Review and Enhancement of “Literature Review and Scientific Synthesis on the Efficacy of Orographic Cloud Seeding”


Dr. Reynolds, the sole author of this monumental review I critique  has done a masterful job of surveying an enormous amount of cloud seeding literature in his “draft” report to his former employer, the Bureau of Reclamation.  The BOR was  the primary sponsor of cloud seeding programs throughout the West in the 1960s to the 1980s.  However, as was also seen in a recent review of the 2009 Springer book,  “Impacts of Aerosols on Precipitation,” such a task appears to be too much despite Reynold’s valiant efforts to “get it right.”   Reynold’s discussions of the benchmark randomized experiments in Colorado that led to the nation’s largest, most costly randomized orographic cloud seeding experiments, the Colorado River Basin Pilot Project, is an example of the problem of having too much to review and not enough time to scrutinize the details of so much literature.

Reynold’s review is well-written, most of the necessary citations are in it that help the reader to understand the topic.    That is,  except for those elements in his review that I am perhaps, a little too familiar with and I feel must be addressed in this VERY belated review of his 2015 draft report.

“Too familiar?”

That’s what happens to someone who has spent thousands of volunteer hours (crackpot alert!) rectifying faulty cloud seeding and cloud claims in peer-reviewed journal articles because he felt, “Someone has to do something about this!” (Second crackpot alert, with possible megalomaniacal implications).   I was employed as a weather forecaster for the Colorado River Basin Pilot Project for all of its five winter operating seasons, 1970/71 through 1974/75.  No one can know about that project and the faulty literature it was based on more than me.  I came in naive and idealistic about the scientific literature on cloud seeding;  I didn’t leave that way

I was not asked to review Reynold’s 2015  review before he posted his report to the BOR  online as a “draft,” the status it retains as of today.   I contacted Dr. Reynolds recently and informed him that I had a few comments on and corrections to his review.  He replied that he was not interested in correcting his review or making changes at this time.  This seemed odd to me, so here we are.

Also, in the spirit of “author disclosure,” I should mention that Dr. Reynolds was also an informal reviewer of my manuscript co-authored with Prof. Dave Schultz, Manchester U., on the history of the BOR-funded Colorado River Basin Project.  It was recently rejected by the J. Appl. Meteor.  due to length.  We are in the process of seeing where that manuscript can be trimmed down without losing  important parts of the story.

The comments pn Reynold’s review would have likely been unnecessary had I reviewed it beforehand, or if Dr. Reynolds  wished to consider my comments and corrections today.    I was well known to Dr. Reynolds as an expert on clouds, cloud seeding, and the weather  in Colorado and Israel ;  he had previously cited my work his 1988 article in the Bull. Amer. Meteor. Soc.   With Professor Peter V.  Hobbs in tow, I dissected those landmark experiments in Colorado and Israel and showed they were, as Foster and Huber (1997) described faulty science,  “scientific mirages.”  They were “low-hanging fruit” that poor peer reviews of manuscripts had let in, mostly, appearing Amer. Meteor. Soc. journals.  It didn’t take a genius to unravel them.

Dr. Reynold’s comprehensive  review can be found here.  It is too long to be a blog post here that includes my embedded comments.  Since I am only commenting on certain sections,  I have extracted only those portions of Reynold’s review where I have made comments.  It may be that only those familiar with this topic, orographic cloud seeding, will be interested, but, oh, well….  It has to be done even if for only ONE person!

My goal is to be objective and not short change Reynolds’ work on what is really an astounding effort.  It would take me two lifetimes to do what Dr. Reynolds has done.

I also believe Dr. Reynold’s made a great effort to be objective in discussing a topic that almost always brings controversy.  The literature in this field is filled with pro-seeding partisans that have often edited results so that cloud seeding has been presented with a happier face than it should have been.  After all, no one got a job saying cloud seeding doesn’t work (is not viable for producing worthwhile amounts of water.)

Considering his background in the cloud seeding arena, Reynolds final conclusion, copied below, must be considered an example of high integrity and his conclusion is one that this author fully agrees as of this date:

“5.3.2 Final Conclusion 

Based on both the historical evidence and the last decade of research, it is reasonable to conclude that artificial enhancement of winter snowpack over mountain barriers is possible. It is very difficult to quantify the seasonal increases to be expected both in snowpack and subsequent spring runoff. This is because each target area has to be investigated as to the meteorology of the winter clouds and their seedability, and the engineering aspects of effectively seeding the clouds to maximize increases. Winter orographic cloud seeding should thus continue to be supported both from the scientific and operational community working together to further the science and operational outcomes. It must be stated however, that as of yet, no rigorous scientific study conducted as a randomized confirmatory seeding experiment with pre-defined primary response variables and requiring an established threshold of statistical significance has demonstrated that seeding winter orographic clouds increases snowfall. As such, the “proof” the scientific community has been seeking for many decades is still not in hand. “

================THE REVIEW=================

My comments, corrections and question on Dr. Reynolds review begins below and are in a red font.   Highly relevant citations are missing  and there are citations in the Reynold’s references that do not appear in the text.  The missing ones, annotated with a “u” ,  have been added at the end of this review.

The portions of Reynold’s review that I examined begins here:


1.0 Introduction 

1.1. Introduction to Winter Orographic Cloud Seeding In its most basic form, artificial seeding of clouds for precipitation enhancement can be divided into two broad categories: 1 – cloud seeding to enhance rainfall i.e. summer convection, 2 – winter orographic cloud seeding to enhance snowfall. The scope of this paper is only concerned with the latter. Winter orographic cloud seeding occurs when very small particles, typically silver iodide, are introduced into a cloud which is below freezing. The cloud moisture collects onto the small particles, freezing the moisture into tiny ice crystals which continue to grow until they become too heavy to remain in the cloud and then fall out as precipitation (typically snow). This process can happen rapidly on the windward slopes of mountains allowing the snow to fall near the crest of the mountain which causes a local enhancement to the amount of precipitation that would have fallen naturally (Figure 1.1).

The situation is complicated if the natural crystals are becoming rimed and due to riming, fall more quickly.  Adding more ice crystals via cloud seeding may result in raising their trajectories by reducing riming and the snow may not be increased snow where it is wanted, or will evaporate in the descending, drying  air on the lee side.  The schematic below would be valid for naturally non-precipitating clouds.  

The properties of clouds that don’t naturally precipitate vary with location.  Maritime clouds along the west coasts of continents generally can precipitate without ice.  Farther inland, where the clouds become impacted by natural and anthropogenic aerosols, ice is generally required for precipitation and may not develop until cloud tops are cooler than about -12°C.  These latter non-precipitating clouds make viable seeding targets.

Figure 1.1 – Simple model of winter orographic cloud seeding. 1 – Introduction of seeding material, 2 forced ascent due to topography, 3 – enhanced precipitation falling out of cloud. 

1.3 Relevance and Need for a Reassessment of the Role of Winter Orographic Cloud Seeding to Enhance Water Supplies in the West 

Weather modification is most commonly conducted through “cloud seeding,” the introduction of chemical agents with the intent of affecting precipitation processes. A number of academic and private entities exist that offer services to states and local governments with the aim of increasing water supplies through inducing precipitation volumes above which would occur naturally. From the 1960s through the 1980s, Reclamation was involved in a variety of weather modification initiatives in the west under Project Skywater. This project included the Colorado River Basin Pilot Project, the High Plains Experiment (summer only), and the Sierra Cooperative Pilot Project. Project Skywater was terminated in 1988, but Reclamation continued to be involved with weather modification efforts. Reclamation participated in the development of the California Department of Water Resource’s design and conduct of the Oroville Reservoir Runoff Enhancement Project from 1988 until 1994. Reclamation also supported other efforts through the mid-2000s, including the Weather Damage Modification Program. 

Based upon scientific literature through 2006 and discussions with experts in the field, the efficacy of weather modification appears to be unsettled. In 2003, the National Research Council (NRC) report “Critical Issues in Weather Modification Research” (NRC 2003), concluded that “there is still no convincing scientific proof of the efficacy of intentional weather modification efforts”. The NRC goes on to state that new technology allows for potential new research to help understand the process of precipitation and if weather modification is a viable means to increase water supplies. 

The NRC 2003 review of cloud seeding cited above  did not measure up to the one the NRC published in 1973.  I reviewed NRC 2003.  If anyone cares, can be found here:         

 A Critical Review of NRC 2003 Critical issues in weather modification NATIONAL ACADEMIES PRESS

As seems to be typical of reviews, there was just too much literature to review for the scientists involved, and also likely, not a top priority for those assigned to this task having other research on their plates.   Prof. Hobbs, with my acquiescence, helped compromise this review by declining an offer by Prof. Garstang, chair of the review committee, to review before it was published, a huge mistake.  Garstang admonished  Peter to not to comment on it AFTER it came out.  But that’s exactly what Peter told me we would do if needed.  I nodded and went back to my desk.  He said we would have “more impact” by doing that.  I hope you appreciate getting stories from behind the scenes.

