WMOD00298

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

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uncertainties as large as a factor of 2 to 3 in diame~er. An example of

cloud droplet spectra at various times after cloud formation point are

shown in Figure 3.3.
Ice crystals are detected by a laser-based detection device

similar to that of Lawson and Stewart (1983). A primary difference,

however, is that the instrument described by Lawson and Stewart uses

transmission/depolarization to detect ice crystals and rolls off to

near. zero response for particles smaller than about 75JSm. Our device

was configured instead to detect single particles by extinction in a

laser beam. Ice crystals which fall through a 3. 8mm diameter hole on

the bottom of the copper liner are collected by a converging air stream

which flows into a funnel-shaped glass sample tube (lOmm o. d. inlet,
3 -1
0.7mm o. d. outlet, flow 15 cm s ). This air stream crosses a HeNe

laser beam (0.7mm diameter), which falls ona 'fast response solid state

photodetector. The extinction signal should be approximately

proportional to the particle cross section area. Experiments have shown

that the technique responds to cloud droplets, ice particles, and

electronic noise. A threshold circuit is used to discriminate against

both noise and small cloud droplets. Cloud droplets rarely reach

diameters larger than 20 to 25~m before settling out of the chamber, so

the threshold is first calibrated coarsely to detect glass beads of

35~m, but not l5~m; finer adjustments were made to discriminate against

droplets in warm cloud formation tests, in which droplets have long

growth lifetimes. The largest droplets in the experiments described

here were l5~m diameter, so it is unlikely that droplets were seen as

ice particles. Other evidence also supports this conclusion (i.e., the

FSSP measurements collected simultaneously with ice formation) .