WMOD00311 ‎(2)‎

• Title: Climatologies and Intercomparisons of Rawinsonde Data Collected During Winter Storm Periods in Utah
• Date: 8/1/1980
• State: UT
• Weather Modification - Doc Type: Report

temperature at the tops of the clouds in a given storm. This
response is attributed to the fact that the cloud top temperature
provides an estimate of the number of na. turally occurr1ng
ice nuclei wi thin a cloud. These nuclei are known to be a
primary requirement for the initiation of the "cold rain"
process (i.e., freezing of supercooled water wi thin a cloud
and subsequent growth and fallout of ice crystals). It is
known that the "cold rain" process is of primary importance
in producing precipi ta tion in the western United States in
wintertime. Grant and Elliott (1974) demonstrated that a
critical cloud top temperature of approximately -30oC ,was
evident in several wintertime research oriented seeding programs,
below which seeding caused decreases instead of increases
in precipitation.

Recent evaluations of the Bureau of Heclama tion t supper
Colorado River Basin Pilot Project (CRBPP) ha.s again emphasized
that both posi tiveand negative, effects can be produced by
seeding wintertime storms in the western United states (Elliott
et aI, 1976). Factors determined to influence seeding effects
in this study included: atmospheric stability, strength of
the wind normal to the barrier, water content of the clouds,
and cloud top temperatures.

An outgrowth from the CRBPP evaluations was the development
of some generalized seedability criteria based upon an analysis
of six winter research programs conducted in the western United
Sta tes. This analysis was performed by Vardiman and Moore
(1977) of the Bureau of Reclamation. Seeding on most of
these programs was accomplished with ground genera tors. This
analysis demonstrated the dependence of seeding effects on
four basic considerations: 1) the atmospheric mixing available
to carry seeding materials into the clouds, 2) the water
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