Techniques and Apparatus

The reported information shows that ground-based equipment was used in 38 activities and airborne equipment in 40. The total is more than 72 because 6 projects used both ground and air techniques.

Liquid-fueled silver iodide generators or burners were the most common type of weather modification apparatus, being used in 15 ground-based projects and 14 aircraft seeding activities. Propane or acetone were normally used as fuel. The arc-type silver iodide burner was used exclusively at ground level in 17 other activities. Pyrotechnic devices including flares and rockets were employed in 21 projects. Other apparatus used in modification activities were dispensers for solids and liquids, spray nozzles, a corona discharge tower, and "cloud busters" consisting of pipes connected to a water supply.

Agents and Dispensing Rates

The active agents used for the reported activities are silver iodide, crushed frozen carbon dioxide (dry ice), polyelectrolytes, propane, charged water droplets, lithium chloride, hexadecanol, 1-5 DN (1-5-dihydroxynaphthalene), and combinations of silver iodide with other liquids or solids. A polyelectrolyte can be defined as a polymeric material having a long chain molecule and usually a cationic (positive ion) form. Chaff (metallic fibers) was dispensed from aircraft in the lightning suppression project. Compressed air was used in one cold fog modification experiment; corona discharge was the "agent" in a warm fog suppression program.

Silver iodide, by itself or in combined form, was the agent in 50 activities. Dry ice was planned for use in 12 activities, propane in 3, and polyelectrolytes in 2. In several activities, more than one agent was

used.

Inspection of the data in appendix A indicates that the dispensing rates for a unit of airborne modification apparatus are considerably higher than the rates for ground-based burners or generators, although the length of dispensing time in aircraft activities is usually shorter. The dispensing rates for silver iodide range from 0.5 grams per hour in one ground-based burner to 24 kilograms per hour in a set of pyrotechnic devices on an aircraft; a flare (pyrotechnic) burns for a few minutes or less. Dispensing rates range up to 1400 kilograms per hour for dry ice, 300 for polyelectrolytes, and 38 liters per hour for propane; these dispensing rates are maintained for only a few seconds to a few minutes at a time.

Interim and Final Report Data

Table 6 is a summary of reported information on weather modification activities that were continued, initiated, or completed in CY 1975. data were obtained from 16 interim and 61 final reports; no data were submitted on 8 projects that began late in the year (after 1 October). Although this summary table is based on incomplete data, it provides the best estimate of Federal and nonfederal weather modification activity for the year.

Table 6.--Summary of interim and final report data

The total number of days on which weather modification activities took place is shown in table 6, item (a). In items (b) through (e), modification days are segregated by the predominant types of weather phenomena involved. The total of entries in (a) does not equal the total of (b) through (e) because the data are incomplete and because on some days more than one cloud system was involved in the operation.

Silver iodide was used as a seeding agent in 50 activities and dry ice in 11. However, the quantity of dry ice dispensed was about 13 times more than the total weight of silver iodide. The dry ice, lithium chloride, and charged water were used chiefly for fog modification; liquid propane and

polyelectrolytes were also dispensed into fog, but to a lesser extent. Although fog dispersal accounted for 23 percent of the weather modification activities, the number of fog modification days is about 5 percent of the cumulative number of modification days. No modification activity was reported for four airports (74-063, 74-064, 74-065, and 74-114), an indication that fog of the type suited to the modification technique was not prevalent at these locations during the reporting period.

Further analysis of the information in table 6 shows that nonfederal efforts in modification were appreciably greater than Federal activities. There were about 13 times more non federal modification days than Federal, 16 times as many missions, and 18 times as many hours of ground-based operations. Nonfederal activities used 14 times as much silver iodide as Federal ground-based projects and 17 times as much as Federal airborne activities.

Project Safety and Environmental Factors

The previous report contained a summary of information on project safety and environmental considerations for CY 1974 (National Oceanic and Atmospheric Administration 1975). No substantial changes in the conclusions concerning these factors were found in analyzing the material reported during CY 1975.

Federal Activities

For the 14 distinct Federal projects discussed herein, 8 reported that Environmental Impact Statements (EIS) had been prepared. The remainder were engaged in projects for which prior analysis indicated that they did not significantly affect the quality of the human environment, and for which an EIS was therefore not required.

In an EIS for a weather modification project, a section is devoted to the possible effects of the seeding agent. The following paragraph from NOAA's environmental statement for Project FACE 1975 (Florida Area Cumulus Experiment) addressed this subject.

