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INTRODUCTION Project Skywater The Bureau of Reclamation's program of Atmospheric Water Resources Management, commonly referred to as Project Skywater, has been in existence since 1962. Its goal is to learn how cloud seeding can be employed efficiently, economically, and on a socially acceptable basis to increase precipitation in water-short areas of the nation; in effect, to manage precipitation. The comprehensive field programs being carried on under Project Skywater have the objective of developing precipitation management technology that has general application. While Project Skywater is a major component in the science of weather. modification, it is to be distinguished from other programs with different objectives. These include hurricane modification, lightning suppression, fog dispersal, and hail suppression. Project Skywater's field activities involve complementary programs of seeding (1) winter storm systems that form over mountain ranges as the air is lifted and cooled by these orographic barriers, and (2) cumulus clouds that form during the warmer months. In the wintertime program, the goal is to increase the mountain snowpack to provide additional runoff during the spring melt season. Cumulus seeding is designed to provide increased rainfall directly on the land. Principles of Cloud Seeding Clouds are made up of billions of tiny ice crystals or water droplets or a combination of both, which form around microscopic particles-soil, dust, smoke, salt crystals, and other materials that are ever present in the atmosphere enveloping the earth. Scientists classify some of these microscopic particles as condensation nuclei on which water vapor condenses to form cloud droplets, and a few as ice nuclei on which condensed water freezes, or ice crystals form directly from water vapor. As a general rule there is an abundance of condensation nuclei, but usually a scarcity of ice nuclei available in the air. The sizes, types, and concentrations of nuclei present in the atmosp~ere play an important role in determining the efficiency with whJch a cloud system precipitates. Tons upon tons of water flow above the United States in these "rivers in the sky," precipitating little or not at all for want of certain required conditions. Of prime importance for both initiation and amount of precipitation from a cloud system are (1) vertical and horizontal dimensions of cloud, (2) lifetime of cloud, and (3) sizes and concentration of cloud droplets and ice particles. Under proper conditions, one or more of these three factors can be favorably modified through seeding the cloud with appropriate nuclei. Basically there are two mechanisms by which precipitation forms in clouds. They are sometimes called the "warm rain" and the "cold rain" processes. The term "warm rain" was derived after scientists noticed that rain in tropical regions often fell from clouds whose temperature throughout was warmer than 320 F. Rain is formed in these "warm" clouds when larger droplets collide with and absorb smaller cloud droplets in a process known as coalescence. The term "warm rain" is a misnomer in that the coalescence mechanism is operative in cold clouds as well. The cold rain process occurs in clouds whose temperature, in all or part, is colder than 320 F. The supercooled (below 320 F) region of the cloud is generally comprised of a combination of water droplets and ice crystals, and oftentimes only the former. Ice crystals that form in the supercooled region grow rapidly, drawing moisture from the surrounding cloud, until their weight causes them to fall. In falling, they coalesce with smaller droplets and fall from the cloud as rain or snow. The atmospheric nuclei present during cloud formation exert a strong influence on the efficiency with which the warm and cold rain processes operate. For instance, giant condensation nuclei are prevalent in the oceanic regions which allows for larger cloud droplets to form and the coalescence process to initiate rain well within the lifetime of the cloud. Conversely, continental regions are characterized by much smaller and more numerous condensation nuclei. Medium-sized clouds formed in these areas normally dissipate before the coalescence mechanism has had a chance to initiate rain. Likewise, many regions have a dearth of ice nuclei which cuts down the efficiency of th.~ cold rain process. Man can assist nature by furnishing appropriate nuclei through "seeding" the cloud at the proper time and place, and in the proper amounts, when he determines through scientific measurement that nature has provided too few nuclei to trigger the precipitation process. Seeding with very large condensation nuclei (hygroscopic particles) can be done to accelerate the warm rain process. And seeding with proper ice nuclei (such as silver iodide) to supply naturally deficient clouds with an optimum concentration of ice crystals will increase rainfall through the colCl rain process. In many cold clouds a combination of hygroscopic and ice nuclei seeding can be done, since as has been previously mentioned, both warm and cold rain mechanisms operate in these clouds. The preceding section has shown the important role played by the sizes and concentration of cloud elements in the initiation and efficiency of