In 2002-2003, Reclamation funded, through earmarks, weather modification studies in the states of Nevada, Utah, California, North Dakota, and Texas. The studies did not provide convincing scientific evidence that weather modification reliably generates additional water. However, there are a number of studies, including from within Reclamation (Hunter 2004 – cited within LBAO (Lahontan Basin Area Office) EA discussed later), that indicate that cloud seeding can significantly increase precipitation amounts for targeted locations. 

Statement on the Application of Winter Orographic Cloud Seeding For Water Supply and Energy Production 

In 2005, Reclamation primarily stopped involvement in weather modification efforts at the program level. As identified within Q&As developed by the Research and Development Office explaining Reclamations abandonment of the practice: 

•Weather modification is not an operational function of Reclamation.

•In a letter dated December 13, 2005, sent to then-Texas Senator Kay Bailey Hutchison(R), the White House Office of Science and Technology Policy (OSTP) said there aresignificant concerns about liability and legal ramifications of weather modification,including whether weather modification can be demonstrated to actually be effective.

Since 2006, continuing drought conditions, and a strong interest amongst some Reclamation stakeholders, Reclamation engaged in two research projects related to weather modification in support of cold-season snowfall enhancement. 

•In 2010 the Mid-Pacific Region’s LBAO finalized an Environmental Assessment (LBAOEA) proposing to provide $1.35 million from Reclamation’s Desert Terminal LakesProgram to the Desert Research Institute (DRI) for a cloud seeding project in the WalkerRiver Basin.

•At a March 12, 2014 meeting of the Upper Colorado River Commission, weather modification was specifically identified as one of three activities that the Upper Basin states propose to include within their drought contingency plans. The Upper Basin states asked that Reclamation provide partial support for Wyoming’s eighth year (2014) of an ongoing weather modification study / program being conducted with the National Center for Atmospheric Research (NCAR). This request resulted in Reclamation’s Upper Colorado Region obligating $200,000 to the State of Wyoming for weather modification research and development efforts conducted by NCAR, with these monies obligated through an amendment to an existing cooperative agreement between Reclamation R&D and Universities Corporation for Atmospheric Research.

The Upper Basin states have noted that state and private entities in Colorado and Utah spend over $1M and $500,000 respectively on weather modification, and estimate efficacy between 6% and 20%. At the low end, the Upper Basin states identify that a benefit of 6% is inexpensive water within the Colorado River Basin. The Upper Basin states have argued that Reclamation’s documents from the 1960s – 1980s identified positive results of weather modification.

This above section needs supporting references for the assertions made about seeding results, the benefit claim, and who are those “Upper Basin States”?  Otherwise these statements should be taken with extra caution.

1.5 Brief History of Federal and State Authorizations for Weather Modification 

The following is taken from Chisolm and Grimes (1979): 

In 1968, the Colorado River Basin Project Act of 1968 (Public Law 90-537) was passed by Congress to provide for the further comprehensive development of water resources of the Colorado River Basin and for the provision of additional and adequate water supplies for use in the upper as well as lower Colorado River Basin. Under Title II of this Act, the Secretary of the Interior was authorized to prepare and implement an augmentation plan to meet the water requirements of the new projects created by the Act (Central Arizona Project and Colorado River Storage Project), existing projects and water allotments, and the 1944 water treaty with Mexico. 

Augmentation was one of the main issues in the deliberation on the Act. The Act defines augmentation as, “ ‘augment’ or ‘augmentation’ when used herein with reference to water means to increase supply of the Colorado River system or its tributaries by introduction of water into the Colorado River system, which is in addition to the natural supply of the system.” The Statement of the Managers on the part of the House with regard to augmentation stated “all possible sources of water must be considered, including water conservation and salvage, weather modification, desalinization and importation from areas of surplus.”

The Colorado River Basin Pilot Project (CRBPP) was the Bureau’s first major effort on weather modification in Colorado under the auspices of Project Skywater and P. L. 90-537. The purpose of the Colorado River Basin Pilot Project was to provide for scientific and economic evaluation of precipitation augmentation technology and to increase precipitation. The specific objectives to be achieved were (l) to establish and operate a ground-based meteorological network in and near the San Juan Mountains of Colorado to provide data input in the selection of suitable storms for seeding, and (2) to establish and operate a ground-based silver iodide seeding system to increase snowfall in the project target area. The field phase of CRBPP began with the winter of 1969-1970 (installation of gauges and seeding generator siting) while the random seeding phase began with the 1970-71 season and ran through the 1974-75 season (not the 1973-74 season as the author stated).

Date corrections are needed from the original text, not a good sign of the author’s knowledge concerning the CRBPP.  What is incomprehensible is that the goal of replicating the large percentage increases in snowfall reported in three randomized experiments by the author’s former home institution, Colorado State University, is left out of this rationale for the CRBPP.  Surely, the author knew, also as a long term BOR cloud seeding division employee, that those experiments were the primary motivation for the BOR to spend $40-50 million (in 2023 dollars) on the CRBPP.

At about the time of completion of CRBPP in Colorado, the Bureau began funding Project Snowman in Utah. Project Snowman was conducted for the Bureau by Utah State University’s Water Research Laboratory. The objective of this four-year project was to develop cold-cloud seeding technology using airborne generators and ground-based generators located in the northern portion of the Wasatch Mountains.

References are also needed here.

The Bureau’s early work on precipitation augmentation in Colorado was based on a fairly extensive background of research activities. Three major research efforts in winter seeding contributed directly to the Bureau’s CRBPP project in the Upper Colorado River Basin. These were: 

  1. The National Science Foundation sponsored research experiments by Colorado State University at Climax, Colorado, during the 1960’s.

“1” above is not a sufficient description of the motivation for the CRBPP.  The Climax experiments were reported on numerous occasions in the peer-reviewed literature as cloud seeding successes when air mass temperatures were high (i. e., high 500 hPa and 700 hPa equivalent temperatures as by Grant and Mielke 1967u, Kahan et al. 1969u, Grant et al. 1969u, Mielke et al. 1970u, 1971u, among others).  The BOR had a LOT of peer-reviewed evidence on which to base the CRBPP and in particular, in the Grant et al. 1969u Interim Report to the BOR that described the results of the Climax I results, and the preliminary results of Climax II and the Wolf Creek Pass experiments.  The findings in these three experiments, as described by Grant et al. 1969u,  were remarkably supportive of one another.  Climax II was a confirmatory experiment; nothing was changed from Climax I.

2. The operational research funded by the State of Colorado during the 1960′ s at several mountain passes, particularly Wolf Creek Pass in the San Juan Mountains, and,

The Wolf Creek Pass experiment mentioned above was a six winter season,  fully RANDOMIZED  experiment where entire winter seasons were randomized.  This experiment was critical to where the CRBPP was located since it appeared to have produced more water than seeding had in northern Colorado where the Climax experiments took place.

3. The Bureau sponsored experiments in the Park Range near Steamboat Springs, Coloradoduring the late 1960’s.

Rhea et al.’s 1969u “Final Report” to the  BOR concerning the Park Range Project  is eventually cited by Reynold’s, but is not in Reynold’s references.  This report was a “heads up” on all the problems that would be “rediscovered” during the CRBPP (e.g, as reported in Willis and Rangno 1971u).

The results of the Colorado River Pilot Project indicated the need for further verification and improvement in technology before a large augmentation program could be undertaken again.

This is a vague description of the CRBPP results, perhaps intentionally so. Why not just say what happened for the reader right here in plain language?   “The results of the earlier CSU experiments could not be replicated in the CRBPP (Elliott et al. 1978u followed by a citation to the “Comments” on Elliott et al’s findings by Rangno and Hobbs 1980-the latter reference is contained in Reynold’s references but is not discussed in his review.   Later, it was discovered that those early optimistic CSU results and the microphysical foundation on which they rested on were all ersatz leaving no real basis for the CRBPP  (e.g., Mielke 1979u, Rangno 1979u, Hobbs and Rangno 1979u, Rhea 1983, Rangno and Hobbs 1987u, 1993, 1995a, u)”

The Wolf Creek Pass seeding effort was designed to test whether a viable signal in runoff from Wolf Creek Pass could be produced by seeding all winter.  The Wolf Creek Pass experiment, conducted from the winters of 1964/65 through 1969/70 produced stunning results when the three randomly chosen seeded seasons were compared with the long historical runoff record (Grant et al. 1969u, Morel-Seytoux and Saheli 1973u).  Furthermore, the results of seeding on  individual days during the seeded winters appeared to replicate the results of Climax I.  It doesn’t get any better than this for a trifecta of apparent cloud seeding successes!  

