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00U171 the locations and spacing of these generators at each site and will also provide for the design of an aircraft seeding program for storms not treatable by the ground-based AgI generators. Key elements in this decision will be the location of the super- cooled water zone and the temperature of that zone because this will largely determine the best seeding method. In some cases, it will be necessary to change the seeding mode as the storm evolves during the treatment period. The general approach toward the type of seeding agent used will be to use the most effective treatment for each type of cloud that occurs during an operational period, even when that means using different agents during the same period. The goal is to get the needed concentration of ice crystals in the supercooled cloud the best way possible. A computerized diffusion model will be employed to compute the expected plume spread of silver iodide from ground sources. If the model predicts that the nuclei plume top will reach supercooled cloud at temperatures equal to or colder than -10 'C, ground silver iodide generators will be used. Seeding will be accomplished by aircraft in those instances where near-surface cloud temperatures are too warm for significant silver iodide nucleation, or where the super- cooled cloud is above the level the ground-generated nuclei are expected to reach. b. Opportunity recognition system. - The presence of supercooled water for seeding operations will be monitored because it is the supercooled water zones that must be treated with the seeding material or resulting ice embryos. Knowledge of the existence and location of the supercooled water will permit a timely response by the appropriate seeding method, be it ground-based or airborne releases. In most past programs, the monitoring of supercooled water, if carried out at all, was limited to aircraft observations more than 600 meters (2000 feet) above the highest terrain. Recent observa- tions have shown the supercooled water to be concentrated near the terrain in the common stable storms. This limits the monitoring role of aircraft, although in unstable situations the convective elements often produce supercooled water through a sufficient vertical extent to permit aircraft sampling. Surface observations have generally been made only at mountain observatories, yielding no information on horizontal or vertical distributions. Clearly, the ability to monitor this key link in the chain of physical events has been severely limited in past experiments. However, three new instrumentation systems have recently been developed which will be of significant value in monitoring super- cooled water in CREST. The first is a microwave radiometer which can remotely sense the liquid water along the path where the antenna is pointing. While no special information is available regarding the distribution of 30