WMOD00071

• Applicant: Western Weather Consultants
• Project Name: Vail Pass
• Date: 11/1/1984
• Weather Modification - Doc Type: Application

BACKGROUND OF CLOUD SEEDING FOR THE SAN JUA.."'{ ECOLOGY
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PROJECT- (W. Howell. U.S. Bureau of Reclamation)
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When winter snowstorms s~eep over the San.Juan Houn-.
tains. not all of the moisture that condenses in the
form of clouds above the mountains falls as snow.
Much of it remains-in particles too small to fall.
Carried beyond the mountains by the wind to where the
airflow sinks once more toward the plains. these par-
ticles re-evaporate. The rate at which snow reaches
.the ground. expressed as a proportion of the rate at
which mo1stur~condenses In the cloud~ is called the
efficiency of precipitation. .

It has been found that in.some.storms. especially
those having relatively deep cloud systems ~lth cloud-
top temperatures below about -27 C. the precipitation
efficiency tends to be relatively high. and in such
situations there i. little that the current knowledge
of weatber =odiftc.tion could do to increase the.sno~-
falL In weak . tl3ms that .condense very little cots-
tu~e. 'there Is l1iltwise little potential for...st1mula-..-
tion. Ho~ver:. to: has been found that when t~ ~_._.
clouds are-deep eno'l'.h and active enough to Cilr.ct4!'t'JI&SQ
r.,;-la:lvely large .l~Ount5 of moisture but the ..:llw::,
top temperature is warmer than about -26 c. seedln~
of the clouds witn artificial ice nuclei often raises
the prec~p1tatlon etI1c1ency tram a rather low va~~e
to one tical of the colder clo~ Under the5~
particular conditions, cloud seed n has the poteli-
tial of substantially increasin~ the rate.of precipi-
tation. probably by as much as a hundred percent.

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The Colorado River Basin Pilot Project vas designed
as a statistical te9t of the capability of the cloud
seeding technology of 1971 to bring about precipita-
tion increases. When the weather forecasters expected
stormcloud cond~tions considered favorable for seeding
_within a twenty-four hour period.beg~nning at 11 A.M.
(and!if established safety criteria were met). an
"experimental day" was declared. A random..1zed deci-
sion then was made Vhether ,th~ experimental day'would
be seeded or left" unseeded as a cootrol-. The experi-
ment was 10tended to run for four consecut.ive 'lrlnters-
and accumUlate 160 experimental days about eqUally
divided between. seeded and unseeded. It actually
ran for fll1e w1Dters and accumulated 71 seeded and 16
unseeded days.

It was thought that the snowfall on seeded days nd~ht
~xceed that 00 unseeded experimental da s b about 15
percent an puss y up to 30 percent,: and that.ex-
perimental cl~~~ might account for as much as half of
the season'n~nowfall. This would have corresponded
to a maximum preci?1tation increase of about 7.5 per-
cent for the season as a whole. Although evaluation
is not yet complete. ~rel~m1nary results indicate
that no such sIzable overall increase was realized.

There appear to have been many "unseeded" days itlll:le-
diately following seeded days when silver iodide
smoke. trapped in the valleys upwind of the mountain
range. affected the clouds and effectively caused
these daY9 to be seeded. On other days strong winds
carried t~e seeding effect over the ~ountain ~rest
and outside the study area. On still other days.
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designated as "seeded," th.e silver iodide failed fa""
one reason or another to ~each the clouds. On stili
other occasions the forecast of favorable conditions
vas unfulfilled or else fulfilled for only a portion
of the 24-hour period. so that any seeding effect was
greatly diluted.

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! Nevertheless there exists an identifiable subse~ of
i experimental days~ free from these disturbinR influ-
I ences. for which substantial snowfall increases
r "could be identitied with ,~ high deRree of confidence.
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The tentative conclusions from the project character-
ize it as a limited success. It revealed serious
weaknesses at the 19/1 tecnnology in the methods used
to identif "seedable" occasions and. to place tne
seedin2 material in the desired place at the eSired
~. It failed alsO- to give a reasonably accurate
measure of the ~otent1al of cloud seeding for in-
creasrng the seasonal precipitation. On the other
hand. it furnished additional evidence that the under-
lyin~ principles are sound and that present veakness
has to do mainly with the practical difficulties of
effective application.

During the ~ive winter seasons of the.PIlot Project.
.32~ k~ ?f ~il~~r iodide. containing 147 kg of silver,
were dispersed. An unknown proportion .of'"1"t"'\i~s .de-:.
..posited on the ground or .on vegetation near the gener-
ator sites; a further unknown proportlon vas carried
to one side or the other "of the stuQY area or deposited
with precipitation either upwind or downvind from the
study area. or escaped precipitation processes and
recaioed suspended in the atmosphere. An estimate that
10 kg of silver actually.accumu1ated..~~thin the 340.000
hectares of the study area appears reasonable. an accu-
mulation of 0.04 grams per hectare season. The current
preliminary estimate of average snow. accumulation in
the study area for tbe five years amQunts to 65 cm
water-equivalent per season. If 25 percent of this
fell from seeded storms. this amounts .to about 0.003
gm of silver per heetare .per centimeter of seeded pre-
e1p.!tation. .

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Dur1Da the five years of the Colorado River Basin Pilot
project. there yas.a midwinter dip in the frequency of
storms suitable for seedins. corresponding to mean ..
upper-air temperatures too 'cold for "seedability." The
experiments did not establish with a reasonable degr~
of confidence whether precipitation increases would
appear as prolongation of snoviall durations or as in-
creases ot lntenSlty. or both. Many such questIons
can be answ~red only by further experimentation.

HoYever. the results sUKgest strongly that the snovfaIl
climate under seedln~ will differ irom the natural cli-
mate only in very subtle ways that can be distinguished,
if at all. only by sophisticated statistical analysis.
That seeded.snowfall will be in any vay strange. either
as perceived or as it interacts with the natural en-
vironment. is now considered extremely unlikely.

The San
for tbe
seeding

Juan Ecology Project. did not include studies.of tbe techniques.o1_cloud
remainder of this volume. we asked the Bureau of Reclamat~on to provide
activities. which they considerately provided.

~~'eding.. .bu~ co. set. the '5:"ta~ii
an overview ~f the. cloud

;18'