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contain a built-in air tank within each generator which, when pressurized, forces the liquid seeding solution through an aperture and nebulizes it into a fine spray. The spray then flows into a combustion chamber where it is vaporized by burning. As the spray bums, very pure particles are formed and are exhausted out the tail-end of the generator into cloud base updrafts where the particles are carned aloft by natural action into the cloud's supercooled region. From 1987 through 1995 the liquid seeding agent was the same: Quantities of the oxidizers sodium perchlorate and anunonium perchlorate were added to a silver iodide-anunonium iodide- acetone-water solution resulting in a liquid solution containing 2% silver iodide by weight. However, in 1997 the formulation was changed to contain amounts of silver iodide, sodium iodide, acetone and paradichlorobenzene (C6H.Cl,). Cloud chamber test results at CSU indicated the total number of ice crystals produced by the new solution at -10C were closely equivalent to the old formulation containing the perchlorates. Also, the particles initially act as hygroscopic condensation nuclei insuring that the formation of vast numbers of water droplets will contain ice nuclei particles. Any ice nuclei initially not trapped in the water droplets can be captured later by other droplets through random collisions within the cloud called "contact nucleation". The entire process of hygroscopic condensation followed by freezing and contact nucleation, forms greater numbers of ice crystals at relatively warmer temperatures within a cloud than by simple contact nucleation. The overriding reason to change to this formulation was due to its clean-burning properties. When this solution burns, little, if any, residue is produced leaving less generator maintenance needed over time---a critically important consideration during persistent, active operational periods. In tlie past two seasons using this formulation, wing generator maintenance due to corroded parts has been nil compared to previous seasons while using the perchlorate additives. In order to obtain the desired effect when seeding clouds, each cloud must be treated within a proper time interval, a "window of opportunity," in order to produce the optimum ice crystal concentrations in clouds naturally deficient in them and to promote supercooled water droplet freezing within clouds. A cloud growing to maturity must be treated with enough time allowed so that the generated ice nuclei can be lifted by natural cloud updraft action into the appropriate temperature and moisture regime and kept there for a sufficient time to interact with the supercooled cloud water. If this opportunity window is missed when attempting rainfall stimulation, clouds may collapse prematurely resulting in wasted effort and resources. "Residence Time" in the supercooled cloud volume is critical to the success of both rain stimulation and hail reduction efforts. The behavior of weakly and moderately growing cumuliform clouds can be altered through . what's called the "dyn~c effect." Under certain atmospheric conditions, clouds may be stimulated to grow larger and rain longer than would be the case if otherwise left unseeded. This is done by getting sufficient amount of seeding agent into the supercooled portion of a cloud to promote the rapid conversion from water droplets to ice crystals. When this water-to-ice conversion process occurs rapidly, the latent heat of fusion is released on a massive scale making the cloud slightly warmer and more buoyant. Updrafts will soon become invigorated drawing in greater amounts of water vapor into the cloud and supplying more moisture to the cloud for subsequent growth and "processing" into rainfall. This process enables a cloud to rain more and rain for a longer time than ifleft unseeded. 6