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<br />~- <br /> <br />~r- - - - <br /> <br />[ <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />! <br /> <br />cloud seeding can be expected to increase the amount of precipita- <br />tion produced when: (1) there is an abundance of moisture in the <br />form of supercooled liquid water; (2) there is a deficiency of <br />natural ice nuclei; (3) the air is slightly to moderately unstable; <br />and (4) the time required for growth and fallout of the precipita- <br />tion is within a range of wind speeds which allows it to fall into <br />the target area. The routine identification of the seedability <br />indices requires further development for practical field use. The <br />bounds for each of the necessary criteria need to be refined con- <br />sidering the specific conditions encountered in each area where <br />seeding operations are planned or are being conducted. The <br />refinement of the bounds for favorable seeding conditions requires <br />that atmospheric data be collected and analyzed in conjunction <br />with cloud seeding programs. <br />Present methods used to seed winter orographic clouds are <br />aircraft and ground-base artifical ice nucleus generators. <br />Aircraft provide a flexible means of seeding, allow the use of <br />either dry ice or artificial ice nuclei, and should result in <br />very accurate targeting of the seeding material. However, aircraft <br />seeding is considerably more expensive than ground-base seeding, <br />the portion of the orographic clouds actually seeded is very <br />limited and there is a high probability of over-seeding if the <br />seeding material is released too near the target area. There <br />also exists severe operational flying restraints including air-' <br />craft icing problems, the proximity of the flight path to moun- <br />tains, and scarcity of navigation aids near the mountain areas <br />to be seeded. <br />Up to the present time, the most prevalent mode of seeding <br />orograhic clouds has been the use of a network of artificial <br />ice nucleus generators on the ground located upwind of the <br />mountain barrier. Turbulence associated with wind flow has been <br />relied upon to disperse the nuclei throughout the clouds over the <br />target area. However, targeting uncertainties exist with ground- <br />based seeding due to a combination of atmospheric stability and <br />wind variation with height. Large mountain barriers distort the <br />flow of air, even to the point of forcing the low-level wind to <br /> <br />-13- <br /> <br />~- <br />