But, it was all a mirage (e.g., Rangno 1979u), which makes this story so interesting from a scientific viewpoint.

Thus, the Bureau’s research program continued.

Winter experiments were conducted outside of the Colorado River Basin at: Elk Mountain, Wyoming (University of Wyoming) Bridger Range, Montana (Montana State University) Jemez Mountains, New Mexico (New Mexico State University) Pyramid Lake Pilot Project (University of Nevada) In addition, the Bureau continued to provide supplemental funds to Colorado State University’s NSF research and to Utah State University’s state -sponsored research project. Through the Emergency Drought Act of 1977 the Bureau granted over $2 million to six states for supplemental support of their cloud seeding projects including over $1 million to the States of Colorado and Utah for cloud seeding in the Colorado River Basin. 

1.6 Current Policy Statements from American Meteorological Society and World Meteorological Organization on Efficacy of Winter Orographic Cloud Seeding 

The two leading organizations representing the atmospheric science scientific establishment, the World Meteorological Organization and the American Meteorological Society, have both issued policy statements on the efficacy of winter orographic cloud seeding. These are relevant to review given the NRC 2003 conclusions. 

The current statement from the World Meteorological Organization (WMO 2010) on weather modification in general and relating specifically to winter orographic cloud seeding efficacy is stated below. 

“The scientific status of weather modification, while steadily improving, still reflects limitations in the detailed understanding of cloud microphysics and precipitation formation, as well as inadequacies in accurate precipitation measurement. Governments and scientific institutions are urged to substantially increase their efforts in basic physics and chemistry research related to weather modification and related programmes in weather modification. Further testing and evaluation of physical concepts and seeding strategies are critically important. The acceptance of weather modification can only be improved by increasing the numbers of well executed experiments and building the base of positive scientific results.” 

“Cloud seeding has been used on both cold clouds, in which glaciogenic seeding aims to induce ice-phase precipitation, and warm clouds, where hygroscopic seeding aims to promote coalescence of water droplets. There is statistical evidence, supported by some observations, of precipitation enhancement from glaciogenic seeding of orographic supercooled liquid and mixed-phase clouds and of some clouds associated with frontal systems that contain supercooled liquid water. “ 

The current AMS policy statement (AMS 2010) does not address specifically the efficacy of winter orographic cloud seeding but much like the NRC 2003 report identifies uncertainty and risk with much the same conclusions. These are listed below. 

UNCERTAINTY – Planned weather modification programs benefit from a comprehensive understanding of the physical processes responsible for desired modification effects. Recent improvements in the composition and techniques for dispersion of seeding agents, observational technology, numerical cloud models, and in physical understanding of cloud processes permit evermore detailed design and targeting of planned weather modification effects, and more accurate specification of the range of anticipated responses. While effects are often immediately evident in simple situations, such as when cloud seeding is used to clear supercooled fog and low stratus cloud decks, in more complex cloud systems it is often difficult to determine a seeding effect on a cloud-by-cloud basis. In these more complex situations, large numbers of events must be analyzed to separate the response to cloud seeding from natural variability in cloud behavior. Rigorous attention to evaluation of both operational and research programs is needed to help develop more effective procedures and to improve understanding of the effects of cloud seeding. Research and operational programs should be designed in a way that will allow their physical and statistical evaluation. Any statistical assessment must be accompanied by physical evaluation to confirm that the statistical results can be attributed to the seeding through a well-understood chain of physical events. It should be noted, though, that in practice large potential benefits can warrant relatively small investments to conduct operational cloud seeding despite some uncertainty in the outcome. 

The text in blue font seems like PR, Dave, and should be updated due to the lack of proof of seeding induced increases in snow we now have. Neiburger (1969u, WMO Tech Note) warned that such thinking usually excludes the idea that seeding might result in decreases in precipitation in addition to mistargeting, faulty operations.  


RISK MANAGEMENT – Unintended consequences of cloud seeding, such as changes in precipitation or other environmental impacts downwind of a target area, have not been clearly demonstrated, but neither can they be ruled out. In addition, cloud seeding materials may not always be successfully targeted and may cause their intended effects in an area different than the desired target area. This brings us to the ethical concern that activities conducted for the benefit of some may have an undesirable impact on others; weather modification programs should be designed to minimize negative impacts.. At times unintended effects may cross political boundaries, so international cooperation may be needed in some regions. Precipitation augmentation through cloud seeding should be viewed cautiously as a drought-relief measure because opportunities to increase precipitation are reduced during droughts. A program of precipitation augmentation is more effective in cushioning the impact of drought if it is used as part of a water management strategy on a long-term basis, with continuity from year to year, whenever opportunities exist to build soil moisture, to improve cropland, and to increase water in storage. From time to time methods have been proposed for modifying extreme weather phenomena, such as seeding severe thunderstorms with aerosols to diminish tornado intensity, or seeding tropical cyclones to cause changes in their dynamics and steer them away from land and/or diminish their intensity. Some experimentation has taken place in these areas, but current knowledge of these complex weather systems is limited, and the physical basis by which seeding might influence their evolution is not well understood. Weather modification techniques other than cloud seeding have been used in various areas of the world for short periods of time to achieve goals similar to those of cloud seeding. Much less is known about the effects of these other techniques, and their scientific basis is even further from being demonstrated, either statistically or physically, than it is for cloud seeding. Application of weather modification methods that are not supported by statistically positive results combined with a well-understood physical chain of processes leading to these results, and that can also be replicated by numerical cloud modeling, should be discouraged.

Other organizations such as the North American Interstate Weather Modification Council, The Weather Modification Association, the American Society of Civil Engineers, and the Western States Water Council have also adopted policy statements or adopted resolutions relating to the use of weather modification for increasing snowpack and water supply. These are referenced in Ryan (2005) and will not be repeated here. Most if not all of these statements are much more positive in their support of the application of weather modification for enhancing snowpack and runoff despite the lack of evidence as reported in NRC 2003.

To the uninitiated reader to the field of weather modification/cloud seeding, it will seem odd that there are government entities that will pay huge sums of tax payer monies for cloud seeding with no viable evidence that it does anything, evidence being in the form of randomized experiments, the “gold standard” of scientific proof.  

Why would those entities take such chances?  

If you haven’t guessed by now, it’s because those government entities are telling their constituents directly or implicitly that they are doing SOMETHING about a drought.  It’s a great ploy, and usually works except in the minds of those who know the science.  

What science?  

Two modern randomized experiments testing to see if cloud seeding can increase precipitation in mountainous regions (Wyoming and in northern Israel) ended with  no indications that cloud seeding increased precipitation.  These null findings have been published by Rasmussen et al. 2018u for Wyoming, and by Benjamini et al. 2023u for Israel.  Now you know.  

Did the “null” finding reported by Rasmussen et al. 2018 terminate cloud seeding in Wyoming?  Of course  not.  It just looks too good to the public that you’re doing something about water needs.  

1.7 Generalized Concepts of Winter Orographic Cloud Seeding 

It is useful to review the general principles of winter orographic snowfall and whether this process could be modified or enhanced by artificial means. The basic physical concepts associated with seeding winter orographic clouds are not debated even though there is considerable debate over weather modification and its efficacy. These basic physical concepts are reviewed in the following section. There are several text books and encyclopedia articles available for a more in-depth discussion or broader overview of the physical basis of cloud seeding (Hess 1974; Dennis 1980; Dennis 1987; and Heymsfield 1992). 

Dennis in his (1980) Academy Press book, “Weather Modification by Cloud Seeding,” relied heavily on the 1977 BOR Monograph Number 1 (yes, it was deemed that important by the BOR to name it as NUMBER ONE),  became outdated almost immediately when external critics (guess who?) found serious flaws in that “meta-analysis.”     The BOR study, published in 1978u (Vardiman and Moore) was retracted in 1980  by Rottner et al. 1980 as critical “Comments” on their paper by, yep, Rangno and Hobbs (1980) were being published.  Thus, Dennis (1980) might be reconsidered as a reference here.  The BOR was too willing to believe in cloud seeding success mirages that led to this major embarrassment.

Too, much of the cloud seeding literature in Hess (1974) has been overturned in reanalyses or has not been replicated, as in the recent Wyoming and Israel experiments.  But, “hey,” you can read about global cooling in Hess (1974), thought to be underway at that time.

