WMOD00400

• Contract/Permit #: 14-06-D-6801
• Title: Structure and Seedability of San Juan Storms
• Date: 11/1/1976
• Weather Modification - Doc Type: Report

41

concentrations. At best, they only indicate the presence of AgI. The AgI

particles produced by the seeding generators probably have a nucleating be-

havior different from the particles we collected on our filters.

b. Scavenging of AgI

AgI from seeding generators is apparently most active as a contact

nucleus (e.g., Davis, 1974). It is considerably less active as a deposition

nucleus, or after immersion in warm water. Cloud bases in these storms were

often as \varm as -lOC, where burner-generated AgI rapidly loses its nucleating

ability (Garvey, 1975). Scavenging of the AgI at warm temperatures may in-

hibit its ability to nucleate in the colder portions of the cloud.

AgI particles from burner generators are typically 0.01-0.03 pm in

diameter (c.f., Rogers and Va1i, 1976). Such particles would be scavenged

rapidly in-cloud; Cooper and Vali (1976) estimate the characteristic collec-

tion time for such particles to be less than one minute in clouds such as

could develop \vithin 200 meters of cloud base. In an average updraft of

2 m/sec, only 5% or fewer of the particles smaller than 0.03 pm would remain

unscavenged 500 meters above cloud base, but the temperature would only have

decreased about 3C. This indicates that much of the AgI may be scavenged at

warm temperatures, where its activity is reduced~

That these scavenging calculations are realistic is supported by our

Aitken nucleus measurements. Several examples were studied where the de-

crease in Aitken nucleus concentration in-cloud could be reliably measured

(V.H.2, V.H.3.b). Reductions of Aitken nucleus concentrations of at least

two orders of magnitude were observed in rising water clouds at heights of

about 700 meters above cloud base.

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