Laserfiche WebLink
<br />of crystals formed too close to a barrier crest, especially if a broad valley exists upwind of the <br />next barrier (Reynolds 1996). <br /> <br />3) Several investigations have demonstrated the best sites for ground-based release of either <br />AgI or liquid propane are at high altitudes, at least midway up the windward slopes from the <br />valley floor to crestline elevations. Propane dispensers must be located high enough to be within <br />or just below liquid cloud base, and at temperatures at least slightly below ooc. While AgI <br />generators are usually run below cloud, it is best to operate them within liquid cloud at -60C or <br />colder so the forced condensation freezing mechanism can function. This mechanism results <br />from high water vapor concentrations immediately downwind of generators, a byproduct of the <br />combustion of propane and acetone (Finnegan and Pitter 19S5, Chai et al. 1993). The mechanism <br />provides large concentrations of seeded crystals just downwind of the generators, maximizing <br />both output and crystal growth times. Operating AgI generators below cloud base can also be <br />effective but only if the seeding agent is vertically transported to altitudes where temperatures are <br />colder than about -Soc. <br /> <br />4. Seeding by Expansion of Liquid Propane <br /> <br />1) It has long been recognized that AgI seeding is ineffective for mildly supercooled cloud, <br />that is, warmer than about -6 to -SoC. The "threshold temperature" at which a tiny fraction of a <br />large population of AgI particles can produce ice crystals has long been stated as near -60C, well <br />below the OoC freezing point of bulk water. Some recent papers suggest that newer formulations <br />of AgI have threshold temperatures of -50C or even -40C. Unfortunately, the Colorado State <br />University (CSU) cloud chamber facility, used for many years as the standard for testing AgI <br />effectiveness as functions of SL W cloud temperature and other variables, is no longer <br />operational. Consequently, such claims are difficult to verify. The authors are unaware of any <br />published observations which show significant AgI nucleation at such warm temperatures in <br />natural (not laboratory) cloud. <br /> <br />2) A far more important point is that some authors have presented threshold temperatures in a <br />manner which erroneously suggests that AgI seeding can be effective at temperatures ~ _60C. <br />That is not the case except for the important exception of forced condensation freezing discussed <br />above. But that mechanism requires that AgI be released directly into SL W cloud, requiring use <br />of high altitude remote-controlled generators. Few operational seeding projects operate AgI <br />generators high enough to be in cloud at temperatures of -60C or colder. Therefore, the following <br />discussion is generally applicable. <br /> <br />3) It is well known that AgI effectiveness is highly temperature dependent, with the percent of <br />the total AgI particle population able to nucleate ice crystals increasing by orders of magnitude <br />from about -6 to -120C. For example, DeMott et al. (1995) used the CSU cloud chamber facility <br />to show that a particular commercial AgI generator with unusually good warmer temperature <br />yield (effectiveness) had an increase in ice crystal production over its -60C output which was 33 <br />times higher at -SoC and 314 times higher at -120C. The same comparison for the type of AgI <br />generator used in the Bridger Range Experiment (Super and Heimbach 19S3) showed the increase <br />from the -60C effectiveness was 770 times higher at -SoC and about 7000 times higher at -120C. <br />The Bridger Range Experiment strongly suggested significant snowfall increases for ridgeline <br />temperatures of -90C and colder. But the half of all experimental periods with ridge temperatures <br />warmer than -9.50C showed no significant seeding effect. And it is likely that much of the <br />success of the Bridger Range Experiment was due to frequent AgI release directly into cloud, <br /> <br />8 <br />