Figure 1.2 from Ludlam (1955), reproduced below, describes the process that remains to this day the fundamental conceptual model associated with winter orographic cloud seeding. Figure 1.2 shows a shallow orographic cloud, where the liquid condensate produced by forced assent over a mountain barrier is unable to be converted to snowfall before the air descends and evaporates in the lee of the mountain. During wintertime the freezing level (height of the 0oC isotherm) varies dependent on the origin of the air mass impinging on the mountain barrier.  This varies from north to south with the freezing level being lower in altitude at the northern latitudes of the western US and the inter-mountain west where the air masses that impact this area are usually modified maritime polar or continental polar.

and height allowing the crystals to grow at the expense of the cloud water that in (a) was lost to the lee, bringing this moisture down on the windward side of the mountain.

1.2a is the non-precipitating cloud that forms the low, demonstrable end of seeding potential.

The text in blue in the body of the paragraph may be true, but….. warm air masses during times of upper level ridges along the West Coast shunt warm air mass storms into the central and northern Rockies, so this “paradigm” often does not hold.  Surface temperatures may be well below freezing, but much higher temperatures usually exist aloft in those warm aloft regions of winter storms.  The Climax I experiment, for example, had numerous warm aloft storms overrunning colder air with west-northwest flow due to this synoptic scenario.   Quantification of this claim would have been very informative and would have pinned it down for the reader…and me!

Freezing levels are usually below ground level in mountainous regions except in the warmest storms. In the Ludlum model, it is assumed the orographic cloud has a significant depth of cloud below 0 oC and thus the cloud moisture is said to be supercooled. The critical uncertainty with regard to successful conversion of the unused cloud condensate to snowfall prior to passing over the crest is the location, duration, temperature and concentration of the supercooled liquid water (SLW).

As Ludlum describes it may take as much as 1500 seconds once artificial ice crystals are initiated to grow and fall out before passing to the lee of the mountain crest.  This can vary by several tens of minutes based on SLW concentration, temperature vertical profile and winds.

The process of crystal growth is almost always much fast than “as much as” 25 minutes to fallout cited by Ludlum  (e.g., Auer et al. 1969, Cooper and Vali 1981).

So the critical factors for achieving success are getting the seeding agent into the cloud at the right location where it will generate enough ice embryos such that they will utilize the available SLW prior to passing over the crest. There are many complex interactions that have made it very difficult to demonstrate the efficacy of winter orographic cloud seeding to the satisfaction of the scientific community. These factors are described in the following paragraphs. 

So true.

 1.7.1 The Initiation, Growth and Fallout of Snow in Winter Orographic Clouds Converting Supercooled Liquid Water (SLW) to Snow

Supercooled liquid water (SLW) in the atmosphere is made up of tiny cloud droplets that are colder than 0 oC. There are two processes in nature by which SLW in the atmosphere can freeze to initiate snowfall: 1. Heterogeneous nucleation or 2. Homogeneous nucleation. Heterogeneous nucleation occurs when the supercooled liquid drop comes in contact with what is called an ice nucleus (IN) that emulates the crystalline structure of ice and causes the droplet to freeze. These can be dust particles, biological particles or a combination of the two. These aerosols can come from as far away as Asia and Africa initiating cloud ice in orographic clouds in the western US (Cremean et al. 2013). They are made of very small particles of tenths of microns in size. They are most active at cloud top and tend to activate the growth of snowflakes from the top of the cloud down. The warmer the cloud top the less percentage of ice makes up the cloud (Cremean et al. 2013).

Sidebar:  An interesting feature, first observed in the 1950s (e.g., Cunningham 1957u) and afterward was the “upside down” storm structure where few ice crystals were found at low cloud top temperatures consisting mostly of supercooled liquid water with increasing ice crystal concentrations below the top. The increasing concentrations of ice crystals were mostly due to the fragmentation of delicate ice crystals. The most recent description of this scenario was by Rauber and Tokay (1991u) and Hobbs and Rangno (1985).

Lower down, within a km of the surface, ground observations have shown that riming and and aggregation occur that increase snowfall rates. This lower region near mountains cannot be sampled by aircraft if precipitation is falling and as a result, has mainly been documented  in ground observations (e.g, Hobbs 1975). Thus, aircraft observations can often be seen as under measuring ice particle concentrations in mountainous regions.  For example, we at the University of Washington often overflew shallow orographic clouds with liquid tops at >-10C with snow falling out underneath, but we couldn’t sample them because the tops were too close to the tops of the Cascade Mountains.

When clouds are dominated by warm rain processes, the aerosol makeup of the cloud is more sea salt and biological particles which act as condensation nuclei producing larger cloud droplets which grow to raindrops via collision coalescence. Homogeneous nucleation occurs when the air temperature drops below -40 oC and the water droplet spontaneously freezes without the aid of a nucleating agent. The most basic hypothesis in winter orographic cloud seeding is that in the presence of SLW droplets, ice crystals will grow at the expense of the drops. This means the drops will convert back to vapor allowing the crystals to grow by vapor deposition unless too many ice crystals have resulted from seeding in which case they might not grow at all. The driver for crystal growth is related to the concentration of SLW and the temperature regime of the SLW (Ryan et al. 1976; Heymsfield 1992; Pruppacher and Klett 1978).

In the presence of moderately high concentrations of SLW and with somewhat preferred growth temperatures (Ryan et al. 1976, Figure 1.3) enough of the initial ice crystals can grow and then begin to aggregate into larger flakes leading to higher fall speeds and earlier fall-out. If these artificial crystals encounter additional SLW as they fall back toward the mountain crest, the individual crystals or aggregates may collect these SLW drops (called riming) which will also increase the crystals fall-speed. If the naturally created ice crystals are unable to utilize all the available SLW, and some SLW evaporates to the lee of the mountain, the cloud is said to be less than 100% efficient. This provides the opportunity for the artificial injection of a nucleating agent to create the additional ice crystals necessary to bring the residual cloud water to the ground before it is lost to the lee of the mountain. This is the basic principles described in Ludlam’s model. 

Aerial Seeding window 

Ground based seeding temperatures 

General seeding window based on Figure 1.5 and observed SLW 

As Super and Heimbach (2005) noted, the frequency of occurrence of SLW is temperature dependent with higher frequencies and amounts at (higher) supercooled temperatures. This is true for all mountain ranges where SLW has been observed. There are two main reasons for this. First, the amount of water vapor in the atmosphere can be higher at warmer (higher) temperatures. Second, as the atmosphere cools and clouds form and reach temperatures lower than -10 oC, and especially at -20 oC, an abundance of natural ice can occur that depletes the supercooled cloud water. Thus, there is less SLW available for cloud seeding to enhance the natural precipitation process as the air approaches these temperatures. It should be noted that studies (Reinking et al. 2000; Super 2005) have found significantly higher amounts of SLW (.5 to 1 mm integrated SLW) in wave clouds during winter storms and noted that others had observed such amounts during brief periods in other western mountain locations. However, the overwhelming amount of observations utilizing microwave radiometers (Heggli and Rauber 1988; Huggins 2009; Super and Heimbach 2005), in-situ aircraft observations, and mountain top icing rate meters indicate that SLW is concentrated in the lowest 1000m along the windward slopes of mountain ranges during passing winter storms. The primary SLW zone rapidly dissipates downwind of the crest because of warming produced by subsidence and by depletion from conversion to snowfall (Boe and Super 1986; Rauber et al. 1986; Rangno 1986 (the rapid dissipation of SLW, one of my main points), Huggins 1995; Super 2005; Huggins 2009).  There are observations that confirm the simple conceptual model espoused by Ludlam when natural ice does not form. The location of many of the research studies referenced in this report along with other locations that will be referenced later in this report are shown in Figure 1.4b. One can compare these locations to Figure 1.4a which shows the location where operational winter orographic cloud seeding is conducted circa 2006 per Griffith et al. 2006. Coastally influenced areas would be west of the Sierra Nevada and Cascades while the intermountain region refers to areas east of these two ranges. 

The actual temperature relationship to SLW occurrence varies geographically. For the intermountain west, where the cloud drop size distributions are more numerous at the smaller drop sizes (10 to 15 microns; what is referred to as a continental drop size distribution), lower temperatures are reached before a sufficient number of natural ice crystals develop to utilize the available SLW. Thus, SLW can exist, at least briefly, at temperatures as low as -15 to -20 oC. Super and Heimbach (2005) provide a comprehensive review of SLW climatology in the intermountain west. 

