WMOD00560

• Project Name: Project Skywater
• Title: Project Skywater - Generalized Criteria for Seeding Winter Orographic Clouds - Summary Report
• Date: 3/1/1977
• Weather Modification - Doc Type: Report

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The basic information used in this analysis was the rawinsonde data
at 6-hour inter~als during experimental events (these soundings give
the vertical distribution of temperature, moisture, and wind), 6-hour
precipitation tiotals summed from 2 hours before the sounding to
4 hours after the sounding, descriptions of the individual mountain
barriers and se!eding networks, and tabulations indicating when the
seeding generat!ors were on and off. If at least one generator was
on during the p'eriod' 1 hour before the start of a 6-hour block to
1 hour before t~e end of the block, the period was considered seeded.
Carryover cont~ination was not considered in this analysis.

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Some of the pro!jects had sounding data at intervals more frequent
than the 6-hou~ interval used in the analysis. Some projects had
other kinds of :data, such as in-cloud measurements and radar obser-
vations. Although not used in the analysis itself, these data pro-
vided insight useful in interpreting results from the statistical
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analysis. !

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An important p~rt of the analysis was to identify where the effects
of seeding occtirred as well as the magnitude of the "effect. Therefore,
the precipi tatilon gages were grouped in similar regions in relation to
the mountain barrier. The crest group included those gages in the
primary target :area of the individual proj ects, generally those along
and on either slide of the ridge crest. Other groups of gages were
considered as upwind or downwind.
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II. Analysis Procedures
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This st~dy con~idered 10 meteorological variables that depend on the
shape and size ;of the mountain barrier and on the characteristics of
the clouds. The variables were grouped into four major physical cat-
egories: (1) the atmospheric mixing available to carry seeding mate-
rial to the apBropriate portion of the cloud, (2) the water available
for conversion :to precipitation, (3) the natural nuclei available to
start the prec~pitation process, and ,(4) the time available for nucle-
ation, growth, :and fallout of the precipitation.

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Using these va~iables, stratifications were performed by project and
groups of proj~cts to determine seed/no-seed ratios and statistical
probabilities, !which suggest whether seeding caused a change in pre-
cipitation. A !nonparametric statistical test (Wilcoxon two-sample)
was used becau~e it required no predeterm~ned assumptions about the
distribution of the data. Each of the variables was stratified to
optimize posidve and negative seeding effects. Some single-variable
stratification~ showed clear demarcation of seeding effects for one
project but corlflicting results for another project. For other proj-
ects, the same Istratification was completely uninformative.

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