Laserfiche WebLink
<br />. Provide recommendations for a future course of action. <br /> <br />2.0 Introduction to Cloud Seeding for Winter Precipitation Augmentation <br /> <br />Since a large percentage of the runoff produced in the Colorado River drainages is <br />produccd from melting sno\\: and since cloud seeding over mountain barriers to increase snowfall <br />is one of the weather modification techniques that have demonstrated the strongest evidence of <br />effectiveness. this white paper is focused on winter seeding programs. Summer cloud seeding <br />programs could be considered in somc areas. although this \..hitc paper docs not address thesc <br />possibilitics. <br /> <br />A basic summary of the concept of how cloud secding is thought to work in wintertimc <br />mountainous (orographic) settings is in order. A numbcr of observational and theoretical studies <br />ha....e suggested that therc is a cold "tempcrature window" of opportunity for cloud sceding. <br />Some information contained in a report from the Weather Modification Association (Orville et <br />a!. 2004) is paraphrased in some of the following discussions. <br /> <br />Numerous observations in the atmospherc and in the laboratory have indicated that cloud <br />water droplets can remain unfrozen at temperatures well below freezing. These droplets are <br />called supercooled. Thus thc phrase supcrcooled liquid water (SL W) has been coined to refer to <br />the presence of sllch \vater droplets in a cloud. In order for watcr droplcts lo freezc at <br />tempcratures between 30.20 F (-I{'OC) and _38.20 F (_390 C) they must come in contact with a <br />foreign particle to calise them to freeze. These particles are called freezing nuclei. The process is <br />known as heterogeneous nucleation. Such nuclei occur in naturc and are primarily composed of <br />tiny soil particles. Numerous observations around the world have indicated that the numbers of <br />naturally occurring freezing nuclei that can cause hcterogeneous nucleation are temperaturc <br />dependcnt. These natural nuclei become increasingly active with decreasing temperatures. Once <br />a sllpereooled watcr droplet is fro/.en. creating an ice crystal. it will grow through vapor <br />deposition from the water droplets surrounding it and. given the right conditions continue to <br />grow through vapor deposition and possibly also aggregation (collection ofwatcr droplets on a <br />snowflake as it falls) to fonn a snowflake large cnough to fall from the cloud and reach the <br />ground. Supercooled \vater droplets in sufficient quanti tics arc the targets of opportunity in order <br />to incrcase prccipitation through seeding. <br /> <br />Studies of both orographic and convective clouds have suggestcd that clouds whose tops <br />are colder than - _130 F (-250C) have sufficiently large concentrations of natural ice crystals such <br />that seeding will have no effect on precipitation (Grant and Elliott. 1974: Grant. 1986: Gagin and <br />Neumann. 1981: Gagin et al.. 1985). There arc also indications that there is a wann temperature <br />limit to seeding effectiveness (Gagin and Neumann. 1981: Grant and Elliott. 1974: Cooper and <br />Lawson. 1984). This is believed to be due to the low efficiency of ice crystal production by <br />silver iodide (the most commonly used seeding material) at temperatures greater than 230 F (-5 0 <br />C). and to the slO\v rates of ice cr)'stal vapor deposition growth at comparatively wann <br />temperatures. Thus there appears to be a '>temperature \vindow" of abolll130 F (_50C) to _130 F (- <br />250C) where clouds respond favorably to silver iodide seeding (i.e.. exhibit seedahility). Dry ice <br />(frozen carbon dioxide) seeding via aircraft can extend this temperature window to temperatures <br />just bdo\v 320 F (OOC). Sceding by venting liquid propane may also present the potential to <br />expand this window to approximately _20 C. <br />