In more coastal regions, such as the Sierra Nevada and Cascades, the drop size distribution can be broad (what is referred to as a maritime drop size distribution). The drops can begin to collide and coalesce because of the varying fall speeds of the drops with a broader distribution of cloud droplets (extending into and above 30 microns diameter). This leads to larger cloud drops (approaching drizzle size) that can be carried upslope into coastal mountains like the Cascades and Sierra Nevada ranges where just a few of these droplets can freeze leading to rime splintering or secondary ice-crystal production (Hallett and Mossop 1974; Dong and Hallett 1989; Mossop 1985). This can, and has been observed to lead to high concentrations of ice crystals with cloud temperatures warmer than -10 oC (Reinking 1978; Cooper 1986; Marwitz 1986; Hobbs and Rangno 1985; Rauber 1992).

Nieman et al 2005u reported  occurrences of the “warm rain” process in Northern California and Oregon were common.  Will cloud seeding increase precipitation if nature is providing rain via collisions with coalescence?

Other factors (Rango  (sic) 1986) can lead to high ice crystal concentrations with relatively high cloud top temperatures. Mixing of very dry air into cloud tops can initiate cloud droplet freezing (e.g., Koenig 1968; Hobbs and Rangno 1985). This has been observed in the Cascades, Sierra Nevada and southern Utah. In the post-frontal airmass, where most of the shallow orographic clouds exist, very dry air can exist above cloud top. This is caused by sinking air parcels in the region behind the upper-level jet-stream that usually passes just ahead of the surface cold front  (Heggli and Reynolds 1985). Thus the coastal mountain clouds will have a lesser degree of supercooling, meaning that the clouds will be only marginally supercooled as natural ice production will utilize the available SLW within moderately supercooled clouds. Reynolds (1995) documented that over an 8 year period in the northern Sierra Nevada, 80% of the hours reporting SLW from mountain-top icing rate meters were at temperatures warmer than -4 oC. Reynolds (1996) also reported that 70% of the hours with precipitation had icing (riming?) reported. Approximately 300 hours of icing were reported per season. However, some seasons had average temperatures during icing warmer than -2 oC which may be too warm for any known seeding agent to work effectively unless seeded aloft using aerial seeding. Studies examining mountain top temperatures in Colorado and Utah revealed that SLW in clouds is mildly supercooled in a large portion of all storm passages, which means clouds are too warm for effective AgI seeding (Super 2005). Refer to Figure 1.5 for activation levels of the various cloud seeding agents currently used or proposed.

Why aren’t supercooled non-precipitating clouds’ occurrences documented for whole seasons as in Ludlams’s simple case? Seems this information would form a great starting point that could determine how much seeding can unequivocally increase snow.  The reader would like to know.

There are many studies (Heggli et al. 1983; Boe and Super 1986; Rauber and Grant 1986; Heggli and Rauber 1988; Super and Huggins 1993; Super 2005; Huggins 2009) that state SLW within a cloud varies rather rapidly with time over any given point. Due to this variability in SLW, identifying seeding potential within winter orographic storms will require identification of the proper seeding agent and delivery technique and applied at the correct time and location (Hunter 2007; Huggins 2009). Huggins (2009) suggests that any cloud seeding program will necessarily be treating clouds that at any given time may not have sufficient SLW (when the seeding agent arrives) given its variability. This begs the question as to whether seeding in these situations may have negative impacts on snowfall production. This will be further discussed in Section 1.7.4. Even though the location of SLW concentrations is known, the exact lower threshold for SLW concentrations to be sufficient for enhancing snowfall has not been quantified. It is believed to be greater than .05 mm integrated in the vertical derived from microwave radiometers (threshold used by Super and Heimbach 2005 and Manton et al 2011). However, Murakami (2013) used .2 mm as the lower threshold for determining cloud seeding feasibility and theorized that .3mm was probably the minimum threshold for viable increases in orographic precipitation enhancement.

For what durations of this SLW threshold? Did those these researchers report how long it lasted?  Did they make any seasonal estimates of these occurrences?  The reader would want to know.

This is a critical question as frequency distributions of SLW concentrations from radiometer data (Reynolds 1988) indicate that 85% of the SLW reported were at concentrations below .2 mm (Figure 1.5).What constitutes a necessary and sufficient concentration of SLW for effective cloud seeding is still in debate. 

Several studies (Rosenfeld 2000; Givati and Rosenfeld 2004; Rosenfeld and Givati, 2006; Griffith et al. 2005; Hunter 2007) have described decreases in orographic precipitation due to pollution.

The above requires some exhaustive comments:

Reynolds was unaware that when the claims of Givati and Rosenfeld concerning air pollution were examined by external skeptics they have not been substantiated.   I think this same view should be taken with Givati and Rosenfeld (2006) for pollution effects on West Coast precipitation.  We need this latter study to be validated by an external skeptic!  Yes, I am excited here.

I suspect, as in the Givati and Rosenfeld’s (2005) Israel study, where more than 500 standard gauges and 82 or so recording gauges were available to cherry-pick whatever result one wants, this may well have been  done in the 2006 study.   Kessler et al. (2006u), in an evaluation of the Israeli operational seeding program wrote: “No supporting evidence was found for the thesis of Givati and Rosenfeld (2005) regarding the decline in the Orographic (sic) precipitations due to the increase of air pollution.”

The air pollution claims, while superficially credible except for their sudden hypothesized appearance in Israel after 1990 when operational seeding produced a slight indication of decreased rainfall (Kessler et al. 2006u), were also evaluated by several independent groups and scientists: Alpert et al. (2008u, 2009u); Halfon et al. (2009u); Levin 2009u.   The Givati and Rosenfeld (2005) claims were also addressed in a review by Ayers and Levin (2009u). All these independent re-analyses and reviews of the hypothesized effect of air pollution on rainfall found the argument that air pollution had canceled seeding-induced increases in rain in Israel unconvincing.   In the few cases that Dr. Rosenfeld’s papers have been reviewed by external skeptics, they don’t hold up.  Ask Prof. Levin, Tel Aviv University, Professor Sandra Yuter, North Carolina State University, Nathan Halfon, Tel Aviv University, or me.   Hence, Caveat Emptor!

This specifically impacts the collision coalescence process and what is called warm rain, i.e. no ice processes involved. These studies discuss that pollution can slow down the collision coalescence process by narrowing the drop-size distribution. This, in turn, slows down the warm rain process and would have the largest impacts in the low-elevation coastal ranges along the west coast where the freezing level is well above the elevations of the coastal mountains, i.e. around Los Angeles where it has been proposed to reduce precipitation. Typically the decrease in orographically enhanced precipitation is greatest downwind of a major metropolitan area that is producing pollution. Givati and Rosenfeld (2004) showed precipitation losses near orographic features downwind of coastal urban centers corresponding to 15-25% of the annual precipitation.

Caveat emptor re Givati and Rosenfeld’s findings!

This loss of precipitation can be greater than the gain claimed by precipitation enhancement techniques in portions of California (Hunter 2007). Hindman et al (2006) noted that the trend over the past 20 years, from cloud droplet measurements at Storm Peak in the northern Rockies, has shown a decrease in CCN and an increase in cloud drop size. The conclusion was a decrease in upwind CCN concentrations (less pollution) but no relationship was found with precipitation rate. Thus, the change in cloud droplet spectra was not impacting riming growth efficiency (Borys et al 2003). It was noted by Creamean (2013) that pollutants, such as from human activity, were found mostly in the boundary layer and with frequently higher concentrations preceding surface cold fronts. The pollutants become trapped in the stable air as the air warms aloft and surface flows tend to be from the southeast to east tapping polluted sources from the central valley of CA. Once the front passed, the air-mass off the ocean did not contain these pollutants. It is the post–frontal cloud systems that have been identified as the most seedable in the northern and central Sierra (Heggli and Reynolds, 1985). It is not anticipated that pollutants play a significant role in these post-frontal shallow orographic clouds.

It should be noted that a more recent survey article by Tanre’ et al (2009), reviewed the impact of aerosols on precipitation and concluded: “Even though we clearly see in measurements and in simulations the strong effect that aerosol particles have in cloud microphysics and development, we are not sure what is the magnitude or direction of the aerosol impact on precipitation and how it varies with meteorological conditions. Even the most informative measurements so far on the effect of aerosols on precipitation do not include simultaneous quantitative measurements of aerosols, cloud properties, precipitation and the full set of meteorological parameters.”

Thanks to Tanre’ et al. (2009)!

The current CALWATER II experiment running this winter in California is an attempt to provide such information.

(Did it?)

