WMOD00002

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

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

When winter snowstorms sweep over the San.Juan Moun-
tains, not all of the moisture that condenses in the
form of elouds 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 moisture 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 with cloud-
top temperatures below about -27 C, the precipitation
efficiency tends to be relatively high, and in such
situations there is little that the current knowledge
of weather modification could do to increase the. snow-
fall. In weak storms that condense very little mois-
ture. there is likewise little potential foi-sti~la-
tion. However, it has been found that when the------:;--
clouds are-deep enol1~h and active enough to condeM8
relatively large amounts of moisture but the clou~
top temperature is warmer than about -26 C. seedJ.n:!.
of the clouds with artificial ice nuclei often raises
th~ precipitation ettlclency trom a ratner low value
to one tical of the colder clo~ Under these
particular conditions, cloud seed n has the poteu-
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 was designed
as a statistical of the capability of the cloud
seedin technolo 1971 to bring about precipita-
tion increases. t e weat er orecasters expected
stormcloud conditions considered favorable for seeding
within a twenty-four hour period beginning at 11 A.M.
(and!if established safety criteria were met), an
"experimental day" was declared. A randomized deci-
sion tben was made whether the experimental day would
be seeded or left. unseeded .. . control. The experi-
ment was intended to run for four consecutive winters
and accumulate 160 experimental days about equally
divided between seeded and unaeeded. It actually
ran for five winters and accumulated 71 seeded and 76
unseeded days.

It was thought that the snowfall on seeded days might
.exceed that on unseeded experimental da s b about 15
percent an pass y u to percent, and that.ex-
per menta ays mig t account for as much as half of
the season's snowfall. This would have corresponded
to a maximum precipitation increase of about 7.5 per-
cent for the season as a whole. Although evaluation
is not yet complete. ~rellm1nary results indicate
that no such sizable overall increase was realized.

There appear to have been many "unseeded" days Itnr.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 days to be seeded. On other days strong winds
carried the seeding effect over the mountain ~rest
and outside the studl_~~ea. On still other days,

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

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Nevertheless there exists an identifiable subset of
experimental days. free from these disturbin~ influ-
ences. for which substantial snowfall increases
could be identified with a high degree of confidence.

The tentative conclusions from the project character-
ize it as a limited success. It revealed serious
weaknesses of the 1~/~ technolo y in the methods used
to identify "seedable occasions and to pace t e
seedin~ material in the desired place at the desired
~. It failed also fO give a reasonabl~ accurate
measure of the potential of cloud seeding for in-
creasing the seasonal precipitation. On the other
hand, it furnished additional evidence that the under-
lying principles are sound and that present weakness
has to do mainly with the practical difficulties of
effective application.

During tbe five winter seasons of the Pilot Project,
.320 kg of silver iodide, containing 147 kg of silver,
were dispersed. An unknown proportion of l"i -was 'd:~
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 upwind or downwind from the
study area. or escaped precipitation processes and
remained suspended in the atmosphere. An estimate that
70 kg o~ silver 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 storms, this amounts to about 0.003
gm of silver per hectare per centimeter of seeded pre-
cipitation.

"

During the five years of the Colorado River Basin Pilot
project, there was a midwinter dip in the frequency of
storms suitable for seeding. corresponding to mean
upper-air temperature. too cold for "seedability." The
experiments did not establish with a reasonable degre;-
of confidence whether precipitation increases would
appear as prolongation of snowfall durations or as in-
creases ot Intensity, or both. Many such questions
can be answered only by further experimentation.

However. the results suggest strongly that the snowfall
climate under seedln~ 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 perceived or as it interacts with the natural en-
vironment. is now considered extremely unlikely.

!I The San Juan Ecology Project did not include studies of the techniques. of cloud seeding, but to set the stage
for the remainder of this volume, we asked the Bureau of Reclamation to provide an overview of the '2.10'11'1
seeding activities, which they considerately providec.

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