"There are no known adverse effects from the silver iodide (AgI) used for seeding. The U.S. Public Health Service Drinking Water Standards of 1962 require that the 'silver content of drinking water be less than 0.05 parts per million'; there are no standards with regard to iodide. On an average day of seeding approximately 1.5 x 10 gm of silver iodide are [3 expended and 3.0 x 10- gm of rain falls. If this silver iodide is evenly. distributed in the rainwater, the silver content of the water would be 0.5 parts per billion---well within the U.S. Public Health Standards for drinking water. Measurements of silver in rainwater during FACE 1973 substantiated this expectation. The silver iodide that does not combine with

De

water should stay in the atmosphere for months or years and be dispersed over a very large area with proportionately smaller concentrations. spite the likelihood that the silver concentrations will remain well below the U.S. Public Health Standards, rainwater will be collected at several locations in the target areas as in FACE 1973 and this water will be analyzed for silver' content."

The Federally sponsored projects continued to monitor current and forecast weather conditions, particularly the severe conditions. In a Florida project, seeding was restricted on days with severe weather potential, postponed during periods of abnormally high rainfall, and curtailed when crops might be damaged by rain. At a hail suppression project in Colorado, one part of the operational plan called for small rockets to dispense sil-. ver iodide. These frangible rockets weighed about a pound and their casings disintegrated before reaching the ground. However, no seeding activity was carried out in 1975.

Nonfederal Activities

This

Some additional information on environmental and safety aspects of nonfederal weather modification activities has been gained from a review of the initial reports of new projects in CY 1975. For example, the possible effect of silver iodide on humans is of growing concern, and part of the EIS for the South Dakota Cloud Seeding Project addresses this matter: "As was stated earlier, silver iodide is highly insoluable in water. extreme insolubility, and the very low level of silver iodide in seeded precipitation are the primary reasons silver iodide from cloud seeding operations offers such little hazard to humans. The .0001 ppm concentration of silver iodide in seeded precipitation is well below the .05 ppm of silver allowed by the Environmental Protection Agency in drinking water, and through normal dilution even unusually higher concentrations of silver iodide in rainwater will be brought well within standards in streamflow.

"Evidence also suggests very little or no hazard to man from bioaccumulation of silver from cloud seeding. Because most land plants do not take up silver actively, there is little likelihood of silver concentrating through terrestrial food chains, nor of danger to terrestrial plants or animals if silver is used as a nucleating agent. This can be said with respect to both immediate effects and effects over a period of perhaps 20 years.

"All available evidence indicates little likelihood of adverse effects from iodine. The role of iodine in physiological processes has been well documented and incidences of toxicity from naturally occurring iodine are very rare. In fact, availability of adequate amounts of iodine in the human diet is necessary to prevent goiter, and because of the lack of naturally occurring iodine in some areas, iodine is purposely added to table salt to ensure adequate consumption. A person would have to drink 130 gallons of precipitation from a storm seeded with silver iodide to obtain as much iodine as he would from salting breakfast eggs with iodized salt."

Although the operators of major cloud seeding projects closely monitor the information provided by the National Weather Service (NWS), some require more detailed data for specific areas and purposes. In Texas,

one operator installed a 3-cm weather radar at project headquarters to supply storm movement and intensity information during seeding operations. The radar data were coordinated with pilot inputs during seeding and observational flights.

Another application of radar was carried out in North Dakota. Rainfall was monitored, and when radar-estimated rainfall exceeded 2 inches, seeding was stopped in that area. On the other hand, if local residents decided that their area had too much rain, operations were suspended.

A different approach to monitoring possible damaging effects of operations was undertaken by about 700 farmers in and around a Texas project. Community officials, representing the farmers, had direct communication with the operations base and had daily contact with the radar meteorologist. Operational constraints and provisions for suspension were based on the threat of floods or excessive nucleating material, and social considerations. Thus, a human observational network was poised to furnish useful information to project decision makers.

The decision-making process in a Kansas project was lodged in a policy Board made up of representatives from each of the 11 participating counties. The Board established the following guidelines for conduct of the project:

"a. Since a high percentage of the seeding opportunities occur during frontal passage, unrestricted seeding will be allowed on a provisional basis as long as no undesirable effects result. This applies to seeding of fronts or squall lines during daylight hours, as long as flying safety is not jeopardized.

"b. Since a high percentage of hail damage occurs during severe thunderstorms, hail suppression will continue when a National Weather Service Thunderstorm Watch or Warning is in effect in all or part of the target area. Cognizance is taken of the sociological aspects of seeding during severe storm situations; however, from a scientific standpoint, there is no reason not to seed for hail suppression.

"C. Seeding activities will terminate at darkness. Since the aircraft will be based in the western portion of the target area, the aircraft may return to base after dark, at the completion of seeding operations."

The subject of seeding for hail suppression during severe storms was also noted in another program. One commercial operator in North Dakota reported that "All severe storms systems will be seeded as they penetrate the target area. The decision for modification of all systems was made by the local authority after having been advised of the possibility of criticism and/or liability should destruction be caused by those systems seeded. Decision concerning rain increase operations will be made on a weekly basis considering crop need throughout the project period."

The County Commissioners in North Dakota were authorized to suspend operations in a seeding project. The project meteorologist could also suspend operations based on information from NWS on river crest and forecast