Executive Summary
This report summarizes past and recent research in cloud seeding to enhance winter snowpack. It is
motivated by the U.S. Bureau of Reclamation’s desire to determine if there is enough evidence in the
scientific literature and other technical reports that would suggest that cloud seeding is a viable
technology for enhancing snowpack and subsequent spring runoff in the western US. Current Bureau
policy is not to engage in funding either research or operational winter cloud seeding programs. This
policy was enacted after the National Research Council report of 2003, “Critical Issues in Weather
Modification Research”, in which it was concluded that there was currently no well documented
scientific evidence that weather modification is a viable method for enhancing precipitation from clouds.
This report went on to identify critical issues and possible technological improvements that were currently
available or being developed that might address these uncertainties and help resolve some of the major
uncertainties that remain on the efficacy of cloud seeding.
This report deals specifically with winter orographic cloud seeding to enhance winter snowpack and
subsequent spring runoff. It begins by summarizing the basic scientific principles of winter orographic
cloud seeding that date back to the late 1940s and early 50s. This began the heyday of cloud seeding in
the US which expanded greatly in the 1960s through the early 1990s when the Bureau of Reclamation
was heavily involved in funding and conducting winter orographic cloud seeding studies. There was a
great deal of knowledge and understanding gained on winter orographic clouds; how natural precipitation
formed, and where the opportunities may exist for enhancing snowfall from these clouds. Exploratory
and some confirmatory statistical seeding experiments were conducted using randomized seeding as a
method to achieve statistical results on the efficacy of seeding. These seeding experiments consisted of
both aerial and ground based seeding using silver iodide, dry ice, and liquid propane as the seeding agent.
The former, silver iodide, is released as an aerosol via combustion, and based on cloud conditions, may
generate sufficient ice crystals to produce meaningful results at cloud temperatures between -5 and -8 oC.
It is by far the seeding agent of choice for almost all winter seeding conducted to date. Dry ice dropped
from an aircraft and ground-based liquid propane released in cloud at temperatures just below freezing,
can generate sufficient ice to produce a meaningful concentration of ice crystals. These are more viable
when cloud liquid water is concentrated at warmer but sub-freezing temperatures. The main uncertainty
that has plagued these studies is the amount of seeding required, both number of seeding generators or
aircraft needed, and knowing the volume of cloud that is actually treated, effective targeting of the
seeding effects, along with knowing the amount, location, and duration of supercooled liquid water
necessary in the cloud that will allow the seeding to produce meaningful results over a given watershed.
This report summarizes some of the key results reported from projects conducted up to the time of the
NRC report, and then reports on some of the major, but one would have to say limited, research that has
been conducted since NRC 2003.
Two well documented randomized confirmatory seeding projects have been conducted in wintertime
orographic cloud seeding since NRC 2003. Both these projects were designed based on the decades of
research that preceded them suggesting that shallow orographic clouds containing supercooled liquid
drops could be treated with silver iodide (AgI) released from ground generators to produce 10-15%
annual increases in snowpack water content. The first was called the Snowy Precipitation Enhancement
Research Project (SPERP) undertaken from May 2005 to June 2009 in the Snowy Mountains of
southeastern Australia. A second follow-on confirmatory randomized seeding project was conducted
from 2010 to 2013. Both studies used a specific target area and an unseeded control area to determine the
statistical significance of the seeding from ground based AgI generators. The project used in-situ
radiometers to measure the amount of supercooled liquid water in the clouds as well as a simple
numerical model that emulates the growth and subsequent fallout of seeded crystals using in-situ profiles
of the winds and temperatures measured in the vertical just upwind of the mountain. This model was to
assure proper targeting of the seeding effects. Both experiments failed to show a statistically significant
effect from seeding. This even though trace chemistry results of AgI in snow and a non-nucleating agent
co-released with the AgI indicated that the AgI was acting as an ice nucleating agent and was falling out
within the target area. A posteriori analysis indicated that when one selected cases with more seeding
generators on and targeting effectively, there was an indication of a 10-15% increase in precipitation.
However these results can only be considered suggestive as they were derived not as part of the predefined statistical study.
