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<br />Two cloud types produce all precipitation: "warm clouds" and "cold clouds". A "warm" <br />cloud, is one in which nowhere in its volume is its temperature below freezing producing ice <br />crystals. The warm cloud is characterized most often by relatively slow growth. Cloud water <br />droplets eventually may grow to a sufficient size and weight to fallout of the cloud if given <br />enough time. While falling, cloud droplets collect other cloud droplets by scavenging them along <br />their downward paths. Although this type of cloud occasionally appears in Western Kansas, it's <br />not the dominant type of cloud producing precipitation here. However, large size warm-rain <br />drops can be important embryo sources in the production of hail when they merge into the sub- <br />freezing cold clouds and are carried aloft rapidly by updrafts whereupon they freeze and grow <br />larger into hail. <br /> <br />Most important to Western Kansas is the "cold" cloud. Cold clouds have a portion of their <br />volume that has grown into a temperatures below freezing. When these clouds form, it is the <br />interaction between the supercociled water drops and ice crystals within the cloud that initiates the <br />process most responsible for producing significant precipitation in Western Kansas. Also, cold <br />clouds tend to be much taller than warm clouds and, therefore, make considerably more moisture <br />available to the eventual precipitation process developing in the cloud. <br /> <br />The widely-accepted hypothesis under which the WKWMP hail suppression operates is <br />called "beneficial competition." The reason hailstones grow to large sizes in thunderstorms is <br />due to the lack of sufficient numbers of IN particles while developing. That insufficiency of IN <br />particles capable of converting to ice crystals causes relatively abundant supercooled cloud water <br />. to collect upon the relatively few numbers of ice crystals formed. Too often those ice particles <br />grow into hailstones so large they can't melt before reaching the earth's surface. By vastly <br />increasing the concentration of ice crystals within these ice crystal-deficient clouds, competition <br />for available cloud water strongly increases, thereby preventing hailstones from growing to sizes <br />large enough to damage crops and property. Hail growth and movement within storms, especially <br />very severe ones, can be very complex. Hail damage is determined by the type of property or <br />crop, what stage of growth the crop was in, hail size and whether or not it was wind-driven. <br /> <br />Most, if not all, long-term hail suppression programs use a similar hypothesis. In 1994 the <br />Kansas Water Office published the most recent evaluation ofthe WKWMP which found a 27% <br />reduction of crop-hail damage, statistically significant at the 5% level, and a Benefit-to-Cost ratio <br />of 3 7.: 1. That.is for every $1 funding the program, $37 is returned to it in increased crop value. <br /> <br />The hail suppression seeding agents used on the WKWM Program either are dry ice or <br />contain silver iodide in its formulation and are delivered to growing clouds by aircraft. Silver <br />iodide (AgI) seeding agents are vaporized in updrafts at cloud base then lifted into the cloud <br />through natural updraft action. However, dry ice is dropped directly into growing clouds at -IOC, <br />about 20,OOOJeet in mid-summer. AgI-based seeding agents produce IN which promote ice <br />crystals to form through heterogeneous nucleation---when ice crystal numbers change (increase) <br />with decreasing temperatures. This contrasts to homogeneous nucleation-nthe instantaneous <br />change of water droplets to ice crystals at all sub-freezing temperatures when contacting dry ice. <br /> <br />5 <br />