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1 Introduction MOTIVATION Societal interest and investment in weather modification have been driven historically by the needs ~~ increased water and for reduced damage from hazardous weather. In many places around the world, freshwater resources are becoming increasingly strained. Recent analyses find that nearly two billion people are currently considered subject to severe water shortage, and this number is projected to increase to over three billion during the next 25 years (Plate 1) Factors such as population growth, economic development, and global climate change are contributing to this expanding starless and leading to ever-increasing water use for domestic, industrials and agricultural purposes. Agriculture alone is responsible for over 70 percent of global freshwater use, primarily for irrigation (Montaigne, 20021. During three-quatters of the last century, increases in withdrawals from ground water reserves ifs the United States exceeded population growth. Economic, environmental, and governmental factors recently have slowed this imbalance, and there are encouraging signs that after a sustained 30-year growth in ground water withdrawals nationwide these to ends now are stabilizing (Figure 1 . 1~. Howeve', a continuing depletion of croundwater reserves is still occurring in song large aquifers (Figure 1.2', and water resource needs are increasing rapidly in many other parts of the world. History is replete with examples of local and regional conflicts over water. Meeting the pressing need for clean, sustainable, and adequate water supplies will require comprehensive resource management strategies that include water conservation and efficiency measures, but there could also be tremendous societal benefits Tom taking actions to increase water supplies in select areas. Hazardous weather such as hail, strong thunderstorm and tornadic winds, hurricanes, lightning, and floods pose a significant threat to life and property. Table 1. slows the costs of severe weather in the United States in terms of fatalities' injuries, and property damage. In developing countries with less protective infrastructure, the toll of severe weather sometimes can be especially devastating; for example, in 1998 Hurricane Mitch spawned mudslides in Honduras that killed over 10~000 people. Clearly it is 9

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10 90 Q 80 70 In o 60 o U) lo - ._ m ._ In 3 - 50 40 30 10 400 :~. 350 300 In 0 ~ _ 250 ~ ._ _ ~ 0 ~ 200 0 0 ._ ._ _ _ ._ ~ m ~ Q 150 ._ O ~ Cal. 3 - ._ 100 50 - . O - Rural E1 Industrial go Public Irrigation ' ': :' 1 . .] 1 . .] 1 ' ' . .] . ., it.- . . 1 t.. 1 CRITICAL ISSUES IN WEA THER MODIFICA TION RESEARCH . l 1.. ... 1.. ... 1 . . . . . . . 1' . . I. .. ; . . . 1950 1955 1960 1965 1970 1975 Year _ Ground Water Surface Water Total Water Population . ~ 1950 1955 1960 1965 1970 1980 1985 1990 1995 .. , , , ,, . . ' 1975 1980 1985 1990 1995 Year FIGURE 1.1 Top figure: Ground-water usage in the United States, by sector. Bottom figure: Trends in water withdrawals in the United States. SOURCE: U.S. Geological Survey (20024.

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INTRODlJ(-TiON 40 a, ._ in o V' ~ a.? 0-] ~:5 a: :5 o o ~ V o _ ._ - a) ~ ~ a;' .> - As -40 -60 - . Storage accretion it, Storage depletion - - 1987 1989 1991 1993 1995 1997 1999 Year FIGURE 1.2 Cumulative changes ill ground-water storage since 1987, Higl1 Plains aquifer. SOURCE: Solve et al. ( 1 998 ). 1 ABLE 1.1 Summary of Natural Hazard Statistics Lloyd 2001 in tile United States Property Crop Damage Weather Event Fatalities Injuries (Millions of $) (Millions of $) Lightning 44 371 43.6 2.0 Tornado 40 743 63 0.1 7.4 Severe thunderstorm 17 341 317.8 6] .0 Hail 0 32 2,368.3 270.4 Floods 48 277 1,220.3 43.0 Coastal storm 53 96 17.7 0 Hurricane 24 7 5,187.8 2 Winter storm 1 8 173 103.6 0.1 67 Fog I.~ o High wind SOURCE: National Oceanic and Atmospheric AdministratiolliNational Weather Selvice Adapted from http://www.nws noaa.gov/om/severe_weather/sum01.pdf. 98 63.8

