Modifying Weather In The Colorado River Basin




October 2005 

Tom Ryan, Metropolitan Water District of Southern California 

Technical Reviewers

Joe Busto, Colorado Water Conservation Board,

Vice-Chairman North American Interstate Weather Modification Council

Arlen W. Huggins, Desert Research Institute,

Nevada, Chairman North American Interstate Weather Modification Council

Steven M. Hunter, U.S. Bureau of Reclamation, Denver

Executive Summary

This paper provides a brief background of weather modification, information on existing programs and current issues, and provides recommendations for the Colorado River Basin States (Basin States) involvement or support of precipitation management through weather modification efforts.

The purpose of winter cloud seeding to increase snowfall in mountainous areas is to increase runoff for hydroelectricity and water supplies for downstream areas. Increases in precipitation can improve soil moisture, stream flows, and reservoir levels. More water storage in reservoirs can allow for increased power generation, irrigation, and municipal and industrial use. Recreation, water quality, salinity reduction, fisheries, forest health, sensitive species, ranching, and tourism can all benefit from additional runoff.

Members of weather modification organizations, public agencies, and private sector companies believe that cloud seeding has reached the point that a well managed program including a proper design component can be implemented to produce cost-effective water resources benefits. More research on the specific cause and effect relationship between cloud seeding and additional water on the ground should be conducted as well. Any proposed operational cloud seeding program should include a strong evaluation component.

It is estimated that cloud seeding six major runoff-producing areas within the Colorado River Basin could produce between 1.1 and 1.8 million acre-feet (maf) in the Upper Basin (approximately 10% of the average annual stream flow) and an additional 830,000 acre-feet in the Lower and adjacent basins. Of the total, it has been estimated that approximately 1.7 maf would be available to reduce deficits and meet new demands.

Although there is wide discussion regarding the effectiveness of weather modification since it began in the 1940s, proponents of ongoing projects believe programs in Utah have resulted in precipitation increases between 7 and 20%, at costs of less that $20 per acre-foot, which compares favorably with traditional water resources projects. The programs currently in operation in Colorado and Utah demonstrate the success of weather modification (WxMod) activities there. Ski areas, water authorities, and agricultural users are the most common project sponsors.

There are several good reasons for the Basin States to continue its research on this topic. One of the most important is the 2000-2004 Colorado River droughts, which is a normal part of the climate of the arid Western United States and Colorado River Basin. Others are a general trend toward reduction in snowpack, increased water demands, as well as the growing concern about reductions in precipitation due to inadvertent anthropogenic modification to weather (air pollution). Factors to Consider in deciding how to proceed are that: new projects take 1-3 years to plan; planning is relatively inexpensive; there is a real need for research and project funding; these projects are very cost-effective, and there are existing programs with data that can be leveraged.

This paper recommends:

  • Further investigation continue as to the availability of WxMod projects in which the Basin States can participate, particularly those focusing on the Upper Basin States of Colorado and Utah;
  • Legislation continue to be monitored, and attempts to influence legislators be considered;
  • An implementation plan be developed with next steps, including a schedule, costs, and deliverables for a feasibility study to increase winter precipitation that would include both operational and evaluation components;
  • The Basin States enlist either the U.S. Bureau of Reclamation (Reclamation) and/or the National Oceanic and Atmospheric Administration as the lead federal agency in the development of a coordinated national research program, and that this program should piggy-back research onto existing and proposed operational programs like Reclamation’s Weather Damage Modification Program;
  • Cost-share with Reclamation in an objective impartial evaluation of existing operational programs, and;
  • For an interim period while the feasibility study is being prepared, expand ongoing operational programs by adding generators and by operating the programs for an expanded period of time each year, including those shown in Table 1.

The additional precipitation can help the Basin States potentially increase water supply in the Colorado River basin, and assist in reducing shortages, or reservoir storage recovery. WxMod should be a standard tool, similar to implementing conservation measures or storing and withdrawing water in groundwater basins for water resources managers to meet demands and assure reliability.


There is an ongoing debate as to the efficacy of cloud seeding as a means of precipitation enhancement. Although much money has been spent over decades in support of cloud seeding to increase precipitation, science has been able to conclusively demonstrate strong evidence of which seeding techniques produce positive effects in only a limited number of weather situations. In the over half-century since cloud seeding demonstrations began, substantial progress has been made in understanding the natural processes of weather. Some voice concern that the scientific challenges of proving seeding effects have been found to be significantly more formidable and complex than initially perceived and proof is elusive (National Research Council (NRC), 2003).

However, many others believe that despite the difficulty in objectively quantifying the absolute values of seeding effects, it has been established that certain aspects of the weather, specifically cloud microphysical and precipitation processes, can be intentionally modified with beneficial effects and without detrimental environmental effects. The large body of positive indications reported by many (Weather Modification Association (WMA), 2004) references in this paper, as well as a multitude of analyses in the literature constitute a collective positive signal. The overarching premise of this paper is that cloud seeding does work, but both sides of the argument are provided here for consideration by decision makers.