A second major randomized cloud seeding program was conducted over two mountain ranges in southern
Wyoming from 2008 through the winter of 2013-14. The Wyoming Weather Modification Pilot Project
(WWMPP) was a randomized experiment using a cross-over design, meaning that the two target
mountain ranges were alternately seeded using a pre-determined randomization scheme. Each mountain
range had a control area associated with it. The idea of a cross-over experiment was to reduce the number
of years needed to reach a statistically significant result assuming a 10 to 15% increase in precipitation
from seeding. Again these results failed to show a statistically significant increase in snowfall from
seeding. A posteriori analysis showed isolating those cases with more generators operating indicted a
positive seeding effect of 10-15%. These a posteriori analyses are only suggestive of a seeding effect and
not proof. This project was one of the first winter orographic cloud seeding projects that used
sophisticated numerical models to simulate the effects of seeding and thus help to inform as to the siting
of the ground based generators for effective seeding. In-situ measurements of cloud liquid water were
again used for deciding when seeding was to be conducted along with wind and temperature profiles in
cloud to assure effective activation of the seeding agent and targeting of seeding effects.
The WWMPP had several additional studies conducted during the 6-year period that included multiple
radar analyses from both downward looking radars aboard the University of Wyoming King Air research
aircraft and from surface based radars of various wavelengths some with Doppler capabilities to try to
document the effects of seeding. Like many studies before it, radar studies were hampered by the natural
variability of precipitation in winter clouds. There were several cases of what appeared to be strong
indications of seeding signatures in seeded clouds over each range, and other cases where the signal was
so weak it could only be extracted through statistical analyses using multiple seeded and non-seeded
cloud volumes. Trace chemistry of Ag in snow was also performed for this project showing small
amounts of Ag in the snowpack in the target area from the seeding. These results suggested that seeding
was targeting the right areas but the signal was weak and variable from year to year. Background
contamination may have been occurring due to very windy conditions noted over the two ranges in
winter. It was concluded by the project scientist conducting the analysis that an “accumulated body of
evidence”combining the physical studies, statistical studies using a posteriori analyses, and numerical
model simulations, indicated a 5-15% seeding effect. Hydrologic modeling studies suggest this would
translate into a 1-3% increase in spring runoff. Again these results can only be considered suggestive of
seeding potential increases. Additional studies were carried out to determine the economic viability of
conducting an operational seeding program over these two barriers. Based on a climatology of seedable
cloud conditions annually and assuming the cost of conducting the seeding with and without additional
modeling and physical studies and estimating the current value of water within this watershed, it was
determined that at least an 8-10% increase in seasonal precipitation was required to have a 1 to 1 benefit
cost ratio.
Given the failure of both these randomized seeding program to demonstrate statistically significant
seeding effects, there is still a lack of “proof” the NRC 2003 report was seeking from programs that could
utilize more sophisticated observing platforms, numerical modeling, and trace chemistry studies to
validate seeding effectiveness. Given the large natural variability in winter precipitation over complex
topography, the large variability of supercooled liquid water in time and space during the passage of
winter storms over the barriers, and the difficulty of effectively treating the clouds with sufficient seeding
material, one can still only assume a 5-15% increase in winter precipitation from seeding is possible but
certainly not a given. The problem is just as much an engineering problem as it is a scientific challenge.
From the science perspective, it is understanding the meteorology of the given mountain barrier and
having the tools such as the right observations and modeling capability to know when it is appropriate to
seed. From the engineering perspective, it comes down to seeding the clouds effectively with enough
nuclei (AgI) or with embryonic ice crystals (propane) to effectively fill the volume of cloud over the
target area with enough ice to deplete the available SLW and bring it to the surface before the seeded
volume passes to the lee of the barrier and may be lost to subsidence. The weather modification
community has spent decades trying to get the engineering right and not with great success, at least from
the scientist perspective.
Climate change and its possible impact for more extreme and persistent droughts in the west, along with
more demands on the limited water supplies given an increasing population, have brought the science of
artificial stimulation of precipitation from winter mountain clouds back into focus of the Bureau of
Reclamation. Improving very high resolution numerical modeling, with explicit physics for simulating
precipitation processes, both natural and artificial, and the cost of running these continuing to decline,
offer a real potential for better designing and conducting winter cloud seeding programs. However models
still need to be validated and it has been demonstrated many times over that making in-situ observations
within clouds at low elevations over complex terrain during snowfall is very difficult. The scientific
community demands both statistical and physical “proof” of the efficacy of winter orographic cloud
seeding. Although this proof is lacking it has not prevented state and local entities from conducting
operational seeding programs, some continuously for over half a century. Given the large expense and
time needed to conduct a viable randomized seeding program, and the additional cost of conducting
detailed in-situ observations, it is unlikely that without federal, state and local funding sources, obtaining
the “proof” the scientific community has been seeking for decades on the efficacy of winter orographic
cloud seeding will remain unfulfilled.