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12 CRITIC,4 L I`SSlJES IN TYE-A Ti-lER AfOD JFI(-~ T10V REtSEARCH important to mitigate society's vulnerability to hazardous weather through actions such as improving construction standards for buildings, relocating, residents from hazard-prone al eas, and providing more accurate warnings. However there might be substantial additional societal benefits to reducing the intensity or occurrence of hazardous weather events through direct interventions in atmospheric processes. Whether or not methods for weather modification ultimately prove effective in providing significant benefits, these expanding, societal stresses and threats will continue to make periodic reassessment of the science and technology underlying weather modification a national need. Searching for ways to enhance precipitation and mitigate hazardous weather is one of the most important challenges that could be tackled by science. Even relatively minor changes in weather could be of profound benefit. This possibility was recognized immediately upon reports of the first cloud-seeding experiments: In congressional hearings in 1951, Dr. Vannevar Bush, president of the Carnegie Institute, testified, "I have become convinced that it is possible under certain circumstances to nuke rain. As it stands today, we are on the threshold of an exceedingly important matter, for man has begun for the first time to affect the weather in which he lives, and no man can tell where such a move finally will end.~, (U.S. H.R., 1953). BOX 1.1 Socio-economic Implications of Weather Modification The Committee's charge calls for this study to focus on research and operational issues and instructs it not to address the policy implications of weather modification. Althougl~ the Committee has not investigated policy and related socio-economic issues (e.g., liability concerns, cost-benefit analyses, societal attitudes), it recognizes that the motivational factors fot applied research and operational activities in weather modification are intimately linked to these issues. For instance, weather modification is aimed primarily at controlling the spatial and temporal distribution of precipitation? which can potentially raise contentious liability issues (i.e., the metaphoric "robbing Peter to pay Paul") Furthermore' societal attitudes toward "tampering with nature" are often linked to steed; people living in drought-prone or water-stressed regions will do what they deem necessary out of desperation. The Committee believes that sound' validated scientific research results can ultimately provide the critical answers needed to address these political and socio-economic issues appropriately. In addition, the Committee recognizes that even if significant, reliable precipitation enhancement techniques were to eventually become feasible (em., if it becomes possible to increase rainfall by up to 20 percent everywhere that is needed) this alone is unlikely to provide a long-ten solution for water resources in areas of the world that are most water stressed. There are a variety of proven, cost-effective societal and technological approaches (e.g., water conservation, precision irrigation, improved building codes in coastal areas) that undoubtedly will continue to play an important role in water resource management and hazard . . . m~t~gat~o~.

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iN7 RODZJ(-TJON 13 This quotation illustrates the initial enthusiasm for cloud seeding As late as 1978, the Department of Commerce Weather Modification Advisory Board (1978) reported that "a usable technology for significantly entrancing rain and snow and ameliorating some weather damage is scientifically possible and within sight.', This conclusion ultimately proved to be too optimistic regarding the time required to realize that possibility, in part because the recommended research program was not pursued (Lambright and Changnon, 19891. The stated goals, however, remain as real today as they were when these statements were first made. Since that time, weather modification has largely been relegated to the realm of promises unfulfilled. Weather modification does not appear as a line item in the budget of any federal agencyalthough closely related topics such as cloud physics, water management, and climate change are being pursued and no work is being done on the complex social and economic implications of attempts to modify weather (see Box 1 14. Yet people in drought-prone areas willingly spend significant resources in support of cloud seeding to increase rain, and commercial operations for increasing mountain snowpack have been supported continuously far many years (Plate 21. But all the while, science is unable to say with assurance which, if any, seeding techniques produce positive effects. In the 55 years following the first cloud-seeding demonstrations, substantial progress has been made in understanding the natural processes that account for our daily weather. Yet scientifically acceptable proof for significant seeding effects has not been achieved, and the scientific challenges have proved to be significantly mole formidable and complex than perceived initially. CLOUD PHYSICS Most attempts at modifying weather in the modern era have aimed at initiating the onset, or accelerating the rates of. the physical-chemical processes involved in precipitation formation. Significant amounts of precipitation can occur only when low- level atmospheric convergence and upward movement of ail parcels provide water vapor for conversion into cloud drops. Thus, a complete u~derstandi~g of the formation of natural precipitation requires understanding the dynamics of atmospheric motions as well as the physical processes governing formation and growth of cloud and precipitation particles The physical processes taking place within a cloud that lead to precipitation are very complex and depend, among other things' on the number and characteristics of aerosol particles in the cloud-forming air. The atmosphere contains a tremendous amount of particulate matter from a wide variety of natural arid anthropogenic sources. These include, for example, soot, sea salt, volcanic ash, wind-blown sand and dust, biogenical]y-derived materials such as pollens and spores, and a variety of sulfur, nitrogen, and carbon compounds (which often result from industrial pollution, biomass burning, and other combustion processes). Soluble and hydrophilic particles absorb water and can eventually act as CCN. Some insoluble particles with wettable surfaces may adsorb water and serve as large cloud drop nuclei or ice nuclei. Some insoluble particles have a crystalline structure that provides an efficient starting place for ice crystals to

