WMOD00427

• Title: Design of Physical Cloud Seeding Experiment for the Arizona Atmospheric Modification Research Program - Final Report
• Date: 2/1/1991
• Weather Modification - Doc Type: Report

randomized seeding experiment to permit statistical evaluation of seeding effectiveness over a
number of winters and over large target areas. Presuming the statistical phase was also successful,
Arizona would then be in a position to evaluate whether to commence with operational seeding
aimed at enhancing the water resources of the Mogollon Rim. By then the achievable magnitude
of precipitation augmentation should be known, along with associated costs and benefits.

1.1.3 Purpose of Physical Experiments. - This report discusses the results of Reclamation
efforts on the Arizona Program from early 1989 to early 1990. The goal of these efforts was to
design comprehensive physical cloud seeding experiments and subsequent analysis approaches for
the Mogollon Rim. The experiments are intended to document each significant link in the chain
of physical events following release of seeding material up to, and including, snowfall at the ground.
Sufficient measurements will be made to permit examination of each experiment concerning its
success or failure to enhance snowfall in a small predetermined target area. As stated by Hobbs
and Radke (1975), "In carrying out a physical evaluation the objective is to record a chain of events,
from cloud to ground, which are consistent with the predicted effects of artificial seeding and which
produce perturbations which are distinct from those to be expected naturally." Hobbs (1975a)
noted that physical experiments are not intended to replace statistical experiments, which are
needed to evaluate the effects of seeding many storms over a large target area, but are a logical
precursor that greatly aid design and evaluation oflater statistical experiments. Braham (1981) also
makes a strong case for improving our understanding of the physical processes in clouds and their
reactions to seeding before conducting any more large field experiments aimed at demonstrating
surface precipitation changes while ignoring intermediate processes.

1.2 Past Attempts At Physical Seeding Experiments

1.2.1 Historic Overview. - Physical changes caused by seeding supercooled clouds were obvious
in the first cold box experiments by Schaefer that started scientific weather modification (Schaefer,
1946). Later the same year Schaefer scattered dry ice along a line over a stratiform supercooled
cloud deck, which quickly converted into snowflakes that fell about 600 m before sublimating
(Schaefer, 1953). Several photographs from the late 1940's show portions of supercooled stratus
cloud converted into ice crystals. Thus, physical evidence that seeding can convert supercooled
cloud into ice particles is available from the earliest work in scientific weather modification.
However, it has proven to be much more difficult to obtain physical evidence that seeding initiates
a complex chain of events which finally affects surface precipitation,

Much cloud seeding experimentation of the 1950's and 1960's was statistical in nature, with clouds
seeded or left as control cases based on random decision. The experiments were usually of the
"black box" type, in which seeding material was released and precipitation was monitored but the
intervening physical processes were not routinely observed because of limitations in instrumentation.
While some of these experiments strongly suggested precipitation changes (increases or decreases)
related to seeding, not one is widely accepted as having provided scientific proof that such
modification occurred, Byers (1974) presented a history of this period.

1.2.2 Overview of Physical Experiments. - A number of reports and publications will be cited
which describe attempts to conduct comprehensive physical cloud seeding experiments. Such
experiments, hereafter called physical experiments, strive to document most or all of the key
physical processes hypothesized to follow the release of the seeding agent. These processes include
the transport and dispersion of the agent into the intended volume of cloud, creation of an
appropriate concentration of ice crystals, initial diffusional growth of the crystals, possible further

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