MIKE MOSS SAYS: Joseph, Several types of cloud seeding operations are still undertaken around the world, although large questions remain unanswered in regards to the effectiveness at producing targeted increases in precipitation. While there is a great deal of variety in results of statistical studies that traces back to the difficulty in knowing what a given cloud or system would have done in the absence of seeding, the consensus seems to be that increases in precipitation on the order of 10% are possible given appropriate conditions. The technique is applied most frequently to orographic clouds that are forced into existence by the flow of moist air up the windward side of mountain ranges, and occasionally to convective cloud systems both to attempt increases in rainfall from cumuliform clouds and also as a hail suppression measure. Over time, this may be effective in increasing snowpack and runoff from mountain systems, and this is a primary way in which seeding is currently employed by several states in the western U.S.
However, in a position statement on cloud seeding to increase rainfall amounts, the American Meteorological Society points out specifically that widespread drought situations are not favorable for the use of cloud seeding, as the large scale weather patterns that limit rainfall in the first place during a serious drought also greatly limit opportunities to effectively seed clouds (in part by promoting in a lack of clouds that are suitable for seeding), so that a seeding program is likely to be ineffective and economically unsound. The AMS recommends that resources be directed toward enhanced conservation and toward expanding facilities for capturing and storing water during non-drought periods.
Here are addresses for a couple of documents you may find interesting that provide more background...
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