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14 CRITICAL ILSSUES IN [YE-A TlIER AlODIFl(.=ATJONREtSEARCT-J grow and thus are referred to as ice nuclei (IN); the exact composition of most IN is not well known. Differences in flee initial population of atmospheric aerosols affect the cloud particle and cloud drop populations, which subsequently affect the amount of precipitation reaching the ground. There is considerable uncertainty as to just how the various IN and CCN activate' how concentrations vary of giant CCN or ultra-giant particles (UGP) and their impact on coalescence broadening, how cloud particles interact and evolve by collision and breakup processes? how winds and electric fields in a cloud evolve and affect the growth and interaction of cloud particles, arid how individual clouds interact among other fundamental questions. There are severa] different physical pathways (often called mechanisms) through which precipitation may form in natural clouds. Local conditions of updraft speed, temperature, pressures initial aerosol characteristics' and cloud and precipitations particle concentrations arid size distributions govern the rates of progress along these pathways. Several mechanisms may be active simultaneously, each affecting the others. Often orate of the mechanisms proceeds fastest than the others and becomes dominant. For the purposes of this report, and at the rislc of oversimplification, it is useful to group these mechanisms into those that involve the formation of ice particles and those that do not. The so-called coalescence mechanism or warm-cloud precipitation mechanism is an all-liquid process wherein raindrops Tom by the merging of the cloud droplets (Bower, 1950; Ludlam, 1951; Young, ]9754. This mechanism proceeds most rapidly in clouds having a high liquid water content (LWC) and a broad spectrum of cloud drops. The sources and characteristics of atmospheric aerosol particles capable of forming drops large enough to initiate the coalescence mechanism are largely unknown and the subject of much research. Typical conditions for the formation of collision- coalescence rain are (a) convective clouds with bases warmer than about +15C and accompanying large LWC and (b) stratified clouds of sufficient lifetimes that are too warm to initiate ice particles on the existing IN. Coalescence rain occurs when drops grow large enough to fall to the Earth before they are carried by the updraft to levels cold enough to cause them to freeze. The so-called Bergeron ( 1 935J mechanismor cold-cloud mechanism - postulates the nucleation of ice particles in supet cooled clouds followed by their growth by vapor diffusion into snow particles. Unde~^ favorable conditions they may aggregate as snow or rime to form low-density graupel or snow pellets. This mechanism was first postulated by Bergeron,' building on earlier work by Alfred Wegener7 and developed into a conceptual model of precipitation by Findeisen (19381. The sources and characteristics of natural IN are largely unknown. In general this mechanism may be important in clouds of all types where temperatures are colder than about -1 5C, including the upper parts of cumulonimbus clouds at all seasons and latitudes. It accounts for most wintertime snow. ~ Bergeron first gave his paper before the Lisbon meeting of the International Union of Geodesy and Geophysics on September 19, 1933, belt it was not published until 1935.

