WMOD00298

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

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content (LWC) can be varied from 0.3 to 3.0 g m without changing the
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droplet size appreciably. Temperature within the 1 m experimental

volume can be maintained to within +/- 0.3.C of the setpoint over a

range from 0 to -25'C.

The cloud density is continuously monitored by means of a

Cambridge Systems dewpoint hygrometer. The technique employed is to

evaporate a cloud sample and measure its dewpoint; the difference

between the saturation mixing ratio corresponding to the dewpoint

temperature and that corresponding to the cloud temperature is taken as

the liquid water content. Temperatures throughout the system are

measured by thermocouples and are recorded continuously. Droplet sizes

have been measured using soot-coated slides with a device patterned

after that of Squires and Gillespie (1952) and using a Particle

Measuring Systems Forward Scattering Spectrometer Probe (FSSP-lOO)

(DeMott et a1., 1983). Mean droplet diameters range between 6 and 9 ~m

on all occasions. Representative mean droplet concentrations are 2100
-3 -3 -3 -3
cm at 0.5 g m and 4300 cm at 1.5 g m LWC. Representative cloud

droplet spectra are shown in Figure 3.8. The cloud simulates a slowly

settling fog or stratus cloud at water saturation, and its quasi-

steady-state nature allows nucleation and ice crystal growth to be

studied as a function of time.

Ice crystals settling from the cloud after injection of artificial

ice nucleating aerosols are collected on microscope slides and are

counted using a cold-box microcope. Slides are sampled from the chamber

periodically until nucleation ceases. The uncertainty in this procedure

is about 30% based on past results. Counts are typically converted to

numbers effective per gram of nucleant dispersed (- Yield), but for