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Weather Damage Modification Program 6 . . . . . . . . . . . . . . . '. . . . . . . . . . '. . . . . . . . . . . . . . . '. . . . Research has demonstrated that certain natural particles in the atmosphere (e.g., clay particles from soils) serve as ice nuclei. These nuclei enable the freezing of supercooled water droplets. This process is referred to as nucleation of the ice phase. Most of the precipitation that occurs in temperate latitudes begins in the cold (<OOC) portion of clouds. Ice nuclei convert the supercooled water droplets into ice crystals that then grow at the expense of water vapor and other supercooled cloud droplets, because the vapor pressure over ice is less than that over water. This cold cloud precipitation process is often referred to as the Bergeron-Findeisen process. Other research has indicated that naturally occurring ice nuclei active in the temperature range between -So to -ISoC are relatively rare. Research has also shown that minute particles of silver iodide begin to act effectively as ice nuclei at temperatures colder than -SEC (Dennis 1980). Since natural ice nuclei active between temperatures of -So to -ISoC are sparse, most clouds are highly inefficient in converting water droplets to ice crystals. The addition of silver iodide- complex nuclei to these supercooled regions can produce additional ice crystals which, under the right conditions, grow into snowflakes and fall out of the cloud as either snow or rain. Rain results from the melting of such snowflakes when they fall through the warmer sub-cloud layer of air near the ground. This increase in efficiency is usually referred to as a static seeding effect. When supercooled water droplets freeze and when water vapor becomes ice, additional heat is added to the cloud due to the release of the latent heat of fusion. This additional heat adds buoyancy to the cloud, which in the presence of atmospheric instability may invigorate the clouds, resulting in a "dynamic" effect. This process was the basis for a National Oceanic and Atmospheric (NOAA) research program conducted in Florida known as the Florida Area Cumulus Experiment (FACE). Two different phases of FACE I, 1978-80 and FACE II, 1978-80 (Woodley, et al. 1983) indicated increases in area wide rainfall but results fell short of strict statistical acceptance criteria. During the research program known as Santa Barbara II Phase I consisted of the release of silver iodide from a ground location near 4,000 feet MSL located in the Santa Ynez Mountains north of Santa Barbara. These silver iodide releases were made into the convective bands as they passed overhead. Treatment was randomized in order to derive some natural (unseeded) cloud information. A large network of recording precipitation gauges was installed for the research program (Figure 2). The amount of precipitation that fell from each seeded or unseeded convection band was determined at each precipitation gauge location.