WMOD00298

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

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required for the unit impulse to settle from the chamber, and At is a

small increment of time.

The transfer function for the procedure in this work is based on

pulse nucleation results in clouds cooling in the -7 to -l2DC

temperature range. Injection of CO at the top inlet to the chamber was
2
controlled so as not to overnucleate the clouds. A small mixing fan was

used to mix the small crystals throughout the cloud volume for a few

seconds. Thus, crystals always grew in a water saturated environment.

The ice crystal signal for a particular pulse nucleation test and the

transfer function fit to the data from several of these experiments are

shown in Figure 3.4. This result is quite consistent with expectations
based on ice crystal growth and fall velocities. The crystal growth and
fall equations of Rogers and Vali (1987) can be used to estimate how

long it takes for growing crystals to fallout of the cloud chamber.

For example, typical observed cloud droplet size extremes, l~m and 15~m

diameter, may be used to bracket initial ice crystal sizes after

nucleation at any point in the cloud chamber. These crystals are

allowed to grow and settle in a water saturated environment at -lODe

and 600mb. The 15~m particle falls 180cm from the top to the bottom of

the chamber 58s later as a skeletal plate crystal 70~min diameter

(the change from spherical to plate habit is assumed to occur at 20~m).
The l~m particle takes 70s and is 68~m in diameter. If these ice
crystals start from the middle of the chamber, the fall times are 37
and 50s, and the crystal lengths at the bottom are 50 and 49~m. These

model results suggest that the crystals should be of detectable size,

and if they all nucleated at once, they should be detected between

about 30 and 80s. These times are very similar to the times for crystal