WMOD00506

• Title: The Persistence of Seeding Effects in a Winter Orographic Cloud Seeded with Silver Iodide Burned in Acetone
• Weather Modification - Doc Type: Report

vollli~e of cloud affected by seeding can be roughly calculated. The result is

7 X 1014 L. Sixteen grams of AgI were released, whic~gives approximately 1015
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\ ~""A.,. -- L~ ,c
nuclei active at: -lOcC .tnet.tott: et a1.. 1983)-." This amounts to an ICe of 1 L-1

from seeding, yet measurements at: 92 min show 10-20 L-1 in the seeded curtain

and ice nuclei still available. These field observations are different than

laboratory rr.easurements in both the activity of the AgI. and the rate at which

it is scavenged by the cloud. The activities measured here are at least an

order of magnitude higher than the laboratory measurements. Also in the cloud

chamber nucleation is complete after 20-30 min (DeMott et al.. 1983), yet the

measurements here show nuclei still available after 92 min. This last

difference could be attributa~le to the lo~er droplet concentration in the

natural cloud sarr:pled here, compared to the cloud chamber where droplet

concentrations are higher by a factor of 100. The collection rate in the

cloud chamber would be significantly faster because of the high droplet

concentration.

Based on these observations AgI is

active

at the warm temperatures

required in the Sierra Nevada, a~d has several advantages over COz.

For CO2,

liquid water is necessary

at: the time

or seeding and all nucleation occurs

instantaneously.

For AgI this is not the case.

Seeding could even be done

cpwind

o~ cloud to allow more time for the material to disperse before t~e AgI

~:.Iclei

encounter liquid water and become active (Hiil,

1980).

Also. wi th AgI,

n~cleation will be spread ou~ over a longer time, thus perhaps making better

use of tp.e liquid water available.

Acknowledgrr.er.ts: This research ~as sponsored by the U.S. Bureau at

Reclamation,

Cepartment of

Ir'lLerio~ .

The support of all Sierra Cooperative

Pilot Project personnel is

appreciated.

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