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Weather Mod Critical Issues Report
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Last modified
10/28/2011 10:18:23 AM
Creation date
9/30/2006 9:03:51 PM
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Template:
Water Conservation
Project Type
General OWC
Project Name
Weather Modification
Title
Critical Issues in Weather Modification Research
Date
1/1/2004
Water Conservation - Doc Type
Final Report
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<br />I <br />I <br />I <br />I <br />II <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />3.2 Cold season orographic cloud seeding programs <br /> <br />3.2.1 General discussion <br /> <br />Although there has been no fully randomized, completely observed chain-of- <br />events, replicated, field experiment in winter cloud seeding, there have been a number of <br />statistically oriented projects, some with thorough physical measurements, that yield <br />considerable evidence of positive effects of cloud seeding (Gagin and Neumann, 1974; <br />Elliott, 1986; Reynolds, 1988; Ryan and King, 1997), Notable examples are the Israeli I <br />experiment, the Tasmanian operation, the Climax I and II projects (Grant, 1986; Mielke, <br />1995), the Lake Almanor experiment, and the Bridger Range experiment, these last two <br />to be discussed below. The NRC report does an adequate job of discussing winter <br />glaciogenic seeding, but leaves out a number of topics and references that, in our opinion, <br />should have been included, particularly those concerning the chemical analysis <br />techniques, which will be discussed later. <br /> <br />A number of observational and theoretical studies have suggested that there is a <br />cold temperature 'window' of opportunity for cloud seeding. Studies of both orographic <br />and convective clouds have suggested that clouds colder than -250C have sufficiently <br />large concentrations of natural ice crystals such that seeding can either have no effect or <br />even reduce precipitation (Grant and Elliot, 1974; Grant, 1986; Gagin and Neumann, <br />1981; Gagin et aI., 1985). It is possible that seeding such cold clouds could reduce <br />precipitation by creating so many ice crystals that they compete for the limited supply of <br />water vapor and result in numerous, slowly settling ice crystals which sublimate before <br />reaching the ground. There are also indications that there is a warm temperature limit to <br />seeding effectiveness (Gagin and Neumann, 1981; Grant and Elliott, 1974; Cooper and <br />Lawson, 1984). This is believed to be due to the low efficiency of ice crystal production <br />by silver iodide at temperatures greater than _40C, and to the slow rates of ice crystal <br />vapor deposition growth at warm temperatures, Thus there appears to be a 'temperature <br />window' of about _50C to -250C where clouds respond favorably to silver iodide seeding <br />(Le" exhibit seedability), Dry ice (frozen carbon dioxide) seeding via aircraft extends <br />this temperature window to temperatures just below OOC, <br /> <br />Orographic clouds are less susceptible to a 'time window' as they are typically <br />quasi-steady state clouds so they offer a greater time opportunity for successful <br />precipitation enhancement than cumulus clouds, A time window of a different type does <br />exist for orographic clouds which is related to the time it takes a parcel of air to condense <br />to form supercooled liquid water and ascend to the mountain crest. If winds are weak, <br />then there may be sufficient time for natural precipitation processes to occur efficiently. <br />Stronger winds may not allow efficient natural precipitation processes but seeding may <br />speed up precipitation formation. Even stronger winds may not provide enough time for <br />even seeded ice crystals to grow to precipitation before being blown over the mountain <br />crest and sublimating in the sinking subsaturated air to the lee of the mountain. A time <br />window related to the ambient winds, however, is much easier to assess in a field setting <br />for orographic clouds than for cumulus clouds. <br /> <br />15 <br />
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