Weather Modification as a Response to Variations in Weather and Climate
Wayne N. Marchant and Arnett S. Dennis
U.S. Bureau of Reclamation
Global warming due to increased concentrations of greenhouse gases, if it occurs, will be the most significant change in weather and climate ever produced by human beings. One can speculate about the possibility of offsetting this inadvertent, long-term weather modification by deliberately modifying the weather through cloud seeding. Because some clouds warm the earth's surface and some cool it (depending upon cloud composition and the temperature at the top of the cloud), it is theoretically possible to modify clouds to offset global warming, at least partially. However, the present knowledge base is not complete enough to provide a working hypothesis.
The use of weather modification to offset some of the undesirable changes in precipitation, temperature, and streamflow that may arise from global warming or other climatic variations will be limited for the foreseeable future. At present, the primary application of weather modification is the augmentation of precipitation and runoff. The Bureau of Reclamation has been conducting research on precipitation augmentation for the past 30 years, with the efforts in recent years concentrated on the seeding of winter orographic clouds to increase snowpack and runoff.
THE SEEDING OF WINTER OROGRAPHIC CLOUDS
In the face of water demands that often outstrip supplies, western water managers have been using weather modification to augment water supplies each year since 1948. The most commonly used method is silver iodide (AgI) seeding of winter orographic clouds to increase precipitation, snowpack, and runoff.
Orographic clouds containing supercooled liquid water (SLW) (liquid water at temperatures below 0°C) offer a unique opportunity for increasing precipitation. They may persist for hours at a time over a mountain range producing little or no precipitation, apparently because they do not contain enough natural ice crystals to form snow efficiently. The introduction of AgI crystals or other artificial ice nuclei into such clouds results in the formation of ice crystals, which grow into snowflakes at the expense of the SLW. The lifting of the air on the upwind side of the mountain barrier suggests that orographic clouds can be seeded cheaply and efficiently by AgI generators on the ground.
A typical winter orographic cloud seeding project involves the deployment of 10 to 20 manually operated AgI generators on the ground upwind to seed a target area of several thousand square kilometers. Some operators use radio-controlled generators to improve coverage of remote areas. Aircraft seeding is conducted on some projects to supplement the effects of the ground-based generators or to reach otherwise inaccessible areas.
FACTORS INFLUENCING THE ACCEPTANCE OF CLOUD SEEDING
The initial acceptance of weather modification technology was due to a perception that large economic benefits would be obtained from it. Subsequent analyses have confirmed that the economic impact of a successful weather modification program could be very large. A Bureau of Reclamation analysis of a hypothetical 10 percent increase in Colorado River streamflow showed average annual benefits of $48 million from increased water supplies for irrigation and for municipal and industrial use; $34 million from increased hydropower generation; and $62 million from improved water quality (chiefly from a reduction in salinity). As a program to seed all the major runoff-producing subbasins of the Colorado could be mounted for some $10 million per year, the potential benefit-cost ratio is large.
One point of concern in considering economic impacts of weather modification is that a weather modification program may harm people receiving few or none of the perceived benefits. The proposed program to increase snowpack in the Colorado River basin is one such case. The program, if implemented, would involve inconvenience and additional costs for snow removal for residents of the high-elevation regions within the basin, notably
in southwestern Colorado, while the perceived benefits would accrue principally to the lower basin states of California, Arizona, and Nevada.
The possibility of undesirable ecological effects is another retarding factor for the adoption of weather modification technology. People have expressed concerns about the seeding agents themselves and about the expected effects of weather modification programs. However, extensive studies over the last 30 years have shown that the ecological effects are very small and would be difficult to detect.
In view of the large stakes involved, it was inevitable that, after several years of operational cloud seeding, the sponsors would want an evaluation of the effects. One of the most comprehensive evaluations was conducted by the Advisory Committee on Weather Control, which was established by the United States government in 1953. The advisory committee's statistical analysis of orographic cloud seeding projects was based on comparisons of precipitation in target and control areas during seeded storms and during storms that had occurred before any seeding was done. On the basis of a study of 299 seeded storms, which occurred over several seasons in different projects in the western United States, in 1957 the advisory committee reported apparent increases in precipitation of 9 to 17 percent above the amounts that would have fallen without seeding. The advisory committee also stated that there was only a very small probability that the apparent increases were due to chance.
Although the advisory committee's report was criticized on various grounds, including the possibility that natural changes may have accounted for some of the precipitation increases, its findings have been supported by later review panels. The latest policy statement of the American Meteorological Society on weather modification states: "Precipitation amounts from certain cold orographic cloud systems apparently can be increased with existing technology in the western United States. Increases of the order of 10 percent in seasonal precipitation are indicated in some areas."
It is difficult to translate indications of precipitation increases at individual gauges into estimates of additional runoff from an entire watershed. However, target-control analyses can be performed with runoff data as well as precipitation data. Several
authors have found evidence of increases—generally ranging from 5 to 15 percent—in seasonal runoff from mountainous basins in the western United States.