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1,~7 ROD{J(-TION Ice may also form in clouds through the freezing of drops. It is well established that the probability of drop freezing is inversely proportional to temperature and directly proportional to drop size. Thus, large drops are more likely to freeze at warmer temperatures than smaller ones. The nature and concentrations of nuclei capable of inducing drop freezing (fieezing nuclei, FN) are largely unknown and the subject of current research. A variant of the warns rain ~necl~anism sometimes called the coa]escence-fireezing mechanismcomes into play in clouds having both an active coalescence n~echanisn~ and an updraft strong enough to carry drizzle drops upward to levels where they freeze through the action of EN In many situations this may occur at temperatures as warm as-5C to-10C. Upon freezing, the drizzle drops become small ice pellets. Further growth through riming with cloud drops produces high-density graupel and small hail These particles then melt into raindrops upon descending below the 0C-level This mechanistic appears to be very important ill convective clouds having bases warmer than about +1 5C arid Title low s~b-cloud CCN concentrations. under certain cloud conditions the process of rinsing Nay result in the creation of small ice particles (so-called secondary ice particles, SIP) in numbers vastly exceeding the original number of ice nuclei. Although the details of this process are still a matter of research, this mechanism may be very important in natural precipitation. The occurrence of SIP was first elucidated from physical measurements obtained in a scientific cloud- seedin~, experiment and is still the subject of research (~Hoffer and B~aham, 1962; Koenig, 1963;Braham, 1964,1986a;HalletandMossop, 19741. Cloud physicists now have relatively cleats pictures of the physics involved in these three precipitation mechanisms. It is possible that the majority of clouds of all types represent mole complex situations, but conceptual cloud-seeding models usually are based on one of these three models. FIRST EXPERIMENTS AND FIRST CONTROVERSIES In the mid-1940s laboratory and field experiments by Drs Vincent Schaefer, Irving Langmuir, and Bernard Vonnegut of the Genera] Electric Laboratory demonstrated that dry-ice and silver-iodide smokes were excellent ice nucleants, and that when released into supercooled stratus clouds, the treated regions were gradually converted into large numbers of tiny ice crystals. These demonstrations appeared to give strong support for the Bergeron mechanism. Even at the time of the 1946-1947 experiments it was well known that the clouds used in those demonstrations contained so little water that even if all of it reached the ground, the amount of rain (or snow) would be insignificant. Meteorologists were aware that useful amounts of precipitation required deep cloud layers with updrafts and continued inflow of moist air' and that natural precipitation results from a progression of and complex interactions between microphysical processes and cloud dynamical processes The unbridled enthusiasm of Dr. Langmu~r for what might be possible through closed seeding and the potential legal liability implications of the early experiments led the General Electric Company to discontinue field experiments, and in 1947 to negotiate a contract for further fieldwork to be carried out by the military with Dr. Langmuir and

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16 CRITIC,4 L [ISSUES IN [YEA TI-IER A10DIFICA TION RELSEARCI-I Dr. Schaefer as technical consultants. This effort came to be called Project Cirrus (Havens et al., 19781. The results of Project Cirrus were widely distributed and the participants were not shy in reporting the potential of cloud seeding. Dr. Langmuir was a world-renowned scientist, and his speculations as to bloat Plight be accomplished by seeding clouds commanded attention. By this time collision and coalescence were recognized as important for producing rain; combined with Langmuir's chain reaction theory, which deems good collection efficiencies as necessary for inducing precipitation from warren clouds (Langmuir, 1948y, it is not surprising that solve scientists and large numbers of the populace accepted the proposition that seeding of clouds might increase rainfall and also perhaps mitigate the vagaries of severe weather. The combination of a few overly enthusiastic scientists, an active press, and a receptive populous (especially in drougl~t-prone areas) quickly resulted in a worldwide commercial industry devoted to cloud seeding, and act era of great interest arid concern among governmental and scientific organizations. These early days of cloud seeding were described by J. C. Oppenheimer of the Advisory Committee on Weather Control (ACWC, 1 957 J as follows: Within two years after Langmuir's and SchaeJer's historic experiment in 1946 of seeding clouds with dry ice, and the beginning of governmental research, a number of commercial cloud-seeding companies were organized. Exorbitant claims by some seeding organizations and scientists led to sharp differences of opinion as to the economic benefits of seeding activities. Various aspects of this controversy came to the attention of Congress. Between 1951 and 1953, Congressional hearings on several bills dealing with cloud seeding revealed that farmers, ranchers, electric utilities, municipalities and other water users were paying 2 cents to 20 cents per acre, and annually were spending between $3 million and $5 million on weather modification activities covering approximately l0 per cent of the land area of the nation....As a result of this lengthy consideration, the Advisory Committee on Weather Control was established by an Act of 13 August 1953. Findings of this committee are considered below. Other details of the history of these early days of cloud seeding can be found in Byers (1974), Elliott (1974), and McDonald (19561. AN EMERGING INDUSTRY AND DEVELOPING PUBLIC CONCERN Initial cloud-seeding experiments were conducted from airplanes flying in or slightly above the cloud target. With the subsequent development of devices for releasing silver iodide particles from ground generators, the cost of seeding operations became quite nominal. This led immediately to widespread efforts to increase rain by operating ground generators upwind of the target areas. With low unit costs and the implicit assumption that cloud seeding could do no harm, and at the worst would be ineffective, the industry grew almost overnight. The commercial operations were paralleled by programs in the Bureau of Reclamation (which

