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<br />\.\.~u <br /> <br />. <br /> <br />. <br /> <br />the Weather Bureau published a short article in the Monthly <br />Weather Revie" ren..cting their negative attitude toward <br />Melbourne and other rainmakers. Melbourne never revealed <br />the constituents of his rainmaking formula. <br /> <br />Charles Hatfield, a contemporary of Melbourne, also was <br />hired by Californians to ease their water problems. He was <br />sued when too much water caused a dam to break. <br /> <br />Another attempt at rainmaking by explosio,' \Vas made during <br />the prolonged drought in New Zealand in 1906-{)? Explosives <br />were fired from clynamite detonators during conditions favor- <br />able for rain fonnulation. The results were inconclusive. <br />The energy produced by the detonation was very small compared <br />with the amount of heat exchange in precipitation formation <br />and other atmospheric processes. <br /> <br />In a 1930 experiment in Holland, Aug. W. Vera art dropped dry <br />ice, among other things, ll1to supercooled clouds. This must <br />have produced ice crystala, though he apparently was not <br />looking for this particular effect. <br /> <br />Undoubtedly, many other techniques have been used in attempt- <br />ing to control or modify the weather. Bergeron foresaw in <br />1938 the possibility of producing rain and suppressing hail <br />from supercooled clouds by the introduction of artificial <br />aublimation nuclei. But it was not until the 1940's that <br />methods based on a knowledge of the physical procesaes of <br />rain formation were used. <br /> <br />Modern teclmiques of cloud seeding are based on three primary <br />assumptions: (1) Either the presence of ice cryatals in a <br />supercooled cloud is necessary to release rain or the presence <br />of comparatively large water droplets ia essential to initiate <br />the coalescence mechanism; (2) some clouds precipitate <br />inefficiently or not at all because these agents are naturally <br />deficient; (3) this deficiency can be remedied by seeding the <br />clouds artificially with an agent designed to produce ice <br />crystals, or by introducing water droplets or large hygroscopic <br />nuclei. <br /> <br />The first assumption reaul ts from the two iD06t prevalent theories <br />of precipitation formation--the Bergeron-Findeisen ice crystal <br />process and the coalescence process. Studies have shown that <br />cloud droplets can exist, under maQy atmospheric conditions, <br />in the liquid state at temperatures much colder than O. C. <br />This is called supercooling. When an ice crystal is introduced <br />into a cloud canposed of supercooled droplets the ice crystal <br />grows at the expense of the droplet. The equilibrium vapor <br /> <br />4 <br />