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1. INTRODUCTION The Weather Modification Assessment: Kingdom of Thailand (Silverman et al., 1986) recommended a comprehensive 5-year developmental program to improve the technical capabilities of the RRRDI (Royal Rainmaking Research Development Institute) through training, additional equipment, and a demonstration cloud seeding project. USAID (U.S. Agency for International Development) accepted these recommendations, and anew, broadly- based program known as the AARRP (Applied Atmospheric Resources Research Program) was established. Subsequent to the report by Silverman et a1. (1986), a core training course was conducted in February and March 1988 to acquaint AARRP participants with the scientific principles, terminology, and technology of weather modification as a water augmentation tool. Simultaneous with and following this training, a number of studies were conducted in preparation for the demonstration cloud seeding project. These studies are described in a report by Medina et al. (1989). The basic concepts to be tested in Thail.and, including hygroscopic particle seeding of warm clouds to increase rainfall and cold cloud seeding to produce dynamic effects and increased rainfall, were investigated using a hierarchy of cloud models. The model runs indicated that both seeding approaches have potential for increasing rainfall in Thailand and that perhaps 35 percent ofthe potential operational days might be suitable for seeding for dynamic effects. Preliminary design work on the demonstration project design extended beyond the numerical studies of possible responses to seeding. Mter visiting potential experimental sites, officials of the RRRDI and Reclamation (Bureau of Reclamation) selected the southern part of the Bhumibol catchment area of northwestern Thailand for the conduct of the demonstration project (see fig. 1.1). The FOC (Fiel.d Operations Center) was located initially at the Bhumibol Dam site, and an S-band (10-centimeter) Doppler weather radar was installed at a site about 9 kilometers southeast of Omkoi on a ridge (height 1,162 m) which provides a good view of the southern catchment area drainage. The FOC was moved to Chiang Mai Airport in Chiang Mai, Thailand, in 1992, and was organized to obtain necessary data for weather forecasting and monitoring. Aircraft were outfitted for hygroscopic ~md glaciogenic seeding. The high-altitude cold cloud seeder aircraft was also equipped for meteorological data collection, including some basic measurements of cloud physics (e.g., temperature, dew point temperature, cloud liquid water, and cloud drafts). A network of rain gauges was installed in the project area, which is being used with the radar to obtain rain gauge adjusted radar rainfall estimates. These data will provide the basis for physical and statistical evaluation of project experiments. The report by Medina et a1. (1989) concluded by recommending that a number of theoretical and related field studies be conducted prior to commencement of the warm and cold cloud demonstration projects because the information sought is important to the project design. Most important were studies to better define cloud seeding potential, including the number and character of seeding opportunities based on physical observations and model runs. The report proposed that the AARRP satellite imagery, rain gauges, radar, rawinsondes, hydrologic data, and a cloud physics aircraft provide the needed observations.