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{NTRODZJCTiON 17 was to become a major supporter of weather modifications studies), the Weather Bureau' the Department of Defense, and others. Almost immediately cloud-seeding programs sprang up in Australia, France' Israel, and South Africa. There also was a renewed interest in hail suppression in Alpine countries where such programs were already under way. By 1951 weather modification programs were active in about 30 countries In the confident belief that seeding would produce a positive effect (such as art increase in rain or decrease in hail), project sponsors required the commercial operators to seed every available opportunity. In commercial operations there was no loom for randomization of cloud treatments. Many projects lasted only one or two seasons. Few if any made provision for measuring the physical variables associated with rain formation in their seeded clouds. As ~ result rigorous proof of a seeding effect in the commercial cloud-seeding projects was very difficult at best, and generally not possible. The commercial seeding operators provided reports to their sponsors. These reports typically contained an estimate of flee seeding effects, usually based on comparison with a pre-seeding period, perhaps with a nearby area not used in their project. The inability of commercial operators to demonstrate positive seeding effects beyond a shadow of doubt gradually led to a skepticism arid demand for more convincing evidence. In a number of hail suppression programs a reduction in damage claims led insurance companies and farmers to continue seeding. Nevertheless, the number and volume of commercial projects began to decline. By about 1 956-1 957 it had reached a leve] of about one-f~urth of its peal<. The rapid expansion of the seeding industry, with claims of seeding effects that could not be rigorously substantiated and for which there was only a sketchy theory and questionable physical evidence, deepened the split between meteorologists and those supporting the seeding efforts. A few of the commercial companies? l~owever, made an effort to deal openly with these problems. These companies survived and contributed substantially to increased knowledge about the seedability of clouds. Yet even today the words "weather modification" and "cloud seeding" conjure up images of alchemy and charlatans. THE PIONEERING EXPERIMENTS In the early 1940s most meteorologists had little background in the physics and chemistry of cloud particles' but some of those who entered the field from other physical and engineering sciences during the wartime training programs saw the possibility that cloud seeding might prove useful as a tool for probing the inner workings of clouds. Recognition of the great potential benefits that might accrue from proven weather modification techniques prompted the Weather Bureau and scientific research units in the U.S Army, Navy, and Air Force to consider experiments to clari: the potential for cloud seeding. In ] 947 the Weather Bureau launched its Cloud Seeding Project, which included 176 non-randomized airplane releases of dry-ice pellets into the tops of supercooled stratified clouds over Ohio and the Sierra Nevadas and into convective clouds over Ohio and along the Gulf Coast Results were inconclusive

