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<br />or by the passage of a cold or a warm <br />front through a given area. <br /> <br />The "life span" of most cumulus clouds <br />generally is about 20 to 30 minutes. They <br />yield little precipitation, because nature <br />provides too little time for the cloud drop- <br />lets to coalesce into drops large enough to <br />fall. <br /> <br />Two seeding techniques are used to pro- <br />mote rainfall from these clouds: Silver <br />iodide (ice-phase) seeding, and hygro- <br />scopic seeding. <br /> <br /> <br />SILVER IODIDE SEEDING is used <br />when the cloud-top temperature of the <br />cumulus formation is 230 to 260 F or <br />colder, or when the cloud gives indications <br />that it will shortly reach that temperature <br />level in the atmosphere. The primary ob- <br />jective here is to initiate the dynamic heat <br />release which will stimulate cloud growth. <br />If cloud updrafts are slow, the objective <br />then is the same as for mountain winter <br />storms. That is, to increase the number <br />of ice crystals in the supercooled part of <br />the cloud to the optimum concentration <br />for the given conditions. The ice crystals <br />will grow rapidly at the expense of the <br />cloud droplets, and become large and <br />heavy enough to fall through the cloud <br />andmelt into raindrops. <br /> <br />1- j 'J~~"~'-"o" '-~'"' '''''l'r'~''T "I' r -,"v'_ <br />I,,' f1 ' ,11 fN'., ,- '~~'_t,..' 'I' 'II <br />I I lJ&fJl' Tflt~,_"'{Jf!1}~~C'lPlC;: -', <br />II.' I , I" 'fIl~ll!f/~/~~~~_! ' ;:, <br /> <br />\ [I 1 ; li', "";;(, '.' , .: <br /> <br />IN HYGROSCOPIC SEEDING, such ma- <br />terials as common salt and a mixture of <br />ammonium nitrate-urea may'be used. <br />Ti}~y~J12lV~, 'the"unique ability" tQ';;--absorb;, <br />wlter vapor from the surrounding afr.' <br /> <br />21 <br /> <br />Finely ground particles of hygroscopic ma- <br />terial, released into the updraft beneath <br />a growing cloud, will grow large enough <br />by condensation, then by coalescence with <br />cloud droplets will produce rainfall. <br /> <br /> <br />This process is regarded as particularly <br />effective in clouds with strong updrafts <br />that might otherwise be inefficient precipi- <br />tators. Left alone, these clouds would blow <br />much of the water out of the top where an <br />ice crystal (cirrus) cloud forms and holds <br />aloft much of the water that has been <br />processed. Hygroscopic seeding usually <br />forms large drops much lower in the cloud, <br />and at warmer temperature levels where <br />there is the greatest supply of moisture. <br />The result is that more of the cloud's <br />water content reaches the ground as rain. <br /> <br />~:;,~~ <br />