A postulate is advanced as an explanation of the ice multiplication observed in the laboratory and field studies.

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