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fell, or indirectly by measuring the change in cloud characteristics. Precipitation can be measured at the ground and at cloudbase, with rain gages and with radar. Rain gage data are adequate for long-term proj- ects but, unless the seeded cloud passes over a rain gage, the data will not be representative of the real effect on individual clouds. Radar provides a measure of the total precipitation that falls over an area, but the radar must be carefully.calibrated using rain gage data. The best evaluation tool may soon be the mathematical model. Models can provide an estimate of the amount of rain a cloud will produce if it is seeded and if it is left alone and allowed to produce precipita- tion naturally. .In research programs, considerable interest is focused on individual clouds and the amount of rain they produce. In operational projects, however, the emphasis is on the total rainfall for the season and upon the timing of the rain, not on how much a given cloud produces. Operational projects, then, tend to be evalua~ed on the basis of sea- sonal precipitation increase and also on the basis of the economic benefits derived from the additional rain. ACTIVITIES AND ACCOMPLIS~1ENTS Research and Development In the Project Skywater of the Bureau of Reclamation, a major part of the research and development effort has been directed toward cloud phys- ics investigations to determine how microphysical and dynamic processes within clouds can be altered to increase precipitation with field experi- ments, laboratory research, and computer modeling. New, more effective and efficient cloud seeding materials and equipment have been developed and tested. There is continuing emphasis on lower cost materials neces- sary for economic large-scale seeding. Integrated weather modification systems have been designed and tested, including centralized operational control with computers for improved direction of seeding activities during rapidly changing weather condi- tions. Instruments and data systems for measuring atmospheric param- eters in the detail and speed required for weather modification opera- tions and evaluations continue to be developed and adapted. New developments include ice-nuclei counters, replicators, lasers, and air- borne instrumentations. Engineering accuracy and reliability are important considerations. Investigations of possible atmospheric side effects of cloud seeding in the atmosphere and on the surface beyond the intended target area have 8