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Weather Modification: Past and Prologue

Robert Haskell Abrams and Alexis Clark


  • Canvasses the basic mechanisms of weather modification, its efficacy, and its regulatory and legal history, while trying to identify its future role in the U.S.
  • Addresses the legal debate over who owns the rights to the water in clouds.
  • Discusses legal issues in weather modification and how the federal government must support activities related to weather modification.
Weather Modification: Past and Prologue
Alan Majchrowicz via Getty Images

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In the western U.S., the over two-decades-old drought has been drying reservoirs to record lows, forcing broader conversations on water scarcity, climate change, and how to mitigate these effects. In the past, weather modification, principally in the form of cloud seeding, has been used to increase precipitation. This article attempts to canvass the basic mechanism at work, its efficacy, and its regulatory and legal history, while trying to identify its future role in the U.S.

Weather Modification’s Emergence from the Fringe to Mainstream

Weather modification is used today to enhance precipitation, protect crops from hail damage, clear fog at airports, and reduce air pollution. These practices rely on promoting the condensation of water vapor in clouds around a nucleating agent—or seed—into either ice crystals (cold-cloud seeding) or coalescence of water vapor into larger droplets (warm-cloud seeding), which then become heavy enough to fall to the ground as precipitation. The practicality and effectiveness of cloud seeding depend on the unique properties of each cloud and the atmospheric conditions in which it forms. The principal study that legitimized the practice was published in the 1940s after researchers for General Electric and the Office of Naval Research stumbled upon the basis for seeding clouds while working to solve problems related to wing-icing of aircraft. The first field-based cloud seeding experiment occurred in 1946, leading to the birth of modern cloud seeding.

Silver iodide is the most commonly used nucleating agent in cloud seeding. Using this method, silver iodide is released into clouds when atmospheric conditions are appropriate using ground generators, drones, or specially equipped aircraft. Seeding activities over mountain ranges generally take place many miles from the target area for precipitation (the downwind side of the mountain range), utilizing the updrafts along mountain ranges to lift nucleating agents into the super-cooled water vapor in the atmosphere. Dispersion of the nucleating agent throughout the cloud via convection causes the formation of ice crystals around the nucleating agent that later fall to the ground as either snow or rain. The seeding of cold-weather clouds that form over mountainous terrain, known as orographic clouds, is the most common precipitation enhancement strategy in the U.S. and has had the most promising results.

Because of the variability in individual weather systems and the difficulty in replicating results, the scientific community has been divided on the efficacy of cloud seeding. In part, this evidences the lack of a coordinated national effort to link understanding of atmospheric science to the processes behind weather modification. Despite a lack of replicable scientific data, practitioners claim great success in enhancing precipitation and water-stressed local and state governments continue to invest in the opportunity to add any additional measure of water to depleted reservoirs.

Save for a few minor reporting requirements, the federal government has largely taken a hands-off approach to regulation of weather modification activities. A congressionally created advisory committee reported in 1957 that, in the early 1950s, water users in the U.S. were spending between $3 and $5 million a year on weather modification activities affecting approximately 10% of the nation’s land, much of which was taking place over federal land in the West. Congress then named the National Science Foundation (NSF) the lead agency for study, research, and evaluation of weather modification practices. When weather modification was reassigned to the Departments of Commerce and Interior, weather modification leadership was transferred to the National Oceanic and Atmospheric Administration (NOAA), where it was not a high priority. Inactivity set in. Congress, in 1972, added modest reporting requirements, but that did not improve understanding of precipitation enhancement practices or their potential. Even though NOAA continues to collect reports and make them publicly available, the reporting requirements, to this day, consist of a one-page form that provides minimal information about operational activities.

