WMOD00298

• Title: Quantifying Ice Nucleation by Silver Iodide Aerosols
• Date: 5/1/1990
• State: CO
• Weather Modification - Doc Type: Report

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4.1.1 Deposition Nucleation Experiments

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The approach to the deposition studies was to perform continuous

expansions to water saturation in the dynamic chamber at a slow ascent

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rate, to cover the range of humidities between ice saturation and water

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saturation for temperatures between -50C and -20oC. In this way.

nucleation of freezing of the liquid phase was limited from influencing

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the results. A thermodynamic path for such an expansion is demonstrated

in Fig. 4.2. The measured cumulative ice crystal flux settling from the

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cloud could be related to a given temperature and percent ice

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supersaturation (S.). S. was determined from the humidity measurements,
1. 1.

with verification provided by the observed thermodynamic cloud point.

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This approach assumes that once the critical supersaturation is

exceeded, nucleation occurs instantaneously; and the time response of

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the experiment is limited by ice crystal growth and fallout. Only

Anderson and Hallett (1976) have noted a finite delay before the onset

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of nucleation at some defined temperature and supersaturation. Edwards

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and Evans (1960) and Schaller and Fukuta (1979) did not find this delay

and neither study observed any nucleation to occur after initial ice

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formation.

To evaluate such potential effects in the dynamic cloud

chamber, one couid conduct intermittent expansion experiments in which

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a set of conditions are established and held to measure the nucleation

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rate and the ultimate number of ice crystals that formed, followed by

another brief expansion to produce a higher value of Si' at a slightly

colder temperature. This has not been done due to the additional time

that would be needed to implement the special coolant control procedure

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required.

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