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IN' CRITIC,4 L [LSSUF,5 IN {YEA TI--IER AJODIFICA 7'10N REtSE-ARCI-f One of the early experiments, organized ilk 1951, was the Artificial Cloud Nucleation Project (Petterssen, 1957~. Results of randomized seeding were generally inconclusive' except for showing that water splay seeding of tropical cumuli speeded the onset oil precipitation. Subsequent studies suggested that total precipitations from fleece clouds may have been decreased, because the seeding and earlier onset of precipitation shortened the time available for creation of cloud water (Braham et ah, 19574. Other projects followed, with meteorologists joined by chemists' physicists, and engineers, and with generous support from tile Departments of Defense, Interior, and Co~nn~erce and flee National Science Foundation (NSF). Under the umbrella of cloud seeding, scientists mounted field and laboratory efforts that led to a breathtaking increased understanding of the microphysics and dynamics of clouds. In an effort to put cloud seeding on a more rigorous foundation, several university and government groups launched major studies of clouds and their reaction to seeding. Some of the most productive studies during this petiod included randomized seeding trials with accompanying physical measurements using the most modern tools available at the time Measurements were made in both seeded and non-seeded clouds Some of these experiments were "double blind," such that the group conducting the seeding did not collect and analyze the rainfall data, while those involved in the analysis had no knowledge of Allen and where seeding had taken place (em, the Missouri Project Whitetop). Typically these experiments ran for several seasons. Results were mixed. None of these experiments provided incontrovertible evidence that seeding was effective; many suggested rainfall increases (or hail decreases) from seeded clouds but a few suggested rainfall decreases. They suggested' but did not prove, that any change in precipitation resulting from seeding would likely be limited to several percent, much less than the original claims by some non-scientific operations The programs of physical measurements greatly expanded knowledge about cloud processes and led to a number of important scientific findings: demonstration of the power of numerical modeling of targeted seeding of cumuli; realization that the coalescence mechanism operated in warm season clouds in mid-latitudes and was not restricted to the tropics; and that drizzle drops that had formed by coalescence often froze and began growth by riming, at temperatures as warm as -5C to ~1 0C (this led to the recognition of a coalescence-freezing mechanism, and in some conditions the production of secondary ice particles). There were early suggestions that the latent heat released by seeding-induced freezing of liquid cloud water could prolong the life of the cloud, leading to more rain than would otherwise have been delivered. These and other observations led to the possibility that increases in cloud downdrafts and sub-cloud outflow caused by seeding may prolong the lifetime of the cloud complex as a whole, although the exact mechanisms for this continue to be unknown. THE NEED FOR IMPARTIAL ASSESSMENT OF SEEDING RESULTS The rapid growth of the commercial cloud-seeding industry, extravagant claims of seeding effects from some commercial operations, and the inherent weaknesses in their assessments raised widespread concern. Thus, the National Academy of Sciences (NAS), the NSF, the American Geophysical Union, and the American Meteorological Society

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iNTRODZJCTJON 19 (AMS) all undertook in-depth examinations of cloud seeding. Papers on cloud physics began to appear at scientific meetings. Entire conferences were devoted to the subject, and many of these became the battleground between seeding proponents and opponents. In virtually every case there was ~ plea for basic ~ esearch to enhance scientific understanding of cloud processes as a prerequisite for intelligent cloud-seeding operations. There was a movement toward independent assessment of the reports of commercial cloud-seeding operations. This involved analyses (or reanalyses) of project findings by persons not involved in the original project and if possible using data collected independently from the original project. The first such assessment was conducted by the President's ACWC. Captain Howard T. Orville, USN (ret.), chaired this committee, and its final report was submitted to the President in December 1957 (AC WC, 19574. The ACWC hired climatologist-statisticia~ Herbert Thom and assisted by a group of outstanding, statisticians to conduct an independent assessment (reanalysis) of 12 sl~ort-term commercial silver-iodide seeding operations. They concluded that winter-season west-coast orographic precipitation was increased an average of 14 percent, significant at the 99 percent level (oc=0.01~. But operations in other seasons and areas did not give conclusive evidence for a seeding effect. The ACWC made a strong plea for increased support of those sciences Blat were basic to understandings, clouds and cloud systems. In 1963 the NAS appointed a Panel on Weather and Climate Modification to "undertake a deliberate and thoughtful review of the present status and activities in this field, and of its potential and limitations for the future." The panel, chaired by Gordon J. F. MacDonald, issued a preliminary report in 1964 (NRC, 1964) in which it concluded that it has not been demonstrated that precipitation from winter orographic storms can be increased significantly by seeding....We conclude that the initiation of large- seale operational weather modification programs would be premature. Many fundamental problems must be answered first. It is unlikely that these problems will be solved by the expansion of present efforts, which emphasize the a posterior) evaluation of largely uncontrolled experiments. We believe that the patient investigation of atmospheric processes coupled with an exploration of the technological applications will eventually lead to useful weather modification, but we must emphasize that the time-scale r equired for success may be measure ed in decades. The panel's final report (NRC, 1966i included a number of recommendations concerning the support and infrastructure needed for research in weather modification. It also sponsored two independent evaluations of a small number of commercial seeding operations Concerning their reanalysis of 14 short-duration, groulld-genel-ator operations in the eastern United States, they found indications of a positive seeding effect. However, "results of these fourteen projects...cannot by themselves be regarded as conclusive evidence of the efficacy of seeding; yet taken together they seem to us to be a new indication of positive effect, warranting optimism." The panel also sponsored an analysis using seasonal runoff data as the test variate in four west-coast winter-season orographic