The most concrete federal action on weather modification was passage of the Weather Modification Policy Act of 1976 (WMPA) (Pub. L. No. 94-490) directing the development of a national policy on weather modification and a national program of weather modification research and development. The WMPA recognized the economic significance of weather modification, not only for production of water, but also for crop protection, energy production, and potential disaster mitigation—and called for resources to assess the “economic, social, environmental, and legal impact … of a national program for managing weather modification activities in the U.S.” The Act sought to develop model codes and agreements to ensure peaceful regulation of domestic and international weather modification activities. The WMPA also sought to establish a national program of research and development and set minimum reporting standards for weather modifiers. Finally, the statute pledged funding for research and development that had “dried up” in recent years and created a congressionally appointed advisory board to provide further recommendations.

That advisory board reported back in 1978 unanimously recommending appointment of a lead agency and a variety of “gap-fillers” for the many deficiencies in the existing system. Report to the Sec. of Com. from the Weather Modification Advisory Board, The Management of Weather Resources Volume I: Proposals for a National Policy and Program, O-268-625 (June 30, 1978). Their report highlighted the lack of required environmental monitoring by weather modifiers during and after an operational or research project and called for going beyond NEPA to address environmental concerns related to federal projects. Id. at 141, 149. This included post-project assessment for evaluation of possible long-range effects of the activity. The board recommended rules requiring monitoring of silver iodide concentrations over time and evaluation of ecological and environmental changes, such as changes in streamflow, migration of ecosystems due to changed annual precipitation patterns, changes in water quality, and invasive species monitoring. Id. at 143–44. The board also recommended regulatory transparency and provisions for public participation to contend with potential fearmongering regarding possible negative outcomes resulting from these activities. Id. at 9. At the programmatic level, the report noted more research was needed before a fully informed national regulatory program could be created but recommended an immediate federal role in licensing (akin to an FAA pilot’s license), federal field research, maintenance of environmentally sensitive operating standards for those regulated by state agencies, improved reporting of nonfederal operations, technical assistance to state and local governments and private operators, and express affirmation of liability for weather modifications under the Federal Tort Claims Act. Id. at 9–11, 147–50.

Weather Modification Activities in the United States

In the U.S., current or past cloud-seeding activities have been reported in 12 states. As of January 2022, activities were reported for the 2021 season in California, Colorado, Idaho, North Dakota, Texas, and Utah related to snowpack augmentation, precipitation enhancement, and hail suppression. All are water-stressed states in which seeding of orographic clouds is possible due to the terrain and where modern precipitation enhancement activities have been occurring consistently since the 1950s. Some states have been taking precipitation enhancement seriously over the past 70 years as part of a larger, integrated water management plan. For example, California’s Water Plan includes precipitation enhancement as a resource management strategy, and Wyoming has been operating precipitation enhancement in the state since the early 2000s for drought mitigation. States and water districts in the Colorado River Basin have formed cooperative agreements to consolidate individual precipitation enhancement efforts to improve overall results.

Cloud seeding has significant limits. If the atmosphere lacks sufficient moisture, as is the case during severe drought conditions like the one the West is currently experiencing, no amount of seeding can force water from the sky. Under more normal conditions, precipitation enhancement increases the overall percentage of precipitation in an area. Any increase in precipitation will be proportionate to what is naturally available to fall in that given season. Long-term, cloud-seeding operations do increase annual snowpack and runoff yields in average years. Efforts at benefit quantification exist but are neither precise nor extensively documented. The Upper Colorado River Commission estimates that a 10–15% increase in snowpack in the Colorado River Basin through existing and expanded seeding programs has the potential for the generation of an additional one million acre-feet of water in a year, which is about 7% of the expected flow of the Colorado River in the current era. Depending on usage, this is enough water to supply between 1 and 3.5 million households each year.

Precipitation enhancement is cost-effective. Precipitation enhancement cost is relatively low ($5–$30 per acre-foot), which compares favorably to other methods of developing water, such as desalination ($1,900–$3,000 per acre-foot). Comparisons to conservation costs are less striking, but precipitation enhancement is meant to complement, not substitute for, conservation. The addition of a cloud-seeding program to a water management portfolio seems like a no-brainer, even if the results of such a program are somewhat speculative in any given year.

