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'" ~ j identified. In many instances ""en the aircraft was operating in close proxiQity to large numuers of clouds or large cloud syste~ it was not possible "'~O -visual.ly '''ci.~6sify,~'eacil' c~oud ,ei.-emeil1:. he,ing.,p.e.n- etrated. These cases were discarded fro~ further consideration except for ligited discussion in the results section. The state of cloud deveiopment was sub- jectively estimated for each ?ass9 using a com- bination of visual appearance anG mean vertical velocity encountered during t:le tlenetration. When a cloud exhibited a hard out:iue and positive average vertical velocity ciuring a particular pass, it was classified as "develo;>i:1g.. II If il: had a relatively firm outline and e~sencially neutral vertical motion it was classified "matun~, II wiliie if it had diffuse edges and G~1arafts predominat- ing it was judged as "deCCl;/in6,. II Individual passes We~e considered the basic unit of the 5tudy. Data tor each pass through each identifiable cloud were u~~raged to o~tain mean prtss values of ice partic:e concentration~ ..~-emp-era-ture, Cin~ ~:.:=ticai. veioc.:..:y. 4. RESULTS 4.1 Cumulus Con~estus C:~~C5 ., The majority of ci.oli(:~ sampled during the season were classified as cu:;:...1.ics congestus. Tilese were separated into two catehry~ies: (1) isolated or semi-isolated cells (oere~:te~ called isolated) and (2) feeder cells associaceci ...,icn cumulonimbus sYB~erns. The data srtmple con~ists of 51 ?8SSeS through a total of 24 isol:1teri ceils and 27 passes through ~9 fe~der cells. Ce~~ ?eoetrations were DV'lde within a few hund't'ed l:1e:ers of cloud top when- ever practical. Data were exa~ned from several aspects and results are su~rized below. 1. Average Ice ?articie Concentrations VB. Cloud TeQ?era~ures. No ice particles aoove tile data threshold ~ere detected in any ofothe sa~?led clouds at temp- eratures warmer than -8 C. Plots of ice crysta~ co~centrations VB. mean pass temperatures revea:ed ao apparent temp- erature-dependent relationshitJ f:lr eithel' the isolated or feeder cells in a~y stage of their development.. Sampling t'?Gl?er:i~llreS ranged from -6 to -lSoC, although 632 of t~e samples were oetween only -10 and _14uC. \:t '.~1.s anticipated that colder cells would tena to 5how higher average ice ~article concentrac: i':)U5 than warmer cells, but the data do not su:'::;;t.antiate this for the admittedly limited r~n~e of cem~eratuLes sampled.. M~ny passes exilioi~eci ,:10 ice crystal!=> aoove the data threshold reg.;;rci..i,~ss of stage of development or temperature. 2.. Maximum Ooserveci \:CI~ Crystal Concen- trations va. Vertil:a.i Velocities. Data for the i~ol;'\te~ C.19CS were, investi- gated to determine if the rnaXl.i1iUl;l observed ice ?article concentration clueir.g e;)':.l pass could be related to the vertical ve locit: for the same 10- second averaging period. '",,-nl;'e:;le range of verti- cal velocitieR was limited. i:-'1XililUm concentrations were observed most frequently witl, downdrafts (range of only 1 to 4 ., s -.I.), which accounted for 52% of the maximums. 1ney w~re associated with near .zero vertical. yelo.ci ty.i n 32:1- of. the .ca.ses t while 16% of the r,l::lximums were <1ssociated with uparafts in the range l.to 3 m s-.1. 3. Time Histories of Ice Particle Con- centrations. A series or passes was made through seven isol!lted <:louds. Of these, two did not exhibit ice particles above the data threshold and an additiona:i. three did not exceed concentt'<1tions above one ice particle per liceI'. One c~oud attained a m~ximum oE 13 particles R.-~ some 19 minutes following the first pafiR, after which concentrntions rapidly decreased, A final cloud was first s3ii.pi..ed during a decay rhUSt"! w~In it showed a maximum concentration of 11 pnrticles 1.. . Ice particle concentrati0ns fell off to thresh- old values on sub$equent passes until the cloud s~arted to red~v~loP'_lA seco~1(..:.1'ry maximum con~entra- tl.on of 5 partl.c.l..es L was tnen observed 19 mUlUtes following the first pass. 4. Average Ice Partic~e Concentrntions vs. Cloud TYlJe ano Stage of Development. Fig. 1 !=thows the perCt:!n::age cumulative frec;- uency for ice particle canc~~r..tration5 for all pa.sses made through both isolated ;IllC; fceder cells. Inspec- tion of the figure revenls tL.<1t developing isolated cells had very low ice pnrtic1.c concentrations. J.!\1:!5e cells represent the types of cuses likely to be pref- erentially selected for seedinG. Over 77~ of t~ese cases had no ice above the d" td th reshold when pene- trated, and the maximum cuncentt'ntion observed was only about 2 particles Jl.-J.. "'ih i s type of cell may have good potential for rAin ,,\.j;mcntation through creation of additional ice ~ncticles. (hIOOI f t lilt ggoL ~ I 7- ~.o~' "'.... 070~ ./' ~ l ....,~:~: ~60(~ ~ I :s~Ol ~ ;:; 'EEOER I ......... 18 MATURE 15Q1..ATEO l - 10 'Uoe:R --.. 15 OleAVING IS04.ATEa - 10 '((0('" - ~30 ' '-'-1..l.U.J-,-~--,...J ~.,., G.S I 2 ";j 0 ,'j 20 <0 (olj (,0100 AVERAGE ICE PARTICLE CONCENTRATION PiR PAS:J h..ITJ;E"'J Figure I Average ice pa'tticlc cOllcentrHtion vs. cloud t:ype and st.,;.;~ of deveiopment. Similarly, .:l m<1joriL:y of the ffi.'1t:uce cells in tile i:;olated c;Jtcgory exil.i;)i.ted little ice. Some 70: of these cnses contaillcJ :,t!~S tllAn 1 pArticl~ 'J..-J.. These may also be suit"ol(!: c:lOdLdates for S\"t"Ji.n~.