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20 CRIT/C,4 L I`SSlJES IN THEA TITER A10DIFI(-~,4 TION RESEARCI-l seeding operations totaling 41 years of operations. It tuned overall runoff increases of about 12 percent, statistically sig~iiicant at the 96 percent level (oc-0.04~. The NSF issued a series of annual reports on weather modification research and evaluation between 1959 and 1964. In 1964 the National Science Board appointed a Special Commission of Weather Modification, chaired by Dr. A. R. Chamberlain, Chicle found that '~supercooled fog on the grouted can be dissipated. No practica] approach to the dissipation of wartn fog is at hand." Also, "while the evidence is still somewhat ambiguous, there is support leer the view that precipitation fiom some types of clouds can be increased by the order of ten percent by seeding. If the results are confirmed by further studies they would have great significance. The question of corresponding decreases of precipitation outside the target area is unresolved." It suggests that 'advanced experimental techniques and application of sophisticated concepts in statistical design promise to reduce the present uncertainty in flee ;nterpretatior~ of field experiments" (NSF, 1965) In 1973 the NAS Review Panel on Weather and Climate Modification (T.F Malone, chair) issued a report titled "Weather and Climate Modification, Problems and Progress." Based on the results of several randomized experimental seeding programs conducted after the 1966 NAS report' the panel concluded that it cc-nuclei seeding can sonnetizes lead to more precipitation, can sometimes lead to less precipitation, arid at other times.. have no effect....lt is concluded that the r ecent demonstration of both positive and negative effects fit om seeding convective clouds emphasizes the complexity of the processes involved.A..A more careful search must be made to determine the seedability criteria that apply to the convective clouds over various climatic regions....The Panel concludes that there is a pressing need for furthet analyses of the areal extent of seeding effects under a variety of meteorological and topographical situations and for investigations into the physical mechanisms that ale responsible for any such effects. Concerning hail reduction and mitigation of severe weather hazards, the panel noted the need for further research (NRC. 19739. Even before these teports were published, papers appealed in the scientific literature pointing to sources of bias and other technical problems that had not been considered that could invalidate conclusions. If anything, the split between those who believed in the immediate application of cloud seeding and those who believed that such actions were premature only widened and deepened. In response to the National Weather Modification Act of 1976 (PL 94-490) the Secretary of the Department of Commerce appointed the Weather Modification Advisory Board' chaired by Harlan Cleveland, to take an in-depth look at cloud seeding. Its two- volume final report was submitted in 1978. That committee found that the major task ahead was to learn more about the atmosphere and processes within it. To this end it urged an increase in federal support for meteorology and other sciences important to this effort. Concerning the status of cloud seeding the Committee found that

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JNTRODZJCTION 21 the experimental evidence for cloud seeding has not yet reached the levels of objectivity, repeatability, and predictability required to establish new knowledge and techniques. There are, however, several lines of evidence suggesting that carefully controlled seeding, using means appropriate to the aims, will result in weather modification effects of useful dimensions. tVol. 1, p. 35.] Several assessments of individual seeding projects, or groups of projects, have been made by individual scientists familiar with cloud physics and cloud seeding but not directly involved with flee projects they assess. Generally speaking, these outlooks cattle to the view that cloud-seeding experiments have not yet provided the evidence required to establish scientific validity, though the prospects are promising and worth pursuing. After dale consideration our Committee finds little reasons to differ from these findings This is due in part to the lacl; of concerted research in weather modification. It has beer three decades since the last NRC report on weather modification In the interim there have beets improvements in the understanding of cloud processes and significant development of tools and techniques, including computational power, statistical analyses, and remote sensing ot: cloud systems. These opportunities mandate a fresh look at the status and potential of weather modification.

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