Climate Change Driving Need for Revival

Changing climate is forcing communities to rethink water supplies on a shorter timeframe than originally imagined. Ongoing drought conditions in the western U.S. make the beginning of this century the worst period since 800 C.E., with at least 19% of this period attributable to anthropogenically induced climate change from 2000–2021, turning what is already a severe drought into a megadrought predicted to continue for several more years. A. P. Williams et al., Rapid Intensification of the Emerging Southwestern North American Megadrought in 2020–2021, 12 Nature Climate Change 232 (2022). Although cloud seeding has minimal effect when atmospheric moisture is extremely low, even in a megadrought there are years in which atmospheric conditions are wetter.

Often overlooked in the discussion is that precipitation enhancement has a potentially positive role to play in more humid regions. On the East Coast, for example, saline intrusion from sea level rise is expected to affect groundwater supplies and rising temperatures will increase evaporative losses. While there is significant rainfall compared to the West, several coastal states are having to cut back on groundwater withdrawals, and even the wettest among them, Florida, is already limiting some groundwater withdrawals. Even in the East, drought conditions act as a double whammy, simultaneously reducing precipitation and increasing evaporative losses in the soil, streams, and reservoirs. Projected hotter temperatures exacerbate water loss even more.

Reviving Interest in Federal Regulation

The overselling of the potential impact and scope of weather modification in the 1950s and 1960s undercut interest in continued federal funding for research and development. Atmospheric modeling, however, was in its infancy in those early years, increasing the difficulty of establishing the evidence needed to prove operational project effectiveness. Additionally, the early scope of weather modification activities and research ranged from small-scale fog suppression research at airports to attempts at altering the trajectory of major hurricanes. The militarization of weather by the U.S. during the Vietnam War negatively affected public perception of the practice, leading to condemnation by Congress and the United Nations in the years that followed. Even so, state and private funding of precipitation enhancement and hail suppression projects continue to thrive in the U.S.

The scientific community in that same 70-year period has made strides in tracking weather. Concern over global warming has spurred advances in the understanding of global atmospheric processes and anthropogenic effects on climate. The discourse is changing, and discussions of large-scale weather modification projects, now termed “geo-engineering,” have crept back into the mainstream as part of climate change mitigation, making renewed interest in the long-dormant Weather Modification Policy Act (WMPA) appropriate.

Those most interested in reviving federal interest in weather modification are stakeholders in the western U.S., and they have gained some support and cooperation from the Bureau of Land Management (BLM). BLM has continued to participate in research (as opposed to operational programs) supporting snowpack enhancement for the Colorado River Basin, and a 2015 BLM literature review and technical synthesis of winter orographic seeding activities urged that technical difficulties in demonstrating the precise effectiveness of seeding activities should not be the death knell to government support of operational and research programs. Congress, however, has moved no legislation on the subject.

State Weather Modification Regulation

In the early- to mid-1950s, states began adopting laws regarding weather modification activities. Twenty-four states have enacted such laws, though six have since been repealed. Of the remaining states, nine allow weather modification as part of emergency response or in a limited capacity. State weather modification statutes have some combination of permitting requirements, licensing and technical qualifications for conducting activities, reporting, public notice, and financial responsibility. The “teeth” behind each element varies from state to state, creating an inconsistent patchwork of regulations for activities that have potential transboundary effects.

Ensuring that those engaging in weather modification activities are qualified to conduct the activities and adequate regulatory oversight of these activities are central to minimizing risks to person, property, and the environment. Nearly all the states that have weather modification rules in place require either permitting of operations or professional licensing of practitioners, or a combination of both. The stringency of these laws varies.

