California Cloud Seeding and Idaho Precipitation

J G MacCracken, J O'Laughlin


Long-standing concerns over the effects of cloud seeding projects in California generated this review article.  The ability to determine the downwind (or extended area) effect of cloud seeding is limited.  Court cases have been inconclusive because it is currently impossible to demonstrate cause and effect due to cloud seeding.  The idea that successful seeding results in less precipitation downwind seems logical at first glance, but is not supported by theory, the characteristics of atmospheric water, the physics of cloud dynamics and the precipitation process, the efficacy of the seeding agents, or the limited data available.  Generally, the extended area effects of seeding are the same as the effects in the target area, and the maximum extent documented for a downwind effect is 180 miles.  Southern Idaho is more than 400 miles from the target areas of California cloud seeding projects.  In addition, the most common situation when seeding winter orographic clouds as practiced in California is an increase in precipitation to support the proposition that cloud seeding in California affects precipitation in Idaho; downwind precipitation could tend to increase rather than decrease.  Because of the lack of definitive information, the subject is still open to debate.

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