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56 JOURNAL OF APPLIED. METEOROLOGY VOLUME 17 (The updrafts in the latter clouds were found to be insufficient to carry the AgI to the sampling level before sampling ended.) An ice crystal survey flight conducted at the'same time by CSI showed negligible ice particle concentrations in most clouds. Very few natural clouds were found that day with concentrations comparable to those in the dry-ice-seeded clouds, and all of them were more than 50 km away from the experimental clouds. The range to naturally glaciated clouds was over 100 km on 8 July 1976, an experiment described in detail below. In general, the natural ice background for young clouds in Montana is very low (McPartland et at., 1977). Large ice concentrations, approaching or exceeding concentrations produced by seeding, have usually been found only in old clouds whose tops have cooled well below - 20oC. The natural ice background in the Australian clouds Date Air tempera- ture at seeding Cloud level no. (oC) 23 June 76 -7 -7 2 3 July 76 -5 4 -8 8 July 76 20 July 76 -10 -8.5 21July 76 7 Dec 72 1 1 2 1 2 -10 -11 -13 -9.5 -;4.5 -12 8 Dec 72 3 Feb. 72 3 -12 15 Mar 73 -10.5 -10.5 -10.5 2 3 16 Mar 73 6 23 Mar 73 -8.5 · TCU, towering cumulus. was highly variable. At one extreme was a cloud cluster on 6 December 1972 which, by its geographical location, may have been continental in nature. Elements of the cloud system were seeded with dry ice, but an analysis of the nose, side and tail time-lapse camera films showed that only natural towers of the cluster were sampled with the impactor. The highest single-frame concentra- tion was 1.7 .e-t, with most below 1 i-I. At the other extreme are old clouds and anvils where concentrations sampled averaged 25 i-I and peaked to 100 t-t, as determined from the foil. Clouds on two of the days with obviously rapid ice multiplication averaged 22 i-I, but clouds were not seeded on such days. As can be seen from Table 4, such high concentrations significantly exceeded those found III the dry-ice-seeded clouds. A natural background averaging 4 i-I was found on 23 March 1973, but the seeded cloud that day was TABLE 5. Seeded cloud conditions and descriptions. Typical temperature and dew points at sampling levels Inside Outside cloud cloud T TD (oC) (oC) eC) -7.5 -5.5 -5.5 -7.0 -7.0 -5.0 -7.0 -5.0 -7.0 -7.0 -9 -9.5 -9.0 -9.0 -6 -7 -9.0 -2.0 -12 -11.5 -10.5 -10.5 -10.5 -8.5 Sampling aircraft -20~0 -13.1 UW UW -31 CSI -39 CSI -20::1::10 CSI UW -16 -18 -36 -36 -14 ? UW CSIRO CSIRO CSIRO . CSIRO CSIRO -16 -16 CSIRO -24 -:- 24 -36 CSIRO CSIRO CSIRO -23 CSIRO e , Comments Possibly descending tall strato- cumulus. Tall stratocumulus, top descend- ing at 1. 7 m S-I; natural glaci- tion may have started. TCU. poorly sampled-plane not allowed to descend be- yond skimming tops. TCU; natural crystals at 0.4 (-1 subtracted from concentration. Final remains of dry ice supply dumped out of plane, tar- geting suspect. Rising, tall altocumulus studied for 53 min. TCU rapidly evaporating, sampled 2 min after seeding only. TCU studied for 24 min. TCU with stationary top. TCU with ascending top. Thin altocumulus. Fire-induced TCU. Flight data no longer recorded. Rising TCU later became Cb. Some natural ice present. Mixed rising and subsiding TCU bubbles. Altocumulus. Altocumulus, poorly sampled, nearly missed on repenetration. Rising TCU, later became Cb. Cloud volume poorly mea- sured; only cloud edges sampled for crys.tal concen- trations quoted. Stagnant TCU poorly sampled. e e