There are several reasons why WxMod has been pursued: fog and stratus dissipation; hurricane strength reduction; lightning reduction, hail suppression, and precipitation augmentation. This document focuses on the use of winter orographic WxMod for precipitation enhancement to augment water supply.


The first laboratory and field experiments by Vince Sharer, Irving Langmuir, and BernardVonnegut began in the mid-1940s and were known as the “Cold Box” experiments. There was much enthusiasm and the positive results from cloud seeding and the potential for producing rain were widely distributed.

The combination of excited scientists, an interested media, and a receptive populous resulted in a worldwide commercial industry focused on cloud seeding, and an era of great interest among scientific organizations and government. By 1951, weather modification programs were operating in about 30 countries.

Wild claims of effectiveness led to differences in opinion as to the economic benefits of cloud seeding and Congress held hearings on the matter between 1951 and 1953. It was learned that millions of dollars were being spent annually by farmers, utilities, ranchers and other users on weather modification activities covering approximately 10 percent of the nation’s area. As a result, the Advisory Committee on Weather Control was established by an Act of Congress of August 13, 1953.

Users were so interested in results that, for commercial operators there was no room for randomization or scientific method. As a result, rigorous proof of a seeding effect in the commercial cloud-seeding projects was not pursued. Even today, the words “weather modification” and “cloud seeding” are met with some skepticism.

In the late 1950s, some projects were developed with support of governmental agencies and although the experiments (e.g., the Missouri Project, Whitetop) ran for several seasons, the results were mixed. None of the experiments provided incontrovertible evidence that seeding was effective. Even in 1964 after many more projects were completed, the National Research Council (NRC) concluded that precipitation from orographic storms would not be increased significantly by seeding and that eventually relevant processes could be understood and usefully applied. “The timescale required for success may be measured in decades” (NRC, 1964). An NRC report in 1966 presented results with an “indication of positive effect.” In general, the authors of the time found that cloud seeding experiments had not yet provided the evidence required to establish scientific validity, though the prospects were promising and worth pursuing.

One of the references for this paper is Critical Issues in Weather Modification Research prepared by the National Research Council of the National Academy of Sciences in 2003 (NRC Report). To paraphrase the 2003 report, the Committee on the Status of and Future Directions in U.S. Weather Modification Research and Operations finds little reason to differ from the findings of the 1964 and 1966 studies. This is due in part to the lack of concerted research in weather modification. In the three decades since the last NRC report there have been improvements in the understanding of cloud processes and significant development in tools and techniques, including remote sensing and computing. These improvements, plus new methods for physically evaluating the impacts of cloud seeding, mandate a fresh look at the status and potential of weather modification.

There are, however, others that do not subscribe to this point of view. List (2005) reviews the NRC Report and finds it flawed in several ways. He submits that the level of accuracy of experiments that WxMod researchers face is higher than that of all other meteorology and atmospheric physics disciplines, which he calls a double standard. Other criticisms of the NRC Report include: the use of old, outdated and misleading criteria; lack of specific criticisms of the science; no discussion of the role of statistics; and the lack of identifying the progress and achievements made in recent decades. The Weather Modification Association takes issue with the NRC Report as well, and its perspective is presented in the section below under Policy Statements.

Meanwhile, numerous short and long-term operational seeding programs have been ongoing with program proponents claiming meaningful, measurable results. These operations programs are described in Section II below.


What follows is a brief description of the major authorities that have been enacted for this subject matter. There are dozens more for the individual states, but this provides an overview of how this subject has been of interest to Congress for decades.

The National Weather Modification Act of 1976. The National Weather Modification Act of 1976 (Public Law 94-490), directed the Secretary of Commerce to develop a comprehensive and coordinated national policy on weather modification and recommended a national weather modification research and development program. The motivation for this legislation was a severe drought in Kansas and annual damage to property and crops caused by severe weather. It was recognized that all the ongoing weather modification activities were not realizing their potential to mitigate such effects. The Secretary of Commerce was directed to prepare a study on the state of scientific knowledge of the atmospheric processes, research needs, economic studies, and funding issues. The study was to be completed in one year and $1 million was appropriated to carry it out. The author could not find a reference to this report being completed.

Colorado River Basin Proiect Act of 1968. The Colorado River Basin Project Act of 1968 (Public Law 90-537) directed the Secretary of the Interior (Secretary) in Section 102(a) “… to provide a program for the further comprehensive development of the 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 the Lower Colorado River Basin.” Under
Title II, the Secretary is authorized to prepare an augmentation plan to meet the water requirements of new projects, existing projects, current water allotments, and the 1944 Water Treaty with Mexico. Section 202 of Public Law 90-537 recognizes a national obligation to annually provide
1.5 million acre-feet of water, together with any associated losses of water from the Colorado River, to meet the requirements of the Mexican Water Treaty. Section 202 also states that “The Congress declares that the satisfaction of the requirements of the Mexican Water Treaty from the Colorado River constitutes a national obligation which shall be the first obligation of any water augmentation project planned pursuant to
Section 201 of this Act and authorized by the Congress.”

