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liter. As expected, there was a tendency for higher concentrations at colder temperatures. These relatively low values also argue against ice multiplication and indicate that the ice crystals sampled by aircraft were generally caused by primary nucleation due to ice nuclei. However, a note of caution is in order. There is reason for concern about the representativeness of the 1987 aircraft sampling. Further, ice multiplication may have sometimes occurred below aircraft sampling levels. Aircraft observations of liquid water contents were usually limited to low values, presenting a less optimistic assessment of cloud seeding potential than the much more abundant microwave radiometer measurements. This is believed to be partially due to a demonstrated bias in the aircraft sampling periods such that the wetter storm periods were not measured. Further, it is suspected that higher amounts of liquid water often existed below permissible aircraft altitudes, but no direct measurements presently exist to test this supposition. Most of the total precipitation at both field sites was from the synoptic stratiform storm class. The heavier snowfall episodes were created by large-scale atmospheric motions that were largely independent of the Mogollon Rim terrain. The precipitation was often initiated at high levels well upwind of the Mogollon Rim but experienced substantial growth in the locally produced cloud liquid water at low levels over the Mogollon Rim. Precipitation rate distributions were highly skewed as is common for mountain snowfall. That is, many hours had light snowfall rates while half the total precipitation fell during only 12 percent of the hours with snowfall. Precipitation generally fell in episodes typically lasting about 1 day with longer periods of dry weather in between. Atmospheric stabilities were examined during 1987 storms using rawinsonde and aircraft observations; in most cases, the stability was neutral. Therefore, ground-released seeding material should mix vertically, given sufficient time and distance, and not be trapped as has been the case with some mountain barriers with stable atmospheres. A series of experiments was conducted that simulated ground-based seeding by release of a tracer gas. These experiments were highly successful in tracking the tracer gas under a variety of conditions; the results showed that seeding material, released from the surface on the windward slope of the barrier, would typically cross the crest with a plume height of over 700 meters. Plume widths were sufficient that a reasonable number of ground-based silver iodide generators could seed a large portion of the Mogollon Rim. The observed tracer gas concentrations and prevailing temperatures during both warm and cold storms were used to estimate effective in-cloud ice nucleus concentrations for typical silver iodide generator outputs. These estimates showed that Arizona stratiform storms are usually too warm, in the layer that could be seeded from the ground, for significant ice crystal production with conventional types of generators and silver iodide. Enhanced vertical mixing to colder levels may occur during periods with embedded convection. Such conditions were not well represented in the tracer gas experiments. Further, recently developed silver iodide solutions, effective at warmer temperatures, and higher output generators should increase the fraction of storms seedable by ground release. Also, the effectiveness of various types of silver iodide requires testing in actual clouds, as the laboratory results used in the simulations may be underestimates. Therefore, while vi