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<br />There are two natural cloud types which produce all rain and <br />other forms of precipitation: "warm clouds" and "cold clouds". <br />Rain from these clouds also are conunonly referred to as "warm cloud <br />rain" and "cold cloud rain". <br /> <br />A "warm" cloud, is one in which cloud temperatures are never <br />below freezing and does not produce ice crystals in its volume. <br />This cloud is generally characterized by the relatively slow growth <br />of cloud water droplets which eventually attain sufficient size and <br />weight to fall from the cloud then collect other cloud droplets, <br />scavenging them along their paths downward. Although this type of <br />cloud occasionally appears in Western Kansas, it doesn't playa <br />dominant role in producing precipitation here. Note: Large size <br />warm-rain drops can be important embryo sources in the production <br />of hail when they merge into sub-freezing clouds (not of the warm- <br />rain variety) ~nd get carried aloft rapidly by updrafts where they <br />can rapidly freeze, grow larger and fall as hail. <br /> <br />"Cold clouds" produce th~ most significant precipitation in <br />Western Kansas. Cold clouds must contain sub-freezing temperatures <br />before it can produce both the supercooled water droplets and ice <br />crystals needed to initiate the cold cloud precipitation process. <br />Once begun, the process which eventually produces precipitation <br />generally evolves relatively quickly in comparison to the warm <br />cloud process. <br /> <br />The prevailing hypothesis under which the WKWM Program hail <br />suppression portion operates is that hailstones grow to large sizes <br />because there are too few ice crystals formed naturally in clouds <br />during vigorous thunderstorm growth, thereby allowing relatively <br />abundant supercooled water to collect upon relatively fewer numbers <br />of ice particles or other hail embryos. Often, those particles grow <br />into hailstones too large to melt before impact at the ground. <br />Current theory is that by increasing the concentrations of ice <br />crystals within ice crystal-deficient clouds, hailstones will be <br />prevented from growing to a size large enough to damage property <br />and crops. Crop type, stage of crop growth and hail size all play <br />an important role in determining crop damage severity. There are <br />other theories about hail development and movement within clouds <br />because hail doesn't always form exactly the same way, every time, <br />in all clouds. Most long-term cloud seeding programs around the <br />world use the same hypothesis and have shown varying degrees of <br />success in both reducing crop damage and increasing rainfall. <br /> <br />The seeding agents used on the WKWM Program are silver iodide <br />and dry ice---both are delivered to the cloud by aircraft. Silver <br />iodide ice nuclei are vaporized in cloud base updrafts, whereas, <br />dry ice is dropped into growing clouds at temperature levels of <br />-lOC to -12C, or about an altitude of 20,000 feet in mid-summer. <br />Over the years, the results from' programs like ours and cloud <br />physics research have been applied to the ~~ Program keeping it <br />reasonably state-of-the-art. <br /> <br />7 <br />