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<br />e <br />~ 3 <br /> <br />= 2291ftirt <br /> <br />, <br />o <br /> <br />X (Km) <br /> <br />X IKml <br /> <br />HgUfte 11. EvoM~on 06 c.1'.oud clJwp- <br />le;t and ftMnMOp J.>pec.:tfta <br />-in di6 6 eftent pa!Lt.6 06 a c.loud. At <br />22.9 m~n double peak diJ.>:tftibution <br />c.an be J.>een, indic.ating gftowth 06 <br />pftec.ipitation-J.>ize paft:tic.leJ.>. The <br />c.1'.oud if.> J.>:tU..l gftow-ing in hught. <br />At 27. 9 min the c.loud hM fteac.hed <br />the 4.8 km level and laftge dftOpJ.> <br />Me in eUf.>tenc.e paft:t-<-C.ufMly <br />0.2 - 0.5 km away 6ftOm c.loud aUf.> <br />and up to a hught 06 3.6 kIn. The <br />ftegioM with the fuftge MOpJ.> Me <br />devo~d 06 c.loud dftople;tJ.> and a down- <br />dfta6t if.> ac.c.ompanying the pftec.ipita- <br />tion. Evapoftation fteduc.~ the <br />numbeft 06 fuftge MOpJ.> to zefto neM <br />:t:he gftOund. [FigUfte ac.c.oftd-ing to <br />ClMk, 1973.) <br /> <br />- 26 - <br /> <br />Weather modification: reached a sub- <br />stantial boost by the finding of Langmuir and <br />Schaefer in 1946 that a layer ~loud could be <br />dissolved by dispersing dry ice, i.e. solid <br />C02 with a temperature of about -700C. One <br />year later Vonnegut of the same group at the <br />General Electric Company, discovered silver <br />iodide, AgI, a substance which. can act as ice <br />nucleus (IN) and induce supercooled water to <br />freeze at -40C and lower. Hence, the ice <br />phase can be triggered. in clouds where not <br />sufficient ice particles are present to <br />initiate the precipitation process. <br /> <br />.- <br /> <br />In order to deal with the modern <br />era of weather modification we will divide <br />its aspects into different groups according <br />to the first sections of this paper. <br /> <br />The modification of the particle <br />evolution processes is more attractive than <br />to try to interfer directly with the energies <br />of clouds or cloud systems; the use of <br />catalysts or triggering substances could be <br />more advantageous. <br /> <br />Following the steps 'of the Warm <br />Rain Process of Figure 2a it can be expected <br />that the growth of larger droplets could be <br />initiated in a cloud either by the dispersal <br />of large CCN like NaCR, particles or by <br />injecting loads of droplets directly from <br />aircraft. Both methods have been tried but <br />no proof of effectiveness has been establish- <br />ed in conformance with today's standards. <br />Further along the chain of evolution breakup <br />could be effected by the use ~f surfactants <br />(Morachevsky and Kirinkhin, 1968). The early <br />production of large fragments .could be speed- <br />ing up rain formation. <br /> <br />The main approach to weather modi- <br />fication is presently employing seeding with <br />AgI or substances like CuS and others. These <br />substances have structures similar to ice (epitaxy) and thus "fool" captured :H20 molecules <br />into forming ice upon them. Another approach to increase the number of ice crystals is by <br />dispersing pellets of dry ice which are so cold that many aerosol particles are activated <br />as IN. If there is a natural defficiency of ice particles, the dissemination of IN can <br />trigger the Cold Rain mechanism. Experiments seem to indicate (Gagin and Neumann, 1976) <br />that it is most effective if the cloud tops have temperatures between -12 and -200C. <br /> <br />Nature has the ability to produce ice particles under conditions ~here there are <br />only few IN available. It compensates by an ice-multiplication mechanism with up to or <br />more than 1000 ice crystals per one single IN as described by Hallet and Mossop, 1974. Its <br />nature is still not proven step by step. Thus, seeding can only be effective if IN are <br /> <br />.~ <br /> <br />~ <br />