Only a handful of states expressly require consideration of environmental impacts as part of the permitting process. Here federal baseline monitoring and reporting would be useful. Despite recent studies showing silver iodide has less potential for harm than originally thought, evaluation of background conditions and monitoring would allay public concerns and ensure that long-term or unexpected consequences do not arise from the use of seeding agents. Likewise, biological monitoring of affected watersheds should be considered as part of the environmental review process because small changes in precipitation and streamflow can impact ecosystems. Similarly, evidence of beneficial impacts of weather modification activities would provide useful information with respect to water quality and habitat restoration, not to mention improving understanding of aggregate cost-effectiveness. Even California, often the most environmentally attuned of states, does not appear to require careful post-project assessments.

Public notice of weather modification activities is a common statutory feature. Giving notice to affected communities may help to ease suspicions of those living downwind of seeding operations, but that same notice can feed claims of flooding or “theft” of rainfall and dissipation of clouds if the course of events disadvantages downwind interests.

All state laws require some form of reporting and recordkeeping, typically consisting of daily activity logs, monthly reports, weather records, and final and/or annual reporting of activities. What is missing from many state requirements, but which seems essential to ensuring that regulatory agencies and the public are adequately informed of the project effects, is a comprehensive evaluation of the project success and shortcomings at completion (or the end of a permit term). The American Meteorological Society’s (AMS) policy statement regarding weather modification activities notes this shortcoming, calling for “rigorous attention to evaluation of both operational and research programs . . . to help develop more effective procedures and to improve understanding of the effects of cloud seeding. AMS, Planned Weather Modification Through Cloud Seeding (adopted by AMS Council Nov. 2, 2010).

Most states require proof of financial responsibility to be submitted by operators. Operators may purchase insurance policies, secure bonds, or provide letters of credit showing their ability to pay any damage claims that may arise because of their operations. The amount of financial responsibility that states require varies considerably.

Legal Issues in Weather Modification

Liability for damages resulting from weather modification activities is seen as a major issue. Here, there are few definitive answers as to how claims will fare in court. Plaintiffs seeking injunctions against weather modifiers typically fail to obtain relief, either because damages would adequately compensate plaintiffs for any injuries or because establishing causation or proving actual harm is often speculative. The current lack of replicable and predictable scientific results for activities leaves the plaintiff without an essential element to their cause of action.

Fourteen states directly address liability in some form in their weather modification statutes. These laws are nonuniform but tend to favor defendants more than plaintiffs. Some add limitations on liability, such as sovereign immunity or forbidding strict liability. Colorado, Idaho, and Utah provide language in their statutes eliminating both public and private nuisance actions against licensed and permitted weather modifiers for dissemination of seeding agents or increased precipitation. These states are joined by North Dakota and Wisconsin in limiting trespass actions for the dispersal of seeding material into the atmosphere as part of an approved project. Other states allow regulatory violations to establish negligence per se for operators acting without a permit or in violation of state regulations, and a few have adopted regulations to hold operators accountable for their actions by expressly negating the permit shield defense for injury to other parties arising from conduct authorized by, or in compliance with, a state permit. None assist plaintiffs on causation.

On the issue of strict liability, moving into an era that is considering geo-engineering, the law pertaining to cloud seeding is not particularly instructive. Cloud seeding for hail suppression and precipitation enhancement was once seen as potentially falling into the category of dangerous, or “ultra-hazardous,” activity, calling for strict liability. Seventy years of permitted activity has brought forward little evidence of harm. That experience, however, has little or no bearing on whether other novel weather modification activities or geo-engineering activities should be excluded from strict liability. Altering the trajectory of a hurricane or altering the atmosphere to stave off the effects of carbon emissions is sufficiently different in kind that such activities may warrant strict liability treatment.

