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". -, * . Decaying isolated clouds also usually had little ice present. In about 70% of the cases o~!erved concentrations were below 2 ice particles .2. o_lMaximum observed concentrations were about 20 R. The low levels of' ice particles observed during all pllases of isolate,d cloud development is notable. II:. sugges'::s that background concentra- tions of natural ice nuclei active between -6 and _16oC are low, and that ice multiplication is not significant. Clouds in thls category should have good ice phase modification potential. Cells in the feeder cell category generally exhibited higher particle concentrations than their isolated counterparts for each stage of development. This is probably because the feeder cells were often rising out of ti,e debris left from older dec.3ying turrets. Some ice particles from the decaylng remnants were probably heing recircul- ated in the new towers. Even so, 50% of the develop- ing mature feeder cel:s displayed no ice concentrn- clans above the data threshold when penetrated. In isolated instanceli", cells of these types showed 5 to 8 particles ~-~, suggesting they could often be candidaces for addition of nuclei. More than half of the decaying feeder_tells penetrated had less than 2 ice. particles L ,while about 25% of the -1 cases had particle concentrations in the 10 - 20 i range. ThIs suggests that many of these cells are somewhat inefficient. In summary, the population of cumulus congestus cells sampled generally exhibited low natural ice concentrations for developing and mature clouds, suggesting these cells may be suitable for ice phase modification. 4.2 Additional Cloue: Types Sampled Two other cloud types we.e sampled during the period of obser/ation. Sample .izes are limited, and only generalized observations a.e possible from the data sets. Each of th.. different types are briefly treated below. 1.. Mature Cumu:~onimbus Systems Much of the Miles City area summer rain- fall during 1976 was assoc:Lated with large cumulo- nimbus systems which had their origins in the mountains to the south and west of the research area (Boatman et al., 1977). These systems freq- uently left th~ntains, crossed the plains. and moved in to the Miles Ci ty ,area in the evening or nighttime hours. Xany wer,~ very large and well organized systems which could not be safely pene- trated and sampled with the project aircraft. Several were hail producers. The edges of the systems were typically obscured by decaying cloud remnants, and few active feeder cells were visible. Active convective areas in the interior portions were observed by radar, but were not.visually distinct or identifIable from the aircraft. On a few occasions it was possible to sample the fringes of these systems. In all instances ice particle concentrations were quite high. Typical concentrations ra~~ed from a few tenB to a few hundredsoof particles ~ at sampling temp- eratures nenr -10 C. These sYBtems do not appear suitable for rain augmentation attempts by the time they have ,.organ.ized..and :reached the Miles City area. 2. General Overcast Rain Situation Occurrences of widespread general rain are often associated vith passage of upper-level troughs or co~d fronts. Several sampling missions were flown in these situations, and all indicated an abundance of natural ice particles. 1'ypical_1 concentrations were approxim3tely 20 particles R. at te~~eratureg of -5 to -10 C, and up to about 100 R. at -15 C, 5. SlW'.ARY The ice particle concentration data obtained during the 1976 HIPLEX field program supports the hypothesis that rain augmentation potential exists for cumulus congestus clouds in the ~iles Ci:y area. Low natural ice particle concentrations were observed during penetrations of both deve:oping and mature isolated cells as well as feecer ce:ls associated with cumulonimbus systems. ~.e5e clouds appear to have good modifi- cation potencial ror seeding with ice nucleating agents. Mature cumulonimbus systems originating over the mountains to the south and west appear to have abundant natural ice when they reach the ~iles City area. It is possible that some poten- tial may exist gor triggering additional systems of this type through judicious seeding of clouds in the mountain areas. Widespread rain occurrences triggered by frontal passages or upper-level troughs appear to have abundant natural ice. 6. RE:'ERENCES Boatman, J., E. Holroyd, III, and A. Super, 1977: Summer rainstorm types and associated rain- fall characteristics in eastern Montana. Pre- prints Sixrl, Conf. on Inadvertent and Planned Weather !.todificacion, Champaign-Urbana, (L, Amer. ~eteor. Soc. Lawson, R., 1977: personal communication. Colo- rado International Corpocation, Boulder, CO. Super, A., ~. Holroyd, III, and B. Silverman, 1977: The practicability of dry ice for on-top seed- ing of convective clouds. Preprin~ Conf. on Inadvertent and Planned Weather Noc!- ification, Champaign-Urbana, IL, ADler. Meteor. Soc.