THE INFLUENCE OF PRECIPITATION VARIABILITY
Commercial cloud seeders try to tailor their operations to the needs of their sponsors. Projects to enhance precipitation are most likely to be undertaken following dry periods, when water supplies are low. The most favorable seeding opportunities in orographic clouds are the shallow clouds, rather than the deep clouds associated with major storms. Nevertheless, suspension criteria are often used in an attempt to avoid contributing to floods, avalanches, or other hazards associated with excessive precipitation. The general tendency, then, is for cloud seeding as commonly practiced to increase precipitation slightly during dry years and normal years, but to have little or no impact during wet years. Therefore, it tends to reduce the variability of precipitation and runoff.
THE SEARCH FOR LARGE-SCALE EFFECTS
The operational projects mentioned above were concerned only with increasing precipitation from relatively small portions of extensive cloud systems. Many people have postulated that such projects may affect precipitation over larger areas. One commonly heard argument is that any additional precipitation in a target area must result in a decrease in precipitation downwind. This argument is flawed, principally because precipitation rates are controlled primarily by vertical motions in the atmosphere, with the moisture content of the affected air mass being of secondary importance. Furthermore, the condensate (the moisture present as clouds at any time) is only a small fraction of the total atmospheric water supply. Only a fraction of the total condensate is precipitated over a mountain barrier, and the increases associated with cloud seeding are much smaller than the natural precipitation. Therefore, drying of the atmosphere downwind of a target area must be slight. Searches for downwind effects using actual precipitation data have not turned up any statistical evidence for them.
RESEARCH ON WINTER OROGRAPHIC CLOUDS
A number of field experiments have been carried out to refine estimates of the effects of cloud seeding on orographic precipitation and to determine which clouds are seedable (that is, which clouds can be made to yield additional precipitation by seeding). Experts agree that seedability is associated with the presence of SLW. In recent years, investigators have searched for SLW with new instruments, including dual-channel microwave radiometers and aircraft equipped with laser-based optical probes for counting, sizing, and classifying cloud and precipitation particles. The research on winter orographic cloud seeding has also involved randomized seeding trials and the use of computer programs to compute the trajectories of snowflakes produced by both natural and artificial ice nuclei.
Present indications are that SLW is most likely to be associated either with shallow stratiform clouds with top temperatures in the -5 to -12°C range or with moderate updrafts within deeper cloud layers. Surprisingly high ice crystal concentrations are observed on occasion—for example, 50 per liter at the -5°C level, which greatly exceeds the concentration of natural ice nuclei active at that temperature. Unusually high ice crystal concentrations are often associated with large cloud droplets and are attributed to a variety of secondary ice crystal production mechanisms. High ice crystal concentrations are important because they suppress the concentration of SLW and thereby reduce the seedability of a cloud.
Results of the various experiments and operational programs suggest that a variety of outcomes is possible and that tailoring a seeding operation to a particular project area requires careful consideration of wind fields, generator siting, seeding agent diffusion and transport, and cloud microphysical characteristics (including the presence or absence of secondary ice formation).
THE AVAILABILITY OF SEEDABLE CLOUDS IN DRY PERIODS
If climate variability were to lead to drier conditions for many years in a given watershed, would there be enough seedable clouds left to offer a chance for practical results?
Research results are of some value in answering this question. As noted above, the most seedable clouds are shallow clouds associated with weak storms rather than the deep clouds associated with
the central pans of intense storms. Therefore, one can expect to find seedable clouds in some areas that are normally dry. Experimental confirmation for this expectation is provided by apparently successful cloud seeding projects in such arid areas as San Diego County, California and northern Israel.
Radiometer data from Arizona show that seedable conditions occur several times each winter and sometimes last for 20 or 30 hours at a stretch, with only intermittent, light snow falling during that period. Data from the Bureau of Reclamation's Sierra Cooperative Pilot Project (SCPP), which was conducted in the Sierra Nevada from 1976 to 1987, showed that seedable clouds existed for several hundred hours each winter and, if anything, were more frequent during dry years than during wet years. This finding is further evidence that a drier climate would not necessarily lead to a decrease in the number of cloud seeding opportunities.
Unavoidably, we will continue to live with climate variability and the uncertainty it produces. Furthermore, that uncertainty is compounded by some predictions of profound, long-term global climate change.
It would be naive to suggest that cloud seeding can eliminate climate variability or overcome uncertainty about climate. Indeed, uncertainty about its effectiveness continues to hinder cloud seeding itself. Nevertheless, precipitation augmentation through cloud seeding has enormous potential to help us moderate the adverse societal and economic costs of wide fluctuations in weather and climate. Given the great potential that our models have shown may be realized through cloud seeding in at least one major basin, it is a tool too attractive to ignore.
Advisory Committee on Weather Control. 1957. Final Report of the Advisory Committee on Weather Control. Washington, D.C.: U.S. Government Printing Office.
Braham, R. R., Jr., ed. 1986. Precipitation Enhancement: A Scientific Challenge. Meteor. Monographs, Vol 21, No. 43. Boston: American Meteorological Society.