Winter Orographic Cloud Seeding Status in the Intermountain West
AbstractThe current status of winter orographic cloud seeding technology was reviewed to assist in lhe consideration of a snowfall enhancement program for the Black Hills of South Dakota. However, this material is relevant to the intermountain west in general. The emphasis is on the physical evidence that key processes occur in the hypothesized chain of events from release of seeding agent to snow reaching the ground. The range of opinions concerning the technology is briefly noted. Some problems related to reliable production of silver iodide are discussed. Evidence is reviewed from a number of locations showing that excess supercooled liquid water often exists during portions of winter storms, though usually in limited amounts. Much of the liquid water is found near the windward slope and crest of the mountains. The main problem with seeding winter orographic clouds is the transport and associated dispersion of the silver iodide between its release and the supercooled water zone. An associated problem is ensuring that the seeding agent reaches a sufficiently cold region of liquid cloud in an appropriate concentration. While adequate transport and dispersion can be difficult with airborne seeding, it is a special problem with low-level ground generators. Evidence is cited showing that ground seeding projects have frequently failed to enhance the silver content of the target area snowfall. It is recommended that the silver content be monitored in all winter orographic projects which use silver iodide as a minimum first step in documenting whether or not the seeding agent was transported to the target. Where practical, additional testing of the physical hypothesis should be accomplished to improve validation of winter orographic cloud seeding.
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