Water Rights

Weather modification also raises interesting questions related to water rights. The legal debate over “who owns the clouds” has been ongoing. In the mid-20th century, when optimism over weather modification was hitting its peak, it was presumed that increased activities would lead to litigation, which would answer these looming questions. The lawsuits did not arise as expected, possibly because so much of the activity takes place in sparsely populated portions of the western U.S. or over federally owned land. There are only three cases that have addressed rights to water in clouds. In Slutsky v. New York, owners of a country club sought to enjoin weather modification activities planned by the City of New York for fear that such activities would cause flooding and impact their business. 197 Misc. 730 (N.Y. Sup. Ct. 1950). The court found no factual or legal basis for their claim, as the injunction was sought speculatively and without any proof that irreparable injury might arise. In dicta, the court stated that the plaintiffs had no vested property rights in the clouds or the moisture therein and that the court will not protect private possible injury at the expense of a positive public advantage. In contrast, ranchers in Texas successfully obtained a lower-court injunction against a weather modification organization hired by farmers on nearby lands to perform hail suppression activities. Sw. Weather Rsch., Inc. v. Jones, 160 Tex. 104 (1959). The Texas Court of Civil Appeals limited the injunction to only restrain hail suppression activities over the plaintiff ranchers’ land. The court took the opposite view of Slutsky, stating, “the landowner is entitled to such precipitation as Nature deigns to bestow” and “that this enjoyment of or entitlement to the benefits of Nature should be protected by the courts if interfered with improperly and unlawfully.” Sw. Weather Rsch., Inc. v. Duncan, 319 S.W.2d 940 (Tex. Civ. App. 1958). This decision predated Texas laws governing weather modification and, while it does indicate that the landowner is entitled to precipitation, it leaves open the possibility of state regulation permitting such activities. Most recently, but still 50+ years ago, a Pennsylvania court held that the clouds and atmospheric moisture were common property held by all, but the right was not unqualified and could be regulated by the state. Pa. Nat. Weather Ass’n v. Blue Ridge Weather Modification Ass’n, 44 Pa. D. & C.2d 749, 759–60 (Com. Pl. 1968).

On this legal issue, seven states expressly declare the right to use the water in the clouds and atmosphere belongs to the state. Except for Louisiana, the states having express provisions for state ownership of atmospheric moisture are in the Colorado River Basin. It is no surprise that those states also follow prior appropriation to allocate water rights. Their users are fully aware that for many water users, the ability to satisfy their appropriations is dependent on annual snowpack and spring runoff from the mountain ranges. In practical effect, these provisions treat water derived from cloud-seeding activities as natural precipitation to be allocated by the state’s prior appropriation laws. That did not stop seeding activities, as some early commentators predicted. Instead, those laws shifted the investment from those who may have sought to “develop” water for profit to those who already had rights to the water, seeking to ensure water was available to fulfill their priority. These interests include municipalities that rely on the river for supply, utilities needing water for cooling thermoelectric generating plants, and the vast agricultural operations in the area.

States where water currently seems to be in abundance should look proactively to the successes of western states and adopt similar statutes to deter the inevitable conflicts that will arise as water resources become scarce. As climate change continues to strain water in the West, exacerbating the ongoing drought and fire seasons alike, the strains on resources will likely increase the potential for climate migration to the less water-stressed states. Concurrently, as other shifts in weather patterns occur, places like Florida, already subject to a “fire season” and where strained aquifers will start coming up short, will want to consider cloud seeding.

The Federal Government Needs to Act on Weather Modification

Federal support of activities related to weather modification should be a vital part of the toolbox for navigating climate change and avoiding past mistakes. Given the apparent nexus this field has with climate change mitigation and calls to consider geo-engineering, the federal government should be taking an active role in this field. Three primary areas the federal government should develop are (1) recordkeeping requirements, reporting, and public transparency; (2) establishment of federal minima, such as environmental assessment and adoption of industry standards; and (3) adequate funding for research and development. The federal government also could be usefully involved in data dissemination. Current reporting requirements are grossly incomplete. Updating the federal reporting requirements and improving public reporting would generate a publicly accessible repository of data and operational records to stave off negative effects of misinformation, keep operators honest, and, potentially, create a database for studying the efficacy of projects.