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<br /> <br />O.o23H <br /> <br />. <br /> <br />. <br /> <br />orographic rainfall occurs in large enough volumes that a 10% increase <br /> <br />r <br /> <br />would be significant. This is not necessarily true with convective <br /> <br /> <br />storms. For example, in most of the arid and semiarid Southwest, a 100% <br /> <br /> <br />increase from .05 to .10 inch from a convective cell would be insignif- <br />, <br /> <br />icant because of high evaporation rates. <br />[,lost atmospheric scientists feel that the present state-of-the-art <br />is too uncertain to seed isolated large convective systems (including <br />squall lines), and the opinions are about evenly divided as to whether <br />such seeding will ever be safely operational. There are several uncer- <br /> <br />tainties in seeding larger convective systems including the fear of increas- <br /> <br /> <br />"ing intense rain or hail damage. Also, experimental design, instrumenta- <br /> <br />tion, and analysis of efforts to modify such systems are extremely costly, <br /> <br /> <br />so information on modification of such systems is lacking. <br /> <br />Droughts <br />There are basically two types of drought--one occurring from several <br />years of below average precipitation, and the other from several dry weeks' <br />during, for example, a critical growing period. Long term droughts are of <br /> <br />greatest interest in regions such as the Pacific-Southwest, where most <br /> <br />irrigation and municipal water supplies come from'snowme1t. Augmentation <br /> <br /> <br />of winter snowfall should help to alleviate moderate long term droughts, <br /> <br /> <br />but water users should understand the magnitude of possible increases <br /> <br /> <br />and the uncertainty that future technology will increase this magnitude. <br /> <br /> <br />Also, the question of whether or not we can increase snowfall from a given <br /> <br />storm system as it crosses each of several mountain ranges has not been <br /> <br />satisfactorily answered. <br />flost atmospheric scientists do not believe that convective rainfall <br /> <br />5 <br />