WMOD00298

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

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since this is an accepted terminology. It should be quite clear,

however, that this terminology truly refers to the macroscopic process

(sometimes multi-stage) by which an ice crystal forms, and not the

microscopic mechanism that describes how the ice phase first appears or

is ordered on a particle surface. Experiments were designed to

systematically isolate nucleation mechanisms operative by direct

measurement and subtraction methods, primarily using continuous

expansions experiments. For the two nucleants employed, certain

nucleation mechanisms are favored or dominant under some cloud

conditions because of chemical and physical properties and the history

of environmental exposure. Flexibility was inherent in the approach,

but not in the basic framework.

In all experiments, the ice nucleating aerosol, generated in the

same way as in operational weather modification projects, was sampled

and diluted as needed with dry air before injection into the DCC. Two

types of seeding materials were used. These were AgI-AgCl and AgI-AgCl-

NaC1 aerosols. These aerosols represent the most efficient AgI-type

aerosols actively used for weather modification, based on current

knowledge. Most experiments utilized narrow size cuts of particles, in

order to differentiate particle size effects. When such nearly

monodisperse nuclei were desired, the sample was first transferred to a

particle size classifier. The aerosols were injected directly into the

DCC before or after cloud formation, depending on the desired

measurement conditions. Total aerosol concentrations were measured

beforehand. Two particle sizes were generated for the experiments:

distributions of particles closely centered at O. 03 ~m and O. 07 ~m

diameter. This covered a sufficient range of dry particle mobility and