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<br />winds between south and west. A line of precipitation gauges was operated along the expected <br />plume trajectory, and one crosswind gauge provided the covariate of natural snowfall. The <br />exploratory statistical analyses presented by Super and Heimbach provided strong suggestions of <br />seasonal seeding increases near 10%. These and other studies have provided convincing evidence <br />that propane seeding can produce useful additional snowfall for mildly supercooled cloud <br />temperatures where Agl is not effective, as well as for colder temperatures where both AgI and <br />propane are effective. Propane seeding requires the use of high altitude dispensers operated at or <br />above the SL W cloud base. Such dispensers are economical to build and operate, and are highly <br />reliable. They can be used in a completely automated mode, using a device to sense SL W <br />presence to determine when to seed. A small operational program in Utah, described by Super et <br />al. (I995), has continued to use this approach for over a decade. The randomized experiment of <br />2003/04 was also completely automated. Both the operational program and the experiment were <br />able to seed on a 24/7 basis whenever SL W cloud was present. This approach provides a major <br />advantage over use of forecasts and calling out personnel to operate manual generators. <br /> <br />.....I 80 <br />';:;-60 <br />o <br />in 40 <br />,.. <br />U 20 <br /> <br />Propane <br /> <br />Agl <br /> <br /> <br />o <br />-< 8000 <br />~ 6000 <br />Cl: <br />l3 4000 <br />z 2000 <br />o <br />~0.8 <br />E <br />E 0.6 <br />::- 0.4 <br />.... <br />.... 0.2 <br />0.0 <br />8.0 8.5 9.0 9.5 10.0 10.5 11.0 <br />MST on 05 March 1995 <br /> <br />Fig. l. Temporal distribution of ice crystal concentration (top) and precipitation intensity (bottom), both <br />calculated from the mountain-top observatory 2D-C probe, and acoustical counter measurements of AgI, <br />effective at -20DC (middle panel). Two 3-min "tags" of AgI were released at the beginning and end of the <br />hour of propane release which appear much longer because of the response characteristics of the acoustical <br />counter. The estimated hour of maximum plume presence is shown by vertical lines for propane seeding <br />(left) and AgI seeding (right). The increase in snowfall rate after 10.5 hrs (10:30 a.m.) MST was associated <br />with a natural shower. <br /> <br />6) Cloud base altitude is an important consideration when siting propane dispensers which must <br />be in-cloud or just below cloud base (at ice saturation) to be effective. Rauber and Grant (I 986) <br />stated that cloud base over the windward slope of northern Colorado's Park Range was typically <br />about 650 to 1000 ft below the 10,370 ft crest line (all elevations above mean sea level), which <br />would place typical bases near 9400 to 9700 ft. Rauber and Grant (ibid.) also noted that <br />occasionally cloud base would lower to as much as 1600 ft below crest line or rise 150 ft above it. <br />This range would place cloud bases from as low as 8800 to as high as 10,500 ft. <br /> <br />10 <br />