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Last modified
7/28/2009 2:41:00 PM
Creation date
4/24/2008 2:56:50 PM
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Weather Modification
Title
Snowpack, Cloud Seeding and the Colorado River
Date
1/1/1974
Weather Modification - Doc Type
Report
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<br />HOW WOSA WORKS <br /> <br />that cold, natural ice crystals <br />and little snow will fall. <br />In the 1940s, scientists found through laboratory experi- <br />ments that the particles in silver iodide smoke will act as ice <br />nuclei at temperatures considerably warmer than -20oC. These <br />artificial nuclei can produce ice crystals in a cloud having cloud <br />top temperatures between -lOoC (140F) and -20oC (-4OF). <br />Therefore, by introducing silver iodide particles of the right size <br />into these clouds, we can cause snow to fall where none would <br />have occurred naturally. <br /> <br />large numbers, <br /> <br />in <br /> <br />form <br /> <br />will not <br /> <br /> <br />How WOSA Works <br /> <br />!j <br /> <br />Cloud-Seeding <br /> <br />The silver iodide particles must reach the insides of the clouds, <br />and in the right numbers, if they are to produce snow. If there <br />are too few nuclei, a few large ice crystals will form but not <br />much snow will fall. If too many particles reach the clouds, a <br />lot of very small ice crystals will be formed that will be carried <br />over the mountain and melted back to water droplets as they <br />reach the downdrafts on the far side; they will not fall as snow. <br />The most common method of producing silver iodide <br />particles in mountainous areas is by ground-based generators. <br />On release from a ground generator the very small particles <br />spread out and rise much like smoke from a chimney. When the <br />storm conditions are favorable for seeding, the low-level winds <br />are stronger than usual and the silver iodide is rapidly dispersed. <br />The silver iodide is lifted to the higher elevations, where it is <br />effective, by the updraft at the upwind side of the mountain <br />barrier (see Figure 3) . <br />In the ground-based silver iodide generators in use today, <br />the silver iodide is usually suspended as a fine dispersion in <br />acetone. This combination is mixed under pressure with liquid <br />propane and burned to produce a silver iodide smoke. The <br />silver iodide produced by this type of generator loses its effec- <br />tiveness' as ice nuclei upon exposure to a few hours of sunlight. <br />This does not limit its effectiveness in seeding winter clouds in <br /> <br />I <br /> <br />DURING WINTER in the Upper Colorado River Basin-and <br />throughout much of the Western United States-clouds are <br />formed over the higher mountain elevations as storms pass over <br />the terrain from a westerly direction. While the storms move <br />generally eastward, the winds move in a cyclonic pattern <br />counterclockwise about the center. In general, the lower the <br />pressure, the stronger the winds around it. <br />Clouds and precipitation occur naturally in certain parts <br />of the storm circulation. A cloud layer consisting of small water <br />droplets exists in the vicinity of the low pressure storm center. <br />These droplets are normally supercooled, remaining in the form <br />of water even though they are colder than. the temperature at <br />which water normally freezes-zero degrees centigrade (OOC) <br />or 32 degrees Farenheit (320F). However, at temperatures <br />colder than -200C (-4OF), small ice crystals begin to form <br />around "seeds" or "nuclei" that are present in the atmosphere. <br />These "seeds" or "nuclei" are very small particles such as dust, <br />smoke particles, or pollen grains given off by trees and flowers. <br />Once ice crystals are formed, they tend to grow rapidly from <br />the nearby water vapor and droplets. During their growth, ice <br />crystals sometimes splinter and form additional ice particles, <br />which can also grow. As the crystals become larger, they fall <br />more rapidly and may eventually reach the ground as snow. <br />The crystals can also grow by colliding and clumping together. <br />If the top of the cloud is warmer than -20oC (-4 OF), then <br />no part of the cloud in winter storms over the mountains will be <br /> <br />/ <br /> <br />11 <br /> <br />t. <br /> <br />10 <br /> <br /> <br />'lI <br /> <br />
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