WMOD00285 ‎(2)‎

• Title: Feasibility Study of Wintertime Cloud Seeding to Augment Arizona Water Supplies
• Date: 1/1/1987
• State: AZ
• Weather Modification - Doc Type: Report

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Arizona, is the isolated convective cloud. It is characterized by
moderate to strong convection leading to the generation of high liquid
water contents. These clouds generally contain high ice c~stal con-
centrations with cold cloud tops, but due to their stronger convection,
appear to often generate excess condensation that may offer cloud treat-
ment potential.

3.3 Treatment methodology

The primary objective of cloud seeding is the initiation of additional
ice c~stals within a volume of cloud containing excess SLW (supercooled
liquid water), upwind of a selected target area. The new ice crystals
grow at the expense of the excess water which would otherwise escape the
barrier and evaporate in the descending airflow over the lee slope.

Water substance escapes to the lee side of the barrier and evaporates
when ice crystals form too late for sufficient growth time to reach the
surface prior to the mountain crest. A seeding strategy must be deve-
loped to create the ice crystals sufficiently upwind to allow adequate
growth for deposition on the upwind portion of the barrier (figure 1).

Current recognized cloud treatment techniques consist of the delivery to
a selected cloud volume of (1) silver iodide complexes (Finnegan, et
~., 1984) by aircraft or turbulent transport via ground release, and/or
(2) dry ice pellets (solid carbon dioxide) by direct injection from
aircraft. Silver iodide complexes and dry ice are currently the pre-
ferred cloud treatment agents. Dry ice pellets create ice crystals more
efficiently at temperatures between 00 andl -6 oC, thus giving preference
to this agent for warmer clouds. Silver iodide complexes are more effi-
cient at creating ice c~stals with temperatures colder than -6 oC.

Silver iodide complexes can be dispersed by ejectable pyrotechnic flares
creating a curtain of treatment agent, and by flying through the cloud
volume and igniting burn-in-place flares or combustion generators con-
taining AgI-NH41-NH4CL04-acetone-water (Finnegan, et ~., 1984) which
lead to an expanding tube of the agent. The droppable units allow the
placement of the curtain of seeding agent within or upwind of the impor-

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