ATMOSPHERIC WATER RESOURCES RESEARCH
in association with the United States Department of the Interior, Bureau of Reclamation, Fresno State College Foundation Fresno State College Fresno, CA
CENSARE Cloud Seeding Manual
The substance most commonly used in creating artificial nuclei is silver iodide (AgI), as its crystaline structure closely approximates that of a natural ice crystal.
Artificial nuclei of AgI crystals are generated by heating the agI, either pure or in combination with other substances, until the vaporization temperature is reached and the microscopic crystals are given off.
The size of crystal, and thus the number of crystals per gram of AgI used, is somewhat dependent upon the temperature of the heat source into which the AgI nuclei is injected.
In turn, the effectiveness of the crystal as a condensation nuclei is somewhat dependent on its size, and also on the presence or absence of other chemicals in the crystal.
In general, the greater the number of effective artificial nuclei at the warmer temperatures (-10C to -5C), the better the cloud seeding generator.
The objectives of a cloud seeding generator system, then, are twofold;
(1) to produce a sufficient number of artificial condensation nuclei of a size and composition which will be effective in attracting and condensing super--cooled cloud water vapor, particularly at the warmer temperatures (<-5C) and (2) to target the resultant effect by producing these nuclei at physical locations from which they will enter the cloud at the desired nucleation temperature level, grow into ice crystals, and then precipitate onto the specified target area. III. SEEDING GENERATOR SYSTEMS Basically, two modes of operation and three types of generator systems were used during the course of this research program, as presented below.
Operational applications of these seeding generators are described in subsequent sections.
A. Ground-Based Seeding
Both manual and remote controlled seeding generators of pyrotechnic and solution types were used for ground based seeding operations.
1. Pyrotechnic Generators.
These generators used pyrotechnic fusees developed and provided by the U.S. Naval Weapons Venter (NWC) at China Lake, California in cooperation with and for the CENSARE project.
In general terms, these fusees were comparable to highway flares with silver iodide (AgI) mixed into the burning compound, and were ignited by a low-voltage electrical current.
The actual construction and composition of these fusees was, however, somewhat more complex. The most commonly used fusee burned for 4 seconds and emitted 20 grams of AgI (silver iodide).
Manually operated pyrotechnic generators were used in the lower elevations (1,200 to 4,700 feet MSL) .
These generators required the services of a person to turn them on, but would then automatically ignite one fusee every eight minutes for a three-hour periods.
Residents in the areas selected as generator sites were utilized as generator operators and received operating instructions by telephone when the AWRR seeding director determined conditions were right to operate that generator.
Remote controlled pyrotechnic generators were also used independently at one lower elevation (3,900) foot) site and at several higher elevation sites in conjunction with remote controlled solution generators, as described below.
The same fusees as above were used, with ignition on remote command.
2. Propane-Burning Solution Generators
These generators were located at high-elevation sites (7,300 to 8,500 feet MSL) and were remote controlled. They used a concentrated silver iodide-sodium iodide acetone solution, and utilized a propane gas flame to vaporize the AgI. (image right of cloud nucleating generator, click for detail, from North American Weather Consultants, Inc.)
These units were turned on and off by radio command, and emitted 30 grams of AgI per hour. They were provided to the CENSARE project by the Pacific Gas and Electric Company (PG&E) for operation under AWRR direction.
3) Acetone-Burning Solution Generators.
A new type of seeding generator developed during 1971-73 by AWRR, relying heavily on advice from NWC (which had developed the prototype unit), was used in the 1973 water year's seeding operations.
This generator utilized a dilute silver iodide-ammonium iodide-acetone solution.
The solution was the only fuel burned to heat the AgI to its vaporization temperature.
The generator was manually ignited, but would then burn for twelve hours unattended if desired.
Results of preliminary tests for effective nuclei performed at the Colorado State University Cloud Simulation and Aerosol laboratory at Fort Collins, Colorado, indicated a substantial increase in effective nuclei output in the warmer ranges (-6C to -10C) over generators utilizing sodium iodide instead of ammonium iodide.
These units were used from November, 1972, to February 1973, at elevations ranging between 2,000 and 5,000 feet MSL. Six units were at fixed locations and one was used in mobile operations.
Over 200 generator-hours were logged by these seven units, and the only malfunction experienced was that on two occasions a generator ran out of fuel.
These units since have been transferred to the custody of the USBR in Denver, Colorado.