WMOD00246

• Contract/Permit #: #95-1
• Applicant: Western Weather Consultants
• Project Name: Vail/Beaver Creek
• Date: 11/1/1995
• Weather Modification - Doc Type: Application

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BACKGROUND OF CLOUIl SEEDING FOR THE SAN JUAN ECOLOGY
PROJECTl! (W~ Howell, U,S. Bureau of Reclamation)

When vinter snowstorms sweep over the San,Juan Moun-
taina, not all. of the moisture that condenses in the
form of cloud. above the mountains falls 8S 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, ~xpressed as a proportion of the rate at
which moisture condenses in the cloud. is called the
efficiency of precipitation. .

It has been f~und that in some storms, especially
those having relatively deep cloud systems with cloud-
top .temperatures below about -27 C, the precipitation
.efficiency..tend. -to berrelatively high, and in such
situations there i. little that the current knowledge
of weather mod1f~cstion could do to increase the. snow-
fall. In weak storms that condense very .little mo1s~ ..
tute. there i. likewise little potential foi~aiimuln.;'
tion" However'~'.it-ha.'bflen found that. when the:'~"-'
clouds are (fee enou h and active enou h to conde".. ;
rela~ively large amounts 0 moisture but the clou~
~op temperature 18 warmer than about -26 C. seedioR
of the cloud. wit.h artitleial ice nuclei often raises
the prec1p1tat10n et11c1ency trom a rather low value
to one t ieal.'of the colder clo~ Under these
articular conditions, cloud seed n has the oten-
tialo substantia y increasin2 the rate.o precipi-
tation. probably by.as much a9 a hundred percent.
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The Colorado'River Basin Pilot Pro ect was designed
as a statistical test 0 the capabi itv 0 the c ou
seedin technolo of 1971 to brin about precipita-
tion increases.-. .When t e veat er arecasters expected
stormcloud-conditlons considered favorable for seeding
within a twenty~four.hour period beginning at 11 A.H.
(and/If established safety criteria were met)~ an
"experimental.; dayll wa. declared. A randOlllized deci-
.1on tben~v.....de whether the experimental day would
be ..eded ,ol<left' unaeeded .. a eoa.trol'. ' :Tbe exped-
. .-ent, _VU j~~ended to run for fotlr:coneecutive winters
and aceuaai.~e::160 experirlental day. about equally
dlvided-betw~en seeded and unseeded. It actually
ran for five/winter. and accUllulated 71 seeded and 76
unseed.ed day~"~t~
- ",f ...... .
It was thought: that the snowfall on seeded days 1IdRht
~xceed~that on unseeded experimental de 8 b about 15
percent a pOB8 y U to ercent and that.ex-
..per menta aYB R t account or a8 much 88 half of
the season I. .nowfal1." This would have corresponded
'to a maximug'precipitatlon increaBe of about 7.5 per-
cent for the aeason a8 e whole. Although evaluat10n
is not yet c~mplete. preliminary results 1ndicnte
that no such .sizable overall increase was realized.

There appear to have been Wlny "unseeded" days ll1'tl:le-
diately following seeded days when silver iodide
smoke. trapped.1n the valleys upwind of the mountain
range. affected the clouds and effectively caused
the.e days to be seeded. On other days strong winds
carried tpe seeding effect over the mountain ~rest
and outside the study_~rea. On sti~~ ~~her dnys.

designated as "seeded." the diver iodide fail~d for
one reason or another to reach the clouds. On still
other occasions the forecaat of favorable conditions
was unfulfilled or else fulfilled for only a portion
of the 24-hour period. so that any seeding effect was
greatly diluted.

Nevertheless there exists an identifiable subset of
experimental days. free from these disturbinR influ-
ences. for which substantial snowfall increases
could be identitied with a hiRh degree of confidence.

~he tentative conclusions .from the project character-
ize it as a l~ited auccess. It revealed serious
weaknesses ot the 1~/l technolo y 1n the methods used
to identlf "seedable occasions and to pace t e
seedinR material in the desired D ace at t e desired
~. It failed also to give a reasonabLy accurate
meaaure of the potential of cloud aeedins for in-
creasing thfl seasonal precipitAtion. On the other
. hand, it furnished additional evidence that the under-
lying principles are sound and that ~res~nt weakness
has to do mainly with the practical ifficulties of
effective application.

During the five winter seasons of the Pilot Project,
,.320" k$ Jf silver iodide, containing 147 kg of silver,
were dlspersed. An unknown proportion,,'of"Yt "'was 'de-
posited on the ground or 'on vegetation near the gener-
ator sites. a further unknown proportion was carried
to one side or the other 'of the study area or deposited
with precipitation either up~Dd or downwind from the
study area~ or escaped precipitation processes and
remained suspended in the atmosphere. An estimate that
. 70 kg of si~ver actually accumulated within 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 the five years amounts to 65 cm
water-equivalent per season.__If 25 percent of this
fell from seeded .torma, this amounts to about 0.003
gm of silver per hectare ~r centimeter of seeded pre-
cipitation.

During the five years of the'Colorado River.Basin Pilot
project, there,vas a midwiDter dip 1n the frequency of
storms suitable for seeding. corresponding to mean
upper-air temperatures too cold for "seedabil1ty." The
experiments did not establish with a reasonable degree-
of confidence whether precipitation increases would
appear as prolongation of snowfall duratIons or_as in-
creases ot intenS1ty, or botn. Many sucn quest10ns
can be answered only by further experimentation.

However. the results SURRest strongly that the sno~fall
climate under seedin~ will differ from 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 way strange, either
as oerceived or aa it interacts with the natural en-
vironment. is now considered extremely unlikely.

!/ The San Juan Ecology Project did not include studies of the techniques,af cloud seeding, but to set the stage
for the remain~er 0; this volume. we asked the Bureau of Reclamatlon to provide an overview of the clnlJd
seeding activities, which they considerately provid~o.

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