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WEATHER MODIFICATION A. S. Dennis u.s. Bureau of Reclamation ';; I. Early Histary 600 II. Claud Seeding Agents and Delivery Systems 601 III. Results of Cloud Seeding to Modify Precipitation 607 IV. Suppression of Weather Hazards 612 V. InadvertenI Weather Modification 616 VI. WeaIher Modification and Human Society 618 GLOSSARY . Artificial cloud nucleation: Release of artificial cloud condensation nuclei or artificial ice nuclei into the atmosphere to influence cloud formation or behavior; the most com- mon form of cloud seeding. Cloud condensation nucleus: Aerosol particle, normally hygroscopic and greater than 0.1 ~m in diameter, which is likely to serve as a center of condensation in cloud formation. Cloud seeding: Any technique carried out with the intent of adding substances to a cloud to alter its natural development. Control area: Region that is selected for com- parison purposes to evaluate cloud seeding effects in a target area; it is expected to be unaffected by the seeding project being evaluated. Downwind effects: Effects of cloud seeding hypothesized to occur in regions downwind of an intended target area. Extra area effects: Effects of cloud seeding outside of an intended target area; hypothe- sized to occur upwind of intended target ar- eas as well as in downwind areas. Glaciogenic seeding: Cloud seeding conducted for the purposes of inducing ice formation in supercooled clouds or in cloud-free re- gions that are supersaturated with respect to ice; includes the release of artificial ice nuclei and the release of chilling agents to promote homogeneous formation of ice par- ticles by freezing supercooled droplets or by deposition from the vapor state. .. ENCYCLOPEDIA OF PHYSICAL SCIENCE AND TECHNOLOGY. VOL. 14 Hygroscopic seeding: Addition of hygroscopic particles or droplets to a cloud to serve as artificial cloud condensation nucleus and thf:reby alter the size spectrum of the cloud droplets; sometimes called warm cloud seeding. Ice nudeus: Insoluble particle that serves as a freezing center for the formation of ice from water or from the vapor; those particles promoting deposition from the vapor were formerly known as sublimation nuclei. Overseeding: Originally, the creation by seed- ing of so many particles, either solid or liq- uid, that none of them would possess suffi- cient fall speed to reach the ground as precipitation; in recent years, any form of cloud seeding that diminishes total precipi- tation at the earth's surface. Persistl~nce: Persistence of activity of cloud seeding agents for periods longer than de- sir,ed by weather modification operators; persistent effects lasting as long as several weeks have been hypothesized. Seedability: Property of a cloud or cloud sys- tem that makes it possible for additional pn:cipitation to be obtained by artificial cloud nucleation or other weather modifica- tion technique. Silver iodide: In its most common form, a yel- low salt of hexagonal crystal structure and density 5.68 g cm-3. It is widely used in weather modification because of its gla- ciogenic properties. Silver iodide generator: Device that produces tiny crystals of silver iodide for use as a cloud seeding agent. Supercooled cloud: Cloud or portion of a cloud in which supercooled water exists; super- cooled water is liquid water existing at tem- peratures below ooc. Copyright IQ 1987 by Academic Press, Inc. All rights of reproduction in any form reserved.