The main limitation is very similar to the problems inherent in quantifying the impacts of artificial seeding of winter orographic clouds. That is the observing systems that we apply to quantifying the impacts have large measurement uncertainties and are of a magnitude similar to the expected aerosol influence on precipitation. Tanre’ notes that satellite and radar measurements have 20-30% errors in the measurement of aerosol optical depth, while aircraft sampling in-cloud can introduce changes in the cloud that can compromise the utility of the aircraft observations. In-deed measurements of surface precipitation, especially snowfall water equivalent can have 10-15% measurement uncertainty given gauge location and thus exposure to wind, minimum threshold/resolution, and such problems as capping. These types of measurement uncertainties require longer term on-going statistical analyses to reduce the random noise in the observations much like is required for cloud seeding experiments, thus reducing the influence of measurement uncertainty so as to extract the small signal that might exist. Artificial Stimulation of Snowfall by Seeding Agents

Artificial stimulation of snowfall is conducted through the application of aerosols that mimic natural ice nuclei to enhance the heterogeneous freezing of available SLW or by chilling the air below -40 oC to initiate homogenous nucleation. It is well known that the effectiveness of the heterogeneous seeding agent is highly temperature dependent. Artificial cloud nucleating substances (AgI, CO2, Liquid propane, SNOWMAX) are dependent on the presence of SLW at temperatures slightly below 0 oC for CO2, propane, and SNOWMAX (Ward and Demott 1989) or below -5 C to -8 oC for AgI mixtures (Figure 1.6). 

Figure 1.6 – Seeding activation versus temperature for seeding agents that have been used or proposed

These seeding agents act in different ways. Solid or liquid CO2 and liquid propane work by homogenous nucleation. These seeding agents need to be directly released in the presence of SLW for them to be effective. AgI and SNOWMAX work by heterogeneous nucleation, meaning they mimic the structure of natural ice nuclei. They do not have to be released directly into cloud or SLW. The aerosol can be carried aloft into clouds and when it encounters SLW at the right temperatures will begin generating ice crystals by contact nucleation. As shown in Figure 1.6, SNOWMAX works at the warmer (higher) end of the SLW temperature spectrum and its effectiveness does not vary greatly with temperature. To the author’s knowledge, SNOWMAX is not used in any operational seeding program but is used almost exclusively for snowmaking at ski resorts. The effectiveness of AgI to nucleate ice crystals increases by orders of magnitude from -5 oC to -12 oC (Super 2005). It should be noted that under transient water supersaturations, AgI can activate more rapidly and at temperatures near -5 oC through the condensation freezing mechanism (Pitter and Finnegan 1987). Chai (1993) explained the only way AgI could have been an effective seeding agent in the Lake Almanor seeding experiment (Moony and Lunn 1969) was through the fast activating condensation freezing process.

Mooney and Lunn (1969)… The westerly case where it was reported there had been large increases in snow reported due to seeding, was not reported for Phase II of the Lake Almanor experiment (Bartlett et al. 1975u). This omission should be unsettling to any objective scientist.

If the AgI is burned below cloud base or at temperatures warmer than -5 o C, the aerosol will not produce sufficient ice embryos until temperatures colder than -8 oC are reached (Super and Heimbach 2005). Huggins (2009) found the best temperatures for SLW in the Bridger Range Experiment occurred at < -9 oC using AgI, which suggests the AgI acted through contact or deposition nucleation. The central reason to explore propane seeding is its characteristic to be effective in mildly supercooled clouds that would be too warm for AgI. Propane dispensers tend to be more reliable, less complicated and less expensive than AgI generators. SLW temperatures in CO frequently range from -4 to -13 oC depending upon location and elevation (Boe and Super 1986; Rauber and Grant 1986; Huggins 1995; Super 2005; Huggins 2009). Due to the mildly supercooled nature of some CO locations, propane could be a useful alternative to AgI generators (Boe and Super 1986; Hindman 1986). The cloud base in California is often warmer than 0 oC while the top of the SLW near the mountain crest is usually > -12 oC (Heggli et al. 1983; Heggli and Rauber 1988; Huggins 2009). This is why propane was adopted by Reynolds (1995) as the seeding agent of choice in the Lake Oroville Runoff Enhancement Program (LOREP) in northern California (see Figure 1.4b). 

Cloud base altitude is an important consideration when siting propane dispensers which must be in-cloud or just below cloud base (at ice saturation) to be effective (Super 2005). Super and Heimbach (2005) indicate that even in the intermountain region, a significant number of hours with SLW are at temperatures where the release of AgI at elevations below -5 oC and out of cloud would not reach elevations cold enough to activate a sufficient quantity of the AgI to effectively “seed” the cloud and produce meaningful increases in snowfall. Thus, the 300 to 600 hours of reported SLW over the intermountain region during the 5 month snowfall season would require a mixture of seeding delivery methods including a mixture of high elevation ground released AgI and liquid propane or seeding from multiple aircraft. 

All weather helicopters with ceilings above 20,000 feet ASL  might be useful for targeting small watersheds when shallower clouds are present.

1.7.2 Transport and Dispersion of Seeding Material Ground Releases 

Flow over complex terrain is not a simple and straightforward problem therefore making targeting a challenge. Trying to disperse AgI from ground based generators has proven to be very difficult (Super and Heimbach 2005). There are two critical issues here. One is whether a parcel of air starting out near the foothills or a valley location will be carried over the mountain in the prevailing wind direction or whether it will flow around the mountain. This is determined by the static stability of the air mass and the strength of the flow perpendicular to the mountain, often noted by the Froude number. When the velocity of the flow is strong enough to overcome the air parcels static stability, a Froude number greater than 1 is produced, meaning the parcel of air will pass over the mountain and not flow around the mountain. The depth of the boundary layer is also very important as ground based cloud seeding efforts are located within this layer. If AgI is released below cloud or at temperatures warmer than -5 oC, the aerosol will have to be carried up into the cloud to a level where the temperature is colder than -8 oC. If the boundary layer is shallow and does not allow the aerosol to reach the appropriate temperature level or that level is reached very near the crest of the mountain, there will be no impact on the windward slopes of the mountain. The depth of the boundary layer is a function of low level wind shear (Xue 2014), which is the change in direction or velocity of wind with height. The stronger the wind shear, the greater the depth of the boundary layer. Strong low level flow perpendicular to the mountain, along with strong wind shear and at times weak embedded convection, will provide the mechanism for lifting the aerosol up the mountain. This allows dispersal of the aerosol to seed more cloud volume. If the temperatures are cold enough and SLW is continuous, an increase in snowfall will occur on the windward slopes and increase the precipitation efficiency of the orographic cloud. The targeting issue has been described by many weather modification researchers (Super and Heimbach 2005; Reynolds 1988; Warburton et al. 1995a and b) as the single most critical issue that has compromised the success of both operational as well as research field projects. Again, reason to emphasize that effective cloud seeding is an engineering problem. 

It has been shown that ample seeded crystals with sufficient concentration need to be dispersed so that a substantial volume of cloud over the target is treated for more than trace snowfall rates to occur (Super 2005; Huggins 2009). The seeding material must be injected into the SLW in sufficient quantities to generate 50 to 100/L or more initial ice embryos. This will then utilize the available SLW and fall out of the cloud prior to the snowflakes passing over the summit of the mountain and sublimating in the lee of the mountain. An example of the use of a rather simple targeting model (GUIDE, Rauber et al , 1988) used in the Lake Oroville Runoff Enhancement Project (LOREP ) to target ground-based liquid propane seeding effects is shown in Figure 1.7. This project used the tracer SF6 co-released with the propane from two sites to validate the GUIDE and assure accurate targeting. The GUIDE plumes as shown both horizontally and vertically along with the vertical motion field from a locally released rawinsonde. Seeding from Valley Locations Many operational cloud seeding projects have placed AgI generators in valley locations as they are easily accessible and can be manually ignited when needed. However, a considerable body of evidence indicates valley released AgI plumes are often trapped by stable air (high static stability), especially when valley-based inversions are present (Langer et al. 1967; Rhea 1969-cited but does not appear in Reynolds references; Super 2005). Often times in past projects AgI plumes from valley located generators were not tracked sufficiently to determine exactly where the aerosol plumes drifted (Smith and Heffernan 1967; Super 2005). As noted earlier, this is a recurring issue that has been raised in many winter orographic cloud seeding articles ( Elliott et al. 1978u, Rangno 1979u; Reynolds 1988; Super 2005; Hunter 2007; Huggins 2009). The aerosols may pool in the valley or may move in a direction around the mountain, only to be carried aloft when the static stability of the airmass decreases and low level winds increase. This usually occurs near and behind the surface cold fronts associated with winter storms. Thus, the AgI aerosol may travel far distances from the target but is unlikely to have appreciable effects far from a target due to the low concentrations that eventuate after many hours or days of travel. Seeding from Airplanes or Helicopters

