Present status of precipitation enhancement by cloud seeding

Atmospheric water in the form of precipitation, is one of the primary sources of fresh water in the world. However, a large amount of water present in clouds never gets transformed into precipitation. This has prompted scientists and engineers to explore the possibility of augmenting water supplies by means of cloud seeding. Although certain successes have been documented, indicating increases in rainfall due to seeding in some projects, others showed a decrease while the majority indicated inconclusive results. The reason for this is that physical mechanisms of cloud and precipitation development in the atmosphere are much more complex than anticipated earlier and the initial optimism in the 1950's and 1960's has given way to a more cautious approach. The earlier experiments treated the physical chain of events from seeding to precipitation at the surface as a ``black box''. However, in the late 1970's and 80's scientists have turned to a physical approach which incorporates observations, modeling and laboratory studies to track the physical chain of events and the effects of seeding from formation of a cloud to precipitation at the surface. The paper will attempt to critically review progress in the past and to address some of the areas where knowledge is presently lacking using the Arizona program as an example.