Colorado River Basin Salinity Control Act of 1974. The Colorado River Basin Salinity Control Act of 1974 (Public Law 93-320) authorized and directed the Secretary to proceed with a program of works of improvement for the enhancement and protection of the quality of water in the Colorado River. Section 101(c) states that replacement of the reject stream from the Yuma desalting plant and of bypassed Wellton-Mohawk drainage water is “recognized as a national obligation as provided in Section 202 of the Colorado River Basin Project Act.” As stated in Section 202, augmented streamflows resulting from cloud seeding would result in decreased salinity concentrations in the Colorado River Basin, and could provide a source of replacement for the Yuma desalting plant reject stream.

Reclamation States Emergency Drought Relief Act of 1991. The Reclamation States Emergency Drought Relief Act of 1991 (Public Law 102-250) authorized the Secretary “to conduct a Precipitation Management Technology Transfer Program to help alleviate problems caused by precipitation variability and droughts in the West, as part of a balanced long-term water resources development and management program.” Section 206(b) states that “in consultation with State, Tribal, and local water, hydropower, water quality and in stream flow interests, areas shall be selected for conducting field studies… to validate and quantify the potential for appropriate precipitation management technology to augment stream flows.” Upon successful completion of such a program, validated technologies will be “transferred to nonFederal [sic] interests for operational implementation.” The 1991 Act was the authorizing legislation for the Weather Damage Modification Program, described in a subsequent section.

Weather Modification Research and Technology Transfer Authorization Act (S. 517) K.B. Hutchison, (R-Texas). On March 3, 2005, Senator Hutchinson re-introduced a bill that is identical to the weather modification bill she introduced in the last Congress. The Act would develop and implement a comprehensive and coordinated national weather modification policy and a national cooperative Federal and State program of weather modification research and development. The work would be accomplished through the Department of Commerce Weather Modification and Advisory Research Board. The duties include promotion of research and development, providing financial assistance, and biennial reporting. The legislation would authorize $10 million for 10 fiscal years. There has been no movement on the bill since its introduction, and her staff is working to get at least some of the provisions of this bill incorporated into the Senate’s version of a NOAA authorization bill, which is again making its way through the legislative process. A copy of the bill may be found in Appendix A.

Weather Modification Research and Technology Transfer Authorization Act of 2005 (HR. 2995). On June 20, 2005, Rep. Udall (D-Colorado) introduced this companion bill to S. 517, which also seeks to develop and implement a comprehensive and coordinated national weather modification policy and a national cooperative Federal and State program of weather modification research and development. The House and Senate bills are nearly identical.

The Primary Winter Cloud and Precipitation Process

Because there is strong evidence that wintertime seeding for snowpack augmentation works and because the high mountains in the Colorado River Basin provide excellent seeding targets, only winter seeding will be addressed here. In general terms, winter cloud seeding attempts to mimic natural snow production processes in clouds that are inefficient in producing ice crystals and snowfall. During winter storms, moist air is forced to ascend over mountain ranges by prevailing winds (orographic ascent). This upward movement causes the air to cool. The rising and cooling of moist air results in water vapor condensing into droplets to form a cloud. The rate of production of liquid water is determined by the air’s temperature, humidity, and upward motion. The tiny cloud water droplets have insignificant fall speeds so they are suspended in the airstream. If not converted to snowflakes while continuing to rise over the mountain, the droplets quickly evaporate because of downward motion on the lee side of the mountain range.

Figure 1 is a generalized depiction of the primary process.

Cloud droplets often remain in the liquid state at temperatures lower than 32 degrees Fahrenheit because of a scarcity of effective ice forming nuclei in the atmosphere. These droplets are called supercooled liquid water (SLW). When effective ice forming nuclei are present, ice crystals form. After formation, the crystals grow from available water vapor. As they fall they collide with SLW droplets that freeze onto them, creating larger crystals. The process continues as the larger crystal falls faster and grows larger into a snowflake. This process is most effective in deep clouds and must occur before reaching the crest of the mountain range. In situations where natural ice nuclei are too scarce to efficiently convert the supercooled liquid water into snow, seeding can assist the conversion process. There is no doubt that the most commonly used agent, silver iodide (AgI), released into sufficiently cold SLW clouds, will produce multitudes of embryonic ice particles. The same result is achieved when liquid propane is expanded into even slightly super-cooled liquid clouds. The challenge is to create seeding-induced ice particles at such locations that their subsequent trajectories will be within SLW clouds for a sufficient time(distance) to permit growth to precipitation sizes (WMA, 1999).

Technical advances have increased the capability to augment precipitation in higher temperature and shallower orographic cloud systems. Numerical modeling has improved understanding of atmospheric transport mechanisms. Improvements in computer, radar, satellite, and communications systems have resulted in better assessments of cloud seeding potential and more effective dispersion of seeding agents from properly positioned cloud nuclei generators. Sensors such as radiometers that continuously monitor SLW amounts greatly improve the chances of successful seeding…

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  1. Pingback: Cloud Seeding– is there more water out there? | Your Water Colorado Blog

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