Seeding by aircraft can be an alternative mechanism in locations where there is insufficient time to activate the seeding agent and grow the crystals to sufficient size for fallout to occur on the windward slopes of the barrier. These situations mainly occur within coastal mountains where the SLW near the crest of the mountain is only slightly sub-cooled. Typically the clouds extend up to a kilometer above and well upwind of the crest such that cloud top temperatures are -6 oC to -8 oC or lower. In these situations, the aircraft or helicopter can fly (hover above) in the tops of the clouds and either drop crushed dry ice, AgI droppable flares, or ignite AgI wing-tip generators or stationary flares that will directly inject the seeding material into the cloud. The dispersion would be especially enhanced in the downwash below a helicopter. Using crushed dry ice or droppable flares will create a curtain of ice crystals some 1000 m below the aircraft. This will spread at a rate of 1-2 m/s dependent upon the amount of vertical wind shear (Borovikov et al. 1961u, Hill 1980; Reynolds 1988). For these seeding curtains to merge together over the intended target area, the length of the seed line cannot be more than 30 to 40 km long (Deshler et al. 1990). However, the watershed of a large river basin can be several hundred kilometers wide. One aircraft will treat only a small portion of the watershed (see Figure 1.8). In addition, the duration of the seeding aircraft is usually about 2 to 4 hours, with the possibility of the aircraft having to descend to deice several times during the seeding mission. Aircraft operations are also expensive. For these reasons, many operational seeding programs use ground based seeding platforms, even if they are only viable a small percentage of the time. 

1.7.4 Extended Area Effects fromWinter Orographic Cloud Seeding (ones that have not been sufficiently investigated for lucky draws/synoptic biases)

The reason why I added this to the title is that I deem this part of the survey the weakest part.  No one talks about how low the concentrations of AgI would be in far away, so-called downwind affected regions, and god save anyone who looks at synoptics for a bias! Meltesen et al.(1978) did look at synoptic bias for the claimed downwind seeding increases from Climax and look what they found;  a synoptic bias that produced the illusion of downwind increases in snow!

Hunter (2009) prepared an extensive literature review of the current state of knowledge on extra or extended area effects from winter orographic cloud seeding. The main impetus for this report was to present any documented evidence that determined that seeding on one mountain barrier resulted in a possible reduction of the amount of precipitation downwind. This has been coined “Robbing Peter to pay Paul”. Hunter provided the following table which is reproduced here (not all references are included in Section 6). In every case, the seeding agent was silver iodide. These results indicate that once the AgI nuclei are released into the atmosphere, they can remain active for many hours, if not several days. If pooled in high concentrations, the AgI nuclei can seed areas well away from the intended target areas. However, the impacts of these extra-area effects are just as uncertain as the increase documented in the primary target areas. That is, without strong physical observations to compare with rigorous statistical analyses, there is still a significant level of uncertainty as to the efficacy of seeding with AgI to increase precipitation within large areas outside the intended target area. 

Table 1.2 here

The little bit of seeding (hour long pulses) or in six h blocks during the last season of the Park Range Project, made this claim ludicrous.  By 1979 it was recognized that a Type I statistical error (Mielke 1979u) affected both Climax experiments.  One of the interesting facets of Climax I that  prevented the seeding researchers from recognizing a Type I error was attributing heavier snow on seeded days upwind of Climax to seeding at Climax (Kahan et al. 1969u) 

1.7.5 Statistical Analyses

Statistical analyses have been a key part of assessing past cloud seeding experiments. Credence has usually only been given to those experiments that have been randomized and run as a confirmatory experiment. Key historical projects such as Climax and the series of Israeli cloud seeding experiments run as confirmatory, and meeting or exceeding the level of statistical significance set out in the experimental design, have come under further scrutiny and found to suffer from what is called Type 1 errors (Mielke 1979u; Rhea 1983; Rangno and Hobbs 1987u, 1993; Rangno and Hobbs 1995a,u,  b; Rosenfeld 1997, Rangno and Hobbs 1997a,u, b u).

Rosenfeld’s (1997) “Comments” cited by Reynolds were replied to by Rangno and Hobbs (1997a,u) in short form, and “Comprehensively” at the Cloud and Aerosol Research Group’s website. This dual approach was favored by Professor Peter V. Hobbs.

Its surprising that Dr. Reynolds did not know of our responses to Dr. Rosenfeld’s many specious comments


Some background on the Israeli work I did: these many exchanges led the Israel National Water Authority to form an independent panel to evaluate its operational seeding program targeting Lake Kinneret (aka, Sea of Galilee), Israel’s primary water source (Y. Goldreich, 2018, personal communication). The expert panel  that was constituted could find no evidence of increased rain the in the catchment of Lake Kinneret between 1975 and 2002  (Kessler et al. 2006).  The independent panel’s findings reversed the optimistic findings of Nirel and Rosenfeld (1995) of a statistically significant 6% increases in rain through 1990. The panel could also not replicate the 6% increase reported by Nirel and Rosenfeld using the same control stations.  Here is what Kessler et al. (2006) reported in graphical form. 

Why is this Israeli discussion important in Reynolds’ review?

The preliminary findings shown in this figure caused Rosenfield to immediately look for an “out,” and with more than 500 standard gauges and 82 recording gauges in Israel (!)  he found it by cherry-picking and claiming what Reynolds suggests in his review: air pollution was decreasing rain as much as cloud seeding was increasing it.  Givati and Rosenfeld’s conclusions did not stand up to independent investigators as was documented earlier, nor by the subsequent investigations for California cited by Reynolds.

This is typical in scientific statistical testing. It is considered less incorrect to not detect a relationship when one exists rather than detect a relationship when one does not exist. Other statistical methods, such as the use of covariates, can be useful in determining the statistical success of seeding operations (Dennis 1980; Mielke et al. 1981; Gabriel 1999; Gabriel 2002; Huggins 2009). A problem common to the statistical method of historical regression is the assumption that climate has been stable over many decades (Hunter 2007) which is called stationarity, and not declaring covariates in advance of experimentation.

1.8 Summary 

From the information summarized above it is worth reviewing the key questions as outlined for winter orographic clouds as listed in Table 1.1. 

What is the location, duration, and degree of supercooling of cloud liquid water in winter orographic clouds? 

•Concentrated in the lowest km on the windward slopes of mountain (Super andHeimbach, 2005)

•Highly variable in space and time given fluctuations in wind speed/direction and naturalprecipitation processes.

•Higher concentrations and higher frequency of SLW at warmer (sic) (higher) temperatures for all mountain ranges•SLW >.05 mm vertical integrated has been used as lower threshold for cloudseeding initiation.

Are their man-made pollutants or natural aerosols/particulates impacting the target clouds that could modify the cloud droplet spectra/IN concentrations to impede seeding effectiveness? 

•Pollutants acting as CCN can narrow the droplet spectrum and slow down the collision coalescenceprocess (warm rain) reducing rainfall downwind of major pollution sources.(Rosenfeld 2000; Givati and Rosenfeld 2004; Givati and Rosenfeld 2005.  See earlier discussion of the latter report.)

•It was found in SCPP that SIP (secondary ice production) produced high ice concentrations at relatively high cloud top temperatures (-5 to -10 oC)

•If pollutants narrow the droplet spectrum, then pollutants in theory should reduce SIP.

Mossop (1978u) found that increases in small (<14 um diameter) droplets combined with those >23 um diameter increased the efficiency of the riming-splintering process.  So it is possible, with the presence of larger droplets say, due to seaborne or other large aerosols combined with pollution sources, that the Hallett-Mossop riming-splintering process is enhanced.

•If high concentrations of pollution produce high concentration of cloud droplets in a narrow size range, this could reduce riming and reduce snowfall on the windward slopes of narrow mountain ranges where growth times are critical.

•A more recent survey article on the role of pollution on clouds and precipitation (Tanre’, 2009) concluded it was still uncertain as to the magnitude of the impact of pollution or whether pollution increases or decreases precipitation based on the meteorological setting.

•Dust (Saharan and Gobi desert) and aerosols (bacteria) acting as IN can enhance natural s

Are their significant enough differences in maritime influenced winter orographic clouds versus continental orographic clouds that strongly influence the natural precipitation process? 

•Yes. Maritime clouds with broader drop size distributions are subject to SIP and thus clouds are more efficient at higher temperatures.

•Continental clouds have relatively more SLW at lower temperatures given the lack of SIP.

•Evidence of this is well-documented when one compares SCPP and Washington studies with interior mountain studies.

====================End of Comments and Corrections  by ALR==========================

References that were uncited in the Reynolds review but were mentioned in this “review and enhancement.”

Alpert, P., N. Halfon, and Z. Levin, 2008: Does air pollution really suppress precipitation in Israel?  J. Appl. Meteor. Climatology, 47, 943-948.

Alpert, P., N. Halfon, and Z. Levin, 2009:  Reply to Givati and Rosenfeld.  J. Appl. Meteor. Climatology, 48, 1751-1754.

Auer, A. H., D. L. Veal, and J. D. Marwitz, 1969: Observations of ice crystals and ice nuclei observations in stable cap clouds.  J. Atmos. Sci., 26, 1342-1343.

Ayers, G., and Levin, 2009:  Air pollution and precipitation.  In Clouds in the Perturbed Climate System.  Their Relationship to Energy Balance, Atmospheric Dynamics, and Precipitation. J. Heintzenberg and R. J. Charlson, Eds.  MIT Press, 369-399.

Bartlett, J. P.,  M. L. Mooney, and W. L. Scott, 1975:  Lake Almanor cloud seeding program.  Preprint, San Francisco Conference on weather modification, Amer. Meteor. Soc., 106-111.

Benjamini, Y, A. Givati, P. Khain, Y. Levi, D. Rosenfeld, U. Shamir, A. Siegel, A. Zipori, B. Ziv, and D. M. Steinberg, 2023:  The Israel 4 Cloud Seeding Experiment: Primary Results.   J. Appl. Meteor. Climate, 62, 317-327.

Borovikov, A. M., I. I. Gaivoronsky, E. G. Zak, V. V. Kostarev, I. P. Mazin, V. E. Minervin, A. Kh. Khrgian and S. M. Shmeter, 1961:  Cloud Physics. Gidrometeor. Izdatel. Leningrad. (Available from Office of Tech. Serv., U. S. Dept. of Commerce.)

Cooper, W. A., and G. Vali, 1981:  The origin of ice in mountain cap clouds.  J. Atmos. Sci., 38, 1244-1259.

Cunningham, R. M., 1957:  A discussion of generating cell observations with respect to the existence of freezing or sublimation nuclei.  In Artificial Stimulation of Rain, H. Weickmann, Ed.  Pergamon Press, NY.,  267-270.

Elliott, R. D., R. W. Shaffer, A. Court, and J. F. Hannaford, 1978:  Randomized cloud seeding in the San Juan Mountains, Colorado.  J. Appl. Meteor., 17, 1298–1318.

Foster, K. R., and P. W. Huber, 1997: Judging  Science–Scientific Knowledge and the Federal Courts.  The MIT Press, Cambridge, MA, 333pp.

Grant, L. O., and P. W. Mielke, Jr., 1967: A randomized cloud seeding experiment at Climax, Colorado 1960-1965.  Proc. Fifth Berkeley Symposium on Mathematical Statistics and Probability, Vol. 5, University of California Press, 115-131.

Grant, L. O., Chappell, C. F., Crow, L. W., Mielke, P. W., Jr., Rasmussen, J. L., Shobe, W. E., Stockwell, H., and R. A. Wykstra, 1969:  An operational adaptation program of weather modification for the Colorado River basin.  Interim report to the Bureau of Reclamation, Department of Atmospheric Sciences, Colorado State University, Fort Collins, 98pp.

Halfon, N., Z. Levin, P. Alpert, 2009:  Temporal rainfall fluctuations in Israel and their possible link to urban and air pollution effects.  Environ, Res. Lett., 4, 12pp. doi:10.1088/1748-9326/4/2/025001

Hobbs, P. V., and A. L. Rangno, 1979: Comments on the Climax randomized cloud seeding experiments.   J. Appl. Meteor., 18, 1233-1237.

Kahan, K. M., J. R. Stinson, and R. L. Eddy, 1969:  Progress in precipitation modification.  Bull. Amer. Meteor. Soc., 50, 206–214.

Kessler, A., A. Cohen, D. Sharon, 2003:  Analysis of the cloud seeding in Northern Israel.  Interim report submitted to the Israel Hydrology Institute and the Israel Water Management of the Ministry of Infrastructure.

Kessler, A., A. Cohen, D. Sharon, 2006:  Analysis of the cloud seeding in Northern Israel. Final report submitted to the Israel Hydrology Institute and the Israel Water Management of the Ministry of Infrastructure, In Hebrew with an English abstract. 117pp.

Levin, Z., 2009:  On the state of cloud seeding for rain enhancement.  Report to the Cyprus Institute on Energy, Environment and Water, 18pp.

Mielke, P. W., Jr., 1979:  Comment on field experimentation in weather modification.  J. Amer. Statist. Assoc., 74, 87–88,

Mielke, P. W., Jr., L. O. Grant, and C. F. Chappell, 1970: Elevation and spatial variation effects of wintertime orographic cloud seeding. J. Appl. Meteor., 9, 476–488; Corrigendum, 10, 842; Corrigendum, 15, 801.

Mielke, P. W., Jr., Grant, L. O., and C. F. Chappell, 1971:  An independent replication of the Climax wintertime orographic cloud seeding experiment.  J. Appl. Meteor., 10, 1198-1212.

Morel-Seytoux, H. J., and F. Saheli, 1973: Test of runoff increase due to precipitation management for the Colorado River Basin Pilot Project.  J. Appl. Meteor., 12, 322-337.

Mossop, S. C., 1978: The influence of the drop size distribution on the production of secondary ice particles during graupel growth. Quart J. Roy. Meteor. Soc., 104, 323-330.

Neiburger, M., 1969: Artificial Modification of Precipitation. W. M. O. Technical Note 105, 33pp.

Neiman, P. J., G. A. Wick, F. M. Ralph, B. E.  Martner, A. B. White, and D. E. Kingsmill, 2005:  Wintertime nonbrightband rain in California and Oregon during CALJET and PACJET:  Geographic, interannual and synoptic variability.  Mon. Wea. Rev., 133, 1199-1223.

Rangno, A. L., 1979:  A reanalysis of the Wolf Creek Pass cloud seeding experiment.   J. Appl. Meteor., 18, 579–605.

Rangno, A. L. and P. V. Hobbs, 1980: Comments on “Randomized cloud seeding in the San Juan Mountains, Colorado,” J. Applied Meteorology, 19, 346-350. 

Rangno, A. L., and P. V. Hobbs, 1987: A re-evaluation of the Climax cloud seeding experiments using NOAA published data.  J. Climate Appl. Meteor., 26,  757-762.

Rangno, A. L., and P. V. Hobbs, 1995a: Reply to Gabriel and Mielke.  J. Appl. Meteor., 34, 1233-1238.

Rangno, A. L. and P. V. Hobbs, 1997a: Reply to Rosenfeld.  J. Appl. Meteor., 36, 272-276.;2

Rangno, A. L. and P. V. Hobbs, 1997b: Comprehensive Reply to Rosenfeld, Cloud and Aerosol Research Group, Department of Atmospheric Sciences, University of Washington, 25pp.

Rasmussen, R. M., S. A. Tessendorf, L. Xue, C. Weeks, K. Ikeda, S. Landolt, D. Breed, T. Deshler, and B. Lawrence, 2018: Evaluation of the Wyoming Weather Modification Pilot Project (WWMPP) using two approaches: Traditional statistics and ensemble modeling. J. Appl. Meteoro. Climatol., 57, 2639–2660, D-17-0335.1.

Rauber, R. M., and A. Tokay, 1991:  An explanation for the existence of supercooled water at the top of cold clouds. J. Appl. Meteor., 48, 1005-1023.

Rhea, J. O., L. G. Davis, and P. T. Willis, 1969:  The Park Range Project.  Final Report to the Bureau of Reclamation, EG&G, Inc., Steamboat Springs, CO. 285 pp.

Rottner, D., L. Vardiman, and J. A. Moore, 1980: Reanalysis of “Generalized Criteria for Seeding Winter Orographic Clouds”, J. Appl. Meteor., 19, 622-626.

Vardiman, L, and J. A. Moore, 1978: Generalized criteria for seeding winter orographic clouds. J. Appl. Meteor., 17, 1769-1777.

By Art Rangno

Retiree from a group specializing in airborne measurements of clouds and aerosols at the University of Washington (Cloud and Aerosol Research Group). The projects in which I participated were in many countries; from the Arctic to Brazil, from the Marshall Islands to South Africa.


  1. Hi Art: The weather in this last week of October here has been nearly perfect: no clouds to be seen today at all with frost in the morning. Hope all is well there and Happy Halloween!

    1. Thanks for the good wishes, Roland! Seattle had a stupendous run of good weather, too.

      We’re both still going, which is something here in 2023 at our ages. These recent posts are mostly bucket list things I’m doing after my web site was resuscitated from some malware problems by professionals early this year. It was out of commission for more than a year.
      Best regards,

      1. Thanks for responding, Art. I had nearly given